Clamping device for plane biaxial loading test of composite material and test method

Abstract

The invention provides a clamping device for a plane biaxial loading test of a composite material, the composite material is made into a regular octagonal test piece, and the clamping device comprises a clamping mechanism for fixedly clamping the test piece in a loading plane, a cross-shaped reinforcing sheet and an anti-loss pressure stabilizing plate, the clamping mechanism comprises four pairs of clamping arms which are spaced from one another and are respectively arranged on four edges of the test piece, four legs of the cross-shaped reinforcing sheet respectively extend towards the edges, without the clamping arms, of the test piece, and a through hole is formed in the center of the cross-shaped reinforcing sheet; the clamping device further comprises a first anti-loss pressure stabilizing plate and a second anti-loss pressure stabilizing plate which are detachably arranged on the opposite surfaces of each pair of clamping arms. And each pair of clamping arms is used for clamping the test piece, the two cross-shaped reinforcing sheets arranged on the upper surface and the lower surface of the test piece respectively, the first anti-loss pressure stabilizing plate and the second anti-loss pressure stabilizing plate between the test piece and the two cross-shaped reinforcing sheets. The invention also relates to a method for carrying out the planar biaxial loading test by using the clamping device.

Description

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 a composite material 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, making the stress state of composite materials more and more complex. In many usage scenarios, they are subjected to loads in two directions. However, most of the existing strength test standard methods only focus on material failure under a single load stress, which is not enough to fully evaluate the mechanical properties of the material. Therefore, testing the mechanical properties of composite structural parts such as composite laminates under complex stress states is crucial to ensure the safety of structural parts in use or service.

[0003] A lot of research has been done on biaxial loading tests of materials at home and abroad, especially for metal materials, whose corresponding biaxial loading tests have formed industry and national standards. However, there is no unified standard for biaxial loading tests of composite materials at present, and composite materials are different from metal materials and are more sensitive to structural characteristics. Therefore, the plane biaxial loading test of composite structural parts is obviously different from the plane biaxial loading test of metal parts, which results in that the clamping device used in the plane biaxial loading test of metal parts cannot be directly applied to the plane biaxial loading test of composite materials. Summary of the invention

[0004] 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.

[0005] This application solves the above technical problems through the following technical solutions:

[0006] 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:

[0007] 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;

[0008] A cross-shaped reinforcement sheet, the cross-shaped reinforcement sheet includes four legs extending in opposite directions, the four legs extending toward the sides of the specimen where no clamping arms are provided, and a through hole is provided in the center of the cross-shaped reinforcement sheet.

[0009] Among them, the clamping device also includes a first anti-instability pressure plate and a second anti-instability pressure plate which are detachably arranged on the opposite surfaces of each pair of clamping arms, and each pair of clamping arms is configured to clamp the specimen, two cross-shaped reinforcement plates 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.

[0010] The present application designs the composite material specimen into a regular octagon, thereby arranging the clamping arms of the clamping device and the chuck of the planar biaxial loading testing machine evenly 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.

[0011] 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 position of the through hole of the composite material specimen corresponding to the through hole, 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.

[0012] 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.

[0013] 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 balls are arranged around the central opening on the surfaces of the first anti-instability pressure plate and the second anti-instability pressure plate that are respectively in contact with the surface of the cross-shaped reinforcement plate, 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.

[0014] 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 the front and back sides of the specimen to simultaneously apply a simply supported constraint on the front and back surfaces of the specimen, thereby achieving a simply supported constraint 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, compression-compression fatigue, and tensile-tensile fatigue in a plane biaxial loading test, thereby preventing the specimen from becoming unstable during the test.

[0015] 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.

[0016] 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 sheet. The annular tracks are arranged around the central opening and are located outside the through hole of the cross-shaped reinforcement sheet, and the ball bearings are arranged in the annular tracks.

[0017] By arranging annular tracks on the first anti-instability pressure plate and the second anti-instability pressure plate and arranging the balls on the annular tracks, it is convenient to obtain the first anti-instability pressure plate and the second anti-instability pressure plate that provide simple support constraints for the specimen in a low-cost and easy-to-manufacture manner. Moreover, arranging the annular tracks outside the through holes of the cross-shaped reinforcement sheet can prevent the balls from damaging the test area of ​​the specimen when squeezing the cross-shaped reinforcement sheet. In addition, the annular tracks can limit the freedom of movement of the balls, so that the balls can only rotate and move at the installation position.

[0018] According to an embodiment of the present application, grooves 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 reinforcement sheet, connecting the annular track and any outer edge thereof, so that the balls can be guided and pushed into the annular track through the grooves. The balls are arranged in the annular track by means of the grooves for easy operation.

[0019] According to one embodiment of the present application, the clamping device further includes a locking slider, which is 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 installation and positioning of the ball in the annular track and the storage of the locking slider when idle are facilitated.

[0020] When the anti-instability platen is idle, the locking slider can be placed in the groove to avoid loss; when preparing to conduct a planar biaxial test on composite materials, before installing the clamping device, take out the locking slider from the groove and push the balls into the annular track through the groove in sequence; after the balls fill the annular track, squeeze the locking slider into the groove and push one end of it to the annular track to lock the locking slider and close the annular track, so as to confine the balls in the annular track. Moreover, when the balls are worn to a certain extent, the locking slider can be taken out again to replace the balls and install new, unworn balls in the annular track.

[0021] According to an embodiment of the present application, the first anti-destabilization pressure plate and the second anti-destabilization pressure plate respectively further include four ears extending outwardly along the center line direction of the side of the clamping arm provided on the specimen. The four ears can increase the contact area between the first anti-destabilization pressure plate and the second anti-destabilization pressure plate and the pair of clamping arms, so that the first anti-destabilization pressure plate and the second anti-destabilization pressure plate are respectively firmly engaged with the pair of clamping arms.

[0022] According to one embodiment of the present application, the clamping mechanism also 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 able to move along the column relative to the fixed clamping arm. By adjusting the distance between the other moving clamping arm and the fixed clamping arm, specimens of different thicknesses can be accommodated to clamp composite specimens for planar biaxial loading tests. Moreover, by adjusting the moving clamping arm, the plane where the specimen is located can be appropriately adjusted to ensure that the specimen is within the plane where the load is applied.

[0023] 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 opened 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 the planar biaxial loading test of composite materials subjected to impact damage. In order to ensure the reasonable and effective introduction of impact damage, it is often necessary to consider the fixture for introducing impact damage. Since the test area of ​​the specimen is usually designed in a circular shape, and the rectangular impact opening will cause stress concentration and uneven damage distribution on the impact damage of the specimen, the impact damage of the specimen is introduced by 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 the impact damage from having a large impact on the non-test area of ​​the specimen.

[0024] According to one embodiment of the present application, the through hole of the cross-shaped reinforcement sheet is circular, and its diameter is at least twice the diameter of the impact damage area. By designing the size relationship between the through hole of the cross-shaped reinforcement sheet and the impact damage area, the expansion of the impact damage during the loading of the test piece and the mutual influence of the edges of the test piece can be effectively prevented.

[0025] According to an 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.

[0026] According to one embodiment of the present application, the chuck of the planar biaxial testing machine is arranged 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 center line of the specimen coincides with the center line of the chuck, thereby ensuring that the load from the chuck is applied at the inertia center of the cross section of the specimen.

[0027] 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.

[0028] 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.

[0029] According to an 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 the circular arc transition makes the stress concentration phenomenon of the specimen at the transition arc during the test weaker, thereby meeting the test requirements of a high and uniform stress level in the test area and a small stress outside the test area, thereby making it easier to achieve a biaxial stress state in the test area of ​​the specimen, thereby ensuring that the initial failure occurs in the test area of ​​the specimen.

[0030] According to an embodiment of the present application, strain gauges are arranged on the front and back sides of the specimen at positions corresponding to the through holes of the cross-shaped reinforcement sheet. The strain of the specimen during loading can be obtained through the strain gauges, thereby obtaining the failure condition of the specimen.

[0031] 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:

[0032] Preheat the plane biaxial loading testing machine;

[0033] Select a chuck with a thickness matching that of the composite material specimen, and install the chuck on a planar biaxial loading test 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 test machine;

[0034] Arrange two cross-shaped reinforcing sheets on the front and back surfaces of the specimen respectively, and make the four legs of each cross-shaped reinforcing sheet extend toward the four sides of the specimen that are separated from each other, and then place the specimen and the legs of the cross-shaped reinforcing sheets arranged on the front and back surfaces of the specimen in the accommodation space of the chuck;

[0035] The four pairs of clamping arms of the clamping mechanism are arranged at intervals from each other on the sides of the legs of the specimen where no cross-shaped reinforcement pieces are arranged, and the first anti-slipping pressure plate and the second anti-slipping pressure plate are respectively installed on the opposite surfaces of each pair of clamping arms by fasteners, and the specimen and the two cross-shaped reinforcement pieces respectively arranged on the front and back surfaces of the specimen are clamped between the first anti-slipping pressure plate and the second anti-slipping pressure plate by each pair of clamping arms, so that the center line of the side of the specimen corresponding to the clamp is coaxial with the center line of the clamp; and

[0036] The strain gauges on the specimen are electrically connected to the dynamic strain gauges to collect values ​​from the strain gauges on the specimen when loading is applied to the specimen.

[0037] 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 scoreline on the specimen, and arranging the clamp on the side of the specimen where no cross-shaped reinforcement plate leg is arranged according to the position of the scoreline.

[0038] 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 the 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.

[0039] According to one embodiment of the present application, the method for performing a planar biaxial loading test using a clamping device further includes applying a planar biaxial load to the specimen through a planar biaxial loading test machine, recording the stress-strain value of the specimen from the strain gauges disposed on the front and back surfaces of the specimen, and calculating the bending percentage of the specimen based on the ratio between the difference between the stress-strain values ​​of the two strain gauges on the front and back surfaces of the specimen and the cumulative value. By analyzing the bending percentage of the specimen in the preloading stage, it is possible to determine whether the clamping position of the specimen is correct, so as to timely perform centering and debugging of the specimen.

[0040] According to one embodiment of the present application, the method for performing 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.

[0041] 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 location and failure mode on the specimen of each loading test, so as to obtain the biaxial failure load of the specimen under the corresponding failure mode.

[0042] On the basis of conforming to the common sense in the art, the above-mentioned preferred conditions can be arbitrarily combined to obtain the preferred embodiments of the present application.

[0043] 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:

[0044] Through the regular octagonal design of the composite material specimen, the specimen can be clamped on the loading plane to ensure that the biaxial load is evenly and effectively applied to the specimen. Through the shape design and position arrangement of the cross-shaped reinforcement plate, the load can be effectively transferred 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 damage 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 be unstable during the plane biaxial loading test. At the same time, impact damage is introduced in the center of the specimen to conduct a plane biaxial loading test of the composite material considering impact damage. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 FIG. 1 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.

[0046] Figure 2 for Figure 2 An exemplary schematic diagram of a composite material specimen is shown in FIG.

[0047] Figure 3 is an exemplary schematic diagram of a fixture for introducing impact damage according to a preferred embodiment of the present application,

[0048] Figure 4 for Figure 1 An exemplary schematic diagram of a cross-shaped reinforcement sheet of a clamping device,

[0049] Figure 5 For two Figure 4 The cross-shaped reinforcement plates are arranged in Figure 2 Schematic diagram of the front and back surfaces of the composite material specimen,

[0050] Figure 6 for Figure 1 An exemplary schematic diagram of the first anti-instability pressure plate of the clamping device,

[0051] Figure 7 It is an exemplary schematic diagram of performing a planar biaxial loading test on 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

[0052] In order to make the purpose, technical solutions and advantages of the present application clearer, the technical solutions in the implementation mode of the present application will be clearly and completely described below in conjunction with the accompanying drawings showing multiple embodiments according to the present application. It should be understood that all other embodiments obtained by ordinary technicians in this field without spending creative work based on the embodiments recorded in the present application will fall within the scope of protection of the present application.

[0053] Existing specimens used for planar biaxial loading tests of composite materials have configurations such as cross, square, and special shapes. Among them, based on the mature biaxial loading test of cross-shaped metal parts, the mechanical properties analysis of cross-shaped composite specimens has been widely studied.

[0054] The cross-shaped composite material specimen is usually cross-shaped as a whole and has four loading arms. For the cross-shaped composite material specimen, the clamping area and the area where the load is applied are both located on the loading arm. In order to ensure that the stress level in the central test area of ​​the specimen is high and uniform, it is necessary to set a loading arm hollow area on each loading arm to weaken the stress on the loading arm. At the same time, an arc transition is set at the intersection of the four loading arms to weaken the stress concentration at the intersection, so as to achieve a biaxial stress state in the central test area and ensure that the initial failure occurs in the expected test area.

[0055] 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 test machines to perform a planar biaxial loading test of the cross-shaped composite material specimen.

[0056] Although the existing loading fixture device can load the specimen through the uniaxial loading test machine, and can effectively control the test cost and reduce the test difficulty, due to the need to clamp and fix while applying the load, most of the stress is concentrated on the chuck position of the loading fixture device rather than the central test area of ​​the specimen, and damage often occurs in the hollow area of ​​the loading arm, which causes the planar biaxial loading test results to be significantly different from the expected results, the test efficiency is low, and even the planar biaxial loading test of composite materials cannot be correctly performed.

[0057] 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.

[0058] like Figure 1As shown, the clamping device includes a clamping mechanism 2 for fixedly clamping the specimen 1 in the loading plane, a cross-shaped reinforcement sheet 3 for transmitting the load to the central test area of ​​the specimen 1, and an anti-instability pressure plate for preventing the specimen 1 from losing stability during the loading test. The clamping mechanism 2 includes four pairs of clamping arms 21, which are arranged on the four sides of the specimen 1 at intervals from each other, and the first anti-instability pressure plate 41 and the second anti-instability pressure plate 42 are also detachably arranged on the opposite surface of each pair of clamping arms 21. Two cross-shaped reinforcement sheets 3 are arranged on the front and back surfaces of the specimen 1, respectively, and the four legs 31 of each cross-shaped reinforcement sheet 3 extend toward the side of the specimen 1 where the clamping arm 21 is not arranged, and a through hole 32 is arranged in the center of the cross-shaped reinforcement sheet 3. Each pair of clamping arms 21 clamps the specimen 1, the two cross-shaped reinforcement sheets 3 arranged on the front and back surfaces of the specimen 1, and the first anti-instability pressure plate 41 and the second anti-instability pressure plate 42 therebetween.

[0059] like Figure 2 As shown, 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 at the positions of the through holes 32 of the cross-shaped reinforcement plate 3 on the front and back sides of the specimen 1.

[0060] like Figures 1 to 7 As shown, 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 test machine, so as to avoid the interference of the biaxial load applied to the specimen 1 by the chuck 5 in clamping and fixing the specimen 1, thereby ensuring that the stress of the specimen 1 in the test area is uniform and maximum, 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.

[0061] 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 and the loading area of ​​the composite material specimen 1 are separated, 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. The clamping device can thus be used to perform a variety of planar biaxial loading tests on the composite material specimen 1, such as planar biaxial tension, planar biaxial compression, planar biaxial tension-compression, planar biaxial compression fatigue, and planar biaxial tension-tension, thereby facilitating the analysis of the mechanical properties of the composite material specimen 1 under complex stress states.

[0062] The overall size of the specimen 1 is usually determined based on the selected planar biaxial loading test machine and the pre-test analysis results to ensure that the specimen 1 is subjected to uniform stress 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.

[0063] Under the conditions that the effective stroke of the test equipment is met, the test area of ​​the test piece 1 is subjected to uniform force, and the test piece does not become unstable before the test area is destroyed, the composite material test piece 1 with a longer size is preferably selected.

[0064] like Figure 5 As shown, the width of the clamping end S of the specimen 1 is usually designed according to the width of the clamping head 5 of the planar biaxial loading test machine. In order to obtain a balanced stress state in the test area of ​​the specimen 1, a wider specimen 1 is preferably selected, but the width of the clamping end S should always be smaller than the width of the clamping head 5.

[0065] The thickness of the specimen 1 is designed according to the ply thickness of the target structure to be verified and the load of the planar biaxial loading test machine, and 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.

[0066] When considering the impact damage of specimen 1, in order to ensure the reasonable and effective introduction of impact damage, it is necessary to consider the fixture for introducing impact damage. Since the test area of ​​specimen 1 is usually designed in a circular shape, in order to prevent the influence of the rectangular impact opening on the impact damage of the specimen, such as Figure 3 As shown, a circular window 61 is provided in the center of the fixture 6 for introducing impact damage in the present application. Moreover, in order to facilitate the clamping of the specimen 1 by the clamp 62 during the introduction of impact damage, the specimen 1 is conveniently fixed. Preferably, the fixture 6 for introducing impact damage adopts a step design.

[0067] Optionally, the test piece 1 may also be a composite material test piece which has not been damaged by impact but has a defect embedded in the center thereof.

[0068] The cross-shaped reinforcing sheet 3 of the clamping device is as follows Figure 4 As shown, it includes four legs 31 extending in opposite directions in pairs, the sides of the four legs 31 are transitionally connected at the cross intersection by arc surfaces 33, and a through hole 32 coaxial with the central axis of the impact damage area is also provided in the center of the cross-shaped reinforcement sheet 3. When a circular window 61 is provided in the center of the impact-damaged fixture 6, the aforementioned through hole 32 is a circular through hole, and the diameter of the circular window 61 is 1.5 times the diameter of the circular through hole 32.

[0069] like Figure 1 and Figure 5 As shown, in the assembled clamping device, two cross-shaped reinforcement sheets 3 are respectively arranged on the front and back surfaces of the composite material specimen 1, and the four legs 31 of each cross-shaped reinforcement sheet 3 extend to the corresponding edge of the specimen 1 where the clamping arm 21 is not provided. The clamp 5 of the planar biaxial loading test machine applies load to the cross-shaped reinforcement sheet 3 and the specimen 1 by clamping the legs 31 of the two cross-shaped reinforcement sheets 3 and the specimen 1 between the two cross-shaped reinforcement sheets 3.

[0070] By designing the cross-shaped reinforcement sheet 3 arranged on the front and back surfaces of the composite specimen 1, the load can be effectively transferred to the specimen 1. By arranging a through hole 32 coaxial with the central axis of the impact damage area in the center of the cross-shaped reinforcement sheet 3, it can be ensured that the load transferred to the central test area of ​​the specimen 1 is greater than that in other areas, thereby ensuring that the stress level in the test area of ​​the composite specimen 1 is higher than that in other areas, so as to ensure that the initial damage occurs in the test area.

[0071] Moreover, the sides of the four legs 31 of the cross-shaped reinforcement sheet 3 are connected by a transitional arc surface 33 at the cross intersection, which can make the load on the specimen 1 at the transition of the arc surface 33 relatively uniform and small, so that the stress concentration phenomenon of the specimen 1 at the transition of the arc surface 33 is weak, thereby meeting the requirement of a high and uniform stress level in the central test area, and further ensuring that the initial failure occurs in the test area. To prevent stress concentration, the curvature of the arc surface 33 of the cross-shaped reinforcement sheet 3 should be greater than a predetermined angle. The predetermined angle can be determined based on the size of the specimen 1 and the applied load.

[0072] The size of the through hole 32 of the cross-shaped reinforcement sheet 3 can be determined according to the size of the specimen 1 and the results of the pre-test analysis, and it is necessary to consider the convenience of pasting 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, otherwise it will cause the specimen 1 to be difficult to fail. The size of the through hole 32 of the cross-shaped reinforcement sheet 3 should not be designed to be too large, otherwise 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.

[0073] In general, the size of the through hole 32 of the cross-shaped reinforcing sheet 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.

[0074] The width of the clamping end N of the cross-shaped reinforcing sheet 3 is slightly smaller than the width of the clamping end S of the specimen. Preferably, the width of the clamping end N of the cross-shaped reinforcing sheet 3 is 10 mm smaller than the width of the clamping end S of the specimen 1. More preferably, the length of the cross-shaped reinforcing sheet 3 is the same as the total length of the specimen 1.

[0075] The first anti-loss stabilization plate 41 of the clamping device is as follows Figure 6As shown, a central opening 43 coaxial with the central axis of the through hole 32 of the cross-shaped reinforcement sheet 3 is formed thereon, and an annular track 44 and a groove 45 connecting the annular track 44 with any outer edge thereof are formed on the surface in contact with the cross-shaped reinforcement sheet 3. The annular track 44 is arranged around the central opening 43 and is located outside the through hole 32 of the cross-shaped reinforcement sheet 3, and the ball 46 can be guided and pushed into the annular track 44 through the groove 45. The ball 46 installed in the annular track 44 protrudes outward from the surface of the first anti-destabilization pressure plate 41 and the second anti-destabilization pressure plate 42, and can press the cross-shaped reinforcement sheet 3 when clamping the test piece 1. The first anti-destabilization pressure plate 41 and the second anti-destabilization pressure plate 42 of the clamping device have the same structure.

[0076] By arranging outwardly protruding balls 46 on the surfaces of the first anti-instability pressure plate 41 and the second anti-instability pressure plate 42 close to the cross-shaped reinforcement sheet 3, the balls 46 can be used to press the cross-shaped reinforcement sheet 3 on both sides of the specimen 1 to simultaneously apply simple support constraints on the front and back surfaces of the specimen 1. By constraining the movement direction of the balls 46, the simple support constraints on the specimen 1 during the loading test are realized, thereby ensuring that the specimen 1 can be protected from instability when subjected to tension, compression, tension-compression fatigue, compression-compression fatigue, tension-tension fatigue and other loads in the plane biaxial loading test, so as to prevent the specimen 1 from becoming unstable during the test.

[0077] 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.

[0078] By setting the annular track 44 on the first anti-instability plate 41 and the second anti-instability plate 42, and setting the ball 46 in the annular track 44, it is convenient to obtain the first anti-instability plate 41 and the second anti-instability plate 42 that provide simple support constraints for the specimen in a low-cost and easy-to-manufacture manner. Moreover, by setting the annular track 44 outside the through hole 32 of the cross-shaped reinforcement sheet 3, it is possible to prevent the ball 46 from damaging the test area of ​​the specimen 1 when squeezing the cross-shaped reinforcement sheet 3. In addition, the annular track 44 can limit the freedom of movement of the ball 46, so that the ball 46 can only move in rotation.

[0079] Moreover, the balls 46 are arranged in the annular track 44 by being guided by the grooves 45, so as to facilitate operation.

[0080] Alternatively, the clamping device further includes a locking slider (not shown), which 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, it is convenient to install and position the ball 46 in the annular track 44 and to store the locking slider when it is idle. Preferably, the locking slider is made of elastic material.

[0081] When the first anti-destabilization pressure plate 41 and the second anti-destabilization pressure plate 42 are idle, the locking slider can be placed and stored in the groove 45 to avoid loss. When preparing to carry out a planar biaxial test of the composite material, before installing the clamping device, take out the locking slider from the groove 45, and push the balls 46 into the annular track 44 through the groove 45 in sequence; after the balls 46 fill the annular track 44, squeeze the locking slider into the groove 45 and push one end of it to the annular track 44 to lock the locking slider and close the annular track 44, thereby restricting the balls 46 in the annular track 44. Moreover, when the balls 46 are worn to a certain extent, the locking slider can be taken out again to replace the balls 46, and install new, unworn balls 46 in the annular track 44.

[0082] Alternatively, if Figure 6 As shown, the first anti-instability pressure plate 41 and the second anti-instability pressure plate 42 also include four ears 47 extending outward along the center line direction of the side where the clamping arm 21 is set in the specimen 1. Through the four ears 47, the first anti-instability pressure plate 41 and the second anti-instability pressure plate 42 can be detachably fixed to the relative surface of the clamping arm 21 by means of fastening means such as threaded fasteners and adhesive bonding, thereby increasing the contact area between the first anti-instability pressure plate 41 and the second anti-instability pressure plate 42 and the clamping arm 21, so that the first anti-instability pressure plate 41 and the second anti-instability pressure plate 42 are respectively firmly engaged with a pair of clamping arms 21. At this time, the first anti-instability pressure plate 41 and the second anti-instability pressure plate 42 provide a simple support constraint for the specimen 1 through the positioning and clamping of the clamping mechanism and the rolling of the ball 46.

[0083] like Figure 1 As shown, 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 test piece 1 between the clamping arms 21.

[0084] Finally, the assembly diagram of the plane biaxial loading test of the composite material specimen 1 using the clamping device and the clamp 5 of the plane biaxial loading test machine is shown in FIG. Figure 7 As shown. Figure 7In the figure, the clamps 5 of the planar biaxial loading test machine are respectively arranged on the sides of the test piece 1 where the clamping arms 21 are not provided, and the length of the clamping of the test piece 1 by the clamps 5 is greater than or equal to 100 mm.

[0085] 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.

[0086] Specifically, the chuck selection and installation step includes selecting a chuck 5 whose width matches the width of the composite material specimen 1, and installing the chuck 5 on the 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.

[0087] 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 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.

[0088] The step of clamping the specimen 1 by using the clamping device comprises arranging four pairs of clamping arms 21 of the clamping mechanism at intervals from each other on the sides of the leg portion 31 of the specimen 1 where the cross-shaped reinforcement sheet 3 is not arranged, and respectively 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 sheets 3 respectively arranged on the front and back 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 center line of the side of the specimen 1 corresponding to the clamp 5 is coaxial with the center line of the clamp 5;

[0089] The step of applying a load and collecting the value of the strain gauge includes electrically connecting the strain gauge on the specimen 1 to the dynamic strain gauge to collect the value from the strain gauge on the specimen 1 when the specimen 1 is loaded.

[0090] Optionally, the method further comprises forming a score line 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 score line.

[0091] The method of performing a planar biaxial loading test using a clamping device can ensure that the specimen 1 is subjected to uniform force in the test area during the loading test and that 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 in line with expectations.

[0092] Optionally, the method further includes applying a planar biaxial load to the specimen 1 through a planar biaxial loading tester, recording stress-strain values ​​from strain gauges on the front and back surfaces of the specimen 1, and calculating the bending percentage of the specimen according to the ratio between the difference and the cumulative value between the stress-strain values ​​from two strain gauges arranged on the front and back surfaces of the specimen 1. By analyzing the bending percentage of the specimen in the preloading stage, it is possible to determine whether the clamping position of the specimen 1 is correct, so as to timely perform centering and debugging of the specimen 1.

[0093] 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.

[0094] Optionally, the method further comprises synchronously loading the biaxial load applied by the planar biaxial loading test machine onto the specimen 1 step by step according to a predetermined load ratio, and recording the load history, the failure position and the failure mode of each loading test, so as to obtain the biaxial failure load of the specimen under the corresponding failure mode. Optionally, for each group of tests, the mean value, standard deviation and dispersion coefficient of each failure load can be calculated for subsequent application.

[0095] In general, the present application can ensure the conduction of tests with different load proportions and different load forms 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 a simply supported constraint, 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.

[0096] 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 tension static testing of composite material specimens 1, such as composite material laminates, in planar biaxial directions, and simultaneously meet the requirements of tension 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 with impact damage.

[0097] Although the specific implementation methods of the present application are described above, those skilled in the art should understand that these are only examples, and the protection scope of the present application is defined by the appended claims. Those skilled in the art may make various changes or modifications to these implementation methods without departing from the principles and essence of the present application, but these changes and modifications all fall within the protection scope of the present application.

Claims

1. A clamping device for planar biaxial loading test of composite materials, in, The composite material is made into a test piece in the shape of a regular octagon, and the clamping device comprises: A clamping mechanism for fixedly clamping the specimen in a 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; A cross-shaped reinforcement sheet, wherein the cross-shaped reinforcement sheet comprises four legs extending in opposite directions in pairs, the four legs of the cross-shaped reinforcement sheet respectively extend toward the sides of the specimen where no clamping arms are provided, and a through hole is provided in the center of the cross-shaped reinforcement sheet, In which, the clamping device also includes a first anti-instability pressure plate and a second anti-instability pressure plate which are detachably arranged on the opposite surfaces of each pair of clamping arms, and each pair of clamping arms is configured to clamp the specimen, two cross-shaped reinforcement plates 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, in, The first anti-destabilization pressure plate and the second anti-destabilization pressure plate are respectively formed with a central opening, 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-destabilization pressure plate and the second anti-destabilization pressure plate that are respectively in contact with the surface of the cross-shaped reinforcement plate are provided with balls surrounding the central opening, and the balls are configured to protrude outward from the surfaces of the first anti-destabilization pressure plate and the second anti-destabilization pressure plate, so as to be able to press against the cross-shaped reinforcement plate when clamping the specimen.

3. The clamping device according to claim 2, in, 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 reinforcement sheet. The annular tracks are arranged around the central opening and are located outside the through hole of the cross-shaped reinforcement sheet, and the ball bearings are arranged in the annular tracks.

4. The clamping device according to claim 3, in, Grooves connecting the annular track and any outer edge thereof 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 reinforcement plate, so that the balls can be guided and pushed into the annular track through the grooves.

5. The clamping device according to claim 4, in, The clamping device further comprises a locking slide which is configured to be pressed into the groove and moved to the annular track to close the annular track.

6. The clamping device according to claim 2, in, The first anti-destabilization pressure plate and the second anti-destabilization pressure plate respectively further include four ears extending outwardly along the center line direction of the side where the clamping arm of the test piece is set.

7. The clamping device according to claim 1, in, The clamping mechanism further comprises four columns for supporting the 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.

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

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

10. The clamping device according to claim 9, in, The diameter of the circular window is 1.5 times the diameter of the through hole of the cross-shaped reinforcement sheet.

11. The clamping device according to claim 10, in, The clamp of the planar biaxial loading test machine is arranged on the side of the specimen where no clamping arm is provided, and the length of the specimen clamped by the clamp is greater than or equal to 100 mm.

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

13. The clamping device according to claim 12, in, The width of each leg was 10 mm less than the length of a single side of the specimen.

14. The clamping device according to claim 1, in, The side surfaces of the four legs of the cross-shaped reinforcement sheet are connected by arc surface transition at the cross intersection.

15. The clamping device according to any one of claims 1 to 14, in, Strain gauges are arranged on the front and back sides of the test piece at positions corresponding to the through holes of the cross-shaped reinforcement sheet.

16. A method for performing a planar biaxial loading test using the clamping device according to any one of claims 1 to 15, in, The method comprises: Preheating plane biaxial loading testing machine Selecting a chuck whose width matches the width of the test piece, and installing the chuck on the planar axis loading test machine, adjusting the orientation of the chuck so that the center line of the chuck is coaxial with the loading axis of the planar biaxial loading test machine; Arrange two cross-shaped reinforcement sheets on the front and back surfaces of the specimen respectively, and make the four legs of each cross-shaped reinforcement sheet extend toward the four sides of the specimen that are separated from each other, and then place the specimen and the legs of the cross-shaped reinforcement sheets arranged on the front and back surfaces of the specimen in the accommodating space of the clamp; The four pairs of clamping arms of the clamping mechanism are arranged at intervals from each other on the sides of the legs of the specimen without the cross-shaped reinforcement sheet, and the first anti-stagnation pressure plate and the second anti-stagnation pressure plate are respectively installed on the opposite surfaces of each pair of clamping arms by fasteners, and the specimen and the two cross-shaped reinforcement sheets respectively arranged on the front and back surfaces of the specimen are clamped between the first anti-stagnation pressure plate and the second anti-stagnation pressure plate by each pair of clamping arms, and the center line of the side of the specimen corresponding to the clamp is made coaxial with the center line of the clamp; and The strain gauges on the front and back surfaces of the specimen are electrically connected to a dynamic strain gauge to collect values ​​from the strain gauges on the specimen when the specimen is loaded.

17. The method according to claim 16, in, The method further comprises: A score line is formed on the test piece, and the clamp is arranged on the edge of the leg portion of the test piece where no cross-shaped reinforcement sheet is arranged according to the position of the score line.

18. The method according to claim 16, in, The method further comprises: A planar biaxial load is applied to the specimen by the planar biaxial loading testing machine, and the stress-strain values ​​of the specimen from the strain gauges on the front and back surfaces of the specimen are recorded. The bending percentage of the specimen is calculated based on the ratio between the difference and the accumulated value between the stress-strain values ​​of the two strain gauges on the front and back surfaces of the specimen.

19. The method according to claim 18, in, The method further comprises: Determining whether the position of the test piece 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 test piece is incorrect, the position of the second anti-instability pressure plate is further adjusted relative to the first anti-instability pressure plate 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, in, The method further comprises: The biaxial load applied by the planar biaxial loading test machine is synchronously and stepwise loaded onto the specimen according to a predetermined load ratio, and the load history, damage location and failure mode on the specimen of each loading test are recorded to obtain the biaxial failure load of the specimen under the corresponding failure mode.

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