Shear fatigue test method and shear fatigue life test method

Abstract

The present application provides a kind of shear fatigue test method. Wherein, carry out V-type opening short beam static shear test, obtain the static test shear strength and static test beam displacement value when test piece reaches predetermined strain in static shear test;Establish the shutdown condition of shear fatigue test, the shutdown condition includes, fatigue test beam displacement amount reaches predetermined displacement amount determined according to static test beam displacement value in shear fatigue test;Carry out V-type opening short beam shear fatigue test, test piece in shear fatigue test is same with test piece in static shear test, wherein, according to static test shear strength, determine the maximum set load load of cyclic loading, when meeting shutdown condition, stop shear fatigue test.The present application also provides a kind of shear fatigue life test method using the shear fatigue test method. The shear fatigue test method described above can be used for resin matrix composite material, help to obtain accurate fatigue life.

Description

TECHNICAL FIELD

[0001] The present application relates to a shear fatigue test method for resin-based composite materials, and a shear fatigue life test method for determining the shear fatigue life of resin-based composite materials. BACKGROUND

[0002] Composite materials have high specific strength and specific stiffness, light weight, easy to form as a whole, good fatigue resistance and corrosion resistance, and have been widely used in the field of aerospace. However, in actual flight, composite aircraft structures often need to withstand the cyclic action of fatigue loads. In order to ensure the safety and reliability of composite aircraft structures, reliable composite fatigue performance must be obtained.

[0003] Composite structures have been applied in the field of aviation in China relatively late. At present, there is no universal test method for composite shear fatigue test at home and abroad. The early shear fatigue test method developed based on the short beam shear in ASTM D2344 standard has limitations in loading mode and applicable material range, and cannot meet the test requirements of the fatigue performance of composite materials for aircraft structures.

[0004] S.S. Wang et al. in Journal of Composite Materials, 1984, No. 1, used V-shaped opening short beam shear test pieces to determine the in-plane shear fatigue performance of resin-based laminated composite materials, and used extensometers to obtain strain and hysteresis loops during the test, so as to determine the fatigue life by calculating the stiffness degradation. Vanderlei O. et al. in Advanced Engineering Forum, Vol. 20, used V-shaped opening short beam shear test pieces to conduct shear fatigue test of carbon fiber-epoxy resin composite materials based on ASTM D5379M standard, and the failure criterion set in this test was the fracture of the test piece or the number of stress cycles reaching 120000.

[0005] The inventors believe that there are three main failure modes for composite fatigue test: "loss of load-carrying capacity", "delamination and damage propagation", and "significant stiffness degradation". Since the composite materials for aircraft structures generally have excellent fatigue performance, the first two failure modes are usually not easy to occur and are easy to identify, and "significant stiffness degradation" is more in line with the design and use requirements of aircraft structures. Generally, the stiffness change during the fatigue test is calculated by strain data, but the existing technical methods for obtaining strain and stiffness have limitations, which cannot obtain accurate fatigue life data and greatly increase the test cycle and test cost.

[0006] Therefore, it is necessary to provide a shear fatigue test method for resin-based composite materials, which is simple and practical, can obtain accurate fatigue life, and shorten the test period and save test cost. SUMMARY

[0007] The purpose of the present application is to provide a shear fatigue test method for resin-based composite materials, which can help obtain accurate fatigue life.

[0008] Another purpose of the present application is to provide a shear fatigue life test method, which can measure the stiffness degradation during the fatigue test, so as to obtain accurate fatigue life data.

[0009] The present application provides a shear fatigue test method for resin-based composite materials. The shear fatigue test method comprises: step S1, performing a V-shaped opening short beam static shear test, obtaining a static test shear strength and a static test beam displacement value when a test piece in the static shear test reaches a predetermined strain value, the test piece in the static shear test being made of resin-based composite material; step S2, determining a stop condition of the shear fatigue test, the stop condition comprising condition T3, i.e. the fatigue test beam displacement amount in the shear fatigue test reaches a predetermined displacement amount, the predetermined displacement amount being determined according to the static test beam displacement value; step S3, performing a V-shaped opening short beam shear fatigue test, the test piece in the shear fatigue test being the same as the test piece in the static shear test, wherein the maximum set load load of the cyclic loading is determined according to the static test shear strength, and the shear fatigue test is stopped when the stop condition is met.

[0010] In one embodiment, in step S1, the static shear test is performed multiple times, and the average static test shear strength and the average static test beam displacement value corresponding to the multiple static shear tests are obtained; in step S2, the predetermined displacement amount is determined according to the average static test beam displacement value; in step S3, the maximum set load load is determined according to the average static test shear strength.

[0011] In one embodiment, in step S1, the static test beam displacement value standard deviation corresponding to the multiple static shear tests is also obtained; in step S2, the predetermined displacement amount is determined according to the average static test beam displacement value and the static test beam displacement value standard deviation together.

[0012] In one embodiment, the predetermined displacement amount is determined by the following formula:

[0013]

[0014] wherein D0 is the predetermined displacement amount, is the average static test crosshead displacement value, and SD is the standard deviation of the static test crosshead displacement values.

[0015] In one embodiment, the predetermined strain amount is 5% strain amount.

[0016] In one embodiment, in step S2, the stoppage condition further comprises: condition T1, the test piece appears fracture; and / or, condition T2, the test piece loses load bearing capacity. In step S3, when any one of the stoppage conditions is satisfied, the shear fatigue test is stopped.

[0017] In one embodiment, condition T2 is determined by determining whether the maximum loading load in each cycle of the cyclic loading process drops by a predetermined degree compared to the maximum set loading load.

[0018] In one embodiment, the predetermined degree of drop is 10% compared to the maximum set loading load.

[0019] The present application also provides a shear fatigue life test method for determining the shear fatigue life of a resin-based composite material. The shear fatigue life test method uses the aforementioned shear fatigue test method, wherein, in step S3, the change of the fatigue test crosshead displacement peak value with the number of loading cycles is obtained. Moreover, when the test is stopped due to condition T3, the fatigue life of the test piece is determined in the following manner: if the growth rate of the fatigue test crosshead displacement peak value after the cyclic loading stabilizes reaches a predetermined rate value, the number of loading cycles corresponding to the time when the growth rate begins to reach the predetermined rate value is taken as the fatigue life of the test piece; if the growth rate of the fatigue test crosshead displacement peak value after the cyclic loading stabilizes has never reached the predetermined rate value, the number of loading cycles corresponding to the time when the test is stopped is taken as the fatigue life of the test piece.

[0020] In one embodiment, when the test is stopped due to any one of the stoppage conditions other than condition T3, the number of loading cycles corresponding to the time when the test is stopped is taken as the fatigue life of the test piece.

[0021] The aforementioned shear fatigue test method uses the shear strength and crosshead displacement data corresponding to the predetermined strain amount as the cyclic loading basis and stoppage condition for the shear fatigue test piece, and thus can well identify the failure mode in which the stiffness greatly degrades, thereby helping to obtain an accurate fatigue life, and is particularly suitable for resin-based composite materials used in aircraft structures.

[0022] The shear fatigue life test method adopts the shear fatigue test method, and by measuring the change of the beam displacement data with the load cycle, the change of the stiffness of the composite material in the fatigue test process can be conveniently determined, so as to obtain accurate fatigue life data. The shear fatigue test method and the shear fatigue life test method are simple and practical, and can shorten the test period and save the test cost. BRIEF DESCRIPTION OF DRAWINGS

[0023] The above and other characteristics, features and advantages of the present application will become apparent from the following description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the application.

[0024] Figure 1 is a schematic view of an exemplary test piece.

[0025] Figure 2 is a schematic view of an exemplary test machine clamping portion, with the test piece shown.

[0026] Figure 3 is a flowchart of an exemplary shear fatigue life test method.

[0027] Figure 4 is an exemplary plot showing beam displacement peak value versus load cycle number, where the test piece experiences significant stiffness degradation during the shear fatigue test.

[0028] Figure 5 is another exemplary plot showing beam displacement peak value versus load cycle number, where the test piece does not experience significant stiffness degradation during the shear fatigue test. DETAILED DESCRIPTION

[0029] The present application is further described by the following specific embodiments and drawings, in which more details are set forth in order to provide a thorough understanding of the present application. However, it will be apparent to those skilled in the art that the present application can be practiced in a variety of other ways without departing from the spirit and scope of the present application, and therefore the present application should not be limited to the specific embodiments described below.

[0030] For example, the first feature is formed above or on the second feature in the description, which can include an embodiment in which the first feature and the second feature are formed by direct contact, and can also include an embodiment in which an additional feature is formed between the first feature and the second feature, so that the first feature and the second feature can not be directly connected. Further, when the first element is described in a manner connected or combined with the second element, the description includes an embodiment in which the first element and the second element are directly connected or combined with each other, and also includes an embodiment in which one or more other intervening elements are added to indirectly connect the first element and the second element or combine with each other.

[0031] The present application provides a shear fatigue test method for resin-based composite materials. That is, the shear fatigue test method is directed to a test piece made of resin-based composite materials. The shear fatigue test method is particularly suitable for composite materials used in aircraft structures, that is, continuous fiber reinforced resin-based composite materials.

[0032] The shear fatigue test method can include the following steps S1, S2, S3, etc. which will be described below. It can be understood that the terms "include", "contain" and the like in the text only suggest the inclusion of explicitly identified steps and elements, and these steps and elements do not constitute an exclusive list, and the method or device can also include other steps or elements. Moreover, the order and numbering of the following steps do not necessarily limit the execution order, and in some cases only for the convenience of description, there can be one or more intervening steps between the steps.

[0033] Step S1, performing a V-notched short beam static shear test.

[0034] The V-notched short beam static shear test is referred to as a static shear test. The static shear test is performed to obtain the shear strength S and the beam displacement value D when the test piece in the static shear test reaches a predetermined strain. The test piece in the static shear test is made of resin-based composite materials, which can also be referred to as a sample. The shear strength S obtained in the static shear test is also referred to as the static test shear strength S, and the beam displacement value D obtained therein is also referred to as the static test beam displacement value D. This description is only for the convenience of distinguishing the parameters obtained at different stages, and has no special meaning if not otherwise stated, and cannot be understood as limiting the scope of protection of the present application. Resin-based composite materials generally refer to composites composed of resin and reinforcing materials such as fibers and fabrics, and are composites composed of resin and particulate, flaky, and fibrous fillers.

[0035] Exemplary test piece 10 and exemplary test machine clamping portion 20 of the V-notched short beam static shear test are shown in Figure 1 and Figure 2

[0036] Referring to Figure 1 , the test piece 10 is a V-notched short beam shear test piece. As Figure 1 ​As shown, the test piece 10 can have a length LI and a width Wl, and V-shaped openings can be formed in the middle of the two long sides. The V-shaped openings can have a V-shaped included angle θl, and can also have a bevel angle θ2, and the vertexes of the V-shaped openings can be rounded, and the rounding can have a radius rl. The vertexes of the two opposite V-shaped openings can have a distance gl, and the vertexes of the V-shaped openings and the corresponding long sides can have a distance g2. In one example, LI = 76 ± 0.3 mm, Wl = 56 ± 0.3 mm, gl = 30.6 ± 0.3 mm, g2 = 12.7 mm, θl = 90°, θ2 = 45°, and rl = 1.3 mm.

[0037] Referring to Figure 2 , the testing machine clamping portion 20 includes a moving crossbeam 50 that applies a load to the test piece 10. The crossbeam displacement value D means the displacement amount of the moving crossbeam 50 during the test. The testing machine clamping portion 20 can also include an upper clamping end 30 and a lower clamping end 40 that clamp the test piece 10, an upper connecting rod 60 and a lower connecting rod 70 that transmit the load, and a testing machine base 80, as shown in Figure 2 . It should be understood that all the drawings are only examples and schematics, and are not drawn according to the condition of the same scale, and should not be used to limit the actual protection scope required by the present application.

[0038] In the preferred embodiment, the aforementioned predetermined strain amount can be 5% strain amount. It is generally believed that 5% is the maximum effective strain amount of the test piece in the V-shaped opening short beam shear test, that is, when the strain amount of the test piece in the V-shaped opening short beam shear test reaches 5%, the test piece is considered to be failed. Thus, the present application provides the following setting in the shear fatigue test method: the test piece fails when it generates 5% strain amount in the shear fatigue test process in step S3 which will be described below without fracture and loss of load bearing capacity.

[0039] Step S2, establishing the stop condition of the shear fatigue test.

[0040] The stop condition includes condition T3. Condition T3 is that the fatigue test crossbeam displacement amount d in the shear fatigue test reaches a predetermined displacement amount D0, which is determined according to the static test crossbeam displacement value D.

[0041] Preferably, the stop condition can also include condition Tl or condition T2. Further preferably, the stop condition can also include both condition Tl and condition T2.

[0042] Condition Tl is that the test piece appears fracture, such as delamination and damage propagation.

[0043] Condition T2 is that the test piece loses load bearing capacity. For example, it is determined that condition T2 is satisfied by determining that the maximum loading load Smax in each cycle during the cyclic loading process decreases by a predetermined degree from the maximum set loading load Fmax. Specifically, for example, the predetermined degree is that the maximum loading load Smax decreases by 10% from the maximum set loading load Fmax, that is, if the maximum loading load Smax in each cycle during the cyclic loading process decreases by 10% from the maximum set loading load Fmax, it is considered that the test piece loses load bearing capacity, that is, it is considered that condition T2 is satisfied.

[0044] Step S3, V-shaped opening short beam shear fatigue test is performed.

[0045] The test piece in the shear fatigue test is the same as that in the static shear test. The shear fatigue test can use the same test machine as that used in the static shear test, preferably, the same test machine is used, that is, the same set of loading rods, clamping tools, etc., only the loading load is different, that is, the hardware is completely the same.

[0046] In the shear fatigue test, the maximum set loading load Fmax of the cyclic loading is determined according to the static test shear strength S. For example, the maximum set loading load Fmax can be set to be less than the static test shear strength S, for example, to be a predetermined proportion of the static test shear strength S, for example, 50%. The minimum set loading load Fmin may, for example, be zero. In an embodiment, the minimum set loading load Fmin of the cyclic loading can also be determined according to the static test shear strength S, for example, the minimum set loading load Fmin can be set to be a predetermined proportion of the maximum set loading load Fmax determined according to the static test shear strength S, the predetermined proportion generally has an absolute value of not more than 1, for example, 10%. After the test piece is clamped on the test machine, it can be cyclically loaded between the maximum set loading load (stress) and the minimum set loading load (stress) according to a specific fatigue loading waveform and frequency.

[0047] When the aforementioned stop condition established in step S2 is satisfied, the shear fatigue test is stopped. As described above, preferably, the stop condition can further include condition T1 and / or condition T2. At this time, in step S3, when any one of the aforementioned stop conditions is satisfied (that is, condition T1, condition T2 or condition T3 is satisfied), the shear fatigue test is stopped.

[0048] In an embodiment, in step S1, multiple static shear tests can be performed to obtain an average shear strength and an average beam displacement value As mentioned above, the shear strength S obtained in the static shear test is also referred to as static test shear strength S, and the beam displacement value D obtained therein is also referred to as static test beam displacement value D. Therefore, the average shear strength and the average beam displacement value are also correspondingly referred to as average static test shear strength and average static test beam displacement value In step S2, the aforementioned predetermined displacement amount D0 can be determined according to the average static test beam displacement value corresponding to the plurality of static shear tests. In step S3, the aforementioned maximum set load Fmax can be determined according to the average static test shear strength corresponding to the plurality of static shear tests.

[0049] In a preferred embodiment, in step S1, the standard deviation SD of the beam displacement values corresponding to the plurality of static shear tests can also be obtained, which is also referred to as static test beam displacement value standard deviation SD. In other words, a plurality of static shear tests can be performed to obtain the average static test shear strength the average static test beam displacement value and the static test beam displacement value standard deviation SD.

[0050] and SD can be calculated by the following formula:

[0051]

[0052] wherein n is the number of static shear tests performed, which can also be considered as the total number of test pieces subjected to static shear tests; i is the test piece number, which can also be considered as the test number of static shear tests performed; S i and D i are the shear strength S and the beam displacement value D corresponding to the test piece numbered i (or, the test numbered i).

[0053] In an embodiment, n≥5, that is, the aforementioned plurality is more than five, for example, five or six. It can be understood that "more than", "within", "less than" and the like in the text all include the endpoint value.

[0054] In step S2, the aforementioned predetermined displacement amount D0 can be determined according to the average static test beam displacement value and the static test beam displacement value standard deviation SD.

[0055] In a preferred embodiment, the aforementioned predetermined displacement amount D0 can be determined by the following formula:

[0056]

[0057] wherein, as previously described, D0 is the aforementioned predetermined displacement amount, is the aforementioned average static test crosshead displacement value, and SD is the aforementioned standard deviation of the static test crosshead displacement values.

[0058] In a comparative example, the failure condition threshold is set as the crosshead displacement amount reaching the average crosshead displacement value However, the inventors analyzed that, in this case, according to the principle of mathematical statistics, there is a 50% risk that the strain amount of the test piece does not reach 5% at the time of shutdown, thereby leading to the risk of inaccurate fatigue life results to be obtained, which will be described in detail later. By setting the failure threshold condition as the crosshead displacement amount reaching D0 determined by formula (4), according to the characteristics of the t-distribution applicable to small samples, there will be 95% of the test pieces whose strain amount is not less than 5%, i.e. when using D0 as the crosshead displacement amount condition for stopping the test, the accuracy of the test pieces meeting the failure condition of reaching a strain amount of 5% can reach 95%.

[0059] It is to be understood that certain terms used in the present disclosure, such as "one embodiment", "another embodiment", and / or "preferred embodiment", mean that a certain feature, structure, or characteristic described in connection with that embodiment is included in at least one implementation of the present application. Therefore, it is emphasized and should be appreciated that a repeated reference to "one embodiment" or "another embodiment" in various places of the specification does not necessarily refer to the same embodiment. In addition, certain features, structures, or characteristics of one or more embodiments of the present application can be suitably combined.

[0060] The present application also provides a shear fatigue life test method for determining the shear fatigue life of a resin-based composite material.

[0061] The shear fatigue life test method can use the shear fatigue test method described above. Moreover, in step S3 of the shear fatigue test method described above, the change of the fatigue test crosshead displacement peak value dmax with the number of load cycles N (also referred to as the number of load cycle times) is obtained. The fatigue test crosshead displacement peak value dmax is the maximum value of the crosshead displacement amount in the direction of the predetermined maximum set load Fmax in each load cycle in the fatigue test.

[0062] For example, a crosshead displacement peak value-load cycle number curve graph can be obtained, as shown in Figure 4 and Figure 5 .

[0063] When the test is stopped due to condition T3, the fatigue life of the test piece is determined in the following manner: 1) when the growth rate Vd of the peak value dmax of the aforementioned fatigue test beam displacement reaches a predetermined rate value V0 after the cyclic loading is stabilized, the fatigue life of the test piece is determined as the number of load cycles N corresponding to the time when the growth rate Vd reaches 10 times the value V0 D As the fatigue life of the test piece, reference can be made to Figure 4 ; 2) if the growth rate Vd of the peak value dmax of the fatigue test beam displacement does not reach the predetermined rate value V0 after the cyclic loading is stabilized, the fatigue life of the test piece is determined as the number of load cycles N0 corresponding to the time when the test is stopped, which can be seen from Figure 5 In one embodiment, the growth rate Vd can be obtained by dividing the increase of the aforementioned peak value dmax of the fatigue test beam displacement by m for every m load cycles. For example, m = 100, i.e., the growth rate Vd can be defined as the value obtained by dividing the increase of the aforementioned peak value dmax of the fatigue test beam displacement by 100 for every 100 load cycles. The predetermined rate value V0 can be determined by the growth rate Vd in a predetermined period after the cyclic loading is stabilized, in particular, in an earlier period. For example, the predetermined rate value V0 can be several times, for example, 5 times, of the growth rate Vd of the peak value dmax of the fatigue test beam displacement for the 101st to 200th load cycles after the cyclic loading is stabilized. In another embodiment, the predetermined rate value V0 can also be a specific numerical value.

[0064] As mentioned above, the stop condition can also include condition T1 and / or condition T2, etc. When the test is stopped due to other conditions (such as the aforementioned condition T1 or condition T2) in the stop condition other than condition T3, the number of load cycles corresponding to the time when the test is stopped is taken as the fatigue life of the test piece.

[0065] In general, based on the aforementioned shear fatigue test method, the aforementioned shear fatigue life test method adds step S4: determining the fatigue life of the test piece. The general process of the aforementioned shear fatigue life test method can be seen from Figure 3 .

[0066] If strain gauges are used to track the strain and hysteresis loop during the test, they are prone to failure under the action of long-term cyclic stress, and the stiffness data during the test cannot be obtained. If extensometers are used to track the strain and hysteresis loop during the test, the test will cause great damage to the extensometers, and the test cost will be too high. If the test is periodically paused during the test to obtain the trend of the stiffness data, this test method has three major problems. First, the test procedure is too complex, and repeated intermittent shutdown and static testing with strain gauges are required, which will result in a long cycle and high cost. Second, the stiffness data measured during the shutdown is actually static stiffness data, which is different from dynamic stiffness data, and the loading mode during the entire test is constantly changing, rather than a single shear cyclic load, which results in inaccurate fatigue life data. Third, the interval of the shutdown is set by humans, and if the interval is set too small, the static test during the shutdown will be too frequent. If the interval is set too large, the degradation of the stiffness may have already occurred when the shutdown occurs, resulting in a higher fatigue life than the actual fatigue life, and thus inaccurate test results, especially the risk of non-conservative life results.

[0067] The above-mentioned shear fatigue life test method uses the beam displacement corresponding to the predetermined strain amount obtained by the static shear test as the shutdown condition of the shear fatigue test piece, and records the data of the beam displacement, especially the peak beam displacement, with the change of the load cycle, to determine the change of the shear stiffness of the resin-based composite material, such as the significant degradation of the stiffness, so as to obtain an accurate fatigue life test method. The above-mentioned shear fatigue life test method does not need to measure the strain data, nor does it need to pause and remove the test piece during the fatigue test to carry out static testing to measure the stiffness degradation. This test method not only greatly simplifies the test process, but also shortens the test period, saves the test cost, and ensures the accuracy of the fatigue life data, overcoming the shortcomings of the existing composite material shear fatigue life test method, and better reflecting the shear fatigue performance of the composite material.

[0068] The following exemplary operation steps of the shear fatigue test method and the shear fatigue life test method according to the present application are exemplarily shown.

[0069] Step S1: Perform a V-shaped opening short beam static shear test.

[0070] For the resin-based composite material that needs to carry out a shear fatigue test, a V-shaped opening short beam static shear test is performed. The ASTM D5379 / D5379M standard is used to measure the shear strength S i and the beam displacement D i corresponding to 5% strain, where i is the sample number. A total of 5 effective static shear tests are carried out, and the average shear strength of the 5 tests is calculated average beam displacement value and beam displacement value standard deviation SD.

[0071] wherein,

[0072] Step S2: Establishing the stop condition of the shear fatigue test.

[0073] Three stop conditions of the shear fatigue test are set.

[0074] Condition T1 : Delamination and damage propagation of the test piece occurs.

[0075] Condition T2: The test piece loses its load bearing capacity. Specifically, the maximum load S max decreases by 10% compared to the maximum set load Fmax.

[0076] Condition T3: The beam displacement reaches D0. Wherein,

[0077] Step S3: Performing the V-shaped opening short beam shear fatigue test.

[0078] The same test pieces and the same set of clamping tooling system used in the V-shaped opening short beam static shear test in step S1 are adopted. After the test pieces are clamped on the testing machine, a specific fatigue loading waveform and frequency are used to load them according to the average shear strength The maximum set load Fmax(stress) and the minimum set load Fmin(stress) are cycled. When any one of the stop conditions set in step S2 is reached, the test is stopped. The data of the beam displacement varying with the number of load cycles are recorded throughout the test.

[0079] Step S4: Determining the fatigue life of the test piece.

[0080] If the test is stopped due to condition T1 or condition T2, the fatigue life of the test piece is the number of load cycles corresponding to the time when the test is stopped.

[0081] If the test is stopped due to condition T3, the data of the peak beam displacement dmax varying with the number of load cycles N are analyzed. If the peak beam displacement dmax increases significantly after the cyclic loading stabilizes Figure 4 , for example, the growth rate Vd Figure 4 increases, the curve slope increases), for example, the growth rate Vd Figure 4If the slope of the curve in the test specimen is greater than or equal to 5 times the peak displacement growth rate Vd of the beam from the 101st to the 200th loading cycle after cyclic loading stabilization, that is, the stiffness of the test specimen has significantly degraded, then record the load cycle number N corresponding to the inflection point P0. D This serves as the fatigue life of the test specimen. If the peak beam displacement versus load cycle count curve is as follows... Figure 5 As shown, the growth rate Vd of the peak displacement dmax of the crossbeam did not increase significantly after the cyclic loading stabilized, that is, the stiffness of the test piece did not degrade significantly. Therefore, the load cycle number N0 corresponding to the test stop was recorded as the fatigue life of the test piece.

[0082] The aforementioned shear fatigue testing method and shear fatigue life testing method use the beam displacement data corresponding to 5% strain as the stopping condition for the shear fatigue test specimen, which can overcome the limitations of existing techniques for obtaining strain and stiffness. Through a V-shaped open short beam static shear test, the beam displacement data corresponding to 5% strain of the test specimen is measured as the test stopping threshold condition for the shear fatigue test. Using the same test specimen and the same clamping fixture system as the static shear test, the shear fatigue test is carried out, and the beam displacement data throughout the process is recorded. By observing the changes in beam displacement data with load cycles, the changes in the stiffness of the composite material during the test can be easily determined, thereby obtaining accurate fatigue life data.

[0083] While the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the invention. Any variations and modifications can be made by those skilled in the art without departing from the spirit and scope of the invention. Therefore, any modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention, without departing from the scope of the invention, fall within the protection scope defined by the claims of the present invention.

Claims

1. A method of shear fatigue testing for resin based composites, characterized in that, The method comprises the following steps: S1, performing a V-shaped opening short beam static shear test to obtain a static test shear strength and a static test beam displacement value when a test piece in the static shear test reaches a predetermined strain value, the test piece in the static shear test being made of a resin-based composite material; S2, determining a stop condition of the shear fatigue test, the stop condition comprising condition T3, i.e., the fatigue test beam displacement value in the shear fatigue test reaches a predetermined displacement value, the predetermined displacement value being determined according to the static test beam displacement value; S3, performing a V-shaped opening short beam shear fatigue test, the test piece in the shear fatigue test being the same as the test piece in the static shear test, wherein a maximum set load load is determined according to the static test shear strength, and the shear fatigue test is stopped when the stop condition is met.

2. The shear fatigue test method of claim 1, wherein in step S1, the average static test shear strength and the average static test beam displacement value corresponding to multiple static shear tests are obtained; in step S2, the predetermined displacement value is determined according to the average static test beam displacement value; in step S3, the maximum set load load is determined according to the average static test shear strength.

3. The shear fatigue test method of claim 2, wherein in step S1, a static test beam displacement value standard deviation corresponding to multiple static shear tests is also obtained; in step S2, the predetermined displacement value is determined according to the average static test beam displacement value and the static test beam displacement value standard deviation together. The predetermined displacement value is determined by the following formula: wherein D0 is the predetermined displacement value, D is the average static test beam displacement value, and SD is the static test beam displacement value standard deviation.

4. The shear fatigue test method according to claim 3, wherein 5. The shear fatigue test method of claim 1, wherein the predetermined strain value is 5% strain value.

6. The shear fatigue test method of any one of claims 1 to 5, wherein in step S2, the stop condition further comprises: condition T1, the test piece is broken; and / or condition T2, the test piece loses load bearing capacity; in step S3, the shear fatigue test is stopped when any one of the stop conditions is met.

7. The shear fatigue test method of claim 6, wherein condition T2 is determined by determining whether the maximum load load in each cycle during the cyclic loading decreases by a predetermined degree compared to the maximum set load load.

8. The shear fatigue test method of claim 7, wherein the decrease by the predetermined degree is a decrease by 10% compared to the maximum set load load.

9. A shear fatigue life test method for determining the shear fatigue life of a resin-based composite material, comprising: adopting the shear fatigue test method of any one of claims 1 to 8, wherein in step S3, a change of a fatigue test beam displacement peak value with the number of load cycles is obtained; and ​ ​ ​ ​ ​ ​ ​ When the test is stopped due to condition T3, the fatigue life of the test piece is determined in the following manner: If the growth rate of the peak value of the displacement of the fatigue test beam after the cyclic loading is stabilized reaches a predetermined rate value, the number of load cycles corresponding to the time when the growth rate reaches the predetermined rate value is taken as the fatigue life of the test piece; If the growth rate of the peak value of the displacement of the fatigue test beam after the cyclic loading is stabilized does not reach the predetermined rate value, the number of load cycles corresponding to the time when the test is stopped is taken as the fatigue life of the test piece.

10. The shear fatigue life test method according to claim 9, wherein, when the test is stopped due to a condition other than condition T3 among the stop conditions, the number of load cycles corresponding to the time when the test is stopped is taken as the fatigue life of the test piece.

10. The shear fatigue life test method according to claim 9, wherein, when the test is stopped due to a condition other than condition T3 among the stop conditions, the number of load cycles corresponding to the time when the test is stopped is taken as the fatigue life of the test piece.

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