Development test method for fatigue crack

Abstract

To provide a development test method for a fatigue crack capable of quantizing a development amount or a development speed of the fatigue crack of a hollow test piece under different environmental conditions and an acceleration rate of the crack development caused by influences of the environments.SOLUTION: A development test method for a fatigue crack formed on an inner surface of a hollow test piece comprising a hollow part, in which a fluid can be introduced, includes: a first fatigue crack development step of developing the fatigue crack by iteratively adding a load to the hollow test piece under a first environmental condition; a second fatigue crack development step of developing the fatigue crack by iteratively adding a load to the hollow test piece under a second environmental condition that at least one of the kind, pressure and temperature of a test fluid filling the inside of the hollow part is different from that of the first environmental condition; and a crack development amount acquisition step of acquiring a development amount of the fatigue crack in each of the first fatigue crack development step and the second fatigue crack development step by observing the fatigue crack formed in the hollow test piece.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a fatigue crack growth testing method. [Background technology]

[0002] There is known an autoclave type fatigue testing machine (for example, Patent Document 1) that performs fatigue testing while placing a test piece, which is a sample for the fatigue test, in a pressure vessel filled with gas such as hydrogen gas. When performing a crack growth 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. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 2014-521103 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, a material testing method using hollow specimens with a hollow part in a hydrogen gas environment has been proposed. In this material testing method, hydrogen gas is sealed inside the hollow specimen, so that material testing can be performed with the inner surface exposed to hydrogen gas. This material testing method has a track record in slow strain rate tensile tests and fatigue tests, but not in fatigue crack growth tests in hydrogen gas. In fatigue crack growth tests in hydrogen gas using hollow specimens, cracks are generated and grow on the inner surface of the hollow specimen, so the quantification method of the crack growth amount based on the change in compliance, which is a parameter that indicates the side length of the crack or the opening amount of the crack, as used in conventional CT test specimens, cannot be used. In addition, when multiple test specimens with different environmental conditions are used, the depth of the fatigue pre-crack varies for each test specimen, so that even if the test is performed under the same loading condition, the test is performed with different stress intensity factors, which makes it difficult to compare the test results under different environmental conditions.

[0005] In view of the above-mentioned 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. [Means for solving the problem]

[0006] A fatigue crack growth test method according to at least one embodiment of the present disclosure includes: A method for testing the propagation of fatigue cracks formed on an inner surface of a hollow test piece having a hollow portion into which a fluid can be introduced, comprising the steps of: a first fatigue crack propagation step of repeatedly applying a load to the hollow test piece under a first environmental condition 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, and 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. Effect of the Invention

[0007] According to at least one embodiment of the present disclosure, a fatigue crack growth testing method is provided that can quantify the amount and rate of fatigue crack growth in a hollow test specimen under different environmental conditions, as well as the acceleration rate of crack growth due to environmental influences. [Brief description of the drawings]

[0008] [Figure 1] FIG. 2 is a flow diagram of a fatigue crack growth test method according to an embodiment of the present disclosure. [Diagram 2] 1 is a schematic cross-sectional view taken along the central axis direction of a hollow test piece that is a test subject of a fatigue crack propagation test method according to an embodiment of the present disclosure. FIG. [Diagram 3]FIG. 3 is a cross-sectional view taken along line AA of the hollow specimen shown in FIG. 2. [Figure 4] FIG. 1 is a schematic diagram of a fatigue testing system used in a fatigue crack growth testing method according to an embodiment of the present disclosure. [Diagram 5] FIG. 2 is an explanatory diagram for explaining a fatigue crack propagation test method according to an embodiment of the present disclosure. [Figure 6] FIG. 2 is an explanatory diagram 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. [Figure 7] FIG. 2 is an explanatory diagram for explaining a crack growth amount acquisition step in a fatigue crack growth test method according to an embodiment of the present disclosure. [Figure 8] FIG. 2 is an explanatory diagram for explaining a fatigue crack propagation test method according to an embodiment of the present disclosure. [Figure 9] FIG. 2 is an explanatory diagram for explaining a fatigue crack propagation test method according to an embodiment of the present disclosure. [Figure 10] FIG. 2 is an explanatory diagram for explaining a fracture surface coloring treatment in a fatigue crack propagation test method according to an embodiment of the present disclosure. [Figure 11] FIG. 2 is an explanatory diagram for explaining a fatigue crack propagation test method according to an embodiment of the present disclosure. [Figure 12] FIG. 1 is an explanatory diagram for explaining cross-sectional observation of a hollow test piece. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, some 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 the embodiments or shown in the drawings are merely illustrative examples and are not intended to limit the scope of the present disclosure.

[0010] 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 direction of a hollow test piece 1 that is a test subject of the fatigue crack growth test method according to an embodiment of the present disclosure. Fig. 3 is an AA cross-sectional view of the hollow test piece 1 shown in Fig. 2. The fatigue crack growth test method according to some embodiments is a fatigue crack growth test method for a fatigue crack formed on an inner peripheral surface (inner surface) 12 of a hollow test piece 1 (see Fig. 2) having a hollow portion 11 into which a fluid can be introduced.

[0011] (Hollow test piece) 2 and 4, the hollow specimen 1 has a longitudinal direction along the direction in which the central axis CA of the hollow specimen 1 extends. The hollow 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 specimen 1. The hollow portion 11 is defined by an inner circumferential surface 12. Hereinafter, the radial direction of the hollow specimen 1 may be simply abbreviated to the radial direction.

[0012] 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 figures), the other end portion 3 formed on the other side in the longitudinal direction (upper side in the figures), 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.

[0013] In the illustrated embodiment, the hollow specimen 1 has a notch (cutout) 13 formed on the inner peripheral surface (inner surface) 12A of the parallel portion 4 and extending along the circumferential direction of the hollow specimen 1, and a fatigue pre-crack 14 extending from the notch 13 toward the outside in the radial direction of the hollow specimen 1. In the embodiment shown in Figs. 2 and 3, the notch 13 is an annular groove extending along the circumferential direction of the hollow specimen 1, but 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 repeatedly applying a load L to the hollow specimen 1, and extends from the outer periphery of the notch 13, which is a stress concentration portion, toward the outside in the radial direction of the hollow specimen 1. The fatigue pre-crack 14 may be formed in an annular shape extending along the circumferential direction of the hollow specimen 1, as shown in Fig. 3.

[0014] The hollow test piece 1 is used for a fatigue crack growth test, and is attached to a fatigue testing machine 110. Specifically, the hollow test piece 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 piece 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 on the lower side in the vertical direction and the other end 3 located on the upper side in the vertical direction.

[0015] (Fatigue Test System) 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 FIG. 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.

[0016] (Fatigue testing machine) 4, the fatigue testing machine 110 includes a one-side supporting part 111, an other-side supporting part 112, a load applying part (actuator) 113, and a load cell 114. The one-side supporting part 111 supports one side in the longitudinal direction of the hollow test piece 1, specifically, one end part 2. The other-side supporting part 112 supports the other side in the longitudinal direction of the hollow test piece 1, specifically, the other end part 3.

[0017] 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 or 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 side in the 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 electric signal.

[0018] (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 end of the first fluid inlet line 121, the second fluid inlet line 122, and the fluid outlet line 123 is 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 for sucking fluids from the hollow portion 11.

[0019] In the illustrated embodiment, the first fluid introduction line 121 and the second fluid introduction line 122 join at a first junction P1, and the downstream side (one end side) of the first junction P1 becomes a shared line 127. The fluid discharge line 123 joins with the shared line 127 at a second junction P2.

[0020] 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 upstream of the second junction P2 of the shared line 127.

[0021] When the test fluid is a first fluid, the first fluid is introduced from a first tank 124 to the hollow portion 11 through a first fluid introduction line 121. When the test fluid is a second fluid, the second fluid is introduced from a second tank 125 to the hollow portion 11 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 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 is 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 vacuum drawing, 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 growth behavior described later can be appropriately given.

[0022] 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 piece 1. 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 guided to the hollow portion 11. In the embodiment shown in Fig. 4, the pressure measuring device 140 is provided downstream (one end side) of the compressor 129 of the shared line 127.

[0023] (Fatigue crack growth test method) As shown in FIG. 1, the fatigue crack growth test method includes a first fatigue crack growth step S10, a second fatigue crack growth step S20, and a crack growth amount acquisition step S30.

[0024] (First fatigue crack growth step) Fig. 5 is an explanatory diagram for explaining a fatigue crack propagation test method according to an embodiment of the present disclosure. Fig. 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 Fig. 5, in a first fatigue crack propagation step S10, a load L is repeatedly applied to the hollow specimen 1 under a first environmental condition 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 specimen 1 by the fatigue testing machine 110 is defined as N1.

[0025] (Second fatigue crack growth step) As shown in Fig. 5, in the second fatigue crack propagation step S20, a load L is repeatedly applied to the hollow 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 specimen 1 by the fatigue testing machine 110 is defined as N2. The second environmental condition is different from the first environmental condition 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 may be performed before the first fatigue crack propagation step S10.

[0026] 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.

[0027] (Crack extension acquisition step) 6 is an explanatory diagram for explaining a crack growth amount acquisition step S30 in a fatigue crack growth test method according to an embodiment of the present disclosure. In the crack growth amount acquisition step S30, 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.

[0028] Fig. 5 shows a schematic representation of the fracture surface of the hollow specimen 1. Fig. 6 shows a schematic representation of an SEM image of the fracture surface of the hollow specimen 1 produced by a scanning electron microscope (SEM). In Figs. 5 and 6, the fracture surface area of ​​the fatigue pre-crack 14 is designated as F0, the fracture surface area caused by the propagation of the fatigue crack in the first fatigue crack propagation step S10 is designated as F1, and the fracture surface area caused by the propagation of the fatigue crack in the second fatigue crack propagation step S20 is designated as F2.

[0029] The fracture boundary formed between the fracture surface region F1 and a fracture surface region (e.g., F0) located radially inward from the fracture surface region F1 and adjacent to the fracture surface region F1 is defined as B0. The fracture boundary formed between the fracture surface region F1 and a fracture surface region F2 located radially outward from the fracture surface region F1 and adjacent to the fracture surface region F1 is defined as B1. The fracture boundary formed between the fracture surface region F2 and a fracture surface region (e.g., F1) located radially outward from the fracture surface region F2 and adjacent to the fracture surface region F2 is defined as 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.

[0030] The fatigue crack propagation amount A1 in the first fatigue crack propagation step S10 is the radial length of the fracture surface region F1, i.e., the radial distance between the fracture surface boundary B0 and the fracture surface boundary B1. The fatigue crack propagation amount A1 can be obtained, for example, by measuring the radial distance between the fracture surface boundary B0 and the fracture surface boundary B1 that appear in an SEM image. The fatigue crack propagation amount A1 may be the average value of the radial distance between the fracture surface boundary B0 and the fracture surface boundary B1 measured at each of a plurality of points in the circumferential direction of the hollow test piece 1.

[0031] The fatigue crack propagation amount A2 in the second fatigue crack propagation step S20 is the radial length of the fracture surface region F2, i.e., the radial distance between the fracture surface boundary B1 and the fracture surface boundary B2. The fatigue crack propagation amount A2 can be obtained, for example, by measuring the radial distance between the fracture surface boundary B1 and the fracture surface boundary B2 appearing in an SEM image. The fatigue crack propagation amount A2 may be the average value of the radial distance between the fracture surface boundary B1 and the fracture surface boundary B2 measured at each of a plurality of points in the circumferential direction of the hollow test piece 1. It is also possible to use a metallurgical (optical) microscope instead of a scanning electron microscope (SEM) for fracture surface observation.

[0032] 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.

[0033] A fatigue crack propagation test method according to some embodiments includes the above-mentioned first fatigue crack propagation step S10, the above-mentioned second fatigue crack propagation step S20, and the above-mentioned crack propagation amount acquisition step S30. In the crack propagation amount acquisition step S30, the fatigue cracks formed in the hollow test piece 1 are observed, and the fatigue crack propagation amounts A1 and A2 in each of the fatigue crack propagation steps S10 and S20 can be acquired. Then, the fatigue crack propagation rates R1 and R2 in each of the fatigue crack propagation steps S10 and S20 can be calculated from the fatigue crack propagation amounts A1 and A2 in each of the fatigue crack propagation steps S10 and S20 and the number of repetitions N1 and N2. Therefore, according to the above-mentioned method, the fatigue crack propagation amounts A1 and A2 and the propagation rates R1 and R2 in the same hollow test piece 1 under multiple different environmental conditions can be quantified. In addition, the acceleration rate AR of the fatigue crack propagation rate due to changes in environmental conditions can also be quantified.

[0034] 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. The acceleration rate AR of the fatigue crack growth rate may be calculated by the following method. FIG. 7 is an explanatory diagram for explaining the relationship between the stress intensity factor range ΔK and the crack growth rates R1 and R2 obtained by the fatigue crack growth test method according to one embodiment of the present disclosure. FIG. 7 shows a graph with the horizontal axis representing the stress intensity factor range ΔK and the vertical axis representing the above-mentioned crack growth rates R1 and R2. The stress intensity factor range ΔK is calculated from the depth of the fatigue crack and the load condition. 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 a plurality of crack growth rates R1 and the stress intensity factor range ΔK obtained by performing the first fatigue crack growth step S10 a plurality of times. The second straight line L2 can be calculated from a plurality of crack growth rates R2 obtained by performing the second fatigue crack growth step S20 a plurality of 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 straight line L1 and the second straight line L2.

[0035] (Fracture surface observation) In some embodiments of the fatigue crack propagation test method, in the above-mentioned crack propagation amount acquisition step S30, the fracture surface of the hollow test piece 1 is observed to obtain the fatigue crack propagation amounts A1, A2 in the first fatigue crack propagation step S10 and the second fatigue crack propagation step S20, respectively.

[0036] 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 piece 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 piece 1.

[0037] In some embodiments of the fatigue crack propagation test method, the second environmental condition is different from the first environmental condition in at least the type of test fluid. The second environmental condition may be the same as the first environmental condition in the pressure and temperature of the test fluid, or may be different from the first environmental condition in at least one of the pressure and the temperature of the test fluid.

[0038] 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 piece 1 under a plurality of environmental conditions in which the type of test fluid filled inside the hollow portion 11 is different.

[0039] 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.

[0040] According to the above method, the fatigue crack propagation amounts A1, A2 and propagation rates R1, R2 of the same hollow test piece 1 can be quantified when the test fluid is hydrogen gas and when the test fluid is an inert gas or air. This allows the acceleration rate of the fatigue crack propagation rate when the test fluid is hydrogen gas to be quantified based on the test fluid being an inert gas or air. The fatigue crack propagation rate R1 in FIG. 7 shows the case where the test fluid is hydrogen gas, and the fatigue crack propagation rate R2 in FIG. 7 shows the case where the test fluid is an inert gas or air. In this case, the above-mentioned acceleration rate AR of the fatigue crack propagation rate is the acceleration rate of the fatigue crack propagation rate when the test fluid is hydrogen gas, based on the test fluid being an inert gas or air.

[0041] 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 piece 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 a nuclear environment, such as cooling water for cooling a nuclear reactor, and seawater containing salt.

[0042] In some embodiments of the fatigue crack propagation test method, the type of test fluid in the first and second environmental conditions is hydrogen gas. In the second environmental conditions, at least one of the pressure and temperature of the test fluid is different from that in the first environmental conditions. When the test fluid is hydrogen gas, the crack propagation rate changes with the change in temperature and pressure of the hydrogen gas, and the fracture surface appearance changes. Therefore, even if the test fluid is limited to hydrogen gas, the propagation amounts A1 and A2 and the propagation rates R1 and R2 of the fatigue crack under each environmental condition can be quantified by changing the temperature and pressure.

[0043] (First Fluid Retention Step) 8 and 9 are explanatory views for explaining a fatigue crack growth test method according to an embodiment of the present disclosure. In the fatigue crack growth test method according to some embodiments, the above-mentioned first fatigue crack growth step S10 includes a first fluid holding step S12 of holding a test fluid under a first environmental condition 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 piece 1, as shown in FIG. 8 and FIG.

[0044] 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 in the case where the first fluid holding step S12 is not performed. As a result, in the first fatigue crack propagation step S10, the fatigue crack is propagated in a manner that better reflects the influence of the first fluid, so that the fatigue crack propagation amount A1 and propagation rate R1 quantified in the crack propagation amount acquisition step S30 better reflect the influence of the first fluid.

[0045] In some embodiments, a heating device (e.g., a heater) 150 is attached to the outer peripheral surface of the parallel portion 4 of the hollow test piece 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 fluid inside the hollow test piece 1 may be held at a predetermined temperature or higher by the heating device 150. Here, the maximum temperature of the fluid inside the hollow test piece 1 is limited to a temperature that does not affect the metal structure of the hollow test piece 1. The higher the temperature of the fluid inside the hollow test piece 1, the more the solid solution of the fluid is promoted, so that 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 piece 1 is maintained at a higher temperature than in the loading step S11.

[0046] (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 a loading step S21 of repeatedly applying a load L to the hollow test specimen 1, as shown in Figures 8 and 9.

[0047] According to the above method, in the second fluid holding step S22, a fluid (second fluid) under the second environmental conditions is held inside the hollow portion 11 for a predetermined period of time, so that a large amount of the second fluid can be dissolved in the hollow test piece 1, compared to the case where the second fluid holding step S22 is not performed. Also, if the first fluid is dissolved in the hollow test piece 1 in the first fluid holding step S12, the first fluid dissolved in the hollow test piece 1 can be discharged in the second fluid holding step S22. As a result, in the second fatigue crack propagation step S20, the fatigue crack is propagated in a manner that better reflects the influence of the second fluid, so that the fatigue crack propagation amount A2 and propagation speed R2 quantified in the crack propagation amount acquisition step S30 better reflect the influence of the second fluid.

[0048] In the second fluid holding step S22, the fluid inside the hollow test piece 1 may be held at a predetermined temperature or higher by the heating device 150. Here, the maximum temperature of the fluid inside the hollow test piece 1 is limited to a temperature that does not affect the metal structure of the hollow test piece 1. The higher the temperature of the fluid inside the hollow test piece 1, the more the solid solution of the fluid is promoted, so that 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 piece 1 is maintained at a higher temperature than in the loading step S21.

[0049] The heating by the heating device 150 is not limited to heater heating, and electric furnace heating or high-frequency heating may be adopted. High-frequency heating can directly heat only the hollow test piece 1 and is unlikely to increase the surrounding atmospheric temperature, so that the risk of explosion can be reduced even if an explosive fluid such as hydrogen gas leaks from the hollow test piece 1.

[0050] (Fracture surface coloring step) In some embodiments, the fatigue crack propagation test method further includes a fracture surface coloring step S40 of holding a gas containing oxygen gas (e.g., air) 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, as shown in Fig. 9. 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 starts, or may be performed after the second fatigue crack propagation step S20 and before the first fatigue crack propagation step S10 starts. The fracture surface coloring step S40 may be performed multiple times as shown in Fig. 9.

[0051] FIG. 10 is an explanatory diagram for explaining 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 held inside the hollow portion 11 for a predetermined period of time, so that the fracture surface caused by the fatigue crack is oxidized (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 is attached to the fracture surface surface. By performing the above-mentioned 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 made clear. As a result, 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 mapping of oxygen atoms or the like. By visualizing this fracture surface boundary, it is possible to reduce the subjectivity in quantification of the propagation amounts A1 and A2 during fracture surface observation, and therefore the accuracy of the propagation amounts A1 and A2 is improved.

[0052] (Environmental conditions set according to the metal material of the hollow test piece) 11 is an explanatory diagram for explaining a fatigue crack propagation test method according to an embodiment of the present disclosure. In the fatigue crack propagation test method according to some embodiments, the environmental conditions of at least one of the first fluid holding step S12 and the second fluid holding step S22 are changed depending on the metal material constituting the hollow test piece 1. In the embodiment shown in FIG. 11, the test fluid in one of the first fluid holding step S12 or the second fluid holding step S22 is hydrogen gas, and the other test fluid is air or an inert gas.

[0053] As shown in FIG. 11, when the hydrogen diffusion rate of the metal material constituting the hollow test piece 1 is low (the hydrogen diffusion rate is less than a predetermined value) ("No" in step S13), the period of the first fluid holding step S12 and the second fluid holding step S22 is made longer and the temperature of the test fluid is made higher than when the hydrogen diffusion rate of the metal material constituting the hollow test piece 1 is high (the hydrogen diffusion rate is equal to or higher than a predetermined value) ("Yes" in step S13). This makes it possible to make the environmental conditions of the first fluid holding step S12 and the second fluid holding step S22 appropriate for the metal material constituting the hollow test piece 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.

[0054] In the fatigue crack growth test method according to some embodiments, the environmental conditions of the fracture surface coloring step S40 are changed depending on the metal material constituting the hollow test piece 1. In the embodiment shown in FIG. 11, the test fluid in one of the first fluid holding step S12 and the second fluid holding step S22 is hydrogen gas, and the test fluid in the other is air or an inert gas. As shown in FIG. 11, when the metal material constituting the hollow test piece 1 is a metal material (a metal material susceptible to the influence of hydrogen, for example, carbon steel or general low alloy steel) with low oxidation resistance ("Yes" in step S41) and high hydrogen embrittlement susceptibility ("Yes" in step S42), the fracture surface boundary is clear, so the fracture surface coloring step S40 is not performed.

[0055] When the metallic material constituting the hollow test piece 1 is a metallic material (first metallic material, for example, low alloy steel with reduced hydrogen sensitivity) having low oxidation resistance ("Yes" in step S41) and low hydrogen embrittlement susceptibility ("No" in step S42), the fracture surface coloring step S40 is performed. When the metallic material constituting the hollow test piece 1 is a metallic material (second metallic material, for example, stainless steel) having high oxidation resistance ("No" in step S41), the fracture surface coloring step S40 is also performed. When the metallic material constituting the hollow test piece 1 is the second metallic material, the temperature of the test fluid in the fracture surface coloring step S40 is made higher than when the metallic material constituting the hollow test piece 1 is the first metallic material. This allows the feasibility of performing the fracture surface coloring step S40 and the environmental conditions to be appropriate according to the metallic material constituting the hollow test piece 1.

[0056] (Hollow specimen preparation steps) 1, a fatigue crack propagation test method according to some embodiments includes a preparation step S1 of preparing a hollow test piece 1 having the above-mentioned notch 13 and fatigue pre-crack 14, prior to a first fatigue crack propagation step S10 and a second fatigue crack propagation step S20. The preparation step S1 includes a notch formation step S2 and a fatigue pre-crack introduction step S3.

[0057] (Notch formation step) 1, a fatigue crack propagation test method according to some embodiments further includes a notch forming step S2 of forming a notch 13 (see FIG. 2) on an inner circumferential surface (inner surface) 12A of the hollow test piece 1 prior to the first fatigue crack propagation step S10 and the second fatigue crack propagation step S20. 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 piece 1.

[0058] According to the above method, the notches 13 that become the starting points of fatigue cracks can be formed in the inner surface 12A of the hollow test piece 1 in the notch forming step S2.

[0059] As shown in FIG. 1, the 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 repeated load is applied to the hollow test piece 1 to generate a fatigue pre-crack 14 extending from a notch 13 formed on the inner surface 12A of the hollow test piece 1. It is preferable that the fatigue pre-crack 14 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. In order to clarify the above-mentioned fracture boundary B0, the repeated load in the fatigue pre-crack introduction step S3 may be smaller than the repeated load L in the first fatigue crack propagation step S10 and the second fatigue crack propagation step S20.

[0060] 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 makes it possible to eliminate the influence of the notch processing on the fatigue crack propagation in each fatigue crack propagation step S10, S20, and therefore makes it possible to quantify the propagation amounts A1, A2 and propagation rates R1, R2 of the fatigue crack relative to the original metal structure of the hollow test piece 1.

[0061] (Fatigue pre-crack acquisition step) 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 of acquiring the depth (radial length) of the fatigue pre-crack 14 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.

[0062] 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 range of influence 15 of the notch processing. Note that the depth of the fatigue pre-crack 14 in the fatigue pre-crack introduction step S3 may be estimated by a known back gauge method.

[0063] In some embodiments, in the above-mentioned fatigue pre-crack acquisition step S4, the propagation amount of the fatigue pre-crack 14 in the hollow specimen 1 during the execution of 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 above-mentioned 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 by a known back gauge method.

[0064] 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.

[0065] (Fatigue crack growth estimation step) A fatigue crack growth test method according to some embodiments further includes a fatigue crack growth estimation step of estimating the fatigue crack growth amounts A1, A2 of the hollow 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 measuring device 130 attached to the outer surface of the hollow 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 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 measuring device 130 by a known back gauge method.

[0066] 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, A2 in each fatigue crack propagation step S10, S20 exceed a predetermined amount. In the fatigue crack propagation amount estimation step, the fatigue crack propagation amounts 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 L is repeatedly applied until the fatigue crack propagation amounts A1, A2 estimated in the fatigue crack propagation amount estimation step exceed a predetermined amount, the fatigue crack propagation under each environmental condition can be clearly grasped.

[0067] (Cross-section observation) In some of the embodiments described above, the fracture surface of the hollow specimen 1 is observed, but a cross section cut along the central axis CA direction of the hollow specimen may be observed. Fig. 12 is an explanatory diagram for explaining cross-sectional observation of the hollow specimen 1. In the fatigue crack growth test method according to some embodiments, in the above-mentioned crack growth amount acquisition step S30, the cross section cut along the central axis CA direction of the hollow 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.

[0068] According to the above method, in a cross section cut along the central axis CA of the hollow test piece 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, so that the fatigue crack propagation area for each environment can be identified by observing the cross section. Therefore, in the crack propagation amount acquisition step S30, by observing the above cross section of the hollow test piece 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 areas in each of the fatigue crack propagation steps S10 and S20 may be measured, and these measured values ​​may be the fatigue crack propagation amounts A1 and A2. Note that a scanning electron microscope (SEM) or a metallurgical (optical) microscope may be used for the cross section observation.

[0069] In this specification, expressions expressing relative or absolute configuration, such as "in a certain direction," "along a certain direction," "parallel," "orthogonal," "center," "concentric," or "coaxial," do not only strictly represent such a configuration, but also represent 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 indicating that things are in an equal state, such as "identical," "equal," and "homogeneous," not only indicate a state of strict equality, but also indicate 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 describing shapes such as a rectangular shape or a cylindrical shape do not only refer to shapes such as a rectangular shape or a cylindrical shape in the strict geometric sense, but also refer to shapes that include uneven portions, chamfered portions, etc., to the extent that the same effect can be obtained. In addition, in this specification, the expressions "comprise," "include," or "have" a certain element are not exclusive expressions that exclude the presence of other elements.

[0070] The present disclosure is not limited to the above-described embodiments, and includes modifications to the above-described embodiments and appropriate combinations of these modifications.

[0071] The contents described in the above-mentioned embodiments can be understood, for example, as follows.

[0072] 1) A fatigue crack propagation test method according to at least one embodiment of the present disclosure, A method for testing the propagation of 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, comprising the steps of: A first fatigue crack propagation step (S10) of propagating a fatigue crack by repeatedly applying a load to the hollow test piece (1) under a first environmental condition; a second fatigue crack propagation step (S20) of repeatedly applying a load to the hollow test piece (1) 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, and temperature of the test fluid filled inside the hollow portion (11); and a crack propagation amount acquisition step (S30) of 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).

[0073] According to the method of 1), in the crack growth amount acquisition step (S30), the fatigue crack formed in the hollow test piece (1) is observed, and the fatigue crack growth amount (A1, A2) in each of the fatigue crack growth steps (S10, S20) can be acquired. Then, the fatigue crack growth rate (R1, R2) in each fatigue crack growth step (S10, S20) can be calculated from the fatigue crack growth amount (A1, A2) in each fatigue crack growth step (S10, S20) and the number of repetitions (N1, N2). Therefore, according to the method of 1), the fatigue crack growth amount (A1, A2) and growth rate (R1, R2) in the same hollow test piece (1) under multiple different environmental conditions can be quantified. In addition, the acceleration rate of the fatigue crack growth rate due to changes in environmental conditions can also be quantified.

[0074] 2) In some embodiments, the fatigue crack growth test method according to 1) above comprises: In the crack growth amount acquisition step (S30), By observing the fracture surface of the hollow test piece (1), the fatigue crack propagation amounts (A1, A2) in the first fatigue crack propagation step (S10) and the second fatigue crack propagation step (S20), respectively, are obtained.

[0075] According to the method of 2) above, since 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 piece (1), 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 piece (1).

[0076] 3) In some embodiments, the fatigue crack growth test method described in 1) above comprises: And, In the crack growth amount acquisition step (S30), By observing a cross section cut along the central axis direction of the hollow test piece (1), the fatigue crack propagation amounts (A1, A2) in the first fatigue crack propagation step (S10) and the second fatigue crack propagation step (S20), respectively, are obtained.

[0077] According to the method of 3) above, since the fatigue crack propagation in each of the multiple fatigue crack propagation steps (S10, S20) appears in the cross section cut along the central axis direction of the hollow test piece (1) in terms of the fatigue crack propagation direction, amplitude in the central axis direction, etc., the fatigue crack propagation area for each environment can be identified by observing the cross section. 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 cross section of the hollow test piece (1).

[0078] 4) In some embodiments, the fatigue crack growth test method according to any one of 1) to 3) above, The second environmental conditions differ from the first environmental conditions at least in the type of the test fluid.

[0079] 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).

[0080] 5) In some embodiments, the fatigue crack growth test method according to 4) above, the type of the test fluid at the first environmental condition is hydrogen gas; The type of test fluid at the second environmental condition is an inert gas or air.

[0081] According to the method of 5) above, it is possible to quantify the amount of fatigue crack growth (A1, A2) and growth 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 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.

[0082] 6) In some embodiments, the fatigue crack growth test method according to 4) or 5) above, The first fatigue crack propagation step (S10) includes: The method 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).

[0083] According to the method of 6) above, 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 in the case where the first fluid holding step (S12) is not performed. As a result, in the first fatigue crack propagation step (S10), the fatigue crack is propagated in a manner that better 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) better reflect the influence of the first fluid.

[0084] 7) In some embodiments, the fatigue crack growth test method according to any one of 4) to 6) above, The second fatigue crack propagation step (S20) includes: The method 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).

[0085] According to the method of 7) above, in the second fluid holding step (S22), a fluid (second fluid) under the second environmental conditions is held inside the hollow portion (11) for a predetermined period of time, so that a large amount of the second fluid can be dissolved in the hollow test piece (1) compared to the case where the second fluid holding step (S22) is not performed. Also, if the first fluid is dissolved in the hollow test piece (1) in the first fluid holding step (S12), the first fluid dissolved in the hollow test piece (1) can be discharged in the second fluid holding step (S22). As a result, in the second fatigue crack propagation step (S20), the fatigue crack is propagated in a manner that more reflects the influence of the second fluid, so that 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.

[0086] 8) In some embodiments, the fatigue crack growth test method according to any one of 4) to 7) above, Between the first fatigue crack propagation step (S10) and the second fatigue crack propagation step (S20), the method further includes a fracture surface coloring step (S40) of retaining a gas including oxygen gas inside the hollow portion (11) for a predetermined period of time.

[0087] According to the method of 8) above, in the fracture surface coloring step (S40), a gas containing oxygen gas is held inside the hollow portion (11) for a predetermined period of time, thereby oxidizing the fracture surface caused by the fatigue crack (coloring treatment). By carrying out the above 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. As a result, when observing the fatigue crack formed in the hollow test piece (1), the fracture surface boundary for each environmental condition can be visualized by performing mapping of oxygen atoms, etc.

[0088] 9) In some embodiments, the fatigue crack growth test method according to any one of 1) to 8) above, The method further includes a notch forming step (S2) of forming a notch (13) in the inner surface (12A) of the hollow test piece (1) prior to the first fatigue crack propagation step (S10) and the second fatigue crack propagation step (S20).

[0089] According to the method of 9) above, in the notch forming step (S2), a notch (13) which becomes the starting point of a fatigue crack can be formed on the inner surface (12A) of the hollow test piece (1).

[0090] 10) In some embodiments, the fatigue crack growth test method according to 9) above, The method 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 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 (12A) of the hollow test piece (1).

[0091] According to the method of 10) above, by generating a fatigue pre-crack (14) in the fatigue pre-crack introduction step (S3), the fatigue crack propagates outside the range of influence (15) of the notch processing in each fatigue crack propagation step (S10, S20). This makes it possible to eliminate the effect of the notch processing 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 piece (1).

[0092] 11) In some embodiments, the fatigue crack growth test method according to 10) above comprises: 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.

[0093] According to the method of 11) above, in the fatigue pre-crack acquisition step (S4), the depth of the fatigue pre-crack (14) is obtained 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 range of influence (15) of the notch processing.

[0094] 12) In some embodiments, the fatigue crack growth test method according to any one of 1) to 11) above, The method 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), 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 fatigue crack propagation amount (A1, A2) estimated in the fatigue crack propagation amount estimation step exceeds a predetermined amount.

[0095] According to the method of 12) above, 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. [Explanation of symbols]

[0096] 1 Hollow test piece 2 One end 3 Other end 4 Parallel section 11 Hollow part 12,12A Inner surface 13 Notches 14 Fatigue Precrack 15. Scope of impact 100 Fatigue Test System 110 Fatigue Testing Machine 111 One side support part 112 Other side support part 113 Load bearing section 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 Release Line 129 Compressor 130 Strain Measuring Device 140 Pressure measuring device 150 Heating device A1,A2 Progress AR acceleration rate B0, B1, B2 fracture boundary CA center axis F0, F1, F2 fracture surface area R1,R2 Progress rate S1 Preparation Steps 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 acquisition step S40 Fracture surface coloring step

Claims

1. A method for testing the propagation of fatigue cracks formed on an inner surface of a hollow test piece having a hollow portion into which a fluid can be introduced, comprising the steps of: a first fatigue crack propagation step of repeatedly applying a load to the hollow test piece under a first environmental condition 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 from the second environmental conditions in terms of at least one of the type, pressure, and temperature of the test fluid filled inside the hollow portion; and a crack growth amount acquisition step of acquiring the growth amount 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. Fatigue crack growth test method.

2. In the crack growth amount acquisition step, Obtaining the propagation amount of the fatigue crack in each of the first fatigue crack propagation step and the second fatigue crack propagation step by observing a fracture surface of the hollow test specimen. The fatigue crack propagation test method according to claim 1.

3. In the crack growth amount acquisition step, Obtaining the propagation amount of the fatigue crack in each of the first fatigue crack propagation step and the second fatigue crack propagation step by observing a cross section cut along the central axis direction of the hollow test specimen. The fatigue crack propagation test method according to claim 1.

4. The second environmental condition is different from the first environmental condition in at least the type of the test fluid. The fatigue crack propagation test method according to any one of claims 1 to 3.

5. the type of test fluid at the first environmental condition is hydrogen gas; The type of the test fluid at the second environmental condition is an inert gas or air; The fatigue crack propagation test method according to claim 4.

6. The first fatigue crack propagation step includes: a first fluid retention step of retaining the test fluid at the first environmental condition inside the hollow portion for a predetermined period of time before repeatedly applying a load to the hollow test specimen; The fatigue crack propagation test method according to claim 4.

7. The second fatigue crack propagation step includes: a second fluid retention step of retaining the test fluid at the second environmental condition within the hollow portion for a predetermined period of time prior to applying a cyclic load to the hollow test specimen; The fatigue crack propagation test method according to claim 4.

8. The method further includes 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. The fatigue crack propagation test method according to claim 4.

9. The method further includes a notch forming step of forming a notch on the inner surface of the hollow test specimen prior to the first fatigue crack propagation step and the second fatigue crack propagation step. The fatigue crack propagation test method according to any one of claims 1 to 3.

10. a fatigue pre-crack introduction step that 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; The fatigue crack growth test method according to claim 9.

11. 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 inspection. The fatigue crack propagation test method according to claim 10.

12. In the fatigue pre-crack acquisition step, a propagation amount of the fatigue pre-crack of the hollow test specimen during the fatigue pre-crack introduction step is estimated based on an output of a strain measuring device attached to an outer surface of the hollow test specimen; In the fatigue pre-crack introduction step, a load is repeatedly applied to the hollow test piece until the extension amount of the fatigue pre-crack acquired in the fatigue pre-crack acquisition step exceeds a predetermined amount. The fatigue crack propagation test method according to claim 11.

13. The method further includes a fatigue crack growth amount estimation step of estimating the growth amount of the fatigue crack in the hollow test specimen during the first fatigue crack growth step or the second fatigue crack growth step based on the output of a strain measuring device attached to the outer surface of the hollow test specimen; In at least one of the first fatigue crack propagation step and the second fatigue crack propagation step, a load is repeatedly applied to the hollow test piece until the propagation amount of the fatigue crack estimated in the fatigue crack propagation amount estimation step exceeds a predetermined amount. The fatigue crack propagation test method according to any one of claims 1 to 3.

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