Fatigue crack growth testing method, and hollow test piece

Abstract

Provided is a method for testing the growth of a fatigue crack formed in the inner surface of a hollow test piece having a hollow portion into which a fluid can be introduced, the method comprising: a first fatigue crack growth step for causing the fatigue crack to grow by repeatedly applying a load to the hollow test piece under first environmental conditions; a second fatigue crack growth step for causing the fatigue crack to grow by repeatedly applying a load to the hollow test piece under second environmental conditions in which at least one of the type, pressure, and temperature of a test fluid with which the inside of the hollow portion is filled differs from that of the first environmental conditions; and a crack growth amount acquisition step for acquiring the amount of growth of the fatigue crack in each of the first fatigue crack growth step and the second fatigue crack growth step by observing the fatigue crack formed in the hollow test piece.

Description

Fatigue crack growth test method and hollow test specimen

[0001] The present disclosure relates to a fatigue crack propagation test method and a hollow test specimen. This application claims priority based on Japanese Patent Application No. 2023-188468 filed with the Japan Patent Office on November 2, 2023, and Japanese Patent Application No. 2023-188447 filed with the Japan Patent Office on November 2, 2023, the contents of which are incorporated herein by reference.

[0002] An autoclave-type fatigue testing machine (for example, Patent Document 1) is known that performs a fatigue test in a state where a test piece, which is a sample for the fatigue test, is placed in a pressure vessel filled with a gas such as hydrogen gas. When performing a crack propagation test in hydrogen gas using an autoclave-type fatigue testing machine, the amount of hydrogen used is large, making it difficult to carry out the test, and therefore the test period is limited.

[0003] Special table 2014-521103 publication

[0004] Recently, a material testing method using hollow specimens with a hollow section in a hydrogen gas environment has been proposed. This material testing method allows material testing to be performed with the inner surface exposed to hydrogen gas by filling the hollow specimen with hydrogen gas. While this material testing method has been used in slow strain rate tensile tests and fatigue tests, it has not been used in hydrogen gas fatigue crack propagation tests. In hydrogen gas fatigue crack propagation tests using hollow specimens, cracks initiate and propagate on the inner surface of the hollow specimen, making it impossible to quantify crack propagation based on changes in compliance, a parameter that indicates the crack side length or crack opening, as used in conventional CT test specimens. Furthermore, using multiple specimens with different environmental conditions results in variations in fatigue pre-crack depth, which means that different stress intensity factors are used for tests under the same loading conditions, making it difficult to compare test results under different environmental conditions.

[0005] In view of the above circumstances, at least one embodiment of the present disclosure aims to provide a fatigue crack propagation testing method that can quantify the amount and rate of fatigue crack propagation in a hollow test specimen under different environmental conditions, as well as the acceleration rate of crack propagation due to environmental influences.

[0006] A fatigue crack propagation test method according to at least one embodiment of the present disclosure is a method for testing the propagation of fatigue cracks formed on the inner surface of a hollow test piece having a hollow portion into which a fluid can be introduced, and comprises: a first fatigue crack propagation step in which a load is repeatedly applied to the hollow test piece under first environmental conditions to cause a fatigue crack to propagate; a second fatigue crack propagation step in which a load is repeatedly applied to the hollow test piece under second environmental conditions in which the first environmental conditions differ in at least one of the type, pressure, or temperature of the test fluid filled inside the hollow portion; and a crack propagation amount acquisition step in which the propagation amount of the fatigue crack in each of the first fatigue crack propagation step and the second fatigue crack propagation step is acquired by observing the fatigue crack formed in the hollow test piece.

[0007] A hollow test piece according to at least one embodiment of the present disclosure is a hollow test piece extending along a longitudinal direction, and includes: a one-side large diameter portion formed on one side in the longitudinal direction; an other-side large diameter portion formed on the other side in the longitudinal direction; and a parallel portion formed between the one-side large diameter portion and the other-side large diameter portion, the parallel portion having an outer diameter smaller than the one-side large diameter portion and the other-side large diameter portion, wherein the parallel portion has an outer peripheral surface, an inner peripheral surface, and a stress concentration portion formed on the inner peripheral surface and extending circumferentially of the inner peripheral surface, and wherein, when the inner diameter of the parallel portion is defined as D1 and the length of the parallel portion in the longitudinal direction is defined as L1, the length L1 of the parallel portion satisfies the condition 0.5×D1≦L1≦3×D1.

[0008] According to at least one embodiment of the present disclosure, a fatigue crack propagation testing method is provided that can quantify the amount and rate of fatigue crack propagation in a hollow test specimen under different environmental conditions, as well as the acceleration rate of crack propagation due to environmental influences.

[0009] FIG. 1 is a flow diagram of a fatigue crack growth test method according to an embodiment of the present disclosure. FIG. 2 is a schematic cross-sectional view along the central axis direction of a hollow test specimen that is a test subject of the fatigue crack growth test method according to an embodiment of the present disclosure. FIG. 3 is an A-A cross-sectional view of the hollow test specimen shown in FIG. 2. FIG. 4 is a schematic diagram of a fatigue test system used in the fatigue crack growth test method according to an embodiment of the present disclosure. FIG. 5 is an explanatory view for explaining the fatigue crack growth test method according to an embodiment of the present disclosure. FIG. 6 is an explanatory view for explaining the relationship between the stress intensity factor range and the crack growth rate obtained by the fatigue crack growth test method according to an embodiment of the present disclosure. FIG. 7 is an explanatory view for explaining a crack growth amount acquisition step in the fatigue crack growth test method according to an embodiment of the present disclosure. FIG. 8 is an explanatory view for explaining the fatigue crack growth test method according to an embodiment of the present disclosure. FIG. 9 is an explanatory view for explaining the fatigue crack growth test method according to an embodiment of the present disclosure. FIG. 10 is an explanatory view for explaining the fracture surface coloring treatment in the fatigue crack growth test method according to an embodiment of the present disclosure. FIG. 11 is an explanatory view for explaining the fatigue crack growth test method according to an embodiment of the present disclosure. FIG. 14 is a schematic cross-sectional view of the notched portion of the hollow test piece shown in FIG. 13 along the central axis direction of the hollow test piece.

[0010] Hereinafter, several embodiments of the present disclosure will be described with reference to the accompanying drawings. However, the dimensions, materials, shapes, relative arrangements, etc. of components described as embodiments or shown in the drawings are merely illustrative examples and are not intended to limit the scope of the present disclosure.

[0011] Fig. 1 is a flow diagram of a fatigue crack growth test method according to an embodiment of the present disclosure. Fig. 2 is a schematic cross-sectional view along the central axis CA of a hollow test specimen 1 that is the test subject of the fatigue crack growth test method according to an embodiment of the present disclosure. Fig. 3 is a cross-sectional view taken along the line A-A of the hollow test specimen 1 shown in Fig. 2. The fatigue crack growth test method according to some embodiments is a method for testing the growth of a fatigue crack formed on the inner peripheral surface (inner surface) 12 of a hollow test specimen 1 (see Fig. 2) that has a hollow portion 11 into which a fluid can be introduced.

[0012] (Hollow Test Specimen) As shown in Figures 2 and 4, the hollow test specimen 1 has a longitudinal direction along the direction in which the central axis CA of the hollow test specimen 1 extends. The hollow test specimen 1 is formed in a cylindrical shape extending along the longitudinal direction, and a hollow portion 11 in which a fluid can exist is formed inside the hollow test specimen 1. The hollow portion 11 is defined by an inner circumferential surface 12. Hereinafter, the radial direction of the hollow test specimen 1 may be simply abbreviated as the radial direction.

[0013] 2 and 4, the hollow test piece 1 includes one end portion 2 formed on one side in the longitudinal direction (lower side in the drawings), the other end portion 3 formed on the other side in the longitudinal direction (upper side in the drawings), and a parallel portion 4 formed in the longitudinal direction between the one end portion 2 and the other end portion 3. In the illustrated embodiment, the parallel portion 4 has a smaller outer diameter than the one end portion 2 and the other end portion 3.

[0014] In the illustrated embodiment, the hollow specimen 1 has a notch 13 formed on the inner peripheral surface (inner surface) 42 of the parallel portion 4, extending circumferentially of the hollow specimen 1, and a fatigue pre-crack 14 extending from the notch 13 toward the radially outward direction of the hollow specimen 1. In the embodiment shown in Figures 2 and 3, the notch 13 is an annular groove extending circumferentially of the hollow specimen 1. However, in other embodiments, the notch 13 may not be annular, and the shape of the notch 13 is not limited to the illustrated example. The fatigue pre-crack 14 is a crack formed by repeated application of a load L to the hollow specimen 1, and extends radially outward from the outer periphery of the notch 13, which is a stress concentration portion. The fatigue pre-crack 14 may be formed in an annular shape extending circumferentially of the hollow specimen 1, as shown in Figure 3.

[0015] The hollow test specimen 1 is used for fatigue crack propagation tests and is attached to a fatigue testing machine 110. Specifically, the hollow test specimen 1 is supported on both longitudinal sides by the fatigue testing machine 110, and a load L is applied along the longitudinal direction by the fatigue testing machine 110. In the illustrated embodiment, the hollow test specimen 1 attached to the fatigue testing machine 110 is arranged so that the longitudinal direction is along the vertical direction, with one end 2 located below in the vertical direction and the other end 3 located above in the vertical direction.

[0016] (Fatigue Testing System) Figure 4 is a schematic diagram of a fatigue testing system 100 used in a fatigue crack propagation testing method according to an embodiment of the present disclosure. As shown in Figure 4, the fatigue testing system 100 includes the hollow test piece 1 described above, the fatigue testing machine 110 described above, and a fluid switching device 120 configured to be able to switch the fluid filled in the hollow portion 11 of the hollow test piece 1.

[0017] 4, the fatigue testing machine 110 includes a one-side support section 111, an other-side support section 112, a load applying section (actuator) 113, and a load cell 114. The one-side support section 111 supports one longitudinal side of the hollow test piece 1, specifically, one end section 2. The other-side support section 112 supports the other longitudinal side of the hollow test piece 1, specifically, the other end section 3.

[0018] The load applying section 113 is configured to apply a load L to the hollow test piece 1 along the longitudinal direction, for example, by separating at least one of the one-side support section 111 and the other-side support section 112 from the other. In the illustrated embodiment, the load L is applied to the hollow test piece 1 by the load applying section 113 toward one side in the longitudinal direction, i.e., toward the downward vertical direction. The load cell 114 is configured to measure the load L applied to the hollow test piece 1 by the load applying section 113. The load cell 114 is configured to convert the load L applied to the hollow test piece 1 into an electrical signal.

[0019] (Fluid Switching Device) In some embodiments, the fluid switching device 120 includes a first fluid inlet line 121 for introducing a first fluid into the hollow portion 11, a second fluid inlet line 122 for introducing a second fluid into the hollow portion 11, and a fluid outlet line 123 for discharging fluids from the hollow portion 11. One ends of the first fluid inlet line 121, the second fluid inlet line 122, and the fluid outlet line 123 are connected to the hollow portion 11. The other end of the first fluid inlet line 121 is connected to a first tank 124 that stores the first fluid. The other end of the second fluid inlet line 122 is connected to a second tank 125 that stores the second fluid. The other end of the fluid outlet line 123 is connected to a vacuum pump 126 that sucks fluids from the hollow portion 11.

[0020] In the illustrated embodiment, the first fluid inlet line 121 and the second fluid inlet line 122 join at a first joining point P1, and the downstream side (one end side) of the first joining point P1 forms a shared line 127. The fluid discharge line 123 joins with the shared line 127 at a second joining point P2.

[0021] In the illustrated embodiment, the fluid switching device 120 further includes a discharge line 128 for discharging the fluid from the shared line 127 to the outside of the fluid switching device 120, and a compressor 129 for pressurizing the fluid guided to the hollow portion 11. In the embodiment shown in Fig. 4, the discharge line 128 has one end connected to the shared line 127 at the second junction P2 and the other end open to the atmosphere. The compressor 129 is provided on the shared line 127 upstream of the second junction P2.

[0022] When the test fluid is a first fluid, the first fluid is introduced into the hollow portion 11 from a first tank 124 through a first fluid introduction line 121. When the test fluid is a second fluid, the second fluid is introduced into the hollow portion 11 from a second tank 125 through a second fluid introduction line 122. When the test fluid is air, the other end of the fluid introduction line (121 or 122) for introducing air into the hollow portion 11 may be open to the atmosphere. When switching the test fluid, it is preferable to drive a vacuum pump 126. The fluid in the hollow portion 11 is drawn into a fluid discharge line 123 by the negative pressure of the vacuum pump 126 and discharged to the outside of the fluid switching device 120 through the fluid discharge line 123. By removing the test fluid before switching from the hollow portion 11 by vacuuming, the purity of the test fluid in the hollow portion 11 after switching can be increased, and the influence of the environment (test fluid) on the crack propagation behavior described below can be appropriately controlled.

[0023] As shown in Fig. 4, a strain measuring device (e.g., a strain gauge) 130 may be attached to the outer surface of the hollow test specimen 1 (in the illustrated example, the outer peripheral surface 41 of the parallel portion 4). As shown in Fig. 4, a pressure measuring device (e.g., a pressure gauge) 140 may be used to monitor the pressure of the fluid introduced into the hollow portion 11. In the embodiment shown in Fig. 4, the pressure measuring device 140 is provided downstream (on one end side) of the compressor 129 of the shared line 127.

[0024] (Fatigue crack propagation test method) As shown in FIG. 1, the fatigue crack propagation test method includes a first fatigue crack propagation step S10, a second fatigue crack propagation step S20, and a crack propagation amount acquisition step S30.

[0025] (First Fatigue Crack Propagation Step) Figure 5 is an explanatory diagram for explaining a fatigue crack propagation test method according to one embodiment of the present disclosure. Figure 5 shows a graph of a test waveform with time T on the horizontal axis and load L on the vertical axis. As shown in Figure 5, in the first fatigue crack propagation step S10, a load L is repeatedly applied to the hollow test specimen 1 under first environmental conditions to propagate a fatigue crack. In the first fatigue crack propagation step S10, the number of repetitions of the load L applied to the hollow test specimen 1 by the fatigue testing machine 110 is defined as N1.

[0026] 5, in the second fatigue crack propagation step S20, a load L is repeatedly applied to the hollow test specimen 1 under second environmental conditions to propagate a fatigue crack. In the second fatigue crack propagation step S20, the number of repetitions of the load L applied to the hollow test specimen 1 by the fatigue testing machine 110 is defined as N2. The second environmental conditions differ from the first environmental conditions in at least one of the type, pressure, and temperature of the test fluid filled inside the hollow portion 11. In the illustrated embodiment, the second fatigue crack propagation step S20 is performed after the first fatigue crack propagation step S10, but it may also be performed before the first fatigue crack propagation step S10.

[0027] As shown in FIG. 1, in the fatigue crack propagation test method, a combination of the first fatigue crack propagation step S10 and the second fatigue crack propagation step S20 may be performed multiple times.

[0028] 6 is an explanatory diagram for explaining the crack growth amount acquisition step S30 in the fatigue crack growth testing method according to an embodiment of the present disclosure. In the crack growth amount acquisition step S30, the fatigue crack growth amounts A1 and A2 in the first fatigue crack growth step S10 and the second fatigue crack growth step S20, respectively, are acquired by observing the fatigue crack formed in the hollow test piece 1.

[0029] Figure 5 schematically shows the fracture surface of the hollow test specimen 1. Figure 6 schematically shows an SEM image of the fracture surface of the hollow test specimen 1 generated by a scanning electron microscope (SEM). In Figures 5 and 6, the fracture surface area of ​​the fatigue pre-crack 14 is designated F0, the fracture surface area caused by the fatigue crack propagation in the first fatigue crack propagation step S10 is designated F1, and the fracture surface area caused by the fatigue crack propagation in the second fatigue crack propagation step S20 is designated F2.

[0030] The fracture boundary formed between the fracture surface region F1 and a fracture surface region (e.g., F0) located radially inward of the fracture surface region F1 and adjacent to the fracture surface region F1 is designated B0. The fracture boundary formed between the fracture surface region F1 and a fracture surface region F2 located radially outward of the fracture surface region F1 and adjacent to the fracture surface region F1 is designated B1. The fracture boundary formed between the fracture surface region F2 and a fracture surface region (e.g., F1) located radially outward of the fracture surface region F2 and adjacent to the fracture surface region F2 is designated B2. By changing the environmental conditions of the first fatigue crack propagation step S10 and the second fatigue crack propagation step S20, the fracture surface appearance of the first fatigue crack propagation step S10 and the second fatigue crack propagation step S20 changes, and the fracture surface regions F0, F1, and F2 and the fracture boundaries B0, B1, and B2 appear in the SEM image.

[0031] The fatigue crack propagation amount A1 in the first fatigue crack propagation step S10 is the radial length of the fractured surface region F1, i.e., the radial distance between the fracture boundary B0 and the fracture boundary B1. The fatigue crack propagation amount A1 can be determined, for example, by measuring the radial distance between the fracture boundary B0 and the fracture boundary B1 that appear in an SEM image. Note that the fatigue crack propagation amount A1 may also be the average value of the radial distances between the fracture boundary B0 and the fracture boundary B1 measured at multiple locations around the circumference of the hollow test piece 1.

[0032] The fatigue crack propagation amount A2 in the second fatigue crack propagation step S20 is the radial length of the fractured surface region F2, i.e., the radial distance between the fracture boundary B1 and the fracture boundary B2. The fatigue crack propagation amount A2 can be determined, for example, by measuring the radial distance between the fracture boundary B1 and the fracture boundary B2 that appear in an SEM image. Note that the fatigue crack propagation amount A2 may also be the average value of the radial distances between the fracture boundary B1 and the fracture boundary B2 measured at multiple locations around the circumference of the hollow test piece 1. Note that a metallurgical (optical) microscope can also be used for fracture surface observation instead of a scanning electron microscope (SEM).

[0033] The fatigue crack growth rate R1 in the fatigue crack growth step S10 is calculated by dividing the fatigue crack growth amount A1 by the number of repetitions N1. The fatigue crack growth rate R2 in the fatigue crack growth step S20 is calculated by dividing the fatigue crack growth amount A2 by the number of repetitions N2.

[0034] A fatigue crack propagation test method according to some embodiments includes the first fatigue crack propagation step S10, the second fatigue crack propagation step S20, and the crack propagation amount acquisition step S30. In the crack propagation amount acquisition step S30, fatigue cracks formed in the hollow test specimen 1 are observed to acquire fatigue crack propagation amounts A1 and A2 for each of the fatigue crack propagation steps S10 and S20. The fatigue crack propagation rates R1 and R2 for each fatigue crack propagation step S10 and S20 can then be calculated from the fatigue crack propagation amounts A1 and A2 for each fatigue crack propagation step S10 and S20 and the number of cycles N1 and N2. Therefore, the above method can quantify the fatigue crack propagation amounts A1 and A2 and the propagation rates R1 and R2 for the same hollow test specimen 1 under multiple different environmental conditions. Furthermore, the acceleration rate AR of the fatigue crack propagation rate due to changes in environmental conditions can also be quantified.

[0035] The acceleration rate AR of the fatigue crack growth rate may be a ratio calculated from the growth rate R1 and the growth rate R2. Alternatively, the acceleration rate AR of the fatigue crack growth rate may be calculated by the following method. FIG. 7 is an explanatory diagram illustrating the relationship between the stress intensity factor range ΔK and the crack growth rates R1 and R2 obtained by a fatigue crack growth test method according to an embodiment of the present disclosure. FIG. 7 shows a graph with the stress intensity factor range ΔK on the horizontal axis and the crack growth rates R1 and R2 on the vertical axis. The stress intensity factor range ΔK is calculated from the fatigue crack depth and the loading conditions. The graph in FIG. 7 depicts a first straight line L1 showing the relationship between the crack growth rate R1 and the stress intensity factor range ΔK, and a second straight line L2 showing the relationship between the crack growth rate R2 and the stress intensity factor range ΔK. The first straight line L1 can be calculated from multiple crack growth rates R1 and stress intensity factor ranges ΔK obtained by performing the first fatigue crack growth step S10 multiple times. The second line L2 can be calculated from multiple crack growth rates R2 obtained by performing the second fatigue crack growth step S20 multiple times and the stress intensity factor range ΔK. The acceleration rate AR of the fatigue crack growth rate may be a ratio calculated from the first line L1 and the second line L2.

[0036] (Fracture surface observation) In some embodiments of the fatigue crack propagation test method, in the crack propagation amount acquisition step S30 described above, the fracture surface of the hollow test piece 1 is observed to obtain the fatigue crack propagation amounts A1 and A2 in the first fatigue crack propagation step S10 and the second fatigue crack propagation step S20, respectively.

[0037] According to the above method, the fatigue crack propagation in each of the multiple fatigue crack propagation steps S10, S20 appears as stripes or the like on the fracture surface of the hollow test specimen 1, and the fatigue crack propagation region (fracture surface region) for each environment can be identified by observing the fracture surface. Therefore, in the crack propagation amount acquisition step S30, the fatigue crack propagation amounts A1, A2 in each of the multiple fatigue crack propagation steps S10, S20 can be quantified by observing the fracture surface of the hollow test specimen 1.

[0038] In some embodiments of the fatigue crack propagation testing method, the second environmental conditions may differ from the first environmental conditions in at least the type of test fluid, and the second environmental conditions may be the same as the first environmental conditions in the pressure and temperature of the test fluid, or may differ from the first environmental conditions in at least one of the pressure and temperature of the test fluid.

[0039] According to the above method, it is possible to quantify the fatigue crack growth amounts A1, A2 and growth rates R1, R2 of the same hollow test specimen 1 under multiple environmental conditions in which different types of test fluid are filled inside the hollow portion 11.

[0040] In some embodiments of the fatigue crack growth test method, the type of test fluid under the first environmental condition is hydrogen gas, and the type of test fluid under the second environmental condition is an inert gas or air.

[0041] The above method makes it possible to quantify the fatigue crack growth amounts A1 and A2 and growth rates R1 and R2 for the same hollow test specimen 1 when the test fluid is hydrogen gas and when the test fluid is an inert gas or air. This also makes it possible to quantify the acceleration rate of the fatigue crack growth rate when the test fluid is hydrogen gas, relative to when the test fluid is an inert gas or air. The fatigue crack growth rate R1 in Figure 7 represents the case when the test fluid is hydrogen gas, and the fatigue crack growth rate R2 in Figure 7 represents the case when the test fluid is an inert gas or air. In this case, the fatigue crack growth acceleration rate AR described above is the acceleration rate of the fatigue crack growth rate when the test fluid is hydrogen gas, relative to when the test fluid is an inert gas or air.

[0042] In some embodiments of the fatigue crack growth test method, the type of test fluid under the first environmental condition is an aqueous solution (environmental aqueous solution) that affects the fatigue crack growth rate of the hollow test specimen 1, and the type of test fluid under the second environmental condition is an inert gas or air. Examples of the environmental aqueous solution include water used in nuclear environments, such as cooling water for cooling a nuclear reactor, and seawater containing salt.

[0043] In some embodiments of the fatigue crack propagation test method, the type of test fluid under the first and second environmental conditions is hydrogen gas. The second environmental conditions differ from the first environmental conditions in at least one of the pressure and temperature of the test fluid. When the test fluid is hydrogen gas, the crack propagation rate and fracture surface appearance change with changes in the temperature and pressure of the hydrogen gas. Therefore, even if the test fluid is limited to hydrogen gas, the fatigue crack propagation amounts A1 and A2 and the propagation rates R1 and R2 under each environmental condition can be quantified by changing the temperature and pressure.

[0044] 8 and 9 are explanatory views for explaining a fatigue crack propagation test method according to an embodiment of the present disclosure. In some embodiments, the first fatigue crack propagation step S10 includes a first fluid retention step S12 of retaining a test fluid under first environmental conditions inside the hollow portion 11 for a predetermined period of time before a loading step S11 of repeatedly applying a load L to the hollow test specimen 1, as shown in FIGS.

[0045] According to the above method, by holding the test fluid (first fluid) under the first environmental conditions inside the hollow portion 11 for a predetermined period in the first fluid holding step S12, a larger amount of the first fluid can be dissolved in the hollow test piece 1 than when the first fluid holding step S12 is not performed. As a result, in the first fatigue crack propagation step S10, fatigue crack propagation is carried out in a manner that more fully reflects the influence of the first fluid, and therefore the fatigue crack propagation amount A1 and propagation rate R1 quantified in the crack propagation amount acquisition step S30 more fully reflect the influence of the first fluid.

[0046] In some embodiments, a heating device (e.g., a heater) 150 is attached to the outer peripheral surface 41 of the parallel portion 4 of the hollow test specimen 1. The parallel portion 4 is heated by the heating device 150, thereby heating the fluid inside the hollow portion 11. In the first fluid holding step S12, the heating device 150 may maintain the fluid inside the hollow test specimen 1 at a predetermined temperature or higher. Here, the maximum temperature of the fluid inside the hollow test specimen 1 is limited to a temperature that does not affect the metal structure of the hollow test specimen 1. The higher the temperature of the fluid inside the hollow test specimen 1, the more the solid solution of the fluid is promoted, and therefore the period of the first fluid holding step S12 can be shortened. In one embodiment, in the first fluid holding step S12, the temperature of the fluid inside the hollow test specimen 1 is maintained at a higher temperature than in the loading step S11.

[0047] (Second fluid retention step) In some embodiments of the fatigue crack propagation test method, the above-mentioned second fatigue crack propagation step S20 includes a second fluid retention step S22 of retaining a test fluid under second environmental conditions inside the hollow portion 11 for a predetermined period of time, prior to the loading step S21 of repeatedly applying a load L to the hollow test specimen 1, as shown in Figures 8 and 9.

[0048] According to the above method, by holding a fluid (second fluid) under the second environmental conditions inside the hollow portion 11 for a predetermined period in the second fluid holding step S22, a larger amount of the second fluid can be dissolved in the hollow test specimen 1 than when the second fluid holding step S22 is not performed. Furthermore, if the first fluid is dissolved in the hollow test specimen 1 in the first fluid holding step S12, the first fluid dissolved inside the hollow test specimen 1 can be discharged in the second fluid holding step S22. As a result, in the second fatigue crack propagation step S20, fatigue crack propagation is performed in a manner that more fully reflects the influence of the second fluid, and therefore the fatigue crack propagation amount A2 and propagation rate R2 quantified in the crack propagation amount acquisition step S30 more fully reflect the influence of the second fluid.

[0049] In the second fluid holding step S22, the fluid inside the hollow test specimen 1 may be maintained at a predetermined temperature or higher by the heating device 150. Here, the maximum temperature of the fluid inside the hollow test specimen 1 is limited to a temperature that does not affect the metal structure of the hollow test specimen 1. The higher the temperature of the fluid inside the hollow test specimen 1, the more the solid solution of the fluid is promoted, and therefore the period of the second fluid holding step S22 can be shortened. In one embodiment, in the second fluid holding step S22, the temperature of the fluid inside the hollow test specimen 1 is maintained at a higher temperature than in the loading step S21.

[0050] The heating by the heating device 150 is not limited to heater heating, and electric furnace heating or high-frequency heating may also be adopted. High-frequency heating can directly heat only the hollow test specimen 1 and is unlikely to raise the temperature of the surrounding atmosphere, so that the risk of explosion can be reduced even if an explosive fluid such as hydrogen gas leaks from the hollow test specimen 1.

[0051] (Fracture Surface Coloring Step) As shown in Fig. 9 , the fatigue crack propagation test method according to some embodiments further includes a fracture surface coloring step S40 in which a gas containing oxygen gas (e.g., air) is maintained inside the hollow portion 11 for a predetermined period of time between the first fatigue crack propagation step S10 and the second fatigue crack propagation step S20. The fracture surface coloring step S40 may be performed after the first fatigue crack propagation step S10 and before the second fatigue crack propagation step S20 begins, or after the second fatigue crack propagation step S20 and before the first fatigue crack propagation step S10 begins. The fracture surface coloring step S40 may be performed multiple times, as shown in Fig. 9 .

[0052] FIG. 10 is an explanatory diagram illustrating a fracture surface coloring process in a fatigue crack propagation test method according to an embodiment of the present disclosure. In the fracture surface coloring step S40, a gas containing oxygen gas is maintained inside the hollow portion 11 for a predetermined period of time, thereby oxidizing the fracture surface caused by the fatigue crack (coloring process) before the fracture surface coloring step S40. In the embodiment shown in FIG. 10, the fracture surface regions F0 and F1 are oxidized, and an oxide scale adheres to the fracture surface. By performing the coloring process between the first fatigue crack propagation step S10 and the second fatigue crack propagation step S20, the fracture surface boundary (B1 in the illustrated example) for each environmental condition can be clearly identified. Thus, when observing a fatigue crack formed in the hollow test specimen 1, the fracture surface boundary for each environmental condition can be visualized by performing oxygen atom mapping or the like. Visualizing the fracture surface boundary reduces the subjectivity in quantifying the propagation amounts A1 and A2 during fracture surface observation, thereby improving the accuracy of the propagation amounts A1 and A2.

[0053] (Setting Environmental Conditions According to the Metallic Material of the Hollow Test Specimen) Figure 11 is an explanatory diagram for explaining a fatigue crack propagation test method according to an embodiment of the present disclosure. In some embodiments of the fatigue crack propagation test method, the environmental conditions of at least one of the first fluid retention step S12 and the second fluid retention step S22 are changed according to the metallic material constituting the hollow test specimen 1. In the embodiment shown in Figure 11, the test fluid in one of the first fluid retention step S12 or the second fluid retention step S22 is hydrogen gas, and the test fluid in the other is air or an inert gas.

[0054] As shown in FIG. 11 , when the hydrogen diffusion rate of the metal material constituting the hollow test specimen 1 is low (the hydrogen diffusion rate is less than a predetermined value) ("No" in step S13), the duration of the first fluid holding step S12 and the second fluid holding step S22 is extended, or the temperature of the test fluid is increased, compared to when the hydrogen diffusion rate of the metal material constituting the hollow test specimen 1 is high (the hydrogen diffusion rate is equal to or greater than a predetermined value) ("Yes" in step S13). This allows the environmental conditions of the first fluid holding step S12 and the second fluid holding step S22 to be appropriate for the metal material constituting the hollow test specimen 1. Examples of metal materials with high hydrogen diffusion rates include BCC metals with a body-centered cubic lattice crystal structure. Examples of metal materials with low hydrogen diffusion rates include FCC metals with a face-centered cubic lattice crystal structure and HCP metals with a hexagonal close-packed crystal structure.

[0055] In some embodiments of the fatigue crack propagation test method, the environmental conditions for the fracture surface coloring step S40 are changed depending on the metallic material constituting the hollow test specimen 1. In the embodiment shown in Fig. 11 , the test fluid for one of the first fluid holding step S12 or the second fluid holding step S22 is hydrogen gas, and the test fluid for the other is air or an inert gas. As shown in Fig. 11 , if the metallic material constituting the hollow test specimen 1 has low oxidation resistance ("Yes" in step S41) and high susceptibility to hydrogen embrittlement ("Yes" in step S42) (a metallic material that is easily affected by hydrogen, such as carbon steel or general low-alloy steel), the fracture surface boundary is clear, and therefore the fracture surface coloring step S40 is not performed.

[0056] If the metallic material constituting the hollow test specimen 1 is a metallic material (first metallic material, e.g., low-alloy steel with reduced hydrogen sensitivity) with low oxidation resistance ("Yes" in step S41) and low hydrogen embrittlement susceptibility ("No" in step S42), the fracture surface coloring step S40 is performed. The fracture surface coloring step S40 is also performed if the metallic material constituting the hollow test specimen 1 is a metallic material (second metallic material, e.g., stainless steel) with high oxidation resistance ("No" in step S41). If the metallic material constituting the hollow test specimen 1 is the second metallic material, the temperature of the test fluid in the fracture surface coloring step S40 is set higher than when the metallic material is the first metallic material. This allows the feasibility of performing the fracture surface coloring step S40 and the environmental conditions to be appropriate for the metallic material constituting the hollow test specimen 1.

[0057] 1, a fatigue crack propagation test method according to some embodiments includes a preparation step S1 of preparing a hollow test specimen 1 having the above-described notch 13 and fatigue pre-crack 14, prior to the first fatigue crack propagation step S10 and the second fatigue crack propagation step S20. The preparation step S1 includes a notch formation step S2 and a fatigue pre-crack introduction step S3.

[0058] (Notch Forming Step) As shown in Fig. 1 , a fatigue crack propagation test method according to some embodiments further includes a notch forming step S2, prior to the first fatigue crack propagation step S10 and the second fatigue crack propagation step S20, in which a notch 13 (see Fig. 2 ) is formed on the inner circumferential surface (inner surface) 42 of the hollow test specimen 1. The notch 13 is formed by inserting a machining tool into the hollow portion 11 and rotating the machining tool along the circumferential direction of the hollow test specimen 1.

[0059] According to the above method, in the notch forming step S2, the notches 13 that become the starting points of fatigue cracks can be formed on the inner peripheral surface (inner surface) 42 of the hollow test piece 1.

[0060] As shown in FIG. 1 , a fatigue crack propagation test method according to some embodiments further includes a fatigue pre-crack introduction step S3, which is performed after the notch formation step S2 and before the first fatigue crack propagation step S10 and the second fatigue crack propagation step S20. In the fatigue pre-crack introduction step S3, a cyclic load is applied to the hollow test specimen 1 to generate a fatigue pre-crack 14 extending from a notch 13 formed in the inner peripheral surface (inner surface) 42 of the hollow test specimen 1. The fatigue pre-crack 14 preferably extends radially outward beyond the range of influence 15 of the notch processing. In the fatigue pre-crack introduction step S3, air is filled inside the hollow portion 11. To clarify the fracture boundary B0 described above, the cyclic load in the fatigue pre-crack introduction step S3 may be smaller than the cyclic load L in the first fatigue crack propagation step S10 and the second fatigue crack propagation step S20.

[0061] According to the above method, by generating a fatigue pre-crack 14 in the fatigue pre-crack introduction step S3, the fatigue crack propagates outside the notch processing influence range 15 in each fatigue crack propagation step S10, S20. This eliminates the influence of notch processing on the fatigue crack propagation in each fatigue crack propagation step S10, S20, making it possible to quantify the fatigue crack propagation amounts A1, A2 and propagation rates R1, R2 relative to the original metal structure of the hollow test specimen 1.

[0062] 1 , in the fatigue crack propagation test method according to some embodiments, the fatigue pre-crack introduction step S3 includes a fatigue pre-crack acquisition step S4 in which the depth (radial length) of the fatigue pre-crack 14 is obtained by non-destructive testing from the outer surface of the hollow test piece 1. The depth of the fatigue pre-crack 14 can be measured non-destructively by ultrasonic testing, electric potential difference method, X-ray CT, or the like.

[0063] According to the above method, in the fatigue pre-crack acquisition step S4, the depth of the fatigue pre-crack 14 is acquired from the outer surface of the hollow test piece 1 by non-destructive testing, thereby making it possible to confirm that the fatigue pre-crack 14 has extended beyond the notch processing influence range 15. Note that the depth of the fatigue pre-crack 14 in the fatigue pre-crack introduction step S3 may also be estimated by a known back gauge method.

[0064] In some embodiments, in the fatigue pre-crack acquisition step S4, the propagation amount of the fatigue pre-crack 14 in the hollow specimen 1 during the fatigue pre-crack introduction step S3 is estimated based on the output of the strain measurement device 130 attached to the outer surface of the hollow specimen 1. In the fatigue pre-crack introduction step S3, a load is repeatedly applied to the hollow specimen 1 until the propagation amount of the fatigue pre-crack 14 acquired in the fatigue pre-crack acquisition step S4 exceeds a predetermined amount. The propagation amount (depth) of the fatigue pre-crack 14 can be estimated from the output of the strain measurement device 130 using a known back gauge method.

[0065] According to the above method, the amount of growth of the fatigue pre-crack 14 can be estimated based on the output of the strain measuring device 130, thereby making it possible to make the fatigue pre-crack 14 have a desired depth.

[0066] (Fatigue crack growth estimation step) Some embodiments of the fatigue crack growth testing method further include a fatigue crack growth estimation step of estimating the fatigue crack growth amounts A1, A2 of the hollow test specimen 1 during the first fatigue crack growth step S10 or the second fatigue crack growth step S20 based on the output of a strain measurement device 130 attached to the outer surface of the hollow test specimen 1. In at least one of the first fatigue crack growth step S10 or the second fatigue crack growth step S20, a load is repeatedly applied to the hollow test specimen 1 until the fatigue crack growth amounts A1, A2 estimated in the fatigue crack growth estimation step exceed a predetermined amount. The fatigue crack growth amounts A1, A2 can be estimated from the output of the strain measurement device 130 using a known back gauge method.

[0067] According to the above method, in order to clearly grasp the fatigue crack propagation under each environmental condition, it is preferable that the fatigue crack propagation amounts A1 and A2 in each fatigue crack propagation step S10 and S20 exceed a predetermined amount. In the fatigue crack propagation amount estimation step, the fatigue crack propagation amounts A1 and A2 of the hollow test specimen 1 during the execution of each fatigue crack propagation step S10 and S20 can be estimated. By observing the hollow test specimen 1 after repeatedly applying a load L to the hollow test specimen 1 until the fatigue crack propagation amounts A1 and A2 estimated in the fatigue crack propagation amount estimation step exceed the predetermined amount, the fatigue crack propagation under each environmental condition can be clearly grasped.

[0068] (Cross-Section Observation) In some of the above-described embodiments, the fracture surface of the hollow test specimen 1 was observed, but a cross section cut along the central axis CA of the hollow test specimen may also be observed. Fig. 12 is an explanatory diagram for explaining cross-sectional observation of the hollow test specimen 1. In the fatigue crack growth test method according to some embodiments, in the above-described crack growth amount acquisition step S30, the cross section cut along the central axis CA of the hollow test specimen 1 is observed to acquire the fatigue crack growth amounts A1 and A2 in the first fatigue crack growth step S10 and the second fatigue crack growth step S20, respectively.

[0069] According to the above method, in a cross section cut along the central axis CA of the hollow test specimen 1, the fatigue crack propagation in each of the multiple fatigue crack propagation steps S10 and S20 appears as amplitudes 16 and 17 in the fatigue crack propagation direction and central axis direction, as shown in FIG. 12 . Therefore, by observing the cross section, the fatigue crack propagation region for each environment can be identified. Therefore, in the crack propagation amount acquisition step S30, by observing the cross section of the hollow test specimen 1, the fatigue crack propagation amounts A1 and A2 in each of the multiple fatigue crack propagation steps S10 and S20 can be quantified. Specifically, the radial lengths of the fatigue crack propagation regions in each of the fatigue crack propagation steps S10 and S20 may be measured, and these measurements may be used as the fatigue crack propagation amounts A1 and A2. Note that a scanning electron microscope (SEM) or a metallurgical (optical) microscope may be used for cross-sectional observation.

[0070] (Shape of Hollow Test Specimen) The hollow test specimen 1 having a notch 13 has the problem that a specific shape that allows for accurate material testing (e.g., fatigue testing) has not been determined. Furthermore, since the notch 13 is machined in a narrow portion, there is a problem of ensuring the machining accuracy of the notch 13. Enlarging the internal shape of the hollow test specimen 1 to machine the notch 13 increases the amount of fluid used in the hollow portion 11. If the fluid is hydrogen gas or the like, which has the risk of explosion, increasing the amount of fluid used will pose a safety management issue. The hollow test specimen 1 disclosed herein can be applied to fatigue tests other than the fatigue crack propagation test method disclosed herein, and to material tests other than fatigue tests.

[0071] 13 is a schematic cross-sectional view taken along the central axis of a hollow test specimen 1 that is the subject of a fatigue crack propagation test method according to an embodiment of the present disclosure. As shown in Fig. 13, the hollow test specimen 1 includes a one-side large diameter portion 2 formed on one side in the longitudinal direction of the hollow test specimen 1 (the lower side in the figure), a other-side large diameter portion 3 formed on the other side in the longitudinal direction of the hollow test specimen 1 (the upper side in the figure), and a parallel portion 4 formed between the one-side large diameter portion 2 and the other-side large diameter portion 3 in the longitudinal direction. The parallel portion 4 has a smaller outer diameter than the one-side large diameter portion 2 and the other-side large diameter portion 3.

[0072] In the illustrated embodiment, the hollow test piece 1 further includes a one-side expanded diameter portion 5 formed between the one-side large diameter portion 2 and the parallel portion 4 in the longitudinal direction, and a second-side expanded diameter portion 6 formed between the other-side large diameter portion 3 and the parallel portion 4 in the longitudinal direction, as shown in Figure 13. The one-side large diameter portion 2, the other-side large diameter portion 3, the parallel portion 4, the one-side expanded diameter portion 5, and the other-side expanded diameter portion 6 each have an outer peripheral surface 21, 31, 41, 51, 61 and an inner peripheral surface 22, 32, 42, 52, 62. The inner peripheral surface 12 includes the inner peripheral surfaces 22, 32, 42, 52, 62, and the hollow portion 11 is defined by the inner peripheral surfaces 22, 32, 42, 52, 62.

[0073] The one-side expanded diameter portion 5 has a diameter that increases toward the one side in the longitudinal direction, i.e., toward the one-side large diameter portion 2. The one-side expanded diameter portion 5 has an outer peripheral surface 51 with an R-shaped configuration that convex radially inward. The other-side expanded diameter portion 6 has a diameter that increases toward the other side in the longitudinal direction, i.e., toward the other-side large diameter portion 3. The other-side expanded diameter portion 6 has an outer peripheral surface 61 with an R-shaped configuration that convex radially inward.

[0074] (Notch) As shown in Fig. 13, the hollow test specimen 1 has a notch (stress concentration portion) 13 formed on the inner peripheral surface 42 of the parallel portion 4 and extending in the circumferential direction of the inner peripheral surface 42. The notch 13 is a portion where stress concentration occurs when a load is applied by a fatigue testing machine (material testing machine) 110. Portions of the hollow test specimen 1 other than the notch 13 are designed to cause less stress concentration than the notch 13 when a load is applied by the fatigue testing machine 110.

[0075] (Length of Parallel Portion) In some embodiments, the parallel portion 4 of the hollow test piece 1 has the above-described outer peripheral surface 41, the above-described inner peripheral surface 42, and the above-described notch 13, as shown in Fig. 13. When the inner diameter of the parallel portion 4 is defined as D1 and the length of the parallel portion 4 in the longitudinal direction is defined as L1, the length L1 of the parallel portion 4 satisfies the condition 0.5 × D1 ≦ L1 ≦ 3 × D1.

[0076] In the embodiment shown in FIG. 13 , the inner diameter D1 of the parallel portion 4 is the diameter of a portion of the inner circumferential surface 42 where the annular cutout 43 (notch 13) is not formed. The inner diameter D1 of the parallel portion 4 of the hollow test specimen 1 is restricted by the machining tool used to form the notch 13 in the inner circumferential surface 42. The length L1 of the parallel portion 4 satisfies the condition 0.5 × D1 ≦ L1 ≦ 3 × D1, thereby homogenizing the stress generated during material testing (fatigue testing). A hollow test specimen 1 including such a parallel portion 4 can have a small internal volume of the parallel portion 4, thereby reducing the volume of fluid introduced into the hollow test specimen 1.

[0077] Furthermore, by satisfying the condition that the length L of the parallel portion 4 is 0.5 x D1 ≦ L1 ≦ 3 x D1, an area can be secured on the outer surface 41 for attaching a strain gauge to balance the axial center of the hollow test piece 1 attached to the material testing machine (fatigue testing machine 110) before the start of the material test (fatigue test).

[0078] (Notch portion) Fig. 14 is a schematic cross-sectional view of the notch portion 43 of the hollow test piece 1 shown in Fig. 13 taken along the central axis direction of the hollow test piece 1. In some embodiments, the notch 13 of the hollow test piece 1 described above includes an annular notch portion 43 having a V-shaped cross section formed on the inner circumferential surface 42 of the parallel portion 4.

[0079] In the illustrated embodiment, the cutout portion 43 includes a first inclined surface 431 that slopes radially outward toward one longitudinal side, a second inclined surface 432 that slopes radially outward toward the other longitudinal side, and an R-shaped portion 433 that connects the outer circumferential end of the first inclined surface 431 and the second inclined surface 432 and curves concavely radially outward. The inclination angle θ between the first inclined surface 431 and the second inclined surface 432 is preferably within the range of 50° to 70°, and more preferably within the range of 55° to 65°. In this case, the cutout portion 43 can be efficiently machined using a machining tool. When the inner diameter of the parallel portion 4 is defined as D1, the radius of curvature RC1 of the R-shaped portion 433 satisfies the condition 0.1 × D1 ≦ RC1 ≦ 0.2 × D1. Changing the radius of curvature RC1 of the R-shaped portion 433 can change the stress concentration factor. In order to change the stress concentration coefficient, the depth ND of the notch 43 may be changed instead of the curvature radius RC1 of the R-shaped portion 433.

[0080] The shape of the notch 43 can be determined according to the desired stress intensity factor to be applied to the hollow test piece 1 during material testing (fatigue testing). The notch 43 having a V-shaped annular cross section can be machined well using a machining tool, making it possible to ensure machining accuracy. Note that the notch 43 is not limited to a V-shaped annular cross section. The notch 43 may be, for example, a U-shaped annular cross section.

[0081] (Outer Diameter of Parallel Portion) In some embodiments, as shown in FIG. 13 , when the inner diameter of the parallel portion 4 of the hollow test specimen 1 described above is defined as D1 and the outer diameter of the parallel portion 4 is defined as D2, the outer diameter D2 of the parallel portion 4 satisfies the condition 1.5 × D1 ≦ D2 ≦ 2 × D1. By ensuring that the outer diameter D2 of the parallel portion 4 satisfies the condition 1.5 × D1 ≦ D2 ≦ 2 × D1, the thickness of the parallel portion 4 required for material testing (fatigue testing) can be ensured while preventing the thickness of the parallel portion 4 from becoming excessive. Here, when a relatively high stress is applied to the notch 13 in material testing (fatigue testing), the thicknesses of the one-side large diameter portion 2 and the other-side large diameter portion 3 must be sufficiently larger than the thickness of the parallel portion 4. By preventing the thickness of the parallel portion 4 from becoming excessive, the thickness of the one-side large diameter portion 2 and the other-side large diameter portion 3 can be prevented from becoming excessively thick, which would result in an increase in the size and weight of the hollow test specimen 1.

[0082] 13 , when the inner diameter of the parallel portion 4 of the hollow test specimen 1 described above is defined as D1 and the outer diameter of the one-side large diameter portion 2 described above is defined as D3, the outer diameter D3 of the one-side large diameter portion 2 satisfies the condition 2×D1≦D3≦3×D1. By making the outer diameter D3 of the one-side large diameter portion 2 satisfy the condition 2×D1≦D3≦3×D1, the thickness of the one-side large diameter portion 2 can be made sufficiently larger than the thickness of the parallel portion 4, and the thickness of the one-side large diameter portion 2 can be prevented from becoming excessively large.

[0083] 13 , when the inner diameter of the parallel portion 4 of the hollow test piece 1 described above is defined as D1 and the outer diameter of the other-side large diameter portion 3 described above is defined as D4, the outer diameter D4 of the other-side large diameter portion 3 satisfies the condition 2×D1≦D4≦3×D1. By making the outer diameter D4 of the other-side large diameter portion 3 satisfy the condition 2×D1≦D4≦3×D1, the thickness of the other-side large diameter portion 3 can be made sufficiently larger than the thickness of the parallel portion 4, and the thickness of the other-side large diameter portion 3 can be prevented from becoming excessively large.

[0084] (Support structure of one side large diameter portion and other side large diameter portion) In some embodiments, as shown in Figure 13, the one side large diameter portion 2 and the other side large diameter portion 3 of the above-mentioned hollow test piece 1 have threaded portions 211, 311 formed on at least a portion of the longitudinal direction of the outer surfaces 21, 31.

[0085] The one-side support portion 111 has a threaded portion that screws into the threaded portion 211 of the one-side large diameter portion 2. The one-side large diameter portion 2 is supported by the one-side support portion 111 by screwing the threaded portion 211 into the threaded portion of the one-side support portion 111. The other-side support portion 112 has a threaded portion that screws into the threaded portion 311 of the other-side large diameter portion 3. The other-side large diameter portion 3 is supported by the other-side support portion 112 by screwing the threaded portion 311 into the threaded portion of the other-side support portion 112.

[0086] In this case, the hollow test specimen 1 is fixed to a material testing machine (fatigue testing machine 110) via threaded portions 211, 311 formed on the outer circumferential surfaces 21, 31 of the one-side large diameter portion 2 and the other-side large diameter portion 3. This hollow test specimen 1 does not have enlarged diameter portions such as flanges for fixing the one-side large diameter portion 2 and the other-side large diameter portion 3 of the hollow test specimen 1 to the material testing machine (fatigue testing machine 110), so that a material with a relatively small outer diameter can be used as the base material for the hollow test specimen 1.

[0087] 13 , when the inner diameter of the parallel portion 4 of the hollow test specimen 1 described above is defined as D1 and the longitudinal length of the one-side large diameter portion 2 described above is defined as L4, the length L4 of the one-side large diameter portion 2 satisfies the condition 3×D1≦L4≦6×D1. By making the length L4 of the one-side large diameter portion 2 satisfy the condition 3×D1≦L4≦6×D1, it is possible to prevent the length L4 of the one-side large diameter portion 2 from becoming excessively large while ensuring the length necessary for supporting the one-side large diameter portion 2 on the one-side support portion 111 of the material testing machine (fatigue testing machine 110).

[0088] 13 , when the inner diameter of the parallel portion 4 of the hollow test piece 1 described above is defined as D1 and the longitudinal length of the other-side large diameter portion 3 described above is defined as L5, the length L5 of the other-side large diameter portion 3 satisfies the condition 3×D1≦L5≦6×D1. By making the length L5 of the other-side large diameter portion 3 satisfy the condition 3×D1≦L5≦6×D1, it is possible to ensure the length necessary for supporting the other-side large diameter portion 3 on the other-side support portion 112 of the material testing machine (fatigue testing machine 110), while preventing the length L5 of the other-side large diameter portion 3 from becoming excessively large.

[0089] (One-side expanded diameter portion, other-side expanded diameter portion) In some embodiments, the hollow test piece 1 described above includes the one-side large diameter portion 2, the other-side large diameter portion 3, the parallel portion 4, the one-side expanded diameter portion 5, and the other-side expanded diameter portion 6, as shown in Figure 13.

[0090] The outer peripheral surface 51 of the one-side expanded diameter portion 5 is smoothly connected to the outer peripheral surface 41 of the parallel portion 4 without any steps, and is connected to the outer peripheral surface 21 of the one-side large diameter portion 2 at a predetermined inclination angle. The outer diameter of the outer peripheral surface 51 is smallest at the connection with the outer peripheral surface 41 and is largest at the connection with the outer peripheral surface 21.

[0091] The outer peripheral surface 61 of the other-side expanded diameter portion 6 smoothly continues without any steps to the outer peripheral surface 41 of the parallel portion 4, and is connected to the outer peripheral surface 31 of the other-side large diameter portion 3 at a predetermined inclination angle. The outer diameter of the outer peripheral surface 61 is smallest at the connection with the outer peripheral surface 41 and is largest at the connection with the outer peripheral surface 31.

[0092] In this case, when a load L is applied to the hollow test piece 1 by a material testing machine (fatigue testing machine 110), stress concentration can be prevented from occurring in areas other than the parallel portion 4 of the hollow test piece 1, such as the one-side expansion portion 5 and the other-side expansion portion 6.

[0093] 13 , when the longitudinal length of the parallel portion 4 of the hollow test piece 1 described above is defined as L1 and the longitudinal length of the one-side expanded diameter portion 5 described above is defined as L2, the length L2 of the one-side expanded diameter portion 5 satisfies the condition 1 / 3 × L1≦L2≦1 / 2 × L1. By making the length L2 of the one-side expanded diameter portion 5 satisfy the condition 1 / 3 × L1≦L2≦1 / 2 × L1, the R-shape of the outer peripheral surface 51 of the one-side expanded diameter portion 5 can be made to have a curvature that does not cause stress concentration, and the length L2 of the one-side expanded diameter portion 5 can be prevented from becoming excessively large.

[0094] 13 , when the longitudinal length of the parallel portion 4 of the hollow test piece 1 described above is defined as L1 and the longitudinal length of the other-side expanded diameter portion 6 described above is defined as L3, the length L3 of the other-side expanded diameter portion 6 satisfies the condition 1 / 3 × L1≦L3≦1 / 2 × L1. By making the length L3 of the other-side expanded diameter portion 6 satisfy the condition 1 / 3 × L1≦L3≦1 / 2 × L1, the R-shape of the outer peripheral surface 61 of the other-side expanded diameter portion 6 can be made to have a curvature that does not cause stress concentration, and the length L3 of the other-side expanded diameter portion 6 can be prevented from becoming excessively large.

[0095] In this specification, expressions expressing relative or absolute arrangements, such as "in a certain direction," "along a certain direction," "parallel," "orthogonal," "center," "concentric," or "coaxial," not only strictly express such arrangements, but also express a state in which there is a relative displacement with a tolerance or an angle or distance to the extent that the same function is obtained. For example, expressions expressing that things are in an equal state, such as "identical," "equal," and "homogeneous," not only express a state in which there is a strict equivalence, but also express a state in which there is a tolerance or a difference to the extent that the same function is obtained. Furthermore, in this specification, expressions expressing shapes such as a rectangular shape or a cylindrical shape not only express shapes such as a rectangular shape or a cylindrical shape in the strict geometric sense, but also express shapes including concave and convex portions, chamfered portions, etc., to the extent that the same effect is obtained. Furthermore, in this specification, the expressions "comprise," "include," or "have" a component are not exclusive expressions that exclude the presence of other components.

[0096] The present disclosure is not limited to the above-described embodiments, but also includes modifications to the above-described embodiments and appropriate combinations of these modifications.

[0097] The contents of the above-described embodiments can be understood, for example, as follows.

[0098] 1) A fatigue crack propagation test method according to at least one embodiment of the present disclosure is a fatigue crack propagation test method for a fatigue crack formed on an inner surface (12) of a hollow test piece (1) having a hollow portion (11) into which a fluid can be introduced, the method comprising: a first fatigue crack propagation step (S10) of repeatedly applying a load to the hollow test piece (1) under first environmental conditions to propagate a fatigue crack; and a second fatigue crack propagation step (S20) of repeatedly applying a load to the hollow test piece (1) under second environmental conditions, the first environmental conditions being different from the second environmental conditions in at least one of the type, pressure, and temperature of the test fluid filled inside the hollow portion (11). and a crack propagation amount acquisition step (S30) for acquiring the propagation amounts (A1, A2) of the fatigue crack in each of the first fatigue crack propagation step (S10) and the second fatigue crack propagation step (S20) by observing the fatigue crack formed in the hollow test piece (1).

[0099] According to the method 1), the fatigue crack propagation amount (A1, A2) in each of the fatigue crack propagation steps (S10, S20) can be obtained by observing the fatigue crack formed in the hollow test specimen (1) in the crack propagation amount acquisition step (S30). The fatigue crack propagation rate (R1, R2) in each fatigue crack propagation step (S10, S20) can then be calculated from the fatigue crack propagation amount (A1, A2) and the number of cycles (N1, N2) in each fatigue crack propagation step (S10, S20). Therefore, according to the method 1), the fatigue crack propagation amount (A1, A2) and propagation rate (R1, R2) can be quantified for the same hollow test specimen (1) under multiple different environmental conditions. Furthermore, the acceleration rate of the fatigue crack propagation rate due to changes in environmental conditions can also be quantified.

[0100] 2) In some embodiments, in the fatigue crack propagation test method described in 1) above, in the crack propagation amount acquisition step (S30), the fatigue crack propagation amounts (A1, A2) in the first fatigue crack propagation step (S10) and the second fatigue crack propagation step (S20) are acquired by observing the fracture surface of the hollow test piece (1).

[0101] According to the method of 2), the fatigue crack propagation in each of the multiple fatigue crack propagation steps (S10, S20) appears as a striped pattern or the like on the fracture surface of the hollow test specimen (1), and the fatigue crack propagation region (fracture surface region) for each environment can be identified by observing the fracture surface. Therefore, in the crack propagation amount acquisition step (S30), the fatigue crack propagation amounts (A1, A2) in each of the multiple fatigue crack propagation steps (S10, S20) can be quantified by observing the fracture surface of the hollow test specimen (1).

[0102] 3) In some embodiments, in the fatigue crack propagation test method described in 1) above, the crack propagation amount acquisition step (S30) involves observing a cross section cut along the central axis direction of the hollow test piece (1) to acquire the fatigue crack propagation amounts (A1, A2) in the first fatigue crack propagation step (S10) and the second fatigue crack propagation step (S20), respectively.

[0103] According to the method 3), the fatigue crack propagation in each of the plurality of fatigue crack propagation steps (S10, S20) appears in the cross section cut along the central axis of the hollow test specimen (1) as a function of the direction of fatigue crack propagation and the amplitude in the central axis direction, and therefore, by observing the cross section, the fatigue crack propagation region for each environment can be identified. Therefore, in the crack propagation amount acquisition step (S30), by observing the cross section of the hollow test specimen (1), the fatigue crack propagation amounts (A1, A2) in each of the plurality of fatigue crack propagation steps (S10, S20) can be quantified.

[0104] 4) In some embodiments, in the fatigue crack propagation test method according to any one of 1) to 3) above, the second environmental conditions are different from the first environmental conditions in at least the type of the test fluid.

[0105] According to the method of 4) above, it is possible to quantify the fatigue crack propagation amount (A1, A2) and propagation rate (R1, R2) of the same hollow test specimen (1) under a plurality of environmental conditions in which different types of test fluid are filled inside the hollow portion (11).

[0106] 5) In some embodiments, in the fatigue crack propagation test method described in 4) above, the type of test fluid under the first environmental condition is hydrogen gas, and the type of test fluid under the second environmental condition is an inert gas or air.

[0107] According to the method of 5) above, it is possible to quantify the fatigue crack propagation amount (A1, A2) and propagation rate (R1, R2) of the same hollow test piece (1) when the test fluid is hydrogen gas and when the test fluid is an inert gas or air. This also makes it possible to quantify the acceleration rate of the fatigue crack propagation rate when the test fluid is hydrogen gas, relative to when the test fluid is an inert gas or air.

[0108] 6) In some embodiments, in the fatigue crack propagation test method described in 4) or 5) above, the first fatigue crack propagation step (S10) includes a first fluid retention step (S12) of retaining the test fluid under the first environmental conditions inside the hollow portion (11) for a predetermined period of time before repeatedly applying a load to the hollow test piece (1).

[0109] According to the method 6) above, by holding the test fluid (first fluid) under first environmental conditions inside the hollow portion (11) for a predetermined period in the first fluid holding step (S12), a larger amount of the first fluid can be dissolved in the hollow test piece (1) than when the first fluid holding step (S12) is not performed. As a result, in the first fatigue crack propagation step (S10), fatigue crack propagation is carried out in a manner that more fully reflects the influence of the first fluid, and the fatigue crack propagation amount (A1) and propagation rate (R1) quantified in the crack propagation amount acquisition step (S30) more fully reflect the influence of the first fluid.

[0110] 7) In some embodiments, in the fatigue crack propagation test method described in any of 4) to 6) above, the second fatigue crack propagation step (S20) includes a second fluid retention step (S22) of retaining the test fluid under the second environmental conditions inside the hollow portion (11) for a predetermined period of time before repeatedly applying a load to the hollow test piece (1).

[0111] According to the method of 7), by holding a fluid (second fluid) under second environmental conditions inside the hollow portion (11) for a predetermined period in the second fluid holding step (S22), a larger amount of the second fluid can be dissolved in the hollow test piece (1) than when the second fluid holding step (S22) is not performed. Furthermore, if the first fluid is dissolved in the hollow test piece (1) in the first fluid holding step (S12), the first fluid dissolved inside the hollow test piece (1) can be discharged in the second fluid holding step (S22). As a result, the fatigue crack propagation in the second fatigue crack propagation step (S20) is more influenced by the second fluid, and the fatigue crack propagation amount (A2) and propagation rate (R2) quantified in the crack propagation amount acquisition step (S30) more reflect the influence of the second fluid.

[0112] 8) In some embodiments, the fatigue crack propagation test method described in any of 4) to 7) above further includes a fracture surface coloring step (S40) between the first fatigue crack propagation step (S10) and the second fatigue crack propagation step (S20) in which a gas containing oxygen gas is maintained inside the hollow portion (11) for a predetermined period of time.

[0113] According to the method of 8) above, in the fracture surface coloring step (S40), a gas containing oxygen gas is maintained inside the hollow portion (11) for a predetermined period of time, thereby oxidizing the fracture surface caused by the fatigue crack (coloring treatment). By performing the coloring treatment between the first fatigue crack propagation step (S10) and the second fatigue crack propagation step (S20), the fracture surface boundary for each environmental condition can be made clear. Therefore, when observing a fatigue crack formed in the hollow test piece (1), the fracture surface boundary for each environmental condition can be visualized by performing oxygen atom mapping or the like.

[0114] 9) In some embodiments, the fatigue crack propagation test method described in any of 1) to 8) above further includes a notch formation step (S2) of forming a notch (13) in the inner surface (42) of the hollow test piece (1) prior to the first fatigue crack propagation step (S10) and the second fatigue crack propagation step (S20).

[0115] According to the method of 9) above, in the notch forming step (S2), a notch (13) that becomes the starting point of a fatigue crack can be formed on the inner surface (42) of the hollow test piece (1).

[0116] 10) In some embodiments, the fatigue crack propagation test method described in 9) above further includes a fatigue pre-crack introduction step (S3) that is performed after the notch formation step (S2) and before the first fatigue crack propagation step (S10) and the second fatigue crack propagation step (S20), in which a load is repeatedly applied to the hollow test piece (1) to generate a fatigue pre-crack (14) extending from the notch (13) formed in the inner surface (42) of the hollow test piece (1).

[0117] According to the method of 10), a fatigue pre-crack (14) is generated in the fatigue pre-crack introduction step (S3), and in each fatigue crack propagation step (S10, S20), the fatigue crack propagates outside the range of influence (15) of notching. This eliminates the influence of notching on the fatigue crack propagation in each fatigue crack propagation step (S10, S20), making it possible to quantify the amount of fatigue crack propagation (A1, A2) and propagation rate (R1, R2) relative to the original metal structure of the hollow test specimen (1).

[0118] 11) In some embodiments, in the fatigue crack propagation test method described in 10) above, the fatigue pre-crack introduction step (S3) includes a fatigue pre-crack acquisition step (S4) of acquiring the depth of the fatigue pre-crack (14) from the outer surface of the hollow test piece (1) by non-destructive testing.

[0119] According to the method of 11) above, in the fatigue pre-crack acquisition step (S4), the depth of the fatigue pre-crack (14) is acquired from the outer surface of the hollow test piece (1) by non-destructive testing, thereby making it possible to confirm that the fatigue pre-crack (14) extends beyond the range of influence (15) of the notch processing.

[0120] 12) In some embodiments, in the fatigue crack propagation test method described in 11) above, in the fatigue pre-crack acquisition step (S4), the propagation amount of the fatigue pre-crack in the hollow test piece (1) during the fatigue pre-crack introduction step (S3) is estimated based on the output of a strain measuring device (130) attached to the outer surface of the hollow test piece (1), and in the fatigue pre-crack introduction step (S3), a load is repeatedly applied to the hollow test piece (1) until the propagation amount of the fatigue pre-crack acquired in the fatigue pre-crack acquisition step (S4) exceeds a predetermined amount.

[0121] According to the method of 12) above, in the fatigue pre-crack acquisition step (S4), the propagation amount of the fatigue pre-crack in the hollow test piece (1) during the fatigue pre-crack introduction step (S3) can be estimated. By repeatedly applying a load to the hollow test piece (1) until the propagation amount of the fatigue pre-crack estimated in the fatigue pre-crack acquisition step (S4) exceeds a predetermined amount, the fatigue pre-crack can be made to a desired depth.

[0122] 13) In some embodiments, the fatigue crack propagation test method described in any of 1) to 12) above further includes a fatigue crack propagation amount estimation step of estimating the propagation amount (A1, A2) of the fatigue crack in the hollow test piece (1) during the first fatigue crack propagation step (S10) or the second fatigue crack propagation step (S20) based on the output of a strain measuring device (130) attached to the outer surface of the hollow test piece (1), and in at least one of the first fatigue crack propagation step (S10) or the second fatigue crack propagation step (S20), a load is repeatedly applied to the hollow test piece (1) until the propagation amount (A1, A2) of the fatigue crack estimated in the fatigue crack propagation amount estimation step exceeds a predetermined amount.

[0123] According to the method of 13), in order to clearly grasp the fatigue crack propagation under each environmental condition, it is preferable that the fatigue crack propagation amount (A1, A2) in each fatigue crack propagation step (S10, S20) exceeds a predetermined amount. In the fatigue crack propagation amount estimation step, the fatigue crack propagation amount (A1, A2) of the hollow test piece (1) during the execution of each fatigue crack propagation step (S10, S20) can be estimated. By observing the hollow test piece (1) to which a load has been repeatedly applied until the fatigue crack propagation amount (A1, A2) estimated in the fatigue crack propagation amount estimation step exceeds a predetermined amount, the fatigue crack propagation under each environmental condition can be clearly grasped.

[0124] 14) In some embodiments, the method for testing fatigue crack propagation described in 9) above, wherein the hollow test piece (1) includes: a one-side large diameter portion (2) formed on one side in the longitudinal direction of the hollow test piece (1); an other-side large diameter portion (3) formed on the other side in the longitudinal direction; and a parallel portion (4) formed between the one-side large diameter portion (2) and the other-side large diameter portion (3) and having an outer diameter smaller than the one-side large diameter portion (2) and the other-side large diameter portion (3), wherein the parallel portion (4) has an outer peripheral surface (41); an inner peripheral surface (42); and the notch (13) formed on the inner peripheral surface (42) and extending along the circumferential direction of the inner peripheral surface (42), When the inner diameter of the parallel portion (4) is defined as D1 and the length of the parallel portion (4) in the longitudinal direction is defined as L1, the length L1 of the parallel portion (4) satisfies the condition 0.5×D1≦L1≦3×D1.

[0125] According to the method of 14) above, the parallel portion (4) of the hollow test piece (1) is restricted in its inner diameter D1 by the machining tool used to form the notch (13) on the inner peripheral surface (42). The parallel portion (4) can homogenize stresses generated during material testing (fatigue testing) by satisfying the condition that the length L is 0.5 × D1 ≦ L1 ≦ 3 × D1. A hollow test piece (1) including such a parallel portion (4) can have a small internal volume of the parallel portion (4), thereby reducing the volume of fluid introduced into the hollow test piece (1).

[0126] 15) In some embodiments, in the fatigue crack propagation test method described in 14) above, the notch (13) of the hollow test piece (1) is an annular cutout portion (43) formed on the inner surface (42) of the parallel portion (4), and includes an annular cutout portion (43) having a V-shaped cross section.

[0127] According to the method of 15), the shape of the notch (43) can be determined according to the desired stress intensity factor to be applied to the hollow test piece (1) during the material test (fatigue test). The annular notch (43) having a V-shaped cross section can be machined well using the machining tool used to form the notch (13), so machining precision can be ensured.

[0128] 16) In some embodiments, in the fatigue crack propagation test method described in 14) or 15) above, when the outer diameter of the parallel portion (4) of the hollow test piece (1) is defined as D2, the outer diameter D2 of the parallel portion (4) satisfies the condition 1.5 × D1 ≦ D2 ≦ 2 × D1.

[0129] According to the method of 16), by setting the outer diameter D2 of the parallel portion 4 to satisfy the condition 1.5 × D1 ≦ D2 ≦ 2 × D1, the thickness of the parallel portion 4 required for the material test (fatigue test) can be ensured while preventing the thickness of the parallel portion 4 from becoming excessive. Here, when a relatively high stress is applied to the notch 13 in the material test (fatigue test), the thicknesses of the one-side large diameter portion 2 and the other-side large diameter portion 3 must be sufficiently larger than the thickness of the parallel portion 4. By preventing the thickness of the parallel portion 4 from becoming excessive, it is possible to prevent the one-side large diameter portion 2 and the other-side large diameter portion 3 from becoming excessively thick, which would result in an increase in the size and weight of the hollow test piece 1.

[0130] 17) In some embodiments, in the fatigue crack propagation test method described in any one of 14) to 16) above, when the outer diameter of the one-side large diameter portion (2) of the hollow test piece (1) is defined as D3, the outer diameter D3 of the one-side large diameter portion (2) satisfies the condition 2 × D1 ≦ D3 ≦ 3 × D1.

[0131] According to the method of 17), by making the outer diameter D3 of the one-side large diameter portion (2) satisfy the condition 2 × D1 ≦ D3 ≦ 3 × D1, the thickness of the one-side large diameter portion (2) can be made sufficiently larger than the thickness of the parallel portion (4), and the thickness of the one-side large diameter portion (2) can be prevented from becoming excessively large.

[0132] 18) In some embodiments, in the fatigue crack propagation test method described in any of 14) to 17) above, the hollow test piece (1) further includes: a one-side expanded diameter portion (5) formed between the one-side large diameter portion (2) and the parallel portion (4), the one-side expanded diameter portion (5) having a diameter that increases toward the one-side large diameter portion (2), the one-side expanded diameter portion (5) having an R-shaped outer peripheral surface (51) that is convex inward in the radial direction; and a other-side expanded diameter portion (6) formed between the other-side large diameter portion (3) and the parallel portion (4), the other-side expanded diameter portion (6) having a diameter that increases toward the other-side large diameter portion (3), the other-side expanded diameter portion (6) having an R-shaped outer peripheral surface (61) that is convex inward in the radial direction.

[0133] According to the method of 18) above, when stress is applied to the hollow test piece (1) in a material test (fatigue test), stress concentration can be suppressed in parts other than the parallel part (4) of the hollow test piece (1), such as the one-side expanded diameter part (5) and the other-side expanded diameter part (6).

[0134] 19) In some embodiments, in the fatigue crack propagation test method described in 18) above, when the longitudinal length of the one-side expanded portion (5) of the hollow test piece (1) is defined as L2, the length L2 of the one-side expanded portion (5) satisfies the condition 1 / 3 × L1 ≦ L2 ≦ 1 / 2 × L1.

[0135] According to the method of 19) above, by making the length L2 of the one-side expanded diameter portion (5) satisfy the condition 1 / 3 × L1≦L2≦1 / 2 × L1, the R-shape of the outer peripheral surface (51) of the one-side expanded diameter portion (5) can be made to have a curvature that does not cause stress concentration, and the length L2 of the one-side expanded diameter portion (5) can be prevented from becoming excessively large.

[0136] 20) In some embodiments, in the fatigue crack propagation test method described in any one of 14) to 19) above, the one-side large diameter portion (2) and the other-side large diameter portion (3) of the hollow test piece (1) have threaded portions (211, 311) formed on at least a portion of the outer circumferential surface (21, 31) in the longitudinal direction.

[0137] According to the method of 20) above, the hollow test piece (1) is fixed to a material testing machine (fatigue testing machine 110) via threaded portions (211, 311) formed on the outer circumferential surfaces (21, 31) of the one-side large diameter portion (2) and the other-side large diameter portion (3). This hollow test piece (1) does not have enlarged diameter portions such as flanges on the one-side large diameter portion (2) and the other-side large diameter portion (3) of the hollow test piece (1) for fixing it to the material testing machine (fatigue testing machine 110), so that a material with a relatively small outer diameter can be used as the base material for the hollow test piece (1).

[0138] 21) A hollow test piece (1) according to at least one embodiment of the present disclosure is a hollow test piece (1) extending along a longitudinal direction, and including: a one-side large diameter portion (2) formed on one side in the longitudinal direction; an other-side large diameter portion (3) formed on the other side in the longitudinal direction; and a parallel portion (4) formed between the one-side large diameter portion (2) and the other-side large diameter portion (3) and having an outer diameter smaller than the one-side large diameter portion (2) and the other-side large diameter portion (3), wherein the parallel portion (4) has an outer peripheral surface (41); an inner peripheral surface (42); and a stress concentration portion (notch 13) formed on the inner peripheral surface (42) and extending along the circumferential direction of the inner peripheral surface (42), When the inner diameter of the parallel portion (4) is defined as D1 and the length of the parallel portion (4) in the longitudinal direction is defined as L1, the length L1 of the parallel portion (4) satisfies the condition 0.5×D1≦L1≦3×D1.

[0139] According to the configuration of 21) above, the parallel portion (4) of the hollow test piece (1) is restricted in its inner diameter D1 by a machining tool used to form the stress concentration portion (notch 13) on the inner peripheral surface (42). The length L of the parallel portion (4) satisfies the condition 0.5 × D1 ≦ L1 ≦ 3 × D1, thereby homogenizing the stress generated during material testing (fatigue testing). The hollow test piece (1) including such a parallel portion (4) can have a small internal volume of the parallel portion (4), thereby reducing the volume of fluid introduced into the hollow test piece (1).

[0140] 22) In some embodiments, the hollow test piece (1) described in 21) above, wherein the stress concentration portion (notch 13) is an annular cutout portion (43) formed on the inner surface (42) of the parallel portion (4), and includes an annular cutout portion (43) having a V-shaped cross section.

[0141] According to the configuration of 22), the shape of the notch (43) can be determined according to the desired stress intensity factor to be applied to the hollow test piece (1) during material testing (fatigue testing). The annular notch (43) with a V-shaped cross section can be machined well using a machining tool for forming the stress concentration portion (notch 13), making it possible to ensure machining precision.

[0142] 23) In some embodiments, in the hollow test piece (1) described in 21) or 22) above, when the outer diameter of the parallel portion (4) is defined as D2, the outer diameter D2 of the parallel portion (4) satisfies the condition 1.5 × D1 ≦ D2 ≦ 2 × D1.

[0143] According to the configuration of 23), by setting the outer diameter D2 of the parallel portion 4 to satisfy the condition 1.5 × D1 ≦ D2 ≦ 2 × D1, the thickness of the parallel portion 4 required for the material test (fatigue test) can be ensured while preventing the thickness of the parallel portion 4 from becoming excessive. Here, when a relatively high stress is applied to the stress concentration portion (notch 13) in the material test (fatigue test), the thicknesses of the one-side large diameter portion 2 and the other-side large diameter portion 3 must be sufficiently larger than the thickness of the parallel portion 4. By preventing the thickness of the parallel portion 4 from becoming excessive, it is possible to prevent the one-side large diameter portion 2 and the other-side large diameter portion 3 from becoming excessively thick, which would result in an increase in size and weight of the hollow test piece 1.

[0144] 24) In some embodiments, in the hollow test piece (1) described in 23) above, when the outer diameter of the one-side large diameter portion (2) is defined as D3, the outer diameter D3 of the one-side large diameter portion (2) satisfies the condition 2 × D1 ≦ D3 ≦ 3 × D1.

[0145] According to the configuration of 24) above, by making the outer diameter D3 of the one-side large diameter portion (2) satisfy the condition 2 × D1 ≦ D3 ≦ 3 × D1, the thickness of the one-side large diameter portion (2) can be made sufficiently larger than the thickness of the parallel portion (4), and the thickness of the one-side large diameter portion (2) can be prevented from becoming excessively large.

[0146] 25) In some embodiments, the hollow test piece (1) described in any of 21) to 24) above further includes a one-side expanded diameter portion (5) formed between the one-side large diameter portion (2) and the parallel portion (4) and whose diameter increases toward the one-side large diameter portion (2), the one-side expanded diameter portion (5) having an R-shaped outer circumferential surface (51) that is convex inward in the radial direction; and a other-side expanded diameter portion (6) formed between the other-side large diameter portion (3) and the parallel portion (4) and whose diameter increases toward the other-side large diameter portion (3), the other-side expanded diameter portion (6) having an R-shaped outer circumferential surface (61) that is convex inward in the radial direction.

[0147] According to the configuration of 25) above, when stress is applied to the hollow test piece (1) in a material test (fatigue test), stress concentration can be suppressed in parts other than the parallel part (4) of the hollow test piece (1), such as the one-side expanded diameter part (5) and the other-side expanded diameter part (6).

[0148] 26) In some embodiments, in the hollow test piece (1) described in 25) above, when the longitudinal length of the one-side expanded portion (5) is defined as L2, the length L2 of the one-side expanded portion (5) satisfies the condition 1 / 3 × L1 ≦ L2 ≦ 1 / 2 × L1.

[0149] According to the configuration of 26) above, by making the length L2 of the one-side expanded diameter portion (5) satisfy the condition 1 / 3 × L1≦L2≦1 / 2 × L1, the R-shape of the outer peripheral surface (51) of the one-side expanded diameter portion (5) can be made to have a curvature that does not cause stress concentration, and the length L2 of the one-side expanded diameter portion (5) can be prevented from becoming excessively large.

[0150] 27) In some embodiments, the hollow test piece (1) is described in any one of 21) to 26) above, wherein the one-side large diameter portion (2) and the other-side large diameter portion (3) have threaded portions (211, 311) formed on at least a portion of the outer circumferential surface (21, 31) in the longitudinal direction.

[0151] According to the configuration of 27) above, the hollow test specimen (1) is fixed to a material testing machine (fatigue testing machine 110) via threaded portions (211, 311) formed on the outer circumferential surfaces (21, 31) of the one-side large diameter portion (2) and the other-side large diameter portion (3). This hollow test specimen (1) does not have enlarged diameter portions such as flanges for fixing the one-side large diameter portion (2) and the other-side large diameter portion (3) of the hollow test specimen (1) to the material testing machine (fatigue testing machine 110), so that a material with a relatively small outer diameter can be used as the base material for the hollow test specimen (1).

[0152] REFERENCE SIGNS LIST 1 Hollow test piece 2 One end (large diameter portion on one side) 3 Other end (large diameter portion on the other side) 4 Parallel portion 5 One-side expanded diameter portion 6 Other-side expanded diameter portion 11 Hollow portion 12 Inner surface 13 Notch 14 Fatigue pre-crack 15 Affected area 21, 31, 41, 51, 61 Outer surface 22, 32, 42, 52, 62 Inner surface 43 Notch portion 100 Fatigue test system 110 Fatigue testing machine 111 One-side support portion 112 Other-side support portion 113 Load application portion 114 Load cell 120 Fluid switching device 121 First fluid introduction line 122 Second fluid introduction line 123 Fluid discharge line 124 First tank 125 Second tank 126 Vacuum pump 127 Shared line 128 Discharge line 129 Compressor 130 Strain measuring device 140 Pressure measuring device 150 Heating device 431 First inclined surface 432 Second inclined surface 433 R-shaped portion A1, A2 Growth amount AR Acceleration rate B0, B1, B2 Fracture surface boundary CA Central axis F0, F1, F2 Fracture surface area R1, R2 Growth rate S1 Preparation step S2 Notch formation step S3 Fatigue pre-crack introduction step S4 Fatigue pre-crack acquisition step S10 First fatigue crack growth step S11, S21 Load step S12 First fluid holding step S20 Second fatigue crack growth step S22 Second fluid holding step S30 Crack growth amount acquisition step S40 Fracture surface coloring step

Claims

1. A fatigue crack propagation test method for a fatigue crack formed on the inner surface of a hollow test piece having a hollow portion into which a fluid can be introduced, comprising: a first fatigue crack propagation step of repeatedly applying a load to the hollow test piece under first environmental conditions to propagate a fatigue crack; a second fatigue crack propagation step of repeatedly applying a load to the hollow test piece to propagate the fatigue crack under second environmental conditions in which the first environmental conditions are different in at least one of the type, pressure, or temperature of the test fluid filled inside the hollow portion; and a crack propagation amount acquisition step of acquiring the amount of propagation of the fatigue crack in each of the first fatigue crack propagation step and the second fatigue crack propagation step by observing the fatigue crack formed in the hollow test piece.

2. A fatigue crack propagation testing method as described in claim 1, wherein in the crack propagation amount acquisition step, the propagation amount of the fatigue crack in each of the first fatigue crack propagation step and the second fatigue crack propagation step is acquired by observing the fracture surface of the hollow test piece.

3. A fatigue crack propagation testing method as described in claim 1, wherein in the crack propagation amount acquisition step, the propagation amount of the fatigue crack in each of the first fatigue crack propagation step and the second fatigue crack propagation step is acquired by observing a cross-section cut along the central axis direction of the hollow test piece.

4. A fatigue crack propagation test method as described in any one of claims 1 to 3, wherein the second environmental conditions are different from the first environmental conditions at least in the type of the test fluid.

5. A fatigue crack propagation test method as described in claim 4, wherein the type of test fluid under the first environmental condition is hydrogen gas, and the type of test fluid under the second environmental condition is an inert gas or air.

6. A fatigue crack propagation test method as described in claim 4, wherein the first fatigue crack propagation step includes a first fluid retention step of retaining the test fluid under the first environmental conditions inside the hollow portion for a predetermined period of time before repeatedly applying a load to the hollow test specimen.

7. A fatigue crack propagation test method as described in claim 4, wherein the second fatigue crack propagation step includes a second fluid retention step of retaining the test fluid under the second environmental conditions inside the hollow portion for a predetermined period of time before repeatedly applying a load to the hollow test specimen.

8. A fatigue crack propagation test method as described in claim 4, further comprising a fracture surface coloring step of retaining a gas containing oxygen gas inside the hollow portion for a predetermined period of time between the first fatigue crack propagation step and the second fatigue crack propagation step.

9. A fatigue crack propagation test method as described in any one of claims 1 to 3, further comprising a notch forming step of forming a notch on the inner surface of the hollow test piece prior to the first fatigue crack propagation step and the second fatigue crack propagation step.

10. A fatigue crack propagation test method as described in claim 9, further comprising a fatigue pre-crack introduction step which is performed after the notch formation step and before the first fatigue crack propagation step and the second fatigue crack propagation step, in which a load is repeatedly applied to the hollow test piece to generate a fatigue pre-crack extending from the notch formed on the inner surface of the hollow test piece.

11. A fatigue crack propagation test method as described in claim 10, wherein the fatigue pre-crack introduction step includes a fatigue pre-crack acquisition step of acquiring the depth of the fatigue pre-crack from the outer surface of the hollow test piece by non-destructive testing.

12. A fatigue crack propagation testing method as described in claim 11, wherein in the fatigue pre-crack acquisition step, the amount of propagation of the fatigue pre-crack in the hollow test specimen during the fatigue pre-crack introduction step is estimated based on the output of a strain measuring device attached to the outer surface of the hollow test specimen, and in the fatigue pre-crack introduction step, a load is repeatedly applied to the hollow test specimen until the amount of propagation of the fatigue pre-crack acquired in the fatigue pre-crack acquisition step exceeds a predetermined amount.

13. A fatigue crack propagation test method as described in any one of claims 1 to 3, further comprising a fatigue crack propagation amount estimation step of estimating the amount of propagation of a fatigue crack in the hollow test specimen during execution of the first fatigue crack propagation step or the second fatigue crack propagation step based on the output of a strain measuring device attached to the outer surface of the hollow test specimen, wherein during at least one of the first fatigue crack propagation step or the second fatigue crack propagation step, a load is repeatedly applied to the hollow test specimen until the amount of propagation of the fatigue crack estimated in the fatigue crack propagation amount estimation step exceeds a predetermined amount.

14. A fatigue crack propagation test method as set forth in claim 9, wherein the hollow test piece includes: a one-side large diameter portion formed on one side in the longitudinal direction of the hollow test piece; an other-side large diameter portion formed on the other side in the longitudinal direction; and a parallel portion formed between the one-side large diameter portion and the other-side large diameter portion and having an outer diameter smaller than the one-side large diameter portion and the other-side large diameter portion, the parallel portion having an outer peripheral surface, an inner peripheral surface, and the notch formed on the inner peripheral surface and extending circumferentially along the inner peripheral surface, wherein, when the inner diameter of the parallel portion is defined as D1 and the longitudinal length of the parallel portion is defined as L1, the length L1 of the parallel portion satisfies the condition 0.5 x D1 ≦ L1 ≦ 3 x D1.

15. A fatigue crack propagation test method as described in claim 14, wherein the notch of the hollow test piece is an annular cutout portion formed on the inner peripheral surface of the parallel portion, and includes an annular cutout portion having a V-shaped cross-sectional shape.

16. A fatigue crack propagation test method as set forth in claim 14, wherein, when the outer diameter of the parallel portion of the hollow test piece is defined as D2, the outer diameter D2 of the parallel portion satisfies the condition 1.5 x D1 ≦ D2 ≦ 2 x D1.

17. A fatigue crack propagation test method as set forth in claim 14, wherein, when the outer diameter of the large diameter portion on one side of the hollow test piece is defined as D3, the outer diameter D3 of the large diameter portion on one side satisfies the condition 2 x D1 ≦ D3 ≦ 3 x D1.

18. A fatigue crack propagation test method as described in claim 14, wherein the hollow test piece further includes: a one-side expansion portion formed between the one-side large diameter portion and the parallel portion, the one-side expansion portion having a diameter that increases toward the one-side large diameter portion, the one-side expansion portion having an outer peripheral surface with an R-shaped configuration that convex radially inward; and a other-side expansion portion formed between the other-side large diameter portion and the parallel portion, the other-side expansion portion having a diameter that increases toward the other-side large diameter portion, the other-side expansion portion having an outer peripheral surface with an R-shaped configuration that convex radially inward.

19. A fatigue crack propagation test method as described in claim 18, wherein, when the longitudinal length of the one side expanded portion of the hollow test piece is defined as L2, the length L2 of the one side expanded portion satisfies the condition 1 / 3 x L1 ≦ L2 ≦ 1 / 2 x L1.

20. A fatigue crack propagation test method as set forth in claim 14, wherein the one-side large diameter portion and the other-side large diameter portion of the hollow test piece have a threaded portion formed on at least a portion of the outer circumferential surface in the longitudinal direction.

21. A hollow test piece extending along the longitudinal direction, comprising: a one-side large diameter portion formed on one side in the longitudinal direction; an other-side large diameter portion formed on the other side in the longitudinal direction; and a parallel portion formed between the one-side large diameter portion and the other-side large diameter portion, the parallel portion having an outer diameter smaller than that of the one-side large diameter portion and the other-side large diameter portion, the parallel portion having an outer peripheral surface, an inner peripheral surface, and a stress concentration portion formed on the inner peripheral surface and extending circumferentially of the inner peripheral surface, wherein when the inner diameter of the parallel portion is defined as D1 and the longitudinal length of the parallel portion is defined as L1, the length L1 of the parallel portion satisfies the condition 0.5 x D1 ≦ L1 ≦ 3 x D1.

22. The hollow test piece according to claim 21, wherein the stress concentration portion is an annular cutout portion formed on the inner peripheral surface of the parallel portion, the cutout portion having a V-shaped cross section.

23. A hollow test piece according to claim 21 or 22, wherein, when the outer diameter of the parallel portion is defined as D2, the outer diameter D2 of the parallel portion satisfies the condition 1.5×D1≦D2≦2×D1.

24. A hollow test piece as set forth in claim 23, wherein, when the outer diameter of the large diameter portion on one side is defined as D3, the outer diameter D3 of the large diameter portion on one side satisfies the condition 2×D1≦D3≦3×D1.

25. A hollow test piece as described in claim 21 or 22, further comprising: a one-side expansion portion formed between the one-side large diameter portion and the parallel portion, the one-side expansion portion having a diameter that increases toward the one-side large diameter portion, the one-side expansion portion having an outer peripheral surface with an R-shaped configuration that convex radially inward; and a other-side expansion portion formed between the other-side large diameter portion and the parallel portion, the other-side expansion portion having a diameter that increases toward the other-side large diameter portion, the other-side expansion portion having an outer peripheral surface with an R-shaped configuration that convex radially inward.

26. A hollow test piece as described in claim 25, wherein, when the length of the longitudinal direction of the one side expanded portion is defined as L2, the length L2 of the one side expanded portion satisfies the condition 1 / 3 x L1 ≦ L2 ≦ 1 / 2 x L1.

27. A hollow test piece according to claim 21 or 22, wherein the one-side large diameter portion and the other-side large diameter portion have a threaded portion formed on at least a portion of the outer circumferential surface in the longitudinal direction.

Images