Clamping apparatus for planar biaxial loading test for composite material and test method

Abstract

The present application provides a clamping apparatus for a planar biaxial loading test for a composite material. The composite material is made into a test piece in the shape of a regular octagon. The clamping apparatus comprises a clamping mechanism used for fixedly clamping the test piece in a loading plane, cross-shaped reinforcing pieces, and destabilization-preventing press plates. The clamping mechanism comprises four pairs of clamping arms which are mutually spaced apart and are respectively arranged on four sides of the test piece. Four legs of each cross-shaped reinforcing piece respectively extend towards sides of the test piece not provided with the clamping arms, and a through hole is formed in the center of each cross-shaped reinforcing piece. The clamping apparatus further comprises a first destabilization-preventing press plate and a second destabilization preventing-press plate detachably arranged on opposite surfaces of each pair of clamping arms, and each pair of clamping arms grips the test piece, two cross-shaped reinforcing pieces respectively arranged on an upper surface and a lower surface of the test piece, the first destabilization-preventing press plate, and the second destabilization-preventing press plate therebetween. The present application further relates to a method for carrying out a planar biaxial loading test by using the clamping apparatus.

Description

Clamping device and test method for planar biaxial loading test of composite materials Technical Field

[0001] The present application relates to the field of strength testing of composite materials, and in particular to a clamping device for a planar biaxial loading test of composite materials and a method for performing a planar biaxial loading test using the clamping device. Background Art

[0002] With the development and application of various advanced composite materials, the proportion of carbon fiber composite materials used in load-bearing structures is increasing. This makes the stress state that composite materials are subjected to increasingly complex, and they are subjected to loads in two directions in many usage scenarios. However, most existing strength testing standard methods only focus on material failure under a single load stress and are insufficient to comprehensively evaluate the mechanical properties of the material. Therefore, testing the mechanical properties of composite structural components such as composite laminates under complex stress states is crucial to ensuring the safety of structural components during use or service.

[0003] Extensive research has been conducted both domestically and internationally on biaxial loading testing of materials, particularly for metals, for which industry and national standards have been established. However, there is currently no unified standard for biaxial loading testing of composite materials. Furthermore, unlike metals, composite materials are more sensitive to structural characteristics. Therefore, planar biaxial loading tests on composite structural components differ significantly from those on metal components, resulting in the clamping devices used in planar biaxial loading tests on metal components being unable to be directly applied to planar biaxial loading tests on composite materials.

[0004] Summary of the Invention

[0005] Therefore, in order to overcome the problem that the existing clamping device for planar biaxial loading test of metal materials cannot be used for planar biaxial loading test of composite materials, the present application proposes a new clamping device for planar biaxial loading test of composite materials.

[0006] This application solves the above technical problems through the following technical solutions:

[0007] Specifically, according to one aspect of the present application, a clamping device for a planar biaxial loading test of a composite material is provided, wherein the composite material is made into a regular octagonal specimen, and the clamping device comprises:

[0008] A clamping mechanism for fixedly clamping the specimen in the loading plane, the clamping mechanism comprising four pairs of clamping arms, the four pairs of clamping arms being spaced apart from each other and respectively arranged on four sides of the specimen;

[0009] The cross-shaped reinforcement piece includes four legs extending in opposite directions, each of which extends toward the side of the specimen where no clamping arm is provided, and a through hole is provided in the center of the cross-shaped reinforcement piece.

[0010] Among them, the clamping device also includes a first anti-instability pressure plate and a second anti-instability pressure plate detachably arranged on the opposite surfaces of each pair of clamping arms, and each pair of clamping arms is configured to clamp the test piece, two cross-shaped reinforcement plates respectively arranged on the front and back surfaces of the test piece, and the first anti-instability pressure plate and the second anti-instability pressure plate therebetween.

[0011] This application designs the composite material specimen into a regular octagon, which facilitates the even arrangement of the clamping arms of the clamping device and the chuck of the planar biaxial loading testing machine at different positions of the specimen, thereby ensuring that the chuck applies the load evenly to the composite material specimen, and avoiding interference with the load applied to the specimen due to the use of the chuck to fix the composite material specimen and load it.

[0012] Moreover, through the design of a cross-shaped reinforcement plate with a through hole in the center, the load can be evenly and effectively transferred to the central test area at the corresponding through hole position of the composite material specimen, and the biaxial stress in the central test area is uniform, so that the stress level in the test area is higher than that in other areas, to ensure that the initial damage occurs in the test area.

[0013] In addition, through the clamping mechanism of the clamping device and the arrangement of the first anti-instability pressure plate and the second anti-instability pressure plate, the specimen with a cross-shaped reinforcement plate arranged thereon can be fixedly clamped in the loading plane to ensure that the load is transferred to the specimen with maximum efficiency and relatively uniformity.

[0014] According to one embodiment of the present application, a central opening is formed on the first anti-instability pressure plate and the second anti-instability pressure plate, respectively, and the central axis of the central opening is coaxial with the central axis of the through hole of the cross-shaped reinforcement plate, and the surfaces of the first anti-instability pressure plate and the second anti-instability pressure plate that are in contact with the surface of the cross-shaped reinforcement plate are provided with balls around the central opening, and the balls are arranged to protrude outward from the surfaces of the first anti-instability pressure plate and the second anti-instability pressure plate, so that they can press against the cross-shaped reinforcement plate when clamping the test piece.

[0015] By arranging outwardly protruding balls on the surfaces of the first anti-instability pressure plate and the second anti-instability pressure plate close to the cross-shaped reinforcement plate, the balls can be used to press the cross-shaped reinforcement plates on both sides of the specimen to apply simple support constraints on both sides of the specimen, thereby achieving simple support constraints on the specimen during the test, thereby ensuring that the specimen can be protected against instability when subjected to loads such as tension, compression, tensile-compression fatigue, compressive-compression fatigue, and tensile-tensile fatigue in a plane biaxial loading test, so as to prevent the specimen from becoming unstable during the test.

[0016] Moreover, the central openings of the first anti-instability pressure plate and the second anti-instability pressure plate are set to be coaxial with the central axis of the through hole of the cross-shaped reinforcement plate, and the balls are arranged with reference to the central opening so that the balls surround the central opening, which can provide uniform and balanced simply supported constraints for the specimen.

[0017] According to one embodiment of the present application, annular tracks are formed on the surfaces of the first anti-instability pressure plate and the second anti-instability pressure plate that are respectively in contact with the cross-shaped reinforcement plate. The annular tracks are arranged around the central opening and are located outside the through hole of the cross-shaped reinforcement plate, and the ball bearings are arranged in the annular tracks.

[0018] By providing annular tracks on the first and second anti-stagnation pressure plates and placing the balls on the tracks, a low-cost and easily manufactured method is achieved to provide simply supported constraints for the specimen. Furthermore, placing the annular tracks outside the through-holes of the cross-shaped reinforcement plates prevents the balls from damaging the specimen's test area when pressing against the cross-shaped reinforcement plates. Furthermore, the annular tracks restrict the balls' freedom of movement, limiting their rotational motion to their installed position.

[0019] According to one embodiment of the present application, grooves are formed on the surfaces of the first and second anti-instability pressure plates, respectively contacting the cross-shaped reinforcement sheet, connecting the annular track to either of its outer edges. This allows the balls to be guided into the annular track via the grooves. Arranging the balls in the annular track using the grooves for guidance facilitates operation.

[0020] According to one embodiment of the present application, the clamping device further includes a locking slider configured to be pressed into the groove and moved to the annular track to close the annular track. By pressing the locking slider into the groove to close the annular track, the ball bearings are easily installed and positioned in the annular track, and the locking slider can be stored when not in use.

[0021] When the anti-destabilization platen is idle, the locking slider can be placed in the groove to prevent loss. When preparing to conduct a planar biaxial test on composite materials, before installing the clamping device, the locking slider is removed from the groove and the balls are pushed sequentially through the groove into the annular track. After the balls fill the annular track, the locking slider is squeezed into the groove and one end is pushed to the annular track to lock the locking slider and close the annular track, thereby confining the balls in the annular track. Moreover, when the balls are worn to a certain extent, the locking slider can be removed again to replace the balls and install new, unworn balls in the annular track.

[0022] According to one embodiment of the present application, the first and second anti-stagnation pressure plates each further include four ears extending outwardly along the centerline of the sides of the specimen where the clamping arms are located. These four ears increase the contact area between the first and second anti-stagnation pressure plates and the pair of clamping arms, ensuring a secure engagement between the first and second anti-stagnation pressure plates and the pair of clamping arms.

[0023] According to one embodiment of the present application, the clamping mechanism further includes four columns for supporting four pairs of clamping arms, wherein one clamping arm in each pair of clamping arms is fixedly mounted on the column, and the other clamping arm is configured to be movable along the column relative to the fixed clamping arm. By adjusting the distance between the other moving clamping arm and the fixed clamping arm, it is possible to adapt to specimens of different thicknesses to clamp composite material specimens for planar biaxial loading tests. Moreover, by adjusting the moving clamping arms, the plane in which the specimen is located can be appropriately adjusted to ensure that the specimen is within the plane where the load is applied.

[0024] According to one embodiment of the present application, a circular impact damage is formed in the center of the specimen, and a circular window is provided in the center of the fixture for introducing the impact damage. Impact damage is introduced in the center of the specimen to meet the needs of analyzing a planar biaxial loading test of a composite material subjected to impact damage. In order to ensure the reasonable and effective introduction of impact damage, it is often necessary to consider a fixture for introducing impact damage. Since the test area of ​​the specimen is usually designed in a circular manner, and the rectangular impact opening will produce stress concentration and uneven damage distribution on the impact damage of the specimen, the impact damage of the specimen is introduced by using a fixture with a circular window in the center, so that the impact damage area can be kept consistent with the test area of ​​the specimen, so as to avoid a large impact of the impact damage on the non-test area of ​​the specimen.

[0025] According to one embodiment of the present application, the through-holes of the cross-shaped reinforcement sheet are circular, and their diameter is at least twice the diameter of the impact damage area. By designing the size relationship between the through-holes of the cross-shaped reinforcement sheet and the impact damage area, the expansion of impact damage and the mutual influence of the specimen edges during the specimen loading process can be effectively prevented.

[0026] According to one embodiment of the present application, the diameter of the circular window is 1.5 times the diameter of the through hole of the cross-shaped reinforcement sheet.

[0027] According to one embodiment of the present application, the chuck of the planar biaxial testing machine is positioned on the side of the specimen where the clamping arm is not provided, and the length of the specimen clamped by the chuck is greater than or equal to 100 mm. By setting the length of the specimen clamped by the chuck within a certain range, the specimen can be fully clamped, thereby ensuring that the chuck effectively applies the load to the specimen. At the same time, the position and angle of the specimen can be observed and adjusted so that the centerline of the specimen coincides with the centerline of the chuck, thereby ensuring that the load from the chuck is applied at the center of inertia of the specimen's cross section.

[0028] According to one embodiment of the present application, the width of each leg is smaller than the length of a single side of the specimen.

[0029] According to one embodiment of the present application, the width of each leg is 10 mm smaller than the length of a single side of the specimen. The width of the leg is designed to be smaller than the side of the specimen to avoid stress concentration at the edge of the specimen.

[0030] According to one embodiment of the present application, the side surfaces of the four legs of the cross-shaped reinforcement sheet are connected by a circular arc transition at the cross intersection. The cross-shaped reinforcement sheet with this circular arc transition reduces stress concentration at the transition arc during testing, thereby meeting the test requirements of a high and uniform stress level in the test area and low stress outside the test area. This makes it easier to achieve a biaxial stress state in the test area of ​​the specimen, thereby ensuring that initial failure occurs within the test area of ​​the specimen.

[0031] According to one embodiment of the present application, strain gauges are arranged on the front and back of the specimen at positions corresponding to the through holes of the cross-shaped reinforcement plate. The strain gauges can be used to obtain the strain of the specimen during loading, thereby obtaining the failure status of the specimen.

[0032] According to another aspect of the present application, there is provided a method for performing a planar biaxial loading test using the clamping device in any of the aforementioned embodiments, the method comprising:

[0033] Preheat the plane biaxial loading testing machine;

[0034] Select a chuck with a thickness that matches the thickness of the composite material specimen, install the chuck on a planar biaxial loading testing machine, and adjust the orientation of the chuck so that the center line of the chuck is coaxial with the loading axis of the planar biaxial loading testing machine;

[0035] Place two cross-shaped reinforcement plates on the front and back surfaces of the specimen, respectively, with the four legs of each cross-shaped reinforcement plate extending toward four mutually spaced sides of the specimen. Place the specimen and the legs of the cross-shaped reinforcement plates on the front and back surfaces of the specimen into the accommodating space of the chuck.

[0036] The four pairs of clamping arms of the clamping mechanism are arranged at intervals from each other on the edges of the legs of the specimen not provided with the cross-shaped reinforcement pieces, and the first and second anti-instability pressure plates are mounted on the opposite surfaces of each pair of clamping arms using fasteners. The specimen and the two cross-shaped reinforcement pieces respectively provided on the front and rear surfaces of the specimen are clamped between the first and second anti-instability pressure plates by each pair of clamping arms, so that the centerline of the edge of the specimen corresponding to the clamping head is coaxial with the centerline of the clamping head; and

[0037] The strain gauges on the specimen are electrically connected to the dynamic strain gauges to collect values ​​from the strain gauges on the specimen as the specimen is loaded.

[0038] According to one embodiment of the present application, the method of performing a planar biaxial loading test using a clamping device further includes forming a line on the specimen and arranging the clamp on the edge of the specimen where no cross-shaped reinforcement plate legs are arranged according to the position of the line.

[0039] The method of conducting a planar biaxial loading test using a clamping device can ensure that the specimen is subjected to uniform force in the test area during the test and that stress concentration outside the test area is small. The specimen will not experience local instability during the test, and the failure mode results are also in line with expectations.

[0040] According to one embodiment of the present application, the method for conducting a planar biaxial loading test using a clamping device further includes applying a planar biaxial load to a specimen using a planar biaxial loading testing machine, recording the specimen stress and strain values ​​from strain gauges disposed on the front and back surfaces of the specimen, and calculating the specimen bending percentage based on the ratio between the difference and the cumulative value of the specimen stress and strain values ​​of the two strain gauges, respectively, on the front and back surfaces of the specimen. By analyzing the specimen bending percentage during the preloading phase, it is possible to determine whether the specimen clamping position is correct, so that the specimen can be aligned and debugged in a timely manner.

[0041] According to one embodiment of the present application, the method for conducting a planar biaxial loading test using a clamping device also includes determining whether the position of the specimen relative to the planar biaxial loading testing machine is correct based on the calculated bending percentage; and when it is determined that the position of the specimen is incorrect, further adjusting the position of the second anti-instability pressure plate relative to the first anti-instability pressure plate, thereby ensuring that the center line of the edge of the specimen corresponding to the chuck is coaxial with the center line of the chuck.

[0042] According to one embodiment of the present application, the method for conducting a planar biaxial loading test using a clamping device also includes synchronously and stepwise loading the applied biaxial load of the planar biaxial loading testing machine onto the specimen according to a predetermined load ratio, and recording the load history, failure position and failure mode of each loading test, so as to obtain the biaxial failure load of the specimen under the corresponding failure mode.

[0043] On the basis of conforming to the common sense in this field, the above-mentioned preferred conditions can be arbitrarily combined to obtain the preferred embodiments of the present application.

[0044] The beneficial technical effects and advantages that can be achieved by the clamping device for planar biaxial testing of composite materials according to the above embodiment of the present application are:

[0045] The regular octagonal design of the composite material specimen allows the specimen to be clamped on the loading plane to ensure that the biaxial load is evenly and effectively applied to the specimen. The shape design and position arrangement of the cross-shaped reinforcement plate can effectively transfer the load to the central test area of ​​the specimen to ensure that the biaxial load on the specimen in the test area is uniform and large, while the stress concentration generated in other areas is small, so that the stress level in the test area is significantly higher than that in other areas, thereby ensuring that the initial failure occurs in the test area. At the same time, through the anti-instability pressure plate design, simple support constraints can be applied to the front and back surfaces of the specimen while loading, thereby ensuring that the specimen will not become unstable during the planar biaxial loading test. At the same time, impact damage is introduced in the center of the specimen to conduct a planar biaxial loading test of the composite material taking impact damage into account. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] FIG1 is an assembly diagram of an exemplary clamping device and a composite material specimen for a planar biaxial loading test of a composite material according to a preferred embodiment of the present application.

[0047] FIG2 is an exemplary schematic diagram of the composite material specimen in FIG2 ,

[0048] FIG3 is an exemplary schematic diagram of a fixture for introducing impact damage according to a preferred embodiment of the present application.

[0049] FIG4 is an exemplary schematic diagram of a cross-shaped reinforcement sheet of the clamping device in FIG1 ,

[0050] FIG5 is an exemplary schematic diagram of two cross-shaped reinforcement sheets in FIG4 arranged on the front and back surfaces of the composite material specimen in FIG2,

[0051] FIG6 is an exemplary schematic diagram of the first anti-destabilization pressure plate of the clamping device in FIG1 ,

[0052] FIG7 is an exemplary schematic diagram of a planar biaxial loading test of a composite material specimen using a clamping device according to a preferred embodiment of the present application and a chuck of a planar biaxial loading testing machine. DETAILED DESCRIPTION

[0053] To make the objectives, technical solutions, and advantages of this application more clear, the following will clearly and completely describe the technical solutions in the embodiments of this application in conjunction with the accompanying drawings showing multiple embodiments according to this application. It should be understood that all other embodiments obtained by ordinary technicians in this field based on the embodiments described in this application without expending creative effort will fall within the scope of protection of this application.

[0054] Existing specimens used for planar biaxial loading tests on composite materials have configurations such as cross, square, and special shapes. Based on the mature biaxial loading tests on cross-shaped metal parts, the mechanical properties analysis of cross-shaped composite specimens has been widely studied.

[0055] Cross-shaped composite specimens are typically cross-shaped overall and have four loading arms. For cross-shaped composite specimens, both the clamping area and the load-applying area are located on the loading arms. To ensure a high and uniform stress level in the central test area of ​​the specimen, a hollow area is provided on each loading arm to reduce the stress on the loading arm. At the same time, a circular arc transition is provided at the intersection of the four loading arms to reduce stress concentration at the intersection, thereby achieving a biaxial stress state in the central test area and ensuring that initial failure occurs in the intended test area.

[0056] The existing loading fixture device for biaxial equal load and non-equal load loading tests of cross-shaped composite material specimens not only clamps the loading arm of the specimen, but also applies load to the loading arm through multiple uniaxial loading testing machines to perform planar biaxial loading tests on the cross-shaped composite material specimens.

[0057] While existing loading fixtures can load specimens using uniaxial testing machines and effectively control testing costs and reduce testing difficulty, the need for simultaneous clamping and load application results results in stress concentration at the clamping head of the loading fixture rather than at the central test area of ​​the specimen. Furthermore, failure often occurs in the hollow area of ​​the loading arm, resulting in significant discrepancies between expected results and planar biaxial loading tests, low test efficiency, and even the inability to correctly conduct planar biaxial loading tests on composite materials.

[0058] Therefore, in order to solve the above problems, the present application provides a new clamping device for planar biaxial testing of composite materials. The clamping device can be used in conjunction with a planar biaxial loading testing machine to perform planar biaxial testing of composite material specimens.

[0059] As shown in Figure 1, the clamping device includes a clamping mechanism 2 for securely holding the specimen 1 within the loading plane, a cross-shaped reinforcement plate 3 for transferring the load to the central test area of ​​the specimen 1, and an anti-destabilization pressure plate to prevent the specimen 1 from buckling during the loading test. The clamping mechanism 2 comprises four pairs of clamping arms 21, spaced apart and arranged on the four sides of the specimen 1. A first anti-destabilization pressure plate 41 and a second anti-destabilization pressure plate 42 are detachably mounted on the opposing surfaces of each pair of clamping arms 21. Two cross-shaped reinforcement plates 3 are positioned on the front and back surfaces of the specimen 1, respectively. Each cross-shaped reinforcement plate 3 has four legs 31 extending toward the side of the specimen 1 not provided with the clamping arms 21, and a through-hole 32 is centrally located within each cross-shaped reinforcement plate 3. Each pair of clamping arms 21 holds the specimen 1, the two cross-shaped reinforcement plates 3 positioned on the front and back surfaces of the specimen 1, and the first and second anti-destabilization pressure plates 41 and 42.

[0060] As shown in FIG2 , in the embodiment, the composite material is made into a specimen 1 in the shape of a regular octagon, and an impact damage area is formed in the center of the specimen 1 , and strain gauges (not shown) are also arranged on the front and back sides of the specimen 1 at positions corresponding to the through holes 32 of the cross-shaped reinforcement plate 3 .

[0061] As shown in Figures 1 to 7, the regular octagonal composite specimen 1 facilitates the evenly spaced arrangement of the clamping mechanism 2 of the clamping device and the chuck 5 of the planar biaxial loading tester, thereby preventing the biaxial load applied to the specimen 1 from being interfered with by the chuck 5 clamping and fixing the specimen 1. This ensures that the stress of the specimen 1 is uniform and maximum in the test area, while the stress concentration in other areas is small, thereby ensuring that the initial failure of the composite specimen 1 occurs in the test area. It should be understood that the composite specimen 1 can also be a regular polygon with multiples of eight sides, such as a regular hexadecagon.

[0062] At the same time, because when the clamping device is used to perform a planar biaxial loading test on a regular octagonal composite material specimen 1, the clamping area of ​​the composite material specimen 1 is separated from the loading area, there is no need to consider the clamping and fixing problem of the specimen 1 when applying a load to the composite material specimen 1. Therefore, the clamping device can be used to perform various planar biaxial loading tests on the composite material specimen 1, such as planar biaxial tension, planar biaxial compression, planar biaxial tension and compression, planar biaxial compression fatigue, and planar biaxial tension and tension, thereby facilitating the analysis of the mechanical properties of the composite material specimen 1 under complex stress states.

[0063] The overall dimensions of the specimen 1 are usually determined based on the selected planar biaxial loading testing machine and the pre-test analysis results to ensure that the specimen 1 is subjected to uniform force in the test area during the loading test, that the specimen 1 does not experience local instability, and that the failure mode results are in line with expectations.

[0064] Under the conditions that the effective stroke of the test equipment is met, the test area of ​​the specimen 1 is uniformly stressed, and the specimen does not become unstable before the test area is destroyed, a composite material specimen 1 with a longer size is preferably selected.

[0065] As shown in Figure 5, the width of the clamping end S of specimen 1 is typically designed based on the width of the planar biaxial loading tester's chuck 5. To achieve a balanced stress state across the entire test area of ​​specimen 1, a wider specimen 1 is preferred, but the width of the clamping end S should always be smaller than the width of the chuck 5.

[0066] The thickness of the specimen 1 is designed based on the ply thickness of the target structure to be verified and the load of the planar biaxial loading test machine. It should be ensured that the upper limit of the output load of the planar biaxial loading test machine is sufficient to destroy the test area of ​​the specimen 1.

[0067] When considering the impact damage of the specimen 1, in order to ensure the reasonable and effective introduction of the impact damage, it is necessary to consider the fixture for introducing the impact damage. Since the test area of ​​the specimen 1 is usually designed to be circular, in order to prevent the influence of the rectangular impact opening on the impact damage of the specimen, as shown in Figure 3, a circular window 61 is opened in the center of the fixture 6 for introducing the impact damage in this application. Moreover, in order to facilitate the clamping of the specimen 1 by the clamp 62 during the introduction of the impact damage, in order to facilitate the purpose of fixing the specimen 1. Preferably, the fixture 6 for introducing the impact damage adopts a stepped design.

[0068] Optionally, the test piece 1 may also be a composite material test piece that has not been damaged by impact but has a defect embedded in its center.

[0069] The cross-shaped reinforcement plate 3 of the clamping device, as shown in Figure 4, comprises four legs 31 extending in opposite directions. The sides of the four legs 31 are connected at the intersection by a circular arc surface 33. A through hole 32 is also provided in the center of the cross-shaped reinforcement plate 3, coaxial with the central axis of the impact-damaged area. When a circular window 61 is provided in the center of the impact-damaged fixture 6, the through hole 32 is a circular hole, and the diameter of the circular window 61 is 1.5 times the diameter of the circular hole 32.

[0070] As shown in Figures 1 and 5, in the assembled clamping device, two cross-shaped reinforcement plates 3 are placed on the front and back surfaces of the composite material specimen 1, respectively. The four legs 31 of each cross-shaped reinforcement plate 3 extend to the corresponding edge of the specimen 1, which is not provided with the clamping arms 21. The clamping head 5 of the planar biaxial loading tester applies load to the cross-shaped reinforcement plates 3 and the specimen 1 by clamping the legs 31 of the two cross-shaped reinforcement plates 3 and the specimen 1 between the two cross-shaped reinforcement plates 3.

[0071] The cross-shaped reinforcement sheet 3, arranged on the front and back surfaces of the composite specimen 1, effectively transfers the load to the specimen 1. By providing a through hole 32 in the center of the cross-shaped reinforcement sheet 3, coaxial with the central axis of the impact damage area, it is possible to ensure that the load transferred to the central test area of ​​the specimen 1 is greater than that in other areas. This ensures that the stress level in the test area of ​​the composite specimen 1 is higher than in other areas, ensuring that initial damage occurs in the test area.

[0072] Furthermore, the sides of the four legs 31 of the cross-shaped reinforcement plate 3 are connected by a transitional arc surface 33 at the intersection. This ensures that the load on the specimen 1 at the transition point of the arc surface 33 is relatively uniform and small, reducing stress concentration in the specimen 1 at the transition point of the arc surface 33. This satisfies the requirement for a high and uniform stress level in the central test area and ensures that initial failure occurs in the test area. To prevent stress concentration, the curvature of the arc surface 33 of the cross-shaped reinforcement plate 3 should be greater than a predetermined angle. This predetermined angle can be determined based on the size of the specimen 1 and the applied load.

[0073] The size of the through hole 32 of the cross-shaped reinforcement sheet 3 can be determined based on the size of the specimen 1 and the results of the pre-test analysis, and it is necessary to take into account the convenience of attaching the strain gauge and observing the stress-strain state of the test area. The size of the through hole 32 of the cross-shaped reinforcement sheet 3 should not be designed to be too small, as it will make it difficult for the specimen 1 to fail. The size of the through hole 32 of the cross-shaped reinforcement sheet 3 should not be designed to be too large, as it will cause the test area to become unstable. Preferably, the through hole 32 of the cross-shaped reinforcement sheet 3 is circular, and its diameter is twice the diameter of the circular impact damage area.

[0074] In general, the size of the through hole 32 of the cross-shaped reinforcement plate 3 is designed to ensure that under all test load conditions, the stress of the test piece 1 in the test area is uniform, and the test area will not be locally unstable before being destroyed.

[0075] The width N of the clamping end of the cross-shaped reinforcement piece 3 is slightly smaller than the width S of the clamping end of the specimen. Preferably, the width N of the clamping end of the cross-shaped reinforcement piece 3 is 10 mm smaller than the width S of the clamping end of the specimen 1. More preferably, the length of the cross-shaped reinforcement piece 3 is the same as the total length of the specimen 1.

[0076] As shown in FIG6 , the first anti-destabilization pressure plate 41 of the clamping device has a central opening 43 formed thereon that is coaxial with the central axis of the through hole 32 of the cross-shaped reinforcement plate 3. Furthermore, a circular track 44 and a groove 45 connecting the circular track 44 to either of its outer edges are formed on the surface in contact with the cross-shaped reinforcement plate 3. The circular track 44 is arranged around the central opening 43 and is located outside the through hole 32 of the cross-shaped reinforcement plate 3. The groove 45 can guide the ball bearings 46 into the circular track 44. The ball bearings 46 installed in the circular track 44 protrude outward from the surfaces of the first and second anti-destabilization pressure plates 41, 42, and can press against the cross-shaped reinforcement plate 3 when clamping the test piece 1. The first and second anti-destabilization pressure plates 41, 42 of the clamping device have the same structure.

[0077] By providing outwardly protruding balls 46 on the surfaces of both the first and second anti-instability pressure plates 41 and 42 near the cross-shaped reinforcement plate 3, the balls 46 can be used to press against the cross-shaped reinforcement plate 3 on both sides of the specimen 1, thereby simultaneously applying a simply supported constraint on both surfaces of the specimen 1. By constraining the movement direction of the balls 46, simply supported constraints are implemented on the specimen 1 during the loading test. This ensures that the specimen 1 is protected from instability when subjected to loads such as tension, compression, tensile-compression fatigue, compressive-compression fatigue, and tensile-tensile fatigue during the planar biaxial loading test, preventing instability of the specimen 1 during the test.

[0078] Moreover, the central opening 43 of the first anti-instability pressure plate 41 and the second anti-instability pressure plate 42 is set to be coaxial with the central axis of the through hole 32 of the cross-shaped reinforcement plate 3, and the balls 46 are arranged with reference to the central opening 43 so that the balls 46 surround the central opening 43, which can provide uniform and balanced simply supported constraints for the specimen 1.

[0079] By providing annular tracks 44 on the first and second anti-instability pressure plates 41, 42, and positioning balls 46 within the annular tracks 44, it is possible to obtain the first and second anti-instability pressure plates 41, 42, providing simply supported constraints for the specimen in a cost-effective and easily manufactured manner. Furthermore, positioning the annular tracks 44 outside the through-holes 32 of the cross-shaped reinforcement plate 3 prevents the balls 46 from damaging the test area of ​​the specimen 1 when compressing the cross-shaped reinforcement plate 3. Furthermore, the annular tracks 44 restrict the freedom of movement of the balls 46, allowing them to move only in rotation.

[0080] Moreover, the balls 46 are arranged in the annular track 44 in a manner guided by the grooves 45 , which facilitates operation.

[0081] Alternatively, the clamping device further includes a locking slider (not shown) that is configured to be pressed into the groove 45 and moved to the annular track 44 to close the annular track 44. By squeezing the locking slider into the groove 45 to close the annular track 44, the ball 46 is easily installed and positioned in the annular track 44, and the locking slider is easily stored when not in use. Preferably, the locking slider is made of an elastic material.

[0082] When the first and second anti-destabilization pressure plates 41, 42 are not in use, the locking slider can be placed in the groove 45 to prevent loss. When preparing for a planar biaxial test on composite materials, before installing the clamping device, the locking slider is removed from the groove 45 and the balls 46 are sequentially pushed through the groove 45 into the annular track 44. After the balls 46 fill the annular track 44, the locking slider is squeezed into the groove 45 and one end is pushed against the annular track 44 to lock the locking slider and close the annular track 44, thereby confining the balls 46 within the annular track 44. Furthermore, if the balls 46 become worn, the locking slider can be removed again to replace the balls 46, and new, unworn balls 46 can be installed in the annular track 44.

[0083] Optionally, as shown in FIG6 , the first and second anti-instability pressure plates 41 and 42 further include four ears 47 extending outwardly along the centerline of the side of the specimen 1 on which the clamping arms 21 are provided. The four ears 47 allow the first and second anti-instability pressure plates 41 and 42 to be removably secured to opposing surfaces of the clamping arms 21 by means of threaded fasteners, adhesive bonding, or other fastening means, thereby increasing the contact area between the first and second anti-instability pressure plates 41 and 42 and the clamping arms 21, thereby allowing the first and second anti-instability pressure plates 41 and 42 to be securely engaged with the pair of clamping arms 21, respectively. At this point, the first and second anti-instability pressure plates 41 and 42 provide simply supported constraints for the specimen 1 through the positioning and clamping of the clamping mechanism and the rolling of the balls 46.

[0084] As shown in Figure 1, the clamping mechanism also includes four columns 22 for supporting four pairs of clamping arms 21, wherein one clamping arm 211 in each pair of clamping arms 21 is fixedly mounted on the column 22, and the other clamping arm 212 can move along the column 22 relative to the fixed clamping arm 211, thereby adjusting the position and angle of the specimen 1 between the clamping arms 21.

[0085] Finally, the assembly diagram for performing a planar biaxial loading test on a composite material specimen 1 using the clamping device and the chuck 5 of the planar biaxial loading testing machine is shown in Figure 7. In Figure 7, the chuck 5 of the planar biaxial loading testing machine is placed on the side of the specimen 1 where the clamping arms 21 are not provided, and the length of the specimen 1 clamped by the chuck 5 is greater than or equal to 100 mm.

[0086] Specifically, the method for performing a planar biaxial loading test using a clamping device and a planar biaxial loading testing machine (not shown) includes preheating the planar biaxial loading testing machine, selecting and installing a chuck 5, arranging a cross-shaped reinforcement plate 3, clamping the specimen 1 using a clamping device, and applying a load and collecting the value of the strain gauge.

[0087] Specifically, the chuck selection and installation step includes selecting a chuck 5 whose width matches the width of the composite material specimen 1, installing the chuck 5 on a planar biaxial loading testing machine, and adjusting the orientation of the chuck 5 so that the center line of the chuck 5 is coaxial with the loading axis of the planar biaxial loading testing machine.

[0088] The step of arranging the cross-shaped reinforcement sheets 3 includes arranging two cross-shaped reinforcement sheets 3 on the front and back surfaces of the specimen 1, respectively, and making the four legs 31 of each cross-shaped reinforcement sheet 3 extend toward the four sides of the specimen that are separated from each other, and then placing the specimen 1 and the legs 31 of the cross-shaped reinforcement sheets 3 arranged on the front and back surfaces of the specimen 1 in the accommodating space of the clamp 5.

[0089] The step of clamping the specimen 1 using the clamping device includes arranging four pairs of clamping arms 21 of the clamping mechanism at intervals from each other on the sides of the legs 31 of the specimen 1 not provided with the cross-shaped reinforcement pieces 3, and installing the first anti-instability pressure plate 41 and the second anti-instability pressure plate 42 on the opposite surfaces of each pair of clamping arms 21 by fasteners, clamping the specimen 1 and the two cross-shaped reinforcement pieces 3 respectively provided on the front and rear surfaces of the specimen 1 between the first anti-instability pressure plate 41 and the second anti-instability pressure plate 42 by each pair of clamping arms 21, and adjusting the position of the second anti-instability pressure plate 42 relative to the first anti-instability pressure plate 41 so that the centerline of the side of the specimen 1 corresponding to the clamp 5 is coaxial with the centerline of the clamp 5;

[0090] The steps of applying a load and collecting values ​​of the strain gauges include electrically connecting the strain gauges on the specimen 1 to the dynamic strain gauges to collect values ​​from the strain gauges on the specimen 1 when the load is applied to the specimen 1 .

[0091] Optionally, the method further comprises forming a scoreline on the specimen 1 , and arranging the clamp 5 on the side of the specimen 1 where the leg 31 of the cross-shaped reinforcement sheet 3 is not arranged according to the position of the scoreline.

[0092] The method of conducting a planar biaxial loading test using a clamping device can ensure that the test area of ​​the specimen 1 is uniformly stressed during the loading test and the stress concentration outside the test area is small. The specimen 1 will not experience local instability during the test and the failure mode results are also in line with expectations.

[0093] Optionally, the method further includes applying a planar biaxial load to the specimen 1 using a planar biaxial loading tester, recording stress and strain values ​​from strain gauges on the front and back surfaces of the specimen 1, and calculating the specimen bending percentage based on the ratio between the difference and the cumulative value of the stress and strain values ​​from the two strain gauges disposed on the front and back surfaces of the specimen 1. By analyzing the specimen bending percentage during the preloading phase, it is possible to determine whether the clamping position of the specimen 1 is correct, so that the specimen 1 can be aligned and debugged in a timely manner.

[0094] Determine whether the position of the specimen relative to the planar biaxial loading testing machine is correct based on the calculated bending percentage; and when it is determined that the position of the specimen is incorrect, further adjust the position of the second anti-instability pressure plate 42 relative to the first anti-instability pressure plate 41 to ensure that the center line of the side of the specimen 1 corresponding to the chuck 5 is coaxial with the center line of the chuck 5.

[0095] Optionally, the method further includes synchronously and stepwise loading the biaxial load applied by the planar biaxial loading tester onto the specimen 1 according to a predetermined load ratio, and recording the load history, failure location, and failure mode of each loading test, thereby obtaining the biaxial failure load of the specimen corresponding to the failure mode. Optionally, for each set of tests, the mean, standard deviation, and coefficient of variation of each failure load can be calculated for subsequent application.

[0096] In general, this application ensures that tests with different load ratios and different load forms are carried out through the design style of the composite material specimen 1, the form and size of the working area, the design of the cross-shaped reinforcement plate 3 of the clamping mechanism, the clamp form of the anti-instability pressure plate with simple support constraints, and the introduction method of impact damage, thereby completing the planar biaxial loading test of the composite material specimen with impact damage under complex stress.

[0097] The clamping device for planar biaxial testing of composite materials and the method for performing planar biaxial loading testing of composite material specimens 1 using the clamping device provided in the present application can realize compression and tensile static testing of composite material specimens 1, such as composite material laminates, in planar biaxial directions, and at the same time meet the requirements of tensile and compression fatigue testing, thereby providing methods and support for verifying the implementation of biaxial load failure tests of composite material laminates and biaxial load failure tests of composite material laminates that introduce impact damage.

[0098] Although specific embodiments of the present application have been described above, those skilled in the art will appreciate that these are merely illustrative and that the scope of protection of the present application is defined by the appended claims. Those skilled in the art may make various changes or modifications to these embodiments without departing from the principles and essence of the present application, and such changes and modifications shall fall within the scope of protection of the present application.

Claims

1. A clamping device for planar biaxial loading tests of composite materials, wherein, the composite material is made into a specimen in the shape of a regular octagon, and the clamping device includes: a clamping mechanism for fixedly clamping the specimen within the loading plane, the clamping mechanism including four pairs of clamping arms, and the four pairs of clamping arms are respectively arranged on four sides of the specimen at intervals from each other; a cross-shaped reinforcing sheet, the cross-shaped reinforcing sheet including four legs extending along opposite directions in pairs, the four legs of the cross-shaped reinforcing sheet respectively extend towards the sides of the specimen where no clamping arms are provided, and a through hole is provided at the center of the cross-shaped reinforcing sheet, wherein, the clamping device further includes a first anti-instability pressure plate and a second anti-instability pressure plate detachably arranged on the opposite surfaces of each pair of clamping arms, and each pair of clamping arms is configured to be able to clamp the specimen, two cross-shaped reinforcing sheets respectively arranged on the front and back surfaces of the specimen, and the first anti-instability pressure plate and the second anti-instability pressure plate therebetween.

2. The clamping device according to claim 1, wherein, central openings are respectively formed on the first anti-instability pressure plate and the second anti-instability pressure plate, the central axis of the central opening is coaxial with the central axis of the through hole of the cross-shaped reinforcing sheet, and balls surrounding the central opening are provided on the surfaces of the first anti-instability pressure plate and the second anti-instability pressure plate respectively in contact with the surface of the cross-shaped reinforcing sheet, and the balls are configured to protrude out of the surfaces of the first anti-instability pressure plate and the second anti-instability pressure plate, so as to be able to press against the cross-shaped reinforcing sheet when clamping the specimen.

3. The clamping device according to claim 2, wherein, annular tracks are formed on the surfaces of the first anti-instability pressure plate and the second anti-instability pressure plate respectively in contact with the cross-shaped reinforcing sheet, the annular tracks are arranged around the central opening and are located outside the through hole of the cross-shaped reinforcing sheet, and the balls are arranged in the annular tracks.

4. The clamping device according to claim 3, wherein, grooves communicating the annular tracks and any edge outside thereof are further formed on the surfaces of the first anti-instability pressure plate and the second anti-instability pressure plate respectively in contact with the cross-shaped reinforcing sheet, so that the balls can be guided and pushed into the annular tracks through the grooves.

5. The clamping device according to claim 4, wherein, the clamping device further includes a locking slider, and the locking slider is configured to be able to be pressed into the grooves and moved to the annular tracks to close the annular tracks.

6. The clamping device according to claim 2, wherein, the first anti-instability pressure plate and the second anti-instability pressure plate respectively further include four ears extending outward along the center line direction of the sides of the specimen where the clamping arms are provided.

7. The clamping device according to claim 1, wherein, the clamping mechanism further includes four columns for supporting the four pairs of clamping arms, wherein, one clamping arm in each pair of clamping arms is fixedly installed on the column, and the other clamping arm is configured to be able to move along the column relative to the fixed clamping arm.

8. The clamping device according to claim 1, wherein, a circular impact damage area is formed in the center of the test piece, and a circular window is provided in the center of the fixture for introducing impact damage.

9. The clamping device according to claim 8, wherein, the through hole of the cross-shaped reinforcing piece is circular and coaxial with the impact damage area, and its diameter is at least twice the diameter of the impact damage area.

10. The clamping device according to claim 9, wherein, the diameter of the circular window is 1.5 times the diameter of the through hole of the cross-shaped reinforcing piece.

11. The clamping device according to claim 10, wherein, the chucks of the planar biaxial loading testing machine are arranged on the side of the test piece where the clamping arms are not provided, and the length of the test piece clamped by the chucks is greater than or equal to 100 mm.

12. The clamping device according to claim 1, wherein, the width of each leg is less than the length of a single side of the test piece.

13. The clamping device according to claim 12, wherein, the width of each leg is 10 mm less than the length of a single side of the test piece.

14. The clamping device according to claim 1, wherein, the sides of the four legs of the cross-shaped reinforcing piece are connected by arc surfaces at the cross intersection.

15. The clamping device according to any one of claims 1 to 14, wherein, strain gauges are arranged on the front and back surfaces of the test piece at positions corresponding to the through holes of the cross-shaped reinforcing piece.

16. A method for performing a planar biaxial loading test using the clamping device according to any one of claims 1 to 15, wherein, the method includes: Preheating the planar biaxial loading testing machine Selecting chucks with a width matching the width of the test piece, installing the chucks on the planar axis loading testing machine, and adjusting the orientation of the chucks so that the center line of the chucks is coaxial with the loading axis of the planar biaxial loading testing machine; Respectively arranging two cross-shaped reinforcing pieces on the front and back surfaces of the test piece, and making the four legs of each cross-shaped reinforcing piece extend towards the four mutually separated sides of the test piece, and then placing the test piece and the legs of the cross-shaped reinforcing pieces arranged on the front and back surfaces of the test piece in the accommodation space of the chucks; Respectively arranging four pairs of clamping arms of the clamping mechanism on the sides of the legs of the test piece where the cross-shaped reinforcing pieces are not arranged, and installing the first anti-instability pressure plate and the second anti-instability pressure plate on the opposite surfaces of each pair of clamping arms through fasteners, and clamping the test piece, and the two cross-shaped reinforcing pieces respectively arranged on the front and back surfaces of the test piece, between the first anti-instability pressure plate and the second anti-instability pressure plate through each pair of clamping arms, and making the center line of the side of the test piece corresponding to the chuck coaxial with the center line of the chuck; and Electrically connecting the strain gauges on the front and back surfaces of the test piece to a dynamic strain gauge to collect the values from the strain gauges on the test piece when loading the test piece.

17. The method according to claim 16, wherein, the method further includes: Form a scribed line on the test piece, and arrange the chuck on the edge of the leg of the test piece where no cross-shaped reinforcement piece is arranged according to the position of the scribed line.

18. The method according to claim 16, wherein, the method further comprises: Applying a planar biaxial load to the test piece through the planar biaxial loading testing machine, recording the stress-strain values of the test piece from the strain gauges on the front and back surfaces of the test piece, and calculating the bending percentage of the test piece according to the ratio between the difference and the sum of the stress-strain values of the test piece from the two strain gauges on the front and back surfaces of the test piece respectively.

19. The method according to claim 18, wherein, the method further comprises: Determining whether the position of the test piece relative to the planar biaxial loading testing machine is correct according to the calculated bending percentage; and When it is determined that the position of the test piece is incorrect, further adjusting the position of the second anti-instability pressing plate relative to the first anti-instability pressing plate, so as to ensure that the center line of the side of the test piece corresponding to the chuck is coaxial with the center line of the chuck.

20. The method according to claim 16, wherein, the method further comprises: Loading the biaxial load applied by the planar biaxial loading testing machine onto the test piece synchronously and step by step according to a predetermined load ratio, and recording the load history, the failure position and the failure mode on the test piece for each loading test, so as to obtain the biaxial failure load of the test piece under the corresponding failure mode.

Images