Anti-buckling clamping device, specimen and assembly method thereof

Abstract

The present invention discloses an anti-buckling clamping device, which includes an outer clamping plate, an inner clamping plate, a spring pressure plate, a spiral spring, a bolt and a nut. The outer clamping plate is provided with an outer state recess, a left observation recess and a right observation recess, and the inner clamping plate is provided with an outer state recess. At the same time, an assembly method of the anti-buckling clamping device of the present invention is disclosed, and a specimen matching the clamping device is designed. The present invention also discloses a method for eliminating the friction effect of the anti-buckling clamping device. The anti-buckling clamping device and the specimen and the assembly method thereof of the present invention can improve the range of deformation measurement of uniaxial compression tests, reduce test costs, and the device is simple and easy to operate. At the same time, the test data obtained by the present invention will overturn the traditional understanding of the Bauschinger effect and expand the depth of scientific research on materials. Therefore, the present invention has important theoretical and engineering practical significance.

Description

Technical Field

[0001] The invention relates to a material mechanical property test, in particular to a uniaxial compression mechanical property test of a metal material, and specifically to an anti-buckling clamping device and a sample and an assembly method thereof. Background Art

[0002] Metal sheets, especially steel sheets and aluminum sheets, as an important raw material, have been widely used in the manufacture of automobile bodies and covering parts. In order to ensure the reliability of materials and the consistency of production, raw material suppliers and OEMs need to fully demonstrate and study the various mechanical properties of metal sheets. Among them, the classic test is the uniaxial tensile test. The corresponding uniaxial compression test is far less popular than the uniaxial tensile test due to the limitations of test conditions and is still in the research stage.

[0003] At present, there are mainly two types of uniaxial compression tests: one is the tension and compression fatigue test of metal sheets; the other is the tension-unloading-compression test, referred to as the TC test, to study the material package Schenker effect. In order to prevent the thin plate specimen from buckling and instability during the compression process, which leads to test failure, it is necessary to add anti-buckling devices on both sides of the thin plate specimen. Anti-buckling devices can be roughly divided into two categories: rigid clamping anti-buckling devices with "unmeasurable lateral loads" and anti-buckling clamping devices with "measurable lateral loads".

[0004] The rigid clamping anti-buckling device with "unmeasurable lateral load" is mainly used for fatigue tests of thin metal plates. GBT26077-2010 "Axial strain control method for fatigue test of metal materials" requires the use of anti-buckling devices for thin plates with a thickness of less than 2.5 mm to prevent the specimen from buckling and instability and affecting the test results. A schematic diagram of the clamping device is given, and the specimen is directly tightened and connected by a pair of clamping steel plates through bolts, and the test method is specified in detail. Since the lateral load of the clamping device is unknown, the friction force is inevitably unknown and can only be ignored. Under large deformation conditions, it is obviously not negligible. However, under fatigue load conditions, the tensile and compressive deformation of the specimen is very small, and the relative movement of the specimen and the clamping device can generally be ignored. Therefore, the test error caused by the friction effect is generally controllable and acceptable.

[0005] The "side load measurable" buckling clamp is mainly used to study the Bauschinger Effect of metal sheets. The Bauschinger Effect refers to the phenomenon that metal materials undergo plastic deformation after pre-loading, and then reverse loading after unloading, resulting in a decrease in yield strength. This phenomenon was first discovered by German engineer Bauschinger in 1886 and was named after him, referred to as BE (Bauschinger effect). With the development of the automobile industry, more and more types of metal materials are used for automobile bodies, and they are all thin plates. Therefore, studying the Bauschinger Effect of metal sheets is becoming more and more important for the stamping of parts.

[0006] The research results of traditional literature on the Bauschinger effect are usually conducted under axial tension and compression test conditions, and there are two loading methods: first stretch-unload-then compress, or first compress-unload-then stretch, that is, TC loading or CT loading. Since the raw materials provided by suppliers to the OEMs are steel coils or aluminum coils, only thin plate tensile specimens can be made. If they are directly used for uniaxial compression tests, buckling instability will easily occur in the thickness direction. RK Boger reported buckling instability modes such as "T-buckling, L-buckling, and W-buckling" [RK Boger, RH Wagoner, Continuous, large strain, tension / compression testing of sheet material, International Journal of Plasticity 21 (2005) 2319–2343], which led to the failure of the uniaxial compression test.

[0007] At present, there are two main methods for controlling and measuring lateral loads: one is the spring system; the other is the hydraulic system. Since the lateral load can be measured, the influence of friction effect can be quantitatively evaluated. Compared with the rigid clamping anti-buckling device with "unmeasurable lateral load", the spring or hydraulic system anti-buckling clamping device with "measurable lateral load" can study a larger degree of compression deformation of the specimen, which is introduced in the following literature.

[0008] F. Yoshida designed a tension and compression test device and method in 2002 [Fusahito Yoshida, Takeshi Uemori, Kenji Fujiwara, Elastic-plastic behavior of steel sheets under in-plane cyclic tension–compression at large strain, International Journal of Plasticity 18 (2002) 633–659]. The system consists of two parts. First, the laminated specimen is made by bonding multiple identical thin plate specimens together. Second, clamping steel plates are applied to both sides of the laminated specimen. The steel plates are connected by four bolts and springs. The clamping force is applied by spring compression, and the displacement is measured on the side of the laminated specimen by an extensometer.

[0009] H. Huh designed an anti-buckling clamping device in 2011 [GH Bae & H. Huh, Tension / compression test of auto-body steel sheets with the variation of the pre-strain and the strain rate, WIT Transactions on Engineering Sciences, Vol 72, 2011]. The clamping device mainly consists of four quadrilateral steel plates, two H-shaped steel plates, eight springs and bolts, which are arranged symmetrically on both sides of the specimen. The clamping force is applied by spring compression, and the displacement measurement is measured on the side of the specimen by the DIC non-contact measurement system.

[0010] AM Beese designed an anti-buckling clamping device in 2011 [Allison M. Beese, Dirk Mohr, Effect of stress triaxiality and Lode angle on the kinetics of strain-induced austenite-to-martensite transformation, Acta Materialia 59 (2011) 2589–2600]. The device consists of three steel plates and 14 coil springs and bolts. The springs are arranged on one side of the specimen, and the clamping force is applied by spring compression. At the same time, a displacement measurement observation hole is opened on the steel plate on the other side of the spring arrangement, and the length change of the gauge length can be measured by the DIC non-contact measurement system.

[0011] RKBoger designed an anti-buckling clamping device in 2005 [RKBoger, RHWagoner, Continuous, large strain, tension / compression testing of sheet material, International Journal of Plasticity 21 (2005) 2319–2343]. Unlike the clamping force provided by a spring, this device applies a lateral load to the specimen through a hydraulic system, and the displacement is measured on the side of the test specimen by an extensometer or a DIC non-contact measurement system. The advantage is that the lateral load can be precisely controlled, but the disadvantage is that the equipment cost of the system is relatively high.

[0012] The above literature involves two major types of buckling prevention devices for uniaxial compression tests. Their common point is that they only solve the problem of buckling prevention failure in the thickness direction. If we want to expand the scientific depth of material research and achieve further test objectives, there are still four core technical issues that must be considered, namely the shortcomings of existing technologies:

[0013] First, buckling instability in the width direction. Although the anti-buckling clamping device in the literature solves the problem of buckling instability in the thickness direction very well, under the condition of large deformation with "local neck expansion" (a concept opposite to local neck contraction in uniaxial tension), the specimen may buckle instability in the width direction. Simha reported this buckling instability mode [Simha, Modeling Springback of Bent Hydroformed Dual-Phase Steel Tubes, GDIS2016]. The main reason for buckling instability in the width direction is the unreasonable design of the specimen, such as the distance between the clamping ends of the specimen is too large, the length-to-width ratio of the specimen is too large, etc.

[0014] Second, uniaxial compression state. Because the specimen is subjected to lateral force on both sides or one side, the gauge area is not in uniaxial compression state, but more accurately in biaxial state or biaxial effect (Biaxial Effect), while the theoretical requirement is a strict uniaxial compression state.

[0015] In the face of this problem, the general approach is to choose to ignore the biaxial effect for different reasons. For example, AM Beese believes that "compared with the axial stress in the specimen (about 800MPa), the applied lateral stress is very small (about 3MPa). Therefore, when processing the test results, ignore its effect on the material response." [Allison M. Beese, Dirk Mohr, Effect of stress triaxiality and Lode angle on the kinetics of strain-induced austenite-to-martensite transformation, Acta Materialia 59 (2011) 2589-2600], Y. Chang also holds a similar view that "embedding a polytetrafluoroethylene film between the support plate and the specimen increases the thickness of the specimen and avoids the lateral plane strain state, so that the biaxial effect can be ignored." [Y. Chang, BT Wang, A new continuous tensile-compressive testing device with friction counteracting and anti-buckling supporting mechanism for large strain, Journal of Materials Processing Tech, 2020] This method is reasonable under small deformation conditions because the lateral load acts on a large area of ​​the specimen and the lateral compressive stress is negligible relative to the tensile stress.

[0016] However, under large deformation conditions, the thickness will increase only in local areas of the specimen (other places will gradually lose contact with the splint due to unchanged thickness). Especially when the gauge length is small and the deformation is large, the local area will be smaller, so that all lateral loads will only act on this local area. Therefore, the lateral compressive stress will increase significantly and cannot be ignored.

[0017] In summary, the general approach is to ignore the biaxial effect for different reasons. The existing anti-buckling devices do not solve this problem technically, but choose to ignore it mathematically.

[0018] Third, the friction effect. Due to the existence of the friction effect, the lateral force that limits the lateral buckling of the specimen will generate friction in the compression direction. Therefore, increasing the compression load measured in the compression direction will significantly affect the compressive mechanical properties of the material. Therefore, the compression load increased by friction, that is, the friction force must be eliminated from the actual measured compression load. In the existing literature, the method of calculating the friction force is to measure the friction coefficient and the lateral load between the specimen and the clamping system. The latter can be measured more accurately, but the former has a large uncertainty. The friction coefficient of specimens of different materials is not the same. RKBoger detailed the correction method for the compression load to eliminate friction [RKBoger, RHWagoner, Continuous, large strain, tension / compression testing of sheet material, International Journal of Plasticity 21 (2005) 2319– 2343]. This method requires the measurement of the friction coefficient and the lateral load. Therefore, the treatment of the friction effect increases the complexity and uncertainty of the uniaxial compression test.

[0019] Fourth, "local neck expansion". Limited to the deformation condition of "uniform neck expansion" (a concept opposite to uniform necking in uniaxial tension), due to uniform deformation, there is no need to measure the change in cross-sectional area, and the Bauschinger effect of the material can also be studied. However, under the condition of "local neck expansion", since the deformation within the effective gauge length of the specimen is no longer uniform, the cross-sectional area of ​​the specimen has undergone a large uneven change, which requires real-time measurement. However, the existing anti-buckling device makes it impossible to measure the cross-sectional change of the specimen in real time, resulting in the inability to accurately calculate the true stress under the deformation condition of "local neck expansion", which generally makes the calculated result of the true stress significantly larger. Therefore, the existing testing technology cannot test the full-process compression mechanical property curve of the material.

[0020] The inventor of this application has found through systematic and in-depth research that the fundamental reason for the above-mentioned deficiencies in the anti-buckling device is that the anti-buckling clamping device and the specimen are not considered as a whole. For a successful uniaxial compression test, the design of the specimen is also a very important factor. The design of the specimen needs to consider at least the following three factors:

[0021] First, prevent the exposed parts of the specimen from buckling failure. The specimen is clamped at both ends by the tensile testing machine and in the middle by the anti-buckling device. If the material strength is low, the difference between the clamping width and the parallel width of the specimen is small, or the length of the exposed part of the specimen is long, the possibility of buckling failure may occur in the exposed parts of the specimen at both ends.

[0022] Second, the specimen and the clamping device can be automatically aligned. Material failure under uniaxial tension or compression begins at the center axis of the specimen. Since the deformation of the gauge length needs to be measured through the opening of the splint, the center axis can be automatically determined without additional means only when the center axis of the splint and the center axis of the specimen coincide.

[0023] Third, the lateral displacement constraint of the specimen clamping end. When the specimen enters the "local neck expansion" deformation stage, the risk of buckling failure in the width direction increases greatly. The main reason is that the difference between the clamping width of the specimen and the parallel part width is small, and the distance between the two clamping ends of the specimen on the tensile testing machine is large.

[0024] Therefore, the buckling restraint clamp and the specimen should be considered as a whole design.

[0025] In addition, existing technologies for dealing with friction effects require measuring the friction coefficient and lateral load, and different materials have different friction coefficients. Therefore, there is uncertainty in the impact of the processing of friction effects on subsequent test data processing.

[0026] Due to the above deficiencies in the existing technology, a deeper understanding of the material itself is limited, which limits its further application in engineering, especially for lightweight metal materials such as aluminum-magnesium alloys with strong tensile-compressive asymmetry. In addition, there are double challenges in theory and experiment to determine the compressive mechanical properties of thin plate materials. Therefore, it is very important to develop a full-process uniaxial compression test device for materials and its application method. Summary of the invention

[0027] 1. Technical Problems Solved by the Invention

[0028] In view of the deficiencies in the prior art, a buckling-preventing clamping device, a specimen and an assembly method thereof are designed, which can avoid buckling instability in the width direction, ensure the uniaxial compression state, eliminate the influence of friction effect, and accurately measure the cross-sectional area. By solving the above technical problems, the present invention can scientifically measure and calculate the full-process compression mechanical performance curve of the material under the premise of strictly meeting the theoretical requirements. Therefore, the buckling-preventing clamping device, the specimen and the assembly method thereof of the present invention can realize tensile and compressive fatigue loading tests under different deformation degrees, TC&C-T loading and its cyclic loading tests and uniaxial compression tests, breaking through the limitations of traditional devices and methods.

[0029] 2. Technical solution of the present invention

[0030] In order to achieve the purpose of the technical problem to be solved by the present invention, the present invention provides an anti-buckling clamping device, including an outer clamping plate, an inner clamping plate, a spring pressure plate, a coil spring, a bolt and a nut. The outer clamping plate, the inner clamping plate and the spring pressure plate are arranged with a plurality of bolt holes. The bolts pass through the bolt holes on the outer clamping plate, the inner clamping plate and the spring pressure plate in sequence. The outer clamping plate and the inner clamping plate are used to clamp the sample, the inner clamping plate and the spring pressure plate are used to clamp the coil spring, the bolt is sleeved in the coil spring, and the tail end of the bolt passes through the bolt hole of the spring pressure plate and is locked with a nut. The device is characterized in that:

[0031] The outer clamping plate is provided with an outer state recessed portion parallel to the uniaxial compression direction;

[0032] The inner clamping plate is provided with an inner state recessed portion parallel to the uniaxial compression direction;

[0033] The outer state recessed portion and the inner state recessed portion are of the same size and shape, and after the outer clamping plate and the inner clamping plate clamp the sample, their positions correspond or overlap.

[0034] Furthermore, the anti-buckling clamping device is characterized in that: the outer clamping plate is provided with a left observation recess and a right observation recess on both sides of the outer state recess.

[0035] Furthermore, the anti-buckling clamping device is characterized in that the outer clamping plate, the inner clamping plate and the spring pressure plate are all the same in length, width and thickness, and are provided with four bolt holes which are symmetrically arranged at the four corners.

[0036] Furthermore, the anti-buckling clamping device is characterized in that: the outer state recessed portion and the inner state recessed portion are in the shape of a "rectangle" or a "rectangle in the middle and semicircles at both ends", and are arranged in the middle of the outer splint and the inner splint, that is, the plane geometric center of the outer state recessed portion coincides with the plane geometric center of the outer splint, and the plane geometric center of the inner state recessed portion coincides with the plane geometric center of the inner splint.

[0037] Furthermore, the anti-buckling clamping device is characterized in that: the left observation recess and the right observation recess are arranged parallel to the external state recess, and their size, shape, and horizontal position of the plane geometric center are consistent with the external state recess, and the horizontal spacing between the left observation recess and the right observation recess and the external state recess is equal.

[0038] Furthermore, the anti-buckling clamping device is characterized in that: the materials of the outer clamping plate, the inner clamping plate and the spring pressure plate are either all made of metal materials or all made of transparent plexiglass materials; the outer state recessed portion, the left observation recessed portion and the right observation recessed portion arranged on the outer clamping plate are through holes; and the inner state recessed portion arranged on the inner clamping plate is a through hole.

[0039] Furthermore, the anti-buckling clamping device is characterized in that: the outer clamping plate, the inner clamping plate and the spring pressure plate are all made of transparent plexiglass material; the outer state recessed portion, the left observation recessed portion and the right observation recessed portion arranged on the outer clamping plate are non-penetrating grooves; the inner state recessed portion arranged on the inner clamping plate is a non-penetrating groove.

[0040] A specimen matched with an anti-buckling clamping device, used in combination with the anti-buckling clamping device, characterized in that:

[0041] The test specimen, including the clamping end, the transition zone and the gauge section, has a geometric shape that is designed to be plane symmetrical;

[0042] The clamping end has a width equal to the width of the outer clamping plate and is provided with a bolt notch;

[0043] The width of the gauge section is equal to the distance between the center axes of the left observation recess and the right observation recess;

[0044] The bolt notch is located corresponding to the bolt hole on the outer clamping plate, and its width is equal to the diameter of the bolt.

[0045] A method for assembling an anti-buckling clamping device, characterized in that the anti-buckling clamping device described in any one of the above items is used to assemble a specimen, and the method comprises the following steps:

[0046] The first step is to insert bolts into the outer splint: the side of the outer splint with the concave portion faces upwards, and a bolt is inserted into each bolt hole of the outer splint, with the bolt passing through the bolt hole facing upwards;

[0047] The second step is to arrange the sample: place the sample on the outer clamping plate, and require that the compression direction of the sample is parallel to the direction of the outer state concave part, the bolt is stuck by the notch of the sample, and the plane geometric center of the sample coincides with the plane geometric center of the outer state concave part;

[0048] The third step is to insert the inner clamp: the side of the inner clamp with the inner state concave part faces downward, and then align the bolt hole of the inner clamp with the bolt and insert it. The sample is clamped by the outer clamp and the inner clamp;

[0049] Step 4: Insert the coil spring: On the inner clamping plate, insert a coil spring into each bolt, and the coil spring is located on the inner clamping plate;

[0050] Step 5: Insert the spring pressure plate: Align the bolt hole of the spring pressure plate with the bolt and insert it. The coil spring is located under the spring pressure plate.

[0051] Step 6. Screw in the nut: On the spring pressure plate, screw each bolt into the nut. Each nut must be screwed in the same distance to ensure that each spring is compressed the same amount. The assembly is complete.

[0052] Furthermore, the assembly method of the anti-buckling clamping device is characterized in that: between the first step and the second step, and between the second step and the third step, polytetrafluoroethylene films are arranged respectively to isolate the outer splint from the sample, and the inner splint from the sample, and the polytetrafluoroethylene film has a plane dimension that is the same as the plane dimension of the outer splint, and the position, size, and number of the openings thereon are consistent with those of the outer splint.

[0053] A method for eliminating the friction effect of an anti-buckling clamping device, characterized in that the anti-buckling clamping device and a test piece described in any one of the above items are used for testing, and the method comprises the following steps:

[0054] The first step is to prepare two identical specimens according to the assembly method of the anti-buckling clamp device and the specimens matched therewith, so that the spring compression amount of the clamp device of one specimen is △d1, and the spring compression amount of the clamp device of the other specimen is △d2;

[0055] In the second step, the same test conditions are used to carry out uniaxial compression tests on the assembled samples of the two spring compression amounts, and the compression loads under the two assembly conditions are measured to be F1 and F2 respectively;

[0056] In the third step, the pure compressive load F of the uniaxial compression specimen is calculated using the following formula: C :

[0057]

[0058] A pure compressive load F will eliminate the friction effect. C Calculation of engineering stress and true stress for uniaxial compression tests.

[0059] 3. Beneficial effects of the present invention

[0060] The present invention avoids the shortcomings of the prior art. Compared with the existing technical solutions, the beneficial effects produced by the present invention are mainly reflected in the following three aspects:

[0061] First, the present invention can complete the study of the Bauschinger effect of materials under the deformation condition of "uniform neck expansion", taking into account the traditional test purposes. At the same time, it can obtain the full-process mechanical property curve of the material under uniaxial compression, or measure the mechanical properties of the material under "local neck expansion", and scientifically evaluate whether the material is tensile-compressive symmetric or tensile-compressive asymmetric, which transcends the traditional test objectives.

[0062] Secondly, the present invention can confirm the tensile-compressive asymmetry or symmetry of the material through experimental measurement, rather than assuming the tensile-compressive symmetry of the material as a prerequisite for research. Therefore, contrary to the traditional understanding of the Bauschinger effect, it is not the Bauschinger effect that causes the tensile-compressive asymmetry of the material, but the tensile-compressive asymmetry of the material that produces the Bauschinger effect, which subverts the traditional understanding of the Bauschinger effect and expands the depth of scientific research on materials.

[0063] Third, the present invention simultaneously solves the five basic problems in the uniaxial compression test, namely, buckling instability in the thickness direction, buckling instability in the width direction, uniaxial compression state, local neck expansion and friction effect. Therefore, it will significantly improve the reliability of test measurement data, reduce test costs, and improve engineering application effects.

[0064] In summary, through the application of the anti-buckling clamping device of the present invention, technical goals that cannot be achieved by the prior art can be achieved, and the structure is simple and easy to operate, and the technical cost is low, which will promote the popularization of the technology of uniaxial compression testing. Therefore, the present invention has important theoretical and engineering practical significance. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0066] Figure 1 It is a schematic diagram of the three-dimensional structure of the anti-buckling device of the present invention in one direction;

[0067] Figure 2 is a schematic diagram of the three-dimensional structure of the anti-buckling device of the present invention in another direction;

[0068] Figure 3 It is a schematic diagram of the structure of clamping the test piece during the test of the present invention;

[0069] Figure 4 yes Figure 3 The main view;

[0070] Figure 5 It is a front view of the outer splint of the present invention;

[0071] Figure 6 It is a front view of the inner splint of the present invention;

[0072] Figure 7 It is a front view of the spring pressure plate of the present invention;

[0073] Figure 8 It is a front view of a sample of the present invention;

[0074] FIG9( a ) is a top view of the outer clamping plate after the bolts are assembled in the present invention;

[0075] FIG9( b ) is a side view of the outer clamping plate after being assembled with bolts in the present invention;

[0076] FIG10( a ) is a top view of the polytetrafluoroethylene film after the first assembly in the present invention;

[0077] FIG10( b ) is a side view of the present invention after the polytetrafluoroethylene film is assembled for the first time;

[0078] FIG. 11( a ) is a top view of the assembled sample in the present invention;

[0079] FIG11( b ) is a side view of the assembled sample of the present invention;

[0080] FIG12( a ) is a top view of the polytetrafluoroethylene film after the second assembly in the present invention;

[0081] FIG12( b ) is a side view of the present invention after the second assembly of the polytetrafluoroethylene film;

[0082] FIG. 13( a ) is a top view of the present invention after the inner clamping plate is assembled;

[0083] FIG13( b ) is a side view of the present invention after the inner clamping plate is assembled;

[0084] FIG. 14( a ) is a top view of the assembled coil spring of the present invention;

[0085] FIG14( b ) is a side view of the present invention after the coil spring is assembled;

[0086] FIG. 15( a ) is a top view of the spring pressure plate after being assembled in the present invention;

[0087] FIG15( b ) is a side view of the spring pressure plate after being assembled in the present invention;

[0088] FIG. 16( a ) is a top view of the present invention after the nut is assembled;

[0089] FIG16( b ) is a side view of the present invention after the nut is assembled;

[0090] Fig.17 It is a front view of the measurable area of ​​the assembled sample in the present invention.

[0091] In the figure: 1—outer clamping plate, 2—inner clamping plate, 3—spring pressure plate, 4—bolt, 5—helical spring, 6—nut, 7—bolt hole, 8—external state recessed portion, 9—left observation recessed portion, 10—right observation recessed portion, 11—inner state recessed portion, 12—specimen, 13—polytetrafluoroethylene film, 14—central axis of outer clamping plate, 15—center horizontal line of outer clamping plate, 16—central axis of specimen in compression direction, 17—central axis of specimen in horizontal direction, 18—measurement area of ​​recessed portion in external state, 19—measurement area of ​​recessed portion in left observation, 20—measurement area of ​​recessed portion in right observation, 21—finite body in uniaxial compression state, 22—clamping end, 23—transition zone, 24—gauge length section, 25—bolt notch. DETAILED DESCRIPTION

[0092] The technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the present invention.

[0093] Example 1

[0094] See also Figure 1-7 The anti-buckling device shown is an embodiment of the present invention, comprising an outer clamping plate 1, an inner clamping plate 2, a spring pressure plate 3, a bolt 4, a coil spring 5 and a nut 6. The bolt 4 passes through the bolt holes 7 of the outer clamping plate 1, the inner clamping plate 2 and the spring pressure plate 3, wherein the outer clamping plate 1 and the inner clamping plate 2 clamp a sample 12 (such as Figure 3 As shown), the inner clamping plate 2 and the spring pressure plate 3 clamp the coil spring 5. The specific method is to insert the coil spring 5 into the bolt 4, and the tail end of the bolt 4 passes through the bolt hole 7 of the spring pressure plate 3, and use the nut 6 to lock (as shown). Figure 2 shown).

[0095] Please read further Figure 3 The outer plate 1 and the inner plate 2 are respectively provided with an outer state recessed portion 8 (such as Figure 5 As shown) and the inner state recess 11 (as shown Figure 6 As shown), the central axis of the outer state recessed portion 8 coincides with the central axis of the outer splint 1, and the central axis of the inner state recessed portion 11 coincides with the central axis of the inner splint 2.

[0096] A left observation recess 9 and a right observation recess 10 are symmetrically provided on both sides of the outer state recess 8 of the outer splint 1, and the edges on both sides of the specimen 12 just coincide with the central axes of the left observation recess 9 and the right observation recess 10, that is, the width of the specimen gauge section is equal to the distance between the two central axes of the left observation recess 9 and the right observation recess 10, so that the change in the cross-sectional area of ​​the specimen caused by "local neck expansion" can be measured in real time through non-contact measurement technology or DIC (digital image correlation) technology, and the data can be processed strictly according to the Cauchy stress definition (the concept of stress-strain was proposed by French mathematician Cauchy in 1822, and the original definition of Cauchy stress is the real-time load F divided by the real-time cross-sectional area A).

[0097] The outer clamping plate 1, the inner clamping plate 2 and the spring pressure plate 3 are symmetrically arranged with bolt holes 7 in the thickness direction. In this embodiment, two bolt holes 7 are vertically opened on each side, that is, four bolt holes 7 are opened on each plate-like member, which are respectively located at the four corners, and the positions of the bolt holes 7 on the outer clamping plate 1, the inner clamping plate 2 and the spring pressure plate 3 correspond to each other. Figure 5 , Figure 6 , Figure 7 shown.

[0098] Since the sample is arranged between the outer clamping plate 1 and the inner clamping plate 2, and the coil spring 5 is arranged between the inner clamping plate 2 and the spring pressure plate 3, the central axis of the sample 12 can be observed through the outer state recess 8, and the boundaries of the two sides of the sample 12 can be observed through the left observation recess 9 and the right observation recess 10, as shown in FIG. Figure 4 shown.

[0099] The outer state recessed portion 8 of the outer plate 1 and the inner state recessed portion 11 of the inner plate 2 are rectangular or rectangular in the middle and semicircular at both ends, and their plane geometric centers and central axes coincide with the plane geometric centers and central axes of the outer plate 1 and the inner plate 2 respectively.

[0100] The size, shape, and horizontal position of the plane geometric center of the left observation recess 9 and the right observation recess 10 arranged on the outer clamping plate 1 are consistent with those of the outer state recess 8, and the horizontal spacing between the left observation recess 9 and the right observation recess 10 and the outer state recess 8 is equal. The central axis of the sample 12 coincides with the central axis of the outer state recess 8.

[0101] The outer clamping plate 1, inner clamping plate 2, and spring pressure plate 3 are of the same length, width and thickness, and are made of steel plates. Alternatively, in order to reduce the cantilever mass on the spring side of the device, the inner clamping plate 2 and the spring pressure plate 3 can be changed to aluminum plates.

[0102] In another embodiment, the material of the outer clamp 1 may be transparent plexiglass, so that the deformation process of the entire specimen 12 can be observed without being limited to the areas observed by the outer state recess 8, the left observation recess 9 and the right observation recess 10. Therefore, the same observation effect as without the anti-buckling clamping device can be achieved.

[0103] In another embodiment, the outer state recess 8, the left observation recess 9 and the right observation recess 10 are arranged on the outer clamping plate 1 made of transparent organic glass, and the size, position and shape are kept unchanged. On the clamping side of the sample 12, the penetrating hole is changed to a non-penetrating groove, for example, the penetration depth is 1 mm, so as to avoid the influence of refraction caused by the transparent material on the measurement.

[0104] Example 2

[0105] This embodiment provides a test specimen matching the buckling prevention clamping device of embodiment 1, characterized in that:

[0106] The sample 12 includes a clamping end 22, a transition zone 23 and a gauge section 24, and its geometric shape is designed to be plane symmetrical, as shown in FIG. 9 .

[0107] The clamping end 22 has a width equal to that of the outer clamping plate 1 and is provided with a bolt notch 25 to enable the specimen 12 and the anti-buckling clamping device to be automatically aligned in the compression direction.

[0108] The width of the gauge section 24 is equal to the distance between the two central axes of the left observation recess 9 and the right observation recess 10 so as to observe the “uniform neck swelling” and “local neck swelling” of the sample 12 .

[0109] The bolt notch 25 is located corresponding to the bolt hole 7 on the outer clamping plate 1 and has a width equal to the diameter of the bolt 6 so as to limit the lateral movement of the clamping end 22 through the bolt 6 and reduce the risk of buckling failure in the width direction.

[0110] In conclusion, the design of specimen 12 should be considered together with the buckling restraint device as a whole.

[0111] Example 3

[0112] This embodiment, based on Embodiment 1 and Embodiment 2, describes in detail the assembly method of the anti-buckling device of the present invention, and the steps include:

[0113] The first step is to insert bolts into the outer clamping plate 1. With the side of the outer clamping plate 1 with the recessed portion facing upward, a bolt 4 is inserted into each bolt hole of the outer clamping plate 1, and the bolts 4 pass through the bolt holes 7 in the same direction, as shown in FIG. 9(a) and FIG. 9(b).

[0114] The second step is to arrange the polytetrafluoroethylene film 13. On the side facing the bolt 4, a polytetrafluoroethylene film 13 is placed on the outer clamping plate 1, as shown in Figures 10(a) and 10(b). The thickness of the polytetrafluoroethylene film 13 is 0.1 mm, the plane size of the polytetrafluoroethylene film 13 is the same as that of the outer clamping plate 1, and the position, size and number of the openings thereon are consistent with those of the outer clamping plate 1.

[0115] The third step is to arrange the sample 12. The sample 12 is placed on the polytetrafluoroethylene film 13, and the compression direction of the sample 12 is required to be arranged parallel to the direction of the outer state recessed portion 8, and the central axis 16 of the sample compression direction (as shown in Figures 11(a) and 11(b)) coincides with the central axis of the outer state recessed portion 8 or the central axis 14 of the outer clamping plate (as shown in Figures 9(a) and 9(b)), and the central axis 17 of the sample in the horizontal direction (as shown in Figures 11(a) and 11(b)) coincides with the horizontal line 15 of the center of the outer clamping plate (as shown in Figures 9(a) and 9(b)), and its boundaries coincide with the central axes of the left observation recessed portion 9 and the right observation recessed portion 10 respectively. The four notches of the sample 12 respectively clamp the four bolts 4, as shown in Figures 11(a) and 11(b).

[0116] Specimen 12 is matched with the anti-buckling clamping device of the present invention. The width of the clamping end of specimen 12 is equal to the width of the outer plate, and the center axis is automatically aligned during the assembly process. At the same time, it can effectively prevent the outer plate 1 from bending and deforming under a large spring preload, especially when the material of the outer plate 1 is made of plexiglass; the width of the parallel portion or the effective gauge area of ​​specimen 12 is equal to the distance between the center axes of the left observation recess and the right observation recess; notches are provided at positions corresponding to the bolts 4 on both sides of the clamping end of specimen 12, and the width of the notch is equal to the diameter of the bolt 4, which shortens the effective length of the specimen for buckling instability in the width direction. Therefore, under the large deformation condition of "local neck expansion", buckling instability of the specimen in the width direction can be effectively prevented.

[0117] Step 4: Arrange the polytetrafluoroethylene film 13. Place another polytetrafluoroethylene film 13 on the sample 12 in the same manner as before, as shown in Figures 12(a) and 12(b). The purpose of inserting the polytetrafluoroethylene film 13 between the outer and inner clamping plates 1 and 2 and the sample 12 is to reduce the friction coefficient and thus reduce the friction force.

[0118] Step 5: Insert the inner clamping plate 2. Place the inner clamping plate 2 with the inner state recessed portion 11 facing downward, and then align the bolt hole 7 of the inner clamping plate 2 with the bolt 4 and insert the bolt 4 at the same time. The specimen 12 is clamped by the outer clamping plate 1 and the inner clamping plate 2, as shown in Figures 13(a) and 13(b). Through the outer state recessed portion 8 and the inner state recessed portion 11, the outer clamping plate 1 and the inner clamping plate 2 are prevented from contacting the specimen 12, ensuring that both sides of the area surrounded by the recessed portion on the specimen 12 are not affected by lateral loads.

[0119] Step 6: insert the coil spring 5. On the inner clamping plate 2, insert each bolt 4 into a coil spring 5, and the coil spring 5 is located on the inner clamping plate 2, as shown in FIG. 14 (a) and FIG. 14 (b).

[0120] Step 7: Insert the spring pressure plate 3. Align the bolt hole 7 of the spring pressure plate 3 with the bolt 4 and insert them simultaneously, so that the coil spring 5 is located under the spring pressure plate 3, as shown in FIG. 15(a) and FIG. 15(b).

[0121] Step 8: Screw in the nut 6. On the spring pressure plate 3, screw each bolt 4 into the nut 6, requiring each nut 6 to be screwed in the same distance to ensure that the compression amount △d of each coil spring 5 is the same, as shown in Figure 16 (a) and Figure 16 (b). At this point, the assembly is completed.

[0122] from Fig.17 In the figure, the outer state depression measurement area 18, the left observation depression measurement area 19 and the right observation depression measurement area 20 of the specimen can be seen. Since the outer state depression 8 and the inner state depression 11 corresponding to the positions are opened on the outer clamping plate 1 and the inner clamping plate 2, a finite body 21 in a uniaxial compression state without lateral load can be determined in the outer state depression measurement area 18, as shown in Figure 16, and its size is generally 1.0 mm ╳ 1.0mm ╳ h0, h0 is the thickness of the sample.

[0123] The inventor Xiao Feng explained in detail the concept of finite body and its measurement method in the new uniaxial tension DIC test [Xiao Feng, An experimental and calculation method for determining the true stress-strain curve of materials, patent application number 201910801077.1]. ​​The anti-buckling clamping device of the present invention is used to perform uniaxial compression tests. The processing of the material's true stress-true strain data can be carried out by referring to the calculation method in this application [201910801077.1].

[0124] Next, it can be installed on a tensile testing machine for a uniaxial compression test. If the sample 12 fails in lateral buckling mode, it needs to be reassembled and the spring compression amount increased and retested. If it still fails, it is necessary to increase the spring compression stiffness or the number of springs until the test is successful.

[0125] Example 4

[0126] This embodiment will explain in detail the method for eliminating the friction effect in the anti-buckling clamping device of the present invention, and the steps include:

[0127] The first step is to prepare two identical uniaxial compression specimens, one specimen clamping device spring compression amount is △d1, and the other specimen clamping device spring compression amount is △d2. In this embodiment, the spring stiffness is k, the number of springs is 4, the friction coefficient at each moment is μ, and the same test conditions are used, requiring that both lateral forces can ensure that the compression specimen does not buckle and become unstable.

[0128] In the second step, the two ends of the assembled specimens of the two spring compression amounts are clamped on the tension and compression testing machine. The exposed part of one end of the specimen is required to be completely inserted into the fixed end of the tension and compression testing machine, that is, the anti-buckling clamping device is kept in contact with the fixed end of the tension and compression testing machine. When the two spring compression amounts are subjected to uniaxial compression tests respectively, the friction force is generated at the same time. The compression loads of the specimens at different times are measured to be F1 and F2 respectively. The friction loads at different times are assumed to be F μ1 and F μ2 , the pure compression load at different times after removing the influence of friction is F C .

[0129] For the sample with spring compression of △d1, the following two equations are established:

[0130] F C +F μ1 =F1

[0131] F μ1 =2·4·μ·k·Δd1

[0132] For the sample with spring compression of △d2, the following two equations are established:

[0133] F C +F μ2 =F2

[0134] F μ2 =2·4·μ·k·Δd2

[0135] The third step is to obtain the pure compression load F from the above four equations. C :

[0136]

[0137] A pure compressive load F will eliminate the friction effect. C Calculation of engineering stress and true stress for uniaxial compression tests.

[0138] The present invention can solve the following technical problems through the above embodiments:

[0139] First, avoid buckling instability in the width direction. A matching specimen is designed for the anti-buckling clamping device. The width of the specimen clamping end is equal to the width of the outer clamping plate, so that the center axis can be automatically aligned during the assembly process. At the same time, it can effectively prevent the outer clamping plate from bending and deforming under a large spring preload, affecting the uniform distribution of the lateral load; the width of the specimen gauge section is equal to the distance between the two center axes of the left observation recess and the right observation recess; notches are opened on both sides of the specimen clamping end at positions corresponding to the bolts, and the width of the notch is equal to the diameter of the bolt to shorten the effective length of the specimen for buckling instability in the width direction. Therefore, under the large deformation condition of "local neck expansion", the specimen can be effectively prevented from buckling failure in the width direction.

[0140] Second, to ensure the uniaxial compression state, holes of the same size and shape are opened in the middle of the clamping steel plates on both sides of the specimen. By introducing the concept of finite body, it is ensured that there is a measurable local area that is in a strict uniaxial compression state, rather than the traditional entire gauge length area.

[0141] Third, eliminate the influence of friction effect. First, prepare two identical uniaxial compression specimens, one specimen clamping device spring compression amount △d1, the other specimen clamping device spring compression amount △d2, use the same test conditions, requiring both lateral forces to ensure that the compression specimen does not have a buckling failure mode; secondly, perform uniaxial compression tests on the two spring compression amounts, and measure the compression loads of the specimens at different times as F1 and F2, respectively. The pure compression load after eliminating the influence of friction is F C ; Finally, the pure compression load at different times is calculated using the following formula: F C :

[0142]

[0143] This data processing method mathematically eliminates the effects of friction and springs in the clamping device on the specimen compression load, and F is calculated in real time at every moment. C .

[0144] Fourth, accurately measure the cross-sectional area. On the basis of opening a hole in the middle of the steel plate on the non-spring side or the measuring side, a hole is opened on each side parallel to the middle hole, so that the edges of the two sides of the specimen coincide with the central axis of the hole. Thus, through non-contact measurement technology or DIC (digital image correlation) technology, the change in the cross-sectional area of ​​the specimen caused by "local neck expansion" can be measured in real time, and the data is processed strictly according to the definition of Cauchy stress (the concept of stress-strain was proposed by French mathematician Cauchy in 1822, and the original definition of Cauchy stress is the ratio of real-time load F to real-time cross-sectional area A). Therefore, in addition to studying the Bauschinger effect under the deformation condition of "uniform neck expansion" of the material, the full-process compression mechanical property curve of the material under the continuous deformation conditions of "uniform neck expansion" and "local neck expansion" can also be tested. At the same time, if the outer splint is replaced with a transparent plexiglass material, the entire deformation process of the specimen under uniaxial compression can be clearly observed.

[0145] By solving the above technical problems, the present invention can scientifically measure and calculate the full-process compression mechanical performance curve of the material under the premise of strictly meeting the theoretical requirements. Therefore, the anti-buckling clamping device of the present invention can realize tensile and compressive fatigue loading tests, TC&C-T loading and its cyclic loading tests and uniaxial compression tests under different deformation degrees, breaking through the limitations of traditional devices and methods.

[0146] The above embodiments are only exemplary embodiments of the present invention and are not intended to limit the present invention. The protection scope of the present invention is defined by the claims. Various modifications or equivalent substitutions made by those skilled in the art to the present invention within the substantial protection scope of the present invention also fall within the protection scope of the present invention.

Claims

1. An anti-buckling clamping device, comprising an outer clamping plate, an inner clamping plate, a spring pressure plate, a coil spring, a bolt and a nut, wherein the outer clamping plate, the inner clamping plate and the spring pressure plate are provided with a plurality of bolt holes, the bolts pass through the bolt holes on the outer clamping plate, the inner clamping plate and the spring pressure plate in sequence, the outer clamping plate and the inner clamping plate are used to clamp the specimen, the inner clamping plate and the spring pressure plate are used to clamp the coil spring, the bolt is sleeved in the coil spring, the tail end of the bolt passes through the bolt hole of the spring pressure plate and is locked with a nut, wherein: The outer clamping plate is provided with an outer state recessed portion parallel to the uniaxial compression direction; The inner clamping plate is provided with an inner state recessed portion parallel to the uniaxial compression direction; The outer state recessed portion and the inner state recessed portion are of the same size and shape, and after the outer clamping plate and the inner clamping plate clamp the sample, the positions of the two plates correspond or overlap; The outer state recessed portion and the inner state recessed portion are in the shape of a "rectangle" or a "rectangle in the middle and semicircles at both ends", and the plane geometric center of the outer state recessed portion coincides with the plane geometric center of the outer splint, and the plane geometric center of the inner state recessed portion coincides with the plane geometric center of the inner splint; The outer state recessed portion, the left observation recessed portion and the right observation recessed portion arranged on the outer clamping plate are penetration holes; the inner state recessed portion arranged on the inner clamping plate is a penetration hole.

2. The anti-buckling clamping device according to claim 1, characterized in that: The outer clamping plate is provided with a left observation recessed portion and a right observation recessed portion on both sides of the outer state recessed portion.

3. The anti-buckling clamping device according to claim 2, characterized in that: The outer clamping plate, the inner clamping plate and the spring pressure plate are all of the same length, width and thickness, and are provided with four bolt holes which are symmetrically arranged at the four corners.

4. The anti-buckling clamping device according to claim 1, characterized in that: The left observation recessed portion and the right observation recessed portion are arranged parallel to the external state recessed portion, and their size, shape, and horizontal position of the plane geometric center are consistent with the external state recessed portion, and the horizontal spacing between the left observation recessed portion and the right observation recessed portion and the external state recessed portion is equal.

5. The anti-buckling clamping device according to claim 4, characterized in that: The outer clamping plate, the inner clamping plate and the spring pressure plate are all made of metal materials or transparent organic glass materials.

6. The anti-buckling clamping device according to claim 4, characterized in that: The outer splint, inner splint and spring pressure plate are all made of transparent organic glass; the outer state recess, left observation recess and right observation recess arranged on the outer splint are non-penetrating grooves; the inner state recess arranged on the inner splint is a non-penetrating groove.

7. A test specimen matched with an anti-buckling clamp, used in combination with the anti-buckling clamp according to any one of claims 1 to 6, characterized in that: The test specimen, including the clamping end, the transition zone and the gauge section, has a geometric shape that is designed to be plane symmetrical; The clamping end has a width equal to the width of the outer clamping plate and is provided with a bolt notch; The width of the gauge section is equal to the distance between the center axes of the left observation recess and the right observation recess; The bolt notch is located corresponding to the bolt hole on the outer clamping plate, and its width is equal to the diameter of the bolt.

8. A method for assembling an anti-buckling clamping device, The method is characterized in that the specimen according to claim 7 is assembled using the anti-buckling clamping device according to any one of claims 1 to 6, and the method comprises the following steps: The first step is to insert bolts into the outer splint: the side of the outer splint with the concave portion faces upwards, and a bolt is inserted into each bolt hole of the outer splint, with the bolt passing through the bolt hole facing upwards; The second step is to arrange the sample: place the sample on the outer clamping plate, and require that the compression direction of the sample is parallel to the direction of the outer state concave part, the bolt is stuck by the notch of the sample, and the plane geometric center of the sample coincides with the plane geometric center of the outer state concave part; The third step is to insert the inner clamp: the side of the inner clamp with the inner state concave part faces downward, and then align the bolt hole of the inner clamp with the bolt and insert it. The sample is clamped by the outer clamp and the inner clamp; Step 4: Insert the coil spring: On the inner clamping plate, insert a coil spring into each bolt, and the coil spring is located on the inner clamping plate; Step 5: Insert the spring pressure plate: Align the bolt hole of the spring pressure plate with the bolt and insert it. The coil spring is located under the spring pressure plate. Step 6. Screw in the nut: On the spring pressure plate, screw each bolt into the nut. Each nut must be screwed in the same distance to ensure that each spring is compressed the same amount. The assembly is complete.

9. The method for assembling the anti-buckling clamp device according to claim 8, characterized in that: Between the first step and the second step, and between the second step and the third step, polytetrafluoroethylene films are arranged to isolate the outer plate from the sample, and the inner plate from the sample. The polytetrafluoroethylene films have the same plane dimensions as those of the outer plate, and the position, size and number of the openings thereon are consistent with those of the outer plate.

10. A method for eliminating the friction effect of an anti-buckling clamping device, characterized by adopting The anti-buckling clamp device according to any one of claims 1 to 6, the test specimen according to claim 7, and the assembly method according to any one of claims 8 to 9 are tested, and the method comprises the following steps: In the first step, according to the assembly method of the anti-buckling clamping device and the matching specimen, two identical specimens are prepared so that the spring compression amount of the clamping device of one specimen is △ d 1. The spring compression of the clamping device of the other sample is △ d 2 ; In the second step, the same test conditions were used to carry out uniaxial compression tests on the assembled samples of the two spring compression amounts. The compression loads under the two assembly conditions were measured to be F 1 and F 2; In the third step, the pure compressive load of the uniaxial compression specimen is calculated using the following formula: F C : ; Pure compressive loads will eliminate friction effects F C Calculation of engineering stress and true stress for uniaxial compression tests.

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