Apparatus for specifying fatigue strength of fiber-reinforced composite material, and method for specifying fatigue strength of fiber-reinforced composite material

Abstract

To provide an apparatus for specifying the fatigue strength of fiber-reinforced composite materials that can quickly and highly accurately measure the fatigue strength of fiber-reinforced composite materials.SOLUTION: An apparatus for specifying the fatigue strength of fiber-reinforced composite materials includes: load application means 11 configured to apply a repeated load at a predetermined frequency to a fiber-reinforced composite material being a measurement target such that the stress amplitude changes sequentially every time the number of load repetitions reaches a predetermined frequency; temperature measurement means 12 configured to measure the temperature change of the measurement target in each predetermined period; first calculation means configured to calculate, in each period, the thermoelastic temperature amplitude and average temperature of the measurement target from data of the temperature change of the measurement target; second calculation means configured to calculate, in each period, a non-dimensional thermoelastic temperature amplitude of the measurement target by a predetermined method; and fatigue strength specification means configured to specify the fatigue strength of the fiber-reinforced composite material by the predetermined method using the non-dimensional thermoelastic temperature amplitude in each period.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an apparatus for determining the fatigue strength of a fiber-reinforced composite material and a method for determining the fatigue strength of a fiber-reinforced composite material. [Background technology]

[0002] Conventionally, various methods for identifying fatigue strength of various materials have been studied for the purpose of examining the reliability of strength during use. For example, JP 2021-131357 A (Patent Document 1) discloses a fatigue strength evaluation method for a composite material sample while repeatedly applying an external load, the fatigue strength evaluation method including: a step (a) of measuring the temperature amplitude generated in the sample by applying the external load with an infrared thermography device and creating a first temperature amplitude distribution; a step (b) of measuring the temperature amplitude generated in the sample by applying the external load after the step (a) with the infrared thermography device and creating a second temperature amplitude distribution; a step (c) of taking the difference between the first temperature amplitude distribution and the second temperature amplitude distribution and creating a temperature amplitude fluctuation distribution; and a step (d) of calculating a damage parameter for determining damage from the temperature amplitude fluctuation distribution to evaluate the fatigue strength of the sample. However, Patent Document 1 states that "The fatigue strength evaluation method of the present invention evaluates fatigue strength probabilistically from the damage rate at the time when an external load is applied a predetermined number of times, using the Weibull distribution corresponding to the composite material."

[0003] In addition, JP 2021-076402 A (Patent Document 2) discloses a fatigue limit stress determination method in which a repeated load of gradually increasing tension and compression is applied to a measurement object at a predetermined frequency, the heat flux is measured with a heat flow sensor attached to the measurement object, the heat flux measured by the heat flow sensor is subjected to frequency analysis, dissipated energy is measured from the frequency component of the second harmonic of the predetermined frequency, and the fatigue limit stress of the measurement object is determined from the measurement result of the dissipated energy. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2021-131357 A [Patent Document 2] Patent Publication No. 2021-076402 Summary of the Invention [Problem to be solved by the invention]

[0005] However, as is clear from the description, the fatigue strength evaluation method for composite materials described in Patent Document 1 measures a characteristic different from the "fatigue strength" that is generally referred to. Note that the "fatigue strength" that is generally referred to in the field of materials corresponds to the fatigue limit (the stress amplitude below which fatigue failure does not occur) which is an index reflected in the design of parts (note that in the field of materials that are considered not to have a clear fatigue limit, such as aluminum alloys and fiber-reinforced composites, for example, the fatigue limit of 10 7 The stress that does not cause fatigue failure even after repeated loading is 10 7 This is sometimes called the cycle fatigue strength, which is an index to be reflected in the design of parts, and is different from that specified in Patent Document 1. In this way, the method described in Patent Document 1 cannot obtain the "fatigue strength" that corresponds to the general fatigue limit.

[0006] Hereinafter, in this specification, the term "fatigue strength" of a fiber-reinforced composite material corresponds to a general fatigue limit (a stress amplitude below which fatigue failure does not occur), and is used consistently to mean the stress amplitude at which irreversible changes due to fatigue begin to occur in the fiber-reinforced composite material.

[0007] In addition, as a method other than the method described in the above patent document, conventionally, when determining the fatigue limit or fatigue strength of a metal material, etc., a repeated load is applied so that the stress amplitude is constant until the test piece breaks, and the number of repetitions until the test piece breaks is determined. Since such a procedure needs to be repeated (a large amount of data needs to be collected) until a stress amplitude at which the test piece does not break is obtained, it is usually necessary to use a large number of test pieces (several dozen depending on the type of material) and spend a huge amount of time (sometimes several months) (for example, see "Tatsuo Sakai, Jun Sugata, "Japan Society of Materials Science Standard: JSMS-SD-6-04 Metallic Material Fatigue Reliability Evaluation Standard [SN Curve Regression Method] Revised Edition and Analysis Examples", Materials, vol. 54, published in 2005, pp. 37-43", etc.). Therefore, when measuring fatigue strength by performing the conventional procedure described above, fatigue strength cannot be measured quickly. In contrast, the measurement method described in Patent Document 2 measures dissipated energy to measure the fatigue limit stress of the measurement object, so that the problems of the large number of test pieces and the huge amount of time required can be improved. However, the measurement method described in Patent Document 2 requires the use of dissipated energy, making it difficult to apply the method to measure the fatigue strength of fiber-reinforced composite materials.

[0008] The present invention has been made in consideration of the problems associated with the above-mentioned conventional technology, and aims to provide an apparatus for determining the fatigue strength of a fiber-reinforced composite material, and a method for determining the fatigue strength of a fiber-reinforced composite material, which enable the fatigue strength of a fiber-reinforced composite material to be measured quickly and accurately. [Means for solving the problem]

[0009] As a result of intensive research into achieving the above-mentioned object, the inventors have found that it is possible to quickly and accurately measure the fatigue strength of a fiber-reinforced composite material (stress amplitude at which irreversible changes begin to occur in the fiber-reinforced composite material due to fatigue) by repeatedly applying a load at a predetermined frequency to a fiber-reinforced composite material, which is an object to be measured, so that the stress amplitude changes sequentially (stepwise) each time the number of load repetitions reaches a predetermined number, measuring the temperature change of the object to be measured for each period in which the stress amplitude is the same until the number of load repetitions reaches a predetermined number, calculating the thermoelastic temperature amplitude of the object to be measured and the average temperature of the object to be measured for each period from the data on the temperature change of the object to be measured for each period obtained by the measurement, non-dimensionalizing the thermoelastic temperature amplitude obtained for each period from the calculation result with the average temperature, and determining the fatigue strength of the fiber-reinforced composite material, which is the object to be measured, based on the change in the relationship between the dimensionless thermoelastic temperature amplitude for each period obtained in this way and the stress amplitude for each period, thereby completing the present invention.

[0010] That is, the present invention provides the following aspects.

[0011] [1] A load applying means for repeatedly applying a load at a predetermined frequency to a fiber-reinforced composite material as a measurement object so that the stress amplitude changes sequentially each time the number of load repetitions reaches a predetermined number; and a temperature measuring means for measuring a temperature change of the measurement object for each period during which the stress amplitude remains constant until the number of load repetitions reaches a predetermined number; A first calculation means for calculating a thermoelastic temperature amplitude of the object and an average temperature of the object for each period from data of a temperature change of the object; A second calculation means for non-dimensionalizing the thermoelastic temperature amplitude for each of the time periods using the average temperature to calculate the non-dimensional thermoelastic temperature amplitude of the object for each of the time periods; a fatigue strength determination means for determining a fatigue strength of the fiber reinforced composite material, which is the object to be measured, based on a change in the relationship between the non-dimensional thermoelastic temperature amplitude for each period and the stress amplitude for each period; An apparatus for determining fatigue strength of a fiber-reinforced composite material, comprising:

[0012] [2] The apparatus for determining fatigue strength of a fiber-reinforced composite material described in [1], wherein the fiber-reinforced composite material is a fiber-reinforced plastic.

[0013] [3] A step of repeatedly applying a load at a predetermined frequency to a fiber-reinforced composite material as a measurement object so that the stress amplitude changes sequentially each time the number of load repetitions reaches a predetermined number; measuring a temperature change of the measurement object for each period during which the stress amplitude remains constant until the number of load repetitions reaches a predetermined number; Calculating a thermoelastic temperature amplitude of the object and an average temperature of the object for each period from data of the temperature change of the object; A step of non-dimensionalizing the thermoelastic temperature amplitude for each of the time periods with the average temperature to calculate a non-dimensional thermoelastic temperature amplitude for each of the time periods of the object to be measured; A step of determining a fatigue strength of the fiber reinforced composite material, which is the measurement object, based on a change in the relationship between the non-dimensional thermoelastic temperature amplitude for each period and the stress amplitude for each period; A method for determining fatigue strength of a fiber-reinforced composite material, comprising:

[0014] [4] The method for determining fatigue strength of a fiber-reinforced composite material according to [3], wherein the fiber-reinforced composite material is a fiber-reinforced plastic. Effect of the Invention

[0015] According to the present invention, it is possible to provide an apparatus for determining the fatigue strength of a fiber-reinforced composite material and a method for determining the fatigue strength of a fiber-reinforced composite material, which enable the fatigue strength of a fiber-reinforced composite material to be measured quickly and accurately. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic diagram showing a preferred example (one embodiment) of the configuration of a measuring unit that can be used in a fatigue strength specifying device of the present invention. [Diagram 2] 1 is a graph showing a schematic relationship between the temperature of a measurement object and the number of repeated loads. [Diagram 3] 3 is a graph showing a schematic diagram of the relationship between frequency and temperature amplitude, which is obtained by performing frequency analysis on time-series temperature fluctuation data (such as the data shown in FIG. 2). [Figure 4] 1 is a graph showing a schematic relationship between the number of load repetitions, the stress amplitude, and the measurement period for temperature measurement in a step load fatigue test. [Diagram 5] 5 is a graph showing a schematic diagram of one form of the relationship between the stress amplitude at each measurement time and the non-dimensional thermoelastic temperature amplitude at each measurement time when the non-dimensional thermoelastic temperature amplitude is calculated from the data of temperature change measured during the measurement period of the temperature measurement shown in FIG. [Figure 6] 1 is a flowchart illustrating a preferred embodiment of a procedure (fatigue strength specification process) that can be adopted when specifying fatigue strength. [Figure 7] 1 is a graph showing a schematic relationship between the number of tests and the stress amplitude in a step load fatigue test. [Figure 8] Figure 8(A) is a graph showing a schematic relationship between the stress amplitude, the number of load repetitions, and the measurement period for temperature measurement for each test run in a step load fatigue test, and Figure 8(B) is a graph showing a schematic relationship between the non-dimensional thermoelastic temperature amplitude obtained for each test run (each measurement run: measurement period for each temperature measurement) in the step load fatigue test of Figure 8(A) and the stress amplitude used in each test run (each measurement run) of the step load fatigue test. [Figure 9] This is a graph that shows a schematic diagram of the relationship between the dimensionless thermoelastic temperature amplitude for each test (measurement) of a step load fatigue test and the stress amplitude for each test, and also shows an example of a boundary line separating a group on the low stress amplitude side from a group on the high stress side. [Figure 10]This is a graph that shows a schematic diagram of the relationship between the dimensionless thermoelastic temperature amplitude for each test (measurement) of a step load fatigue test and the stress amplitude for each test, and also shows an example of a boundary line separating a group on the low stress amplitude side from a group on the high stress amplitude side (an example of a boundary line at a position different from that shown in Figure 9). [Figure 11] This is a graph that shows a schematic diagram of the relationship between the dimensionless thermoelastic temperature amplitude for each test (measurement) of a step load fatigue test and the stress amplitude for each test, and also shows all the boundary lines that can be used to separate a group on the low stress amplitude side from a group on the high stress side. [Figure 12] In a graph showing the relationship between the non-dimensional thermoelastic temperature amplitude for each test (measurement) of a step load fatigue test and the stress amplitude for each test, this graph shows an approximate function (linear function) of the non-dimensional thermoelastic temperature amplitude within each group, based on the boundary line separating the low stress amplitude group from the high stress amplitude group. [Figure 13] 12 is a graph showing a schematic relationship between the stress amplitude at the boundary (boundary line) position shown in FIG. 11 and the sum of the squares of the residuals (sum of squared residuals) SSR between the actual measured value and the approximated value of the non-dimensional thermoelastic temperature amplitude obtained for each boundary. [Figure 14] FIG. 2 is a schematic diagram showing a surface of a test piece of a measurement object (CFRP) used in Examples 1 and 2. [Figure 15] 1 is a graph showing the relationship between the stress amplitude for each test run in the step load fatigue tests carried out in Examples 1 and 2 and the non-dimensional thermoelastic temperature amplitude for each test run. [Figure 16] 16 is a graph showing the relationship between the stress amplitude at each boundary (candidate change point) and the sum of the squares of the residuals (sum of squared residuals) SSR between the actual measured value and the approximated value of the non-dimensional thermoelastic temperature amplitude, obtained based on the results shown in FIG. [Figure 17] 1 is a graph showing the relationship between stress amplitude and the number of repetitions until breakage (SN curve) obtained in Reference Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In the following description and drawings, the same or corresponding elements are denoted by the same reference numerals, and duplicated explanations will be omitted. In addition, in the following, for convenience, the above-mentioned "device for determining fatigue strength of fiber-reinforced composite material" of the present invention may be simply referred to as the "device for determining fatigue strength", and the above-mentioned "method for determining fatigue strength of fiber-reinforced composite material" of the present invention may be simply referred to as the "method for determining fatigue strength".

[0018] A preferred embodiment of the apparatus for specifying fatigue strength of a fiber-reinforced composite material (fatigue strength specifying apparatus) of the present invention will be described below with reference to Fig. 1, but the apparatus for specifying fatigue strength of a fiber-reinforced composite material of the present invention is not limited to the embodiment shown below. In addition, the apparatus for specifying fatigue strength of a fiber-reinforced composite material of the present invention can be suitably used when carrying out the method for specifying fatigue strength of a fiber-reinforced composite material of the present invention. Therefore, a preferred embodiment of the method for specifying fatigue strength of a fiber-reinforced composite material of the present invention will also be described below by describing a method that can be suitably adopted when specifying fatigue strength using such an apparatus.

[0019] FIG. 1 is a schematic diagram showing a preferred example (one embodiment) of the configuration of a measuring unit that can be used in the fatigue strength specification device of the fiber-reinforced composite material of the present invention. The measuring unit 1 shown in FIG. 1 includes a test piece 10 of the measurement object, which is a fiber-reinforced composite material, a load applying means 11 for applying a repeated load to the test piece 10 of the measurement object at a predetermined frequency while controlling the stress amplitude to change sequentially (increase stepwise) every time the load repetition number reaches a predetermined number, and a temperature measuring means 12 for measuring the temperature change of the test piece 10 of the measurement object every period in which the stress amplitude is the same until the load repetition number reaches a predetermined number. In FIG. 1, the arrows indicating both the up and down directions conceptually depict the repeated load F applied by the load applying means 11. In addition, in the measuring unit 1 shown in FIG. 1, the load applying means 11 and the temperature measuring means 12 are each connected to an external computer (an appropriate combination of known peripheral devices such as a CPU, ROM, RAM, etc. necessary to enable desired calculations) not shown in the figure, and control the operation thereof.

[0020] In such a measuring section 1, the load applying means 11 is configured to be capable of applying a repeated load to the test piece 10 at a predetermined frequency while controlling it by a connected external computer so that the stress amplitude changes sequentially (increases stepwise) every time the number of load repetitions reaches a predetermined number. In other words, by controlling the operation by the external computer, the load applying means 11 is capable of applying a repeated load to the test piece 10, which is the measurement object, at a predetermined frequency so that the stress amplitude changes sequentially (increases stepwise) every time the number of load repetitions reaches a predetermined number.

[0021] In the measurement unit 1, the temperature measurement means 12 controls the operation of the temperature measurement means 12 by a connected external computer so as to measure information (data) of temperature change for each period during which the stress amplitude remains constant until the number of load repetitions reaches a predetermined number, and appropriately controls the measurement timing, etc. In this way, by controlling the operation by the external computer, the temperature measurement means 12 is capable of measuring the temperature change of the test piece 10, which is the measurement object, for each period during which the stress amplitude remains constant until the number of load repetitions reaches a predetermined number. The data of "temperature change of the measurement object" measured by the temperature measurement means 12 for each period may be data obtained by continuously measuring the temperature change throughout the entire time period for each period (for the entire period). However, for example, the temperature change of the measurement object may be measured at least for a specific time at a specific timing within each period, and the obtained data (time-series temperature change data within the specific time period) may be used as data representative of the temperature change of the measurement object for that period, thereby obtaining temperature change data (representative value) for each period, and using this as data of "temperature change of the measurement object" measured for each period.

[0022] In addition, in the measurement unit 1 of this embodiment, an external computer (not shown) connected to the temperature measurement means 12 is equipped with a first calculation means (a calculation unit within a computer) configured to be able to calculate (calculate) the thermoelastic temperature amplitude of the measurement object and the average temperature of the measurement object for each period from the temperature change data when information on the temperature change (information (data) related to temperature information) measured by the temperature measurement means 12 is input; a second calculation means (a calculation unit within a computer) that calculates (calculates) a dimensionless thermoelastic temperature amplitude of the measurement object for each period, in which the thermoelastic temperature amplitude is non-dimensionalized by the average temperature, based on the thermoelastic temperature amplitude and the average temperature for each period calculated by the first calculation means; and a fatigue strength identification means (a calculation unit within a computer) that identifies (calculates) the fatigue strength of the measurement object from the change in the relationship between the dimensionless thermoelastic temperature amplitude of the measurement object for each period and the stress amplitude, using the dimensionless thermoelastic temperature amplitude of the measurement object calculated by the second calculation means.

[0023] In addition, the computer used for such various calculations may be configured to appropriately combine known peripheral devices such as a necessary CPU, ROM, RAM, and programs necessary for various calculations (such programs may be recorded in the ROM and used, or may be recorded in another recording medium and used) to enable the desired calculations to be performed, and the specific configuration is not particularly limited. For example, a computer equipped with hardware consisting of a CPU and memory for performing the above-mentioned calculations and a computer program (software) installed to perform the necessary calculations may be used. In addition, examples of such CPUs include a central processing unit, processing unit, arithmetic unit, processor, microprocessor, microcomputer, DSP (Digital Signal Processor), etc.

[0024] The object to be measured may be a fiber-reinforced composite material, and is not particularly limited, but is preferably a fiber-reinforced plastic. In this way, the test piece 10 of the object to be measured is preferably made of a fiber-reinforced plastic. In addition, examples of fibers used in the fiber-reinforced composite material include carbon fiber, glass fiber, natural fiber, recycled fiber, etc. As a result, examples of the fiber-reinforced plastics that are preferably used in the fiber-reinforced composite material include carbon fiber-reinforced plastic, glass fiber-reinforced plastic, natural fiber-reinforced plastic, recycled fiber-reinforced plastic, etc. Among such fiber-reinforced plastics, carbon fiber-reinforced plastic is more preferable from the viewpoint of enabling more accurate measurement.

[0025] The shape of the test piece 10 to be measured is not particularly limited, and can be appropriately designed according to the type of the load applying means 11. The test piece 10 to be measured may be used by applying black body paint to a surface area including a region where the temperature is to be measured, so that the test piece 10 to be measured has a known emissivity, for example, when a radiation thermometer is used as the temperature measuring means 12 to measure the temperature change during repeated load application. In this way, when the type of the temperature measuring means 12 is a radiation thermometer, applying black body paint to a surface area including a region where the temperature of the test piece to be measured (for example, the fiber reinforced plastic) can be measured more efficiently.

[0026] Furthermore, the load applying means 11 is not particularly limited as long as it can apply a repeated load to the test piece 10 of the measurement object at a predetermined frequency so that the stress amplitude changes sequentially every time the load repetition number reaches a predetermined number, and a known dynamic fatigue testing machine (for example, a hydraulic servo type fatigue testing machine that applies a repeated load by moving a hydraulic cylinder up and down) can be appropriately used. In order to gradually change (increase) the stress amplitude generated by the repeated load, it is sufficient to gradually change (increase) the applied repeated load every time the load repetition number reaches a predetermined number. In this way, in order to make the load applying means 11 capable of applying a repeated load to the test piece 10 of the measurement object at a predetermined frequency so that the stress amplitude changes sequentially every time the load repetition number reaches a predetermined number, it is necessary to control the operation of the load applying means 11 so that the applied repeated load is gradually changed (increased) every time the load repetition number reaches a predetermined number (for example, every 1300 times (1300 cycles)). From this viewpoint, the load applying means 11 is connected to an external computer (not shown) as described above, and the operation is controlled by the computer so that the stress amplitude changes sequentially each time the number of load repetitions reaches a predetermined number. Note that the method of controlling the operation is not particularly limited as long as it is a method capable of gradually changing (increasing) the applied repetitive load each time the number of load repetitions reaches a predetermined number.

[0027] The temperature measuring means 12 is not particularly limited, and may be any device configured to measure the temperature change of the object to be measured for each period during which the stress amplitude is the same until the number of repeated loads reaches a predetermined number, and any known temperature measuring device that can be used to obtain the temperature change of the object to be measured (the temperature change during the application of repeated loads at a predetermined frequency) may be appropriately used. As such a temperature measuring means 12, a device capable of measuring temperature without contact (for example, an infrared camera (infrared thermography camera), a non-contact temperature sensor such as a radiation thermometer) may be suitably used, and an infrared thermography camera is used in the embodiment of FIG. 1. When an infrared thermography camera is used as the temperature measuring means 12, it is possible to detect the amount of infrared rays emitted from the object according to the temperature of the object, and measure the temperature for each unit area (for example, each pixel), and it is possible to measure the temperature change for each pixel of an arbitrary measurement area (the measurement area may be appropriately set according to the size of the test piece, the type of load applying means 11, the characteristics of the camera, etc.) on the surface of the test piece 10 of the object to be measured.

[0028] 1, when a device (infrared thermography camera) that measures the temperature of the test piece 10 as the measurement object using infrared rays is used as the temperature measurement means 12, the measurement unit 1 may be placed in a darkroom so that the infrared measurement can be performed with higher accuracy. Note that there is no particular limitation on the method of controlling the operation of the temperature measurement means 12 so as to measure the temperature change of the measurement object for each period, and as described above, a method may be adopted in which the temperature measurement means 12 is connected to an external computer (not shown) and the operation of the temperature measurement means 12 is controlled so that the measurement can be performed at a predetermined timing by the computer.

[0029] 1 is connected to an external computer as described above, and various calculation units in the external computer are used to input data on the temperature change of the test piece 10 as the measurement object for each period measured by the temperature measuring means 12 to perform various calculations, obtain a non-dimensional thermoelastic temperature amplitude for each period, and specify the fatigue strength based on the change in the relationship between the non-dimensional thermoelastic temperature amplitude and the stress amplitude. Before describing the first and second calculation means and the fatigue damage degree specifying means, each of which serves as a calculation unit in the computer, a brief description will be given of the reason (principle) why it is possible to specify the fatigue strength based on the change in the relationship between the non-dimensional thermoelastic temperature amplitude and the stress amplitude.

[0030] First, when a load is applied to a material, the material exhibits a reversible temperature change. The phenomenon that causes such a temperature change is generally known as the thermoelastic effect. When a load is repeatedly applied to a material, the material exhibits a temperature change (thermoelastic temperature change) due to the thermoelastic effect, and the temperature change has the same frequency as the repeated load. Here, in the case of a material that exhibits anisotropy in the direction parallel to the fibers (L direction) and the direction perpendicular to the fibers (T direction), such as a fiber-reinforced composite material, the amplitude ΔT of the thermoelastic temperature change is expressed by the following formula (1):

[0031]

number

[0032] (In equation (1), ΔT is the amplitude (absolute temperature) of temperature fluctuations due to the thermoelastic effect, ρ is the density, C is the specific heat of the fiber-reinforced composite material being measured, T0 is the mean absolute temperature, and Δσ L indicates the stress amplitude acting in the fiber direction (L direction), and Δσ T denotes the stress amplitude acting in the direction perpendicular to the fiber (T direction), and α L indicates the linear expansion coefficient in the fiber direction (L direction), and α T indicates the linear expansion coefficient in the direction perpendicular to the fiber (T direction). (For example, G. Pitarresi, U. Galietti, “A quantitative analysis of the thermoelastic effect in CFRP composite materials”, Strain, Vol. 46, 2010, pp. 446-459). In the present invention, the amplitude of temperature fluctuation caused by the thermoelastic effect (thermoelastic temperature amplitude ΔT) is calculated based on the absolute temperature.

[0033] Here, in the case where a repeated load is applied at a predetermined frequency so that the stress amplitude changes sequentially each time the number of repeated loads reaches a predetermined number, the temperature fluctuation of the material under the repeated load condition is measured for a predetermined time (a specific measurement time: for example, a time for a predetermined number of frames when measuring with an infrared camera) at a specific timing during the period in which the stress amplitude is the same until the number of repeated loads reaches a predetermined number, and temperature fluctuation data is acquired. In this way, when the temperature fluctuation of the test piece (measurement object) caused by the thermoelastic effect during the period in which the repeated load is applied at a predetermined frequency so that the stress amplitude is the same, it is found that the temperature fluctuation has the same frequency as the frequency of the repeated load. In this regard, the relationship between the temperature fluctuation and the frequency will be described in more detail with reference to FIG. 2. FIG. 2 is an example of a graph that shows a schematic relationship between the temperature of the measurement object and the number of repeated loads. For example, when an infrared camera is used as the temperature measuring means 12, such a graph can be obtained by setting the load frequency of the repeated load to a predetermined frequency (e.g., 10Hz), setting the frame rate of the infrared camera as the temperature measuring means 12 to a predetermined value (e.g., 211HZ), setting the measurement time to a predetermined number of frames (e.g., 4009 frames), and measuring the temperature change of the test piece 10 of the measurement object (Note that, in the measurement using an infrared camera, a specific measurement area may be set on the surface of the test piece (measurement object) 10, and the temperature change between frames may be obtained in time series from the temperature of each pixel of the specific measurement area in the image for each frame). In addition, in such a measurement, the measurement time is the value obtained by dividing the number of frames by the frame rate (for example, if the frame rate is 211HZ and the number of frames is 4009 frames, the measurement time is 4009 / 211=19 seconds). Furthermore, the number obtained by multiplying the measurement time by the frequency of the repeated load is the number of load repetitions (for example, when a measurement is performed for 19 seconds and the frequency of the repeated load is 10 Hz, the load is applied 10 times per second, so the total number of load repetitions during the measurement time for determining the temperature change is 19 x 10 = 190 cycles).Here, by arranging the data of all frames of the measurement time (measurement period) in chronological order in correspondence with the number of repeated loads, a graph such as that shown in Figure 2 can be obtained as a graph showing the relationship between the temperature of the object to be measured at a specific time during the period when the stress amplitude is the same and the number of repeated loads. Note that, after determining the temperature of all pixels within a specific measurement area for each temperature image of each frame, it is desirable to determine the average temperature of all pixels and use this average value as the temperature of the object to be measured at the measurement time of that frame. From such a graph, it can be confirmed that the temperature fluctuation caused by the thermoelastic effect has the same frequency as the frequency of the repeated load.

[0034] As explained here, since the temperature fluctuation caused by the thermoelastic effect has the same frequency as the frequency of the repeated load, the relationship between the frequency and the temperature amplitude can be obtained by frequency analysis (for example, Fourier transform) of the time-series temperature fluctuation data (data as shown in FIG. 2). An example of such a relationship between frequency and temperature amplitude is shown in FIG. 3. Since the temperature fluctuation caused by the thermoelastic effect basically has the same frequency as the repeated load, the temperature amplitude at the same frequency as the frequency of the repeated load (load frequency) can be obtained from the data (graph) as shown in FIG. 3 obtained by frequency analysis of the temperature fluctuation data (time-series data), and the temperature amplitude can be obtained as the thermoelastic temperature amplitude ΔT (absolute temperature: hereinafter, for convenience, the notation "absolute temperature" is omitted in some cases) of the measurement object. In addition, the average temperature (average surface temperature) of the measurement object can be obtained by dividing the sum of the temperature of the measurement object obtained for each image of each frame (the temperature (absolute temperature) of the measurement object obtained for each image of the time series) from the time-series temperature fluctuation data by the number of frames. The calculation formula for obtaining such an average temperature (average surface temperature) is the following formula (2):

[0035]

number

[0036] (In formula (2), T indicates the average temperature (absolute temperature) of the object to be measured, N indicates the number of frames (total number of frames) in which the temperature fluctuation is measured, and Ti indicates the temperature in a specific frame i (where i is a natural number indicating the frame number (number) and is a value from 1 to N) (in other words, the temperature in the i-th frame).) It becomes.

[0037] Here, the average absolute temperature T0 is replaced with the average surface temperature T (absolute temperature) of the object to be measured, and the equation is transformed to obtain the non-dimensional thermoelastic temperature amplitude (ΔT / T) which is non-dimensionalized by the average temperature:

[0038]

number

[0039] (ρ, C, and α in Equation (3) L , α T , Δσ L , and Δσ T are the same as those in formula (1), ΔT indicates the thermoelastic temperature amplitude (absolute temperature) of the object to be measured, and T indicates the average temperature (absolute temperature) of the object to be measured.) can be sought.

[0040] Therefore, a load fatigue test (hereinafter sometimes referred to as simply a "step load fatigue test") is performed in which the magnitude of the repeated load is changed (increased) stepwise so that the stress amplitude is changed (increased) stepwise each time the number of load repetitions reaches a specified number (predetermined number), and the temperature fluctuation is measured at each measurement timing for each stress amplitude (for each period in which the stress amplitude is the same until the number of load repetitions reaches a specified number (predetermined number): for each test round of the step load fatigue test from the start of application of the repeated load until the number of load repetitions reaches a specified number (predetermined number)). By using the measurement data to perform the calculations described above, it is possible to obtain the thermoelastic temperature amplitude ΔT from the temperature amplitude of the same frequency component as the load frequency for each period in which the stress amplitude is the same, and to obtain the average surface temperature T from the average value of the temperature fluctuations over time (see Figures 2 and 3).

[0041] Using these values, the non-dimensional thermoelastic temperature amplitude (ΔT / T) can be calculated for each period in which the stress amplitude is the same until the number of load repetitions reaches a specified number (predetermined number). It is known that when fatigue damage occurs in fiber-reinforced composite materials such as fiber-reinforced plastics (FRP), the load sharing between the fiber and the resin changes due to the fatigue damage (for example, D. Shiozawa et al., “Fatigue damage evaluation of short carbon fiber reinforced plastics based on phase information of thermoelastic temperature change”, Sensors, Vol. 17, 2017, P. 2824 (hereinafter sometimes simply referred to as “Reference 1”)). Therefore, it is considered that the stress state in the material changes in fiber-reinforced composite materials such as fiber-reinforced plastics (FRP) as fatigue damage progresses. Furthermore, in fiber-reinforced composites such as fiber-reinforced plastics (FRP), if the load distribution between the fiber and the resin changes as fatigue damage progresses, it is thought that the behavior of the dimensionless thermoelastic temperature amplitude obtained in the step load fatigue test will change.

[0042] From this viewpoint, when a load F is applied to the fiber reinforced composite (more preferably fiber reinforced plastic) using an apparatus as shown in FIG. 1, it is clear that the change in the state of the fibers in the material (fiber reinforced composite) (the change in load sharing based on the change) appears as a change in the non-dimensional thermoelastic temperature amplitude of the material (the thermoelastic temperature amplitude ΔT non-dimensionalized by the average temperature T). Here, when the stress amplitude generated by the repeated load is increased stepwise in a range below the fatigue strength, there is no change in the load sharing between the fibers and the resin in the material (fiber reinforced composite) (for example, when the fiber reinforced composite is fiber reinforced plastic, damage such as peeling between the fibers and the resin does not occur, and it is clear that the load sharing between the fibers and the resin does not change), so when the relationship between the stepwise changed stress amplitude and the non-dimensional thermoelastic temperature amplitude at that stress amplitude is obtained, it is considered that these relationships change linearly. On the other hand, when the repeated load is increased stepwise, the behavior of the non-dimensional thermoelastic temperature amplitude changes at a certain stress amplitude. This is because when a stress amplitude occurs that exceeds the magnitude of the fatigue strength, the state of the fibers in the material (fiber-reinforced composite) changes irreversibly due to peeling, etc., and the load sharing between the fibers and the resin changes. Therefore, it is believed that by determining the magnitude of the stress amplitude at the position where the behavior changes from the relationship between the dimensionless thermoelastic temperature amplitude and stress amplitude obtained in the step load fatigue test, it is possible to determine the magnitude of the stress at which the state of the fibers in the material (fiber-reinforced composite) changes irreversibly due to peeling, etc.

[0043] From this viewpoint, in the present invention, the fatigue strength is determined from the magnitude of the stress amplitude at the change point of the behavior of the non-dimensional thermoelastic temperature amplitude (the point where the relationship between the non-dimensional thermoelastic temperature amplitude and the stress amplitude changes). If the stress amplitude at the change point of the behavior of such a non-dimensional thermoelastic temperature amplitude is taken as the fatigue strength, the measured value is actually almost the same as the fatigue strength that can be determined by a conventional fatigue test using multiple test pieces. Therefore, the present inventors believe that the present invention can quickly determine the fatigue strength with high accuracy and can fully contribute to the study of the reliability of materials. In this way, the present inventors have found that it is possible to accurately and quickly determine the fatigue strength by focusing on the change point of the relationship between the non-dimensional thermoelastic temperature amplitude and the stress amplitude when the stress amplitude is changed stepwise, and by determining the fatigue strength from the change point, the present invention has been completed.

[0044] Here, the method of determining the fatigue strength will be described more specifically with reference to the drawings. When determining the fatigue strength, as described above, a step load fatigue test is performed in which a load is repeatedly applied to a test object at a predetermined frequency while the magnitude of the repeated load is changed stepwise so that the stress amplitude increases stepwise every time the number of load repetitions reaches a predetermined number, and the fatigue strength is obtained. In such a test, the stress amplitude is the same until the number of load repetitions reaches a predetermined number, so that the measurement time of the temperature change is set at a predetermined timing for each period in which the stress amplitude is the same, and the data of the temperature change of the test object is measured for each period (the temperature change is measured for each period). Such a relationship (graph) between the number of load repetitions, the stress amplitude, and the timing (measurement time) of the temperature measurement is shown in FIG. 4. First, when the magnitude of the stress amplitude is increased stepwise every time the number of load repetitions reaches a predetermined number, it can be seen that the graph of the relationship between the number of load repetitions and the stress amplitude becomes a step-like graph as shown in FIG. 4. When the non-dimensional thermoelastic temperature amplitude is calculated from the data of temperature change at each measurement timing (measurement time) as shown in Fig. 4, one form of the relationship between the stress amplitude at that measurement time and the non-dimensional thermoelastic temperature amplitude can be illustrated, for example, as shown in the graph in Fig. 5. The straight line in the graph in Fig. 5 is an "approximation line" showing the relationship between the non-dimensional thermoelastic temperature amplitude of the first four measurement points (measurement times) of all the measurement points (seven points) shown in Fig. 4 and the stress amplitude used in that measurement, and the dotted curve is an "approximation curve" calculated from the data of all points showing the relationship between the non-dimensional thermoelastic temperature amplitude and the stress amplitude. Here, referring to Reference 1 mentioned above, if we assume that the load sharing between the fiber and resin changes at a stress amplitude equal to or greater than the fatigue strength, and determine the stress amplitude at the point of change, it is thought that in a step load fatigue test, up to a stress amplitude below the fatigue strength (up to the point of change), the non-dimensional thermoelastic temperature amplitude increases linearly as the stress amplitude increases (see Figure 5), and then, at the point (point of change) where a change in the load sharing between the fiber and resin due to fatigue damage occurs, the increasing behavior of the non-dimensional thermoelastic temperature amplitude changes.Therefore, in the present invention, the position where the increasing behavior of the non-dimensional thermoelastic temperature amplitude (the relationship between the non-dimensional thermoelastic temperature amplitude and the stress amplitude) changes, i.e., the inflection point (sometimes referred to as the "knee point") of the increasing behavior of the non-dimensional thermoelastic temperature amplitude, is determined, and the "fatigue strength" is determined from the stress amplitude at the inflection point (knee point).

[0045] In this way, the above-mentioned measurement is performed on a measurement object made of a fiber-reinforced composite material (more preferably a fiber-reinforced plastic), and the fatigue strength is determined based on the change in the relationship between the non-dimensional thermoelastic temperature amplitude and the stress amplitude, assuming that the load distribution between the fiber and the resin changes at a stress amplitude equal to or greater than the fatigue strength, thereby making it possible to measure the fatigue strength easily and with high accuracy. Furthermore, when the stress amplitude at the change point of the non-dimensional thermoelastic temperature amplitude is measured as the fatigue strength, the determined fatigue strength has a value almost equal in magnitude to the fatigue strength determined by the conventional fatigue test method using multiple test pieces, making it possible to perform a measurement with sufficiently high accuracy (a preferred embodiment of a method for determining the change point of the increasing behavior of the non-dimensional thermoelastic temperature amplitude will be described later).

[0046] The above briefly explains the reason (principle) why it is possible to identify fatigue strength based on the change in the relationship between the non-dimensional thermoelastic temperature amplitude and the stress amplitude. Below, we will explain the first calculation means and second calculation means in the external computer connected to the measurement unit 1, which perform the calculation to determine such a non-dimensional thermoelastic temperature amplitude (ΔT / T).

[0047] As a preferred embodiment of the first calculation means and the second calculation means in such an external computer, the first calculation means is a calculation unit configured to, when data on temperature changes for a predetermined number of repeated loads obtained by the temperature measurement means 12 performing measurements for a predetermined period of time at a predetermined measurement timing for each period during which the stress amplitude is the same until the load repetition number reaches a predetermined number, perform frequency analysis of the temperature change data to calculate (calculate) the thermoelastic temperature amplitude ΔT of the object to be measured, and calculate (calculate) the average temperature T of the object to be measured based on the temperature change data, using the formula (2) based on the temperature change data, and the second calculation means is a calculation unit configured to, when the thermoelastic temperature amplitude ΔT for each period and the average temperature T for each period calculated by the first calculation unit are input, non-dimensionalize the thermoelastic temperature amplitude ΔT with the average temperature T and calculate (calculate) the non-dimensional thermoelastic temperature amplitude (ΔT / T) of the object to be measured for each period. When the first calculation means and the second calculation means configured in this manner are used, it is possible to efficiently calculate the dimensionless thermoelastic temperature amplitude (ΔT / T). Note that the method of frequency analysis of the time series temperature data (temperature fluctuation data) is not particularly limited, but a suitable method is, for example, Fourier transform.

[0048] The fatigue strength determination means may be a calculation unit configured to use the non-dimensional thermoelastic temperature amplitude (ΔT / T) for each period obtained by the first calculation means and the second calculation means, and to determine (calculate) the fatigue strength of the measurement object based on the change in the relationship between the stress amplitude during the period during which the non-dimensional thermoelastic temperature amplitude (ΔT / T) was obtained and the non-dimensional thermoelastic temperature amplitude (ΔT / T). Therefore, the fatigue strength determination means provided in the external computer may be a calculation unit configured to input the non-dimensional thermoelastic temperature amplitude (ΔT / T) of the measurement object calculated (calculated) by the first and second calculation means and the magnitude of the stress amplitude at the time of measuring the non-dimensional thermoelastic temperature amplitude (ΔT / T), to obtain a graph (such as the graph shown in FIG. 5) of the relationship between the non-dimensional thermoelastic temperature amplitude (ΔT / T) and the stress amplitude, and to determine the change point of the behavior of the non-dimensional thermoelastic temperature amplitude (ΔT / T) from the graph, thereby to determine (calculate) the fatigue damage degree of the measurement object (configured to enable desired calculation processing).

[0049] In this way, in the present invention, the non-dimensional thermoelastic temperature amplitude (ΔT / T) for each period obtained by the first calculation means and the second calculation means is used, and the fatigue strength determination means (the calculation unit in the computer described above) determines the change point of the behavior of the non-dimensional thermoelastic temperature amplitude (ΔT / T) based on the change in the relationship between the non-dimensional thermoelastic temperature amplitude (ΔT / T) for each period and the stress amplitude for the period corresponding to the period in which the non-dimensional thermoelastic temperature amplitude (ΔT / T) was obtained, thereby determining (calculating) the fatigue strength of the measurement object. Note that the method for determining the change point of the behavior of the non-dimensional thermoelastic temperature amplitude (ΔT / T) is not particularly limited, and various methods can be used (note that a preferred embodiment of the method for determining the change point of the behavior of the non-dimensional thermoelastic temperature amplitude (ΔT / T) will be described later).

[0050] Above, we have described a preferred embodiment of the fatigue strength determination device of the present invention with reference to Figure 1. Below, we will also describe a preferred embodiment of the fatigue strength determination method of the present invention by describing a method (a preferred example of a procedure for determining fatigue strength) that can be preferably adopted when determining fatigue strength using the fatigue strength determination device of this embodiment (a fatigue strength determination device having a measuring unit as shown in Figure 1).

[0051] Fig. 6 is a flow chart showing an example of a procedure (fatigue strength determination process) that can be suitably adopted when determining fatigue strength. The fatigue strength determination process is a process in which a step load fatigue test (in a test in which a repeated load is applied for each test, a repeated load is applied for a predetermined period (a period until the number of load repetitions reaches a specified number) with a constant repeated load applied to the test piece 10 while gradually increasing the magnitude of the repeated load so that the stress amplitude σ increases by a predetermined amount (Δσ) each time the number of tests increases) is performed according to the flow chart shown in Fig. 6, temperature distribution data is continuously measured in a time series for a predetermined time at a predetermined timing for each test (each period), and then the obtained data is analyzed to obtain a non-dimensional thermoelastic temperature amplitude (ΔT / T) for each test (a period until the number of load repetitions N reaches a specified number), and fatigue strength is determined from a change in the relationship between the non-dimensional thermoelastic temperature amplitude (ΔT / T) and the stress amplitude for each test.

[0052] In this process, the magnitude of the stress amplitude is increased each time the number of tests (for the number of tests here, the repeated load application test performed during the period until the load repetition number N reaches a specified number (the period during which the same stress amplitude is repeatedly applied) is considered to be one test) increases. In this embodiment, however, the magnitude of the maximum stress amplitude to be increased and applied is set in advance, and the operation of the load application means 11 is controlled so that the step load fatigue test in which the stress amplitude is increased in stages is terminated when the application of the repeated load at the set maximum stress amplitude is completed. In this way, in the step load fatigue test, the initial stress amplitude (magnitude of the initial stress amplitude), the maximum stress amplitude (magnitude of the stress amplitude at the final test round), the amount of stress amplitude to be increased each time (increase in stress amplitude: Δσ), and the specified number of load repetitions N for each test round (specified number as the ending condition for each repetitive load test) are preset depending on the type of material constituting the test piece, and the operation of the load application means 11 is controlled so that the stress amplitude is increased by a specified amount (Δσ) each time the number of load repetitions for each round reaches the specified number, and the load application means 11 is controlled so that operation is stopped when the application of the repetitive load at the set maximum stress amplitude is completed.

[0053] In addition, in order to measure the surface temperature of the test piece during such a step load fatigue test, the specified number of load repetitions N at which to start measuring the temperature distribution (specified number of load repetitions as a condition for starting temperature measurement), the period during which the temperature distribution is measured (measurement time), etc. are set in advance, and the operation of the temperature measurement means 12 is controlled so that the surface temperature of the test piece is measured for a specified period after the load repetitions N reaches the set number (specified number of load repetitions) for starting temperature measurement.

[0054] The operation of the load applying means 11 and the temperature measuring means 12 and the counting of the number of load repetitions N during the test may be controlled by an external computer connected to these means. For reference, a graph showing the relationship between the number of tests and the stress amplitude in the step load fatigue test is shown in FIG. 7 (note that the actual waveform of the stress amplitude is more detailed, but in FIG. 7, a waveform different from the actual waveform is drawn for convenience in explaining the correspondence between the stress amplitude and the number of tests. Also, in FIG. 7, the waveforms of the stress amplitude from the first test (first time) to the third test are shown for convenience). Note that the measurement period shown in FIG. 7 is a schematic showing the measurement time (for a predetermined number of repetitions) for measuring the temperature variation data, which starts from the point when the set number of load repetitions is reached.

[0055] In the process of identifying fatigue strength, first, as a preliminary preparation, a test piece is set at the position where the test piece 10 of the measurement object of the measurement unit 1 shown in Fig. 1 is depicted. Then, when performing a test according to the flow shown in Fig. 6, in the first step S101, the application of a cyclic load of a predetermined magnitude at a predetermined frequency (e.g., 10 Hz) is started to the test piece, starting a cyclic load test for the first test run, and counting the number of load repetitions N from the start of the application of the cyclic load is started.

[0056] Next, proceed to step S102, and in step S102, the time when the number of load repetitions N reaches a specified number for starting the measurement of the temperature distribution (for example, 1000 times (1000 cycles) in the example shown in FIG. 7) is used as the starting point, and a specified measurement time (a specified number of repetitions: for example, 190 times (190 cycles) as shown in FIG. 2 can be exemplified, and in this case, for example, the number of repetitions may be used as a reference, and the measurement may be performed for a specified time by measuring the time-series temperature change from the 1000th repetition to the 1190th repetition (the section from the 1000th cycle to the 1190th cycle)). In this way, in step S102, when the number of load repetitions N reaches a specified number, the time-series surface temperature of the test piece is measured for the specified number of repetitions, and data on the relationship between the number of load repetitions and the temperature of the measurement object (temperature change) is obtained (thereby, for example, a graph as shown in FIG. 2 can be obtained).

[0057] In this way, when performing the measurement according to the flow shown in FIG. 6, the specified number of load repetitions N (for example, 1000 times) for starting the temperature measurement is set in advance, and the load repetitions N are counted from the start of the repetitive load application in step S101 (in this case, for example, one cycle may be counted as one time), and when the count reaches the specified number for starting the measurement of the temperature distribution, the operation of the measurement unit 1 shown in FIG. 1 may be controlled so that the measurement of the surface temperature of the test piece 10 is started (Note that, for such control, an external computer may be used to grasp the operating status of the load application means 11 (grasp the count status of the load repetitions, etc.) and control the operating status of the temperature measurement means 12 according to the count number of the load repetitions). In this way, when the count number of the load repetitions reaches the specified number for starting the temperature measurement, the measurement of the surface temperature of the test piece is started. When measuring such surface temperatures, for example, data on changes in the surface temperature of the test piece over time may be obtained by measuring the surface temperature of the test piece a predetermined number of times (a predetermined period of time: in the case of measurement using an infrared camera, this may be a predetermined number of frames) from a specified number of times after the start of measurement (e.g., the 1000th time).

[0058] As a method for measuring the test piece using such an infrared camera, for example, when an infrared camera is used for the temperature measuring means 12, the frame rate is set to a predetermined value in advance, and the temperature fluctuation is measured from the start of measurement until a predetermined number of frames is reached, thereby measuring the change in the surface temperature of the test piece over time. Here, for example, if the frame rate of the infrared camera is set to 211 Hz and the number of measurement frames is set to 4009 frames, the measurement time for the temperature fluctuation is 19 seconds (=4009 / 211) (in this case, if the load frequency is 10 Hz, the number of load repetitions during the measurement period is 190 cycles).

[0059] In addition, the method of specifying the temperature of the test piece for each image of each frame is not particularly limited. For example, when a specific area of ​​the test piece is set as the measurement area (note that in the embodiment described later, a test piece having the form shown in FIG. 14 is used, and a rectangular area A2 on the surface is set as the measurement area), a method may be adopted in which the temperature of each pixel in the specified measurement area is obtained for each image of each frame, the average value of the temperatures of all pixels is calculated, and the obtained average value of the temperatures of all pixels is set as the temperature of the test piece for the image of that frame. Then, by arranging the temperatures of the test piece for each image of each frame in chronological order, the relationship between the number of load repetitions and the temperature (for example, the relationship shown in FIG. 2) can be calculated and obtained. Note that such calculations (analysis) may be appropriately performed using an external computer (connected to the measurement unit 1) equipped with a calculation unit designed to enable such calculations.

[0060] Then, after measuring the temperature fluctuation data in step S102, proceed to step S103, and when the count of the load repetition number N reaches the specified number for the end of the test in that test run (the end condition for each repeated load test: for example, in the example shown in Figure 7, the set number of load repetition numbers N for the end of the test in each test run is 1300 times (1300 cycles)), the application of the repeated load is stopped and it is determined whether the stress amplitude in that test run has reached a preset specified value (maximum stress amplitude value).

[0061] Then, if the result of the judgment in step S103 is that the stress amplitude in the test does not reach the specified value (if it has not been reached), the process proceeds to step S104, where the magnitude of the repeated load applied to the test piece is changed so that the stress amplitude is increased by a predetermined amount (Δσ) from the previous test, and a repeated load test in a new test is started (the application of repeated load is started), and at the same time, the count of the number of repeated loads in the test is newly started (for example, in order to start counting anew, the number of repeated loads may be reset and counted from 0 again at the stage of increasing the stress amplitude by a predetermined amount). After starting the test in a new test in step S104 in this way, the process proceeds to the above-mentioned step S102, and the process of step S102 is performed, and then the process proceeds to step S103 again and judgment is made in S103. In this way, after proceeding to step S104, the processes described in steps S102 and S103 are performed in sequence. Then, in step S103, steps S104, S102, and S103 are sequentially repeated until it is determined that the stress amplitude in that test has reached a preset specified value (maximum stress amplitude value). Note that, for each test, the test is performed such that the stress amplitude is constant from the start of the application of the repeated load until the count number N of load repetitions reaches a specified number, which is the end condition of the repeated load test for that test.

[0062] This point will be further explained with reference to FIG. 7. FIG. 7 shows a graph of the stress amplitude for each test (graphs of three stress amplitudes) of the first to third repeated load tests. The test for which the graph shown in FIG. 7 is obtained is assumed to be a test in which the specified number of load repetitions N, which is the end condition of the repeated load test, is set to 1300 (one cycle is counted as one), the specified number of times for starting the measurement of temperature fluctuation is set to 1000, and the measurement time for the temperature measurement is set to 190 load repetitions (the period from the 1000th to the 1190th). In the step load fatigue test of the form shown in FIG. 7, the application of the repeated load is stopped every time the number of load repetitions N reaches the specified number of 1300, and the application of the repeated load is newly started so that the stress amplitude is increased by Δσ (a constant value) in the next test. Although only data for three tests is shown in FIG. 7, the test is repeated until the stress amplitude, which is increased as described above, reaches the specified value. In this way, in the graph shown in FIG. 7, the application of a cyclic load is started anew for each test, and data on temperature fluctuations of the test piece are measured for 190 load repetitions each time the number of load repetitions N reaches the specified number (1000th) for starting temperature measurement. Since the test is performed in this way, the stress amplitude is the same (for each test) during the period until the number of load repetitions N reaches the specified number for the end of the test in that test. In this way, the test is performed so that the stress amplitude increases stepwise, and in each test, the cyclic load is applied so that the stress amplitude is the same during the period from the start of the application of the cyclic load until the number of load repetitions N reaches the specified number that is the test end condition, and data on temperature fluctuations over time can be obtained for a predetermined period at a specified timing during that period.

[0063] In this manner, in the flow shown in FIG. 6, steps S104, S102, and S103 are repeated until it is determined in step S103 that the stress amplitude has reached the specified value, and then, if it is determined in step S103 that the stress amplitude has reached the specified value, the process proceeds to step S105.

[0064] In step S105, the thermoelastic temperature amplitude ΔT in the xth test (measurement) (x is a natural number) is x The average temperature T of the xth test (measurement) is T x In this case, the thermoelastic temperature amplitude ΔT for each test (measurement) is calculated by the first calculation means based on the time series temperature change data (temperature data for all frames of a predetermined number of frames, etc.) for each test (measurement) obtained by the measurement in step S102 described above. x and the average temperature T x (where x is a value (natural number) indicating any one of the first to final measurements). That is, the data of the temperature change over time for each test (measurement) obtained by the measurement in step S102 above is input to the first calculation means, and the thermoelastic temperature amplitude ΔT x (See FIG. 3) and calculate the average temperature T x ( x is a number indicating the number of measurements for which the average temperature was calculated).

[0065] In this manner, in step S105, the thermoelastic temperature amplitude ΔT x and average temperature T x After calculating the temperature amplitude ΔT x and the average temperature T x Based on the value of x / T x ) is calculated. x / T x ) is the thermoelastic temperature amplitude ΔT calculated by the first calculation means x and the average temperature T x The value of is input to the second calculation means, and the non-dimensional thermoelastic temperature amplitude (ΔT x / T x ) can be calculated.

[0066] In this way, after step S106 is executed, the process proceeds to step S107, in which the fatigue strength determination means calculates the non-dimensional thermoelastic temperature amplitude (ΔT x / T x ) and the stress amplitude (σ x : The magnitude of the stress amplitude for the xth test is σ x The fatigue strength is determined by obtaining a graph of the relationship between the stress amplitude and the non-dimensional thermoelastic temperature amplitude (hereinafter referred to as stress amplitude) and determining the change point of the increasing behavior of the non-dimensional thermoelastic temperature amplitude with respect to the stress amplitude based on the change in the relationship.

[0067] Here, the non-dimensional thermoelastic temperature amplitude (ΔT x / T x ) and the stress amplitude (σ x : The magnitude of the stress amplitude for the xth test is σ x Based on the change in the relationship with the stress amplitude, the method for determining the change point of the increase behavior of the non-dimensional thermoelastic temperature amplitude with respect to the stress amplitude is not particularly limited, and may be, for example, a method applied to the method described in A. Akai, Y. Sato et al., “Rapid determination of fatigue limit using temperature second harmonic”, Experimental Mechanics, vol. 63, 2023, pp. 349-362 (hereinafter, sometimes simply referred to as “Reference 2”). As a method for determining the change point of the increase behavior of the non-dimensional thermoelastic temperature amplitude with respect to such stress amplitude, a method including the first to fifth steps described below can be mentioned as a suitable method. Hereinafter, a suitable embodiment of the method for determining the change point of the behavior (increase intensity) of the non-dimensional thermoelastic temperature amplitude (one example of a method for measuring the change point) will be described.

[0068] <A preferred embodiment of a method for determining the change point of the behavior (increase intensity) of the non-dimensional thermoelastic temperature amplitude (a preferred method for measuring the change point of the increase behavior of the non-dimensional thermoelastic temperature amplitude)> A suitable method for measuring the change point of the increasing behavior of the non-dimensional thermoelastic temperature amplitude can include a method including the following first to fifth steps. Each step will be described below with reference to Figs. 8 to 13.

[0069] [First step] First, a step load fatigue test is carried out according to the flow (procedure) shown in Figure 6, and the non-dimensional thermoelastic temperature amplitude (ΔT x / T x ) and the stress amplitude (σ x ) is obtained (first step). A graph showing a schematic diagram of the relationship between the stress amplitude and the number of load repetitions for each test in such a step load fatigue test is shown in FIG. 8(A) (for convenience, the measurement period of temperature measurement is conceptually shown by a circle symbol in FIG. 8(A)). The dimensionless thermoelastic temperature amplitude (ΔT x / T x ) and the stress amplitude (σ x ) is shown in a graph in Fig. 8(B). The example shown in Fig. 8 is an example of a case where a step load fatigue test was performed so that the measurement was performed seven times in total.

[0070] [Second process] Next, the non-dimensional thermoelastic temperature amplitude (ΔT x / T x ) and stress amplitude (σ x ) The boundary that divides the non-dimensional thermoelastic temperature amplitude into two groups, a low stress amplitude group and a high stress amplitude group, is determined by the following conditions 1 to 2: [Condition 1] The low stress amplitude group and the high stress amplitude group each contain at least two non-dimensional thermoelastic temperature amplitudes; [Condition 2] The boundary between the two groups is the position of the average stress amplitude between the maximum stress amplitude among the non-dimensional thermoelastic temperature amplitudes of the low stress amplitude group and the minimum stress amplitude among the non-dimensional thermoelastic temperature amplitudes of the high stress amplitude group; The boundary (boundary line) determined in the second step is described below with reference to the drawings.

[0071] 9 and 10 respectively show the non-dimensional thermoelastic temperature amplitude (ΔT x / T x ) and the stress amplitude (σ x 9 is an example of drawing boundaries for the graph so as to satisfy the above conditions 1 and 2 when a graph of the relationship between the temperature and the stress amplitude is obtained. Here, first, these drawings will be described. In FIG. 9, the low stress amplitude group is composed of two points, non-dimensional thermoelastic temperature amplitudes a and b, and the high stress amplitude group is composed of five points, non-dimensional thermoelastic temperature amplitudes c to g, so that condition 1 is satisfied. The boundary (boundary 1) shown in FIG. 9 is the position of the average stress amplitude of the maximum stress amplitude (stress amplitude at the position of non-dimensional thermoelastic temperature amplitude b) among the non-dimensional thermoelastic temperature amplitudes of the low stress amplitude group and the minimum stress amplitude (stress amplitude at the position of non-dimensional thermoelastic temperature amplitude c) among the non-dimensional thermoelastic temperature amplitudes of the high stress amplitude group, so that condition 2 is satisfied.

[0072] 10, the low stress amplitude group consists of three points of non-dimensional thermoelastic temperature amplitudes a to c, and the high stress amplitude group consists of four points of non-dimensional thermoelastic temperature amplitudes d to g, which satisfy condition 1. The boundary (boundary line 2) shown in Fig. 10 is the position of the average stress amplitude between the maximum stress amplitude (stress amplitude at the position of non-dimensional thermoelastic temperature amplitude c) of the non-dimensional thermoelastic temperature amplitudes of the low stress amplitude group and the minimum stress amplitude (stress amplitude at the position of non-dimensional thermoelastic temperature amplitude d) of the non-dimensional thermoelastic temperature amplitudes of the high stress amplitude group, which satisfies condition 2.

[0073] In this way, in the second step, the boundary dividing the non-dimensional thermoelastic temperature amplitude into a group on the low stress amplitude side and a group on the high stress amplitude side is obtained by grouping so as to satisfy the above conditions 1 to 2. In the second step, all boundaries are obtained within a range that can be set for all the measured measurement points (seven in the embodiment shown in FIG. 8). Therefore, the combination of groups to which the boundary can be set must be obtained so as to satisfy the condition that the low stress amplitude side group and the high stress amplitude side group each contain at least two non-dimensional thermoelastic temperature amplitudes. Therefore, when the non-dimensional thermoelastic temperature amplitude is measured n times (when the number of tests is n), there are (n-3) combinations, and the number of boundaries is (n-3). In this regard, when a test is performed to obtain a graph such as that shown in FIG. 8(B), seven measurements are performed in the step load fatigue test, so that the number of boundaries (boundaries) is four (four lines) as shown in FIG. 11. In the second step, all boundaries are obtained within a range that can be set for all the measured measurement points (seven measurement points in the embodiment shown in FIG. 8) as shown in FIG. 11. It should be noted that all such boundaries are provisionally set as candidates for change points to be identified in the fifth step described below.

[0074] [Third step] Next, using all the boundaries (candidates for change points) obtained in the second step, an approximation function is obtained for each group by the least squares method using all the non-dimensional thermoelastic temperature amplitudes in each group divided by the boundaries (third step). Note that all of the approximation functions obtained in this way are linear functions. Here, the third step will be described with reference to FIG. 12. FIG. 12 is a schematic graph showing a graph of an approximation function (linear function) obtained from the data of the non-dimensional thermoelastic temperature amplitude in each group for the low stress amplitude side group and the high stress amplitude side group of the non-dimensional thermoelastic temperature amplitude when the position of the boundary line 2 in the graph shown in FIG. 11 is the boundary. In the graph shown in FIG. 12, "approximation line 1" is an approximation function of the non-dimensional thermoelastic temperature amplitude of the low stress amplitude side group, and "approximation line 2" is an approximation function of the non-dimensional thermoelastic temperature amplitude of the high stress amplitude side group. In this way, in the third step, all the boundaries (four boundaries in the embodiment shown in FIG. 11) obtained in the second step are used to obtain an approximation function (linear function) of the non-dimensional thermoelastic temperature amplitude on the low stress amplitude side and the high stress amplitude side for each boundary. When the non-dimensional thermoelastic temperature amplitude of the low stress amplitude side group or the high stress amplitude side group is two points, the equation of the straight line connecting the two points is regarded as the equation (approximation function) of the approximation line (approximation line). Since the approximation function (linear function) is obtained in this way in the third step, one of the conditions for obtaining the boundaries in the second step is that the low stress amplitude side group and the high stress side group each include at least two points of non-dimensional thermoelastic temperature amplitude.

[0075] (Fourth step) Next, in the fourth step, the approximation functions (linear functions) obtained in the third step are used to calculate the sum of squared residuals (SSR) between the actual and approximate values ​​of the non-dimensional thermoelastic temperature amplitude for each boundary (for example, four boundaries in the embodiment shown in FIG. 11) (fourth step). For example, when the non-dimensional thermoelastic temperature amplitudes measured in the 1st to ith test runs belong to the low stress amplitude group, and the non-dimensional thermoelastic temperature amplitudes measured in the (i+1) to nth test runs belong to the high stress amplitude group (i and n are natural numbers), the sum of squared residuals SSR is calculated using the following formula (4):

[0076]

number

[0077] [In the formula, j is the same value as the test number x of each repeated load test performed during the step load fatigue test to measure the dimensionless thermoelastic temperature amplitude (stress amplitude (σ x ) from small to large, the non-dimensional thermoelastic temperature amplitude (ΔT x / T x ) is a natural number that indicates the order when i indicates the value of test number x at which the maximum non-dimensional thermoelastic temperature amplitude was measured in the group on the low stress amplitude side divided by the boundary for calculating SSR (for example, when grouping is performed at the boundary (boundary line 2) shown in Figure 10, point c is the maximum non-dimensional thermoelastic temperature amplitude in the group on the low stress amplitude side, but since the measured value c of the non-dimensional thermoelastic temperature amplitude is the measured value from the third test, when calculating using boundary line 2 as shown in Figure 10 as the boundary, i is 3). n indicates the total number of repeated load tests performed during the step load fatigue test in which the non-dimensional thermoelastic temperature amplitude is measured (for example, in the step load fatigue test shown in Figure 10, the total number of tests in which the non-dimensional thermoelastic temperature amplitude is measured is 7, so n is 7). A j denotes the non-dimensional thermoelastic temperature amplitude measured in the jth test run (x=j), A' j is an approximation of the non-dimensional thermoelastic temperature amplitude of the low stress amplitude group, which is an approximation of the non-dimensional thermoelastic temperature amplitude of the j-th test run (for example, in the example shown in FIG. 12, an approximation obtained using approximation line 1 of the low stress amplitude group), A '' j indicates an approximation value of the non-dimensional thermoelastic temperature amplitude of the high stress amplitude group, which is an approximation value of the non-dimensional thermoelastic temperature amplitude of the jth test run (for example, in the example shown in FIG. 12, an approximation value obtained by using approximation line 2 of the low stress amplitude group). In this case, the average of the i-th stress amplitude and the (i+1)-th stress amplitude becomes the stress amplitude at the boundary position. Such SSR values ​​are calculated for each of all boundaries calculated in the second step (for each stress amplitude position of boundary lines 1 to 4 in the example shown in FIG. 11).

[0078] (Fifth step) Next, a graph of the relationship between the stress amplitude at each boundary and the SSR value at each boundary is obtained, and the position where the SSR is minimum is set as the change point (knee point) to specify the fatigue strength (specific fatigue limit). This point will be explained with reference to FIG. 13. FIG. 13 is a graph obtained by plotting the relationship between the magnitude of the stress amplitude at the boundary position shown in FIG. 11 and the SSR obtained for each boundary. Considering that the SSR value is a value obtained by the above formula (4), the position where the SSR value is minimum can be determined to be the boundary where the most appropriate approximation line (approximation line with smaller approximation error) is obtained on the low stress amplitude side and the high stress amplitude side, respectively, and can be recognized (specified) as the point where the increase behavior of the non-dimensional thermoelastic temperature amplitude of the test piece starts to fluctuate. Therefore, by finding such a boundary where the SSR value is minimum, the change point (knee point) of the increase behavior of the non-dimensional thermoelastic temperature amplitude can be found, and the magnitude of the stress amplitude at the change point (knee point) can be specified as the fatigue limit (fatigue strength).

[0079] In this way, by using the measurement unit in the form shown in Figure 1 and determining the fatigue strength of a fiber-reinforced composite material using the flow shown in Figure 6, it is possible to measure the fatigue strength quickly and accurately.

[0080] Above, with reference to the drawings, a preferred embodiment of the fatigue strength specifying device and method of the present invention for a fiber-reinforced composite material has been described. However, the fatigue strength specifying device and method of the present invention are not limited to the above embodiment.

[0081] For example, in the embodiment shown in Fig. 1, an infrared camera is used as the temperature measuring means 12, but the temperature measuring means 12 is not limited to an infrared camera and may be a thermocouple. Note that even when a thermocouple is used as the temperature measuring means 12, it is possible to easily measure the degree of fatigue damage.

[0082] In addition, in the flow shown in Figure 6, the step load fatigue test is terminated when the magnitude of the stress amplitude reaches a specified value, but the termination condition of the step load fatigue test is not limited to this. For example, the number of test runs as the termination condition may be set in advance, and the step load fatigue test may be terminated when the counted number of test runs reaches a specified number.

[0083] Furthermore, the method for determining the change point of the behavior (increased strength) of the non-dimensional thermoelastic temperature amplitude that can be adopted in the present invention is not limited to the above embodiment (the method including the first to fifth steps), and for example, a method may be adopted in which, in a graph showing the relationship between the stress amplitude obtained in a step load fatigue test and the behavior (increased strength) of the non-dimensional thermoelastic temperature amplitude, a straight line is obtained connecting the non-dimensional thermoelastic temperature amplitude at the position of the lowest stress amplitude (stress amplitude of the first test) and the non-dimensional thermoelastic temperature amplitude at the position of the next stress amplitude (stress amplitude of the second test), the magnitude of deviation of the non-dimensional thermoelastic temperature amplitude of each test run (the third test run or later) from that straight line is obtained, and the change point is evaluated based on the change in the magnitude of that deviation. EXAMPLES

[0084] The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0085] (Examples 1 to 2) Preparation of test specimens Two strip-shaped test pieces as shown in Fig. 14 were made from a 3 mm thick sheet (plate) made of carbon fiber reinforced plastic (CFRP). s ):200mm, width (W s Two strip-shaped test pieces 10 each having a surface of 25 mm and a thickness of 3 mm were prepared. Next, a part of the area A1 (length (L A1 The black body paint was applied to the test piece on the test piece's length (L x W x H) and width (L x W x H) of 80 mm. B In addition, when measuring the temperature data, the area A2 (length (L)) shown by the broken line in the area A1 of the test piece 10 shown in FIG. A2 ):70mm, width (W A2 ): an area of ​​23 mm) was taken as the area for measuring temperature.

[0086] In addition, four samples for tensile tests were prepared using the CFRP constituting the sheet used to prepare the above two test pieces (for convenience, one is sometimes referred to as "SP1" and the other as "SP2"), and the ultimate tensile strength of the CFRP was separately determined, and the average value was adopted as the "tensile strength (UTS)" of the CFRP. Hereinafter, for convenience, the average value of the ultimate tensile strength thus determined is sometimes simply referred to as "tensile strength (UTS)" or "UTS".

[0087] <Equipment used for measurement> For the measurements, a device with the same configuration as the measurement unit 1 shown in Fig. 1 (a device simulating the device in Fig. 1) was used. Here, a hydraulic servo type fatigue testing machine was used as the load application means 11, and an infrared thermography camera was used as the temperature measurement means 12. As shown in Fig. 1, the measurement unit 1 is a device configured such that an infrared camera 12 is installed in front of the load application means 11 (hydraulic servo type fatigue testing machine) so that the surface temperature distribution of the test piece 10 can be measured using the infrared camera 12. Note that all of the test pieces 10 were used in such a way that the areas other than the paint application area A1 were fixed by the gripping tools of the hydraulic servo type fatigue testing machine.

[0088] <Step load fatigue test> In order to confirm the reproducibility of the results, step load fatigue tests were performed using each of the above two test pieces (SP1 and SP2), changing the test conditions for each test piece as shown in Table 1 (hereinafter, an example of the step load fatigue test performed using test piece SP1 will be referred to as "Example 1", and an example of the step load fatigue test performed using test piece SP2 will be referred to as "Example 2").

[0089] [Table 1]

[0090] The specific steps employed in each step load fatigue test are described below. First, in each step load fatigue test, a repeated load was applied to each test piece under the conditions of a frequency of 10 Hz (constant) and a stress ratio of 0.1, while controlling the stress amplitude to increase stepwise at the rate (increase: 4% UTS) shown in Table 1 each time the number of load repetitions reached the specified number shown in Table 1 (SP1: 1300 times (cycles), SP2: 3300 times (cycles)). Here, the repeated load was applied so that the stress amplitude remained the same during the period until the number of load repetitions reached the specified number, and the stress amplitude in the first measurement (test) was set to 4% (4% UTS) of the tensile strength (UTS) of the CFRP constituting the test piece. Each step load fatigue test was performed according to the flow (procedure) shown in Figure 6, and each time the load repetition count reached the specified number (SP1: 1300 times, SP2: 3300 times), the stress amplitude was increased by 4% UTS in sequence until the stress amplitude value reached 32% UTS (32% of the tensile strength (UTS) of CFRP). In this way, 32% UTS was adopted as the specified value of the stress amplitude, which is the condition for ending the step load fatigue test, and the flow shown in Figure 6 was performed. In addition, in each step load fatigue test, for each period when the stress amplitude was the same (each test run), counting was started from the start of that test run, and each time the load repetition count reached the set number for starting temperature measurement (SP1: 1000 cycles, SP2: 3000 cycles), the surface temperature distribution image of the test specimen was measured using an infrared thermography camera for 190 load repetitions (frame rate: 211 Hz, number of frames: 4009 frames, measurement time: 19 seconds).

[0091] Then, for each test, time series temperature fluctuation data was obtained based on the surface temperature distribution image of each frame of the test piece obtained during the measurement period (4009 frames: 19 seconds) of the surface temperature distribution image, and the non-dimensional thermoelastic temperature amplitude (ΔT / T) was calculated for each test as follows. That is, in order to calculate the non-dimensional thermoelastic temperature amplitude (ΔT / T), first, for each frame of the surface temperature distribution image, all the temperatures of the pixels in the area A2 of the test piece were obtained, and the average value of the temperatures of all the pixels was obtained, and the obtained value was adopted as the temperature of the test piece in that frame. Then, after the temperature of the test piece in each frame was obtained, the data was used to calculate the above formula (2) to obtain the average temperature (average surface temperature) T of the test piece (note that N in the formula (2) used in such calculation is 4009). Next, the temperature data of each test piece was subjected to frequency analysis using a discrete Fourier transform to obtain the relationship between frequency and temperature amplitude (as shown in FIG. 3). The temperature amplitude at the same frequency (10 Hz) as the frequency of the repeated load (frequency of the repeated load) was obtained as the thermoelastic temperature amplitude ΔT (absolute temperature) of the test piece. In this way, the thermoelastic temperature amplitude ΔT x (absolute temperature) and the average temperature (average surface temperature) of the test piece T x (absolute temperature) for each test, and the non-dimensional thermoelastic temperature amplitude (ΔT x / T x ) were obtained. The stress amplitude and non-dimensional thermoelastic temperature amplitude (ΔT x / T x ) is shown in Fig. 15. In Fig. 15, the stress amplitude is shown as a ratio (percentage) to the UTS. In addition, considering the calculation method of the dimensionless thermoelastic temperature amplitude, Fig. 15 shows the area A2 (length (L A2 ):70mm, width (W A2It can also be said that the graph (result) is a plot of the non-dimensional thermoelastic temperature amplitude, which is calculated using the average temperature of each pixel in the region (area 23 mm in diameter) as a representative value. Using the graph shown in Fig. 15, the "method including the first to fifth steps" described above as the "preferable method for measuring the change point in the increasing behavior of the non-dimensional thermoelastic temperature amplitude" was carried out, and a graph showing the relationship between the stress amplitude at each boundary (candidate for the change point) and the SSR value was obtained. The obtained graph is shown in Fig. 16.

[0092] As a result of such measurements, in the results shown in Figure 16, the boundary at which SSR becomes minimum was located at a stress amplitude position that was 22% of UTS (22% UTS) for both test pieces SP1 and SP2. Therefore, when the fatigue strength is specified based on the "method including the first to fifth steps" that was described as a suitable method for measuring the change point of the increasing behavior of the non-dimensional thermoelastic temperature amplitude, the fatigue strength can be specified as 22% UTS (note that the above-mentioned "method including the first to fifth steps" is a method for specifying the stress amplitude at the boundary at which the SSR value becomes minimum as the fatigue strength).

[0093] (Reference example 1) In order to confirm the accuracy and the like in comparison with the conventional method, 15 test pieces were prepared using the same material (CFRP) as the material used to form the test pieces, and the relationship between the load stress (stress amplitude) and the number of repetitions until fracture (SN curve) was obtained. The results are shown in Figure 17. Here, as a conventional method for determining fatigue strength, a method was adopted in which the average value of the minimum stress amplitude among the fracture data and the maximum stress amplitude among the non-fracture data with a stress amplitude lower than the minimum stress amplitude among the fracture data was defined as the fracture strength, with reference to the method described in "Tatsuo Sakai and Jun Sugata, "JSMS-SD-6-04 Metallic Material Fatigue Reliability Evaluation Standard [SN Curve Regression Method] Revised Edition Publication and Analysis Example," Materials, vol. 54, 2005, pp. 37-43." 17, it can be confirmed that the minimum stress amplitude among the fracture data is 24% UTS, and the maximum stress amplitude among the non-fracture data (data of unfractured samples when the number of repetitions exceeded 2.0E+6) at stress amplitudes lower than 24% UTS is 22% UTS, and therefore it was found that the fatigue strength of CFRP is 23% of UTS (23% UTS: shown by the dotted line in FIG. 17). Thus, it was confirmed from Reference Example 1 that when fatigue strength is determined by the conventional method, the fatigue strength is 23% UTS.

[0094] [Discussion: Comparison of results obtained in Examples 1 and 2 and Reference Example 1] From the results shown in Fig. 15, it can be seen that in Examples 1 and 2 as well, the non-dimensional thermoelastic temperature amplitude changes almost linearly from the start of the test until the stress amplitude reaches the vicinity of the fatigue strength (23% UTS) specified based on the SN curve shown in Fig. 17, and then changes nonlinearly from the vicinity of the fatigue strength (23% UTS) specified based on the SN curve shown in Fig. 17. In addition, by adopting the above-mentioned "method including the first to fifth steps", all boundaries (candidates for change points) are set, SSR is calculated for each boundary, and the relationship between the stress amplitude and SSR at each boundary is plotted to obtain a graph (Fig. 16). Based on this, the stress amplitude at the boundary where SSR is minimum is specified as the fatigue strength, as described above. In Fig. 16, it can be confirmed that the stress amplitude at the boundary where SSR is minimum is 22% UTS in both the tests for the test piece SP1 and the test piece SP2. Therefore, in Examples 1 and 2, the fatigue strength of the CFRP constituting the test piece was specified to be 22% UTS.

[0095] On the other hand, since the fatigue strength determined based on the SN curve by adopting the conventional method as described in Reference Example 1 is 23% UTS, the difference between the fatigue strength determined by the method described in Examples 1-2 (corresponding to the method for determining fatigue strength of the present invention) and the fatigue strength determined based on the SN curve by adopting the conventional method is sufficiently small (when considering the difference in the magnitude of the stress amplitude of the actual fatigue strength, the magnitude of the stress amplitude of the fatigue strength determined in Examples 1-2 is about 4% smaller than the magnitude of the stress amplitude of the fatigue strength determined by the conventional method), and it was revealed that the fatigue strength can be determined with sufficiently high accuracy when the method described in Examples 1-2 is adopted. Here, in the method described in Examples 1-2 (corresponding to the method for determining fatigue strength of the present invention), the fatigue strength can be determined by using one test piece each, and the specified value is the same even if the test conditions are changed, so it is clear that a highly reproducible result is obtained, and the actual test time was sufficiently short, about 1 hour, etc., and therefore it was found that the fatigue strength can be determined quickly with a small number of test pieces and the specified value is also sufficiently reliable, compared to the conventional method using the SN curve.

[0096] In Examples 1 and 2, the temperature distribution is measured while increasing the stress amplitude stepwise by 4% UTS to determine the relationship between the stress amplitude at the boundary (candidate for the change point) and the SSR, but if the test is performed with a smaller increase in the stress amplitude (e.g., 2% UTS), it is possible to determine more boundaries (candidates for the change point) and specify the fatigue strength, and it is clear that it is possible to perform measurements with even higher accuracy. Even in this case, it is clear that the present invention does not require the use of multiple test pieces and can specify the fatigue strength with one test piece, making it possible to perform measurements significantly faster than with conventional methods. [Industrial Applicability]

[0097] As described above, according to the present invention, it is possible to provide an apparatus and a method for determining the fatigue strength of a fiber-reinforced composite material, which enable the fatigue strength of the fiber-reinforced composite material to be measured quickly and accurately. Therefore, the apparatus for determining the fatigue strength of a fiber-reinforced composite material of the present invention is useful as a technique for estimating the fatigue strength of a fiber-reinforced composite material whose fatigue strength is unknown. [Explanation of symbols]

[0098] 1... measuring part, 10... test piece, 11... load applying means, 12... temperature measuring means.

Claims

1. a load applying means for repeatedly applying a load at a predetermined frequency to a fiber-reinforced composite material as a measurement object so that the stress amplitude changes sequentially each time the number of load repetitions reaches a predetermined number; a temperature measuring means for measuring a temperature change of the measurement object for each period during which the stress amplitude remains constant until the number of load repetitions reaches a predetermined number; A first calculation means for calculating a thermoelastic temperature amplitude of the object and an average temperature of the object for each period from data of a temperature change of the object; A second calculation means for non-dimensionalizing the thermoelastic temperature amplitude for each of the time periods using the average temperature to calculate the non-dimensional thermoelastic temperature amplitude of the object for each of the time periods; a fatigue strength determination means for determining a fatigue strength of the fiber reinforced composite material, which is the object to be measured, based on a change in the relationship between the non-dimensional thermoelastic temperature amplitude for each period and the stress amplitude for each period; An apparatus for determining fatigue strength of a fiber-reinforced composite material, comprising:

2. 2. The apparatus for determining fatigue strength of a fiber-reinforced composite material according to claim 1, wherein the fiber-reinforced composite material is a fiber-reinforced plastic.

3. a step of repeatedly applying a load at a predetermined frequency to a fiber-reinforced composite material as a measurement object such that the stress amplitude changes sequentially each time the number of load repetitions reaches a predetermined number; measuring a temperature change of the measurement object for each period during which the stress amplitude remains constant until the number of load repetitions reaches a predetermined number; Calculating a thermoelastic temperature amplitude of the object and an average temperature of the object for each period from data of the temperature change of the object; A step of non-dimensionalizing the thermoelastic temperature amplitude for each of the time periods with the average temperature to calculate a non-dimensional thermoelastic temperature amplitude for each of the time periods of the object to be measured; A step of determining a fatigue strength of the fiber reinforced composite material, which is the measurement object, based on a change in the relationship between the non-dimensional thermoelastic temperature amplitude for each period and the stress amplitude for each period; A method for determining fatigue strength of a fiber-reinforced composite material, comprising:

4. 4. The method for determining fatigue strength of a fiber-reinforced composite material according to claim 3, wherein the fiber-reinforced composite material is a fiber-reinforced plastic.

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