Novel biaxial tensile test sample and simulation test method

Abstract

The invention relates to the technical field of biaxial tensile tests, and discloses a novel sample for a biaxial tensile test and a simulation test method.The novel sample for the biaxial tensile test is integrally in a cross shape and comprises a center connecting part and four loading arms, and the thickness of the center connecting part is gradually reduced from the outer edge to the center position; the two side faces, located in the width direction, of each loading arm are connecting side faces, each connecting side face sequentially comprises a first arc face, a second arc face and a vertical face from the side close to the center connecting part to the side away from the center connecting part, and the first arc face is sunken towards the side where the center connecting part is located. And the second arc surface sinks towards one side far away from the central connecting part. The four arc chamfers are formed on the loading arm, and compared with two arc chamfers, the shear stress of the chamfer positions can be greatly reduced, so that on one hand, the calculation error of the stress at the central position can be reduced, and on the other hand, the phenomenon of tearing at the arc chamfers during a test can be avoided.

Description

Technical Field

[0001] The present invention relates to the technical field of biaxial tensile testing, and in particular to a novel biaxial tensile testing specimen and a simulation testing method. Background Art

[0002] Solid propellant is a type of propellant widely used in aerospace, missiles, and rocket engines. Compared to liquid propellants, it offers advantages such as convenient storage, reliable operation, and simple structure, leading to its widespread adoption in many applications. The performance and reliability of solid propellants are crucial for critical sectors such as aerospace and defense. HTPB solid propellant, due to its excellent mechanical properties, is widely used in launch vehicles and guided missiles. HTPB materials are subject to complex loads during manufacturing, storage, and transportation. Therefore, the mechanical properties of HTPB have a significant impact on its structural safety and service life.

[0003] At present, there are two main types of test specimens for biaxial tensile tests: the first is the strip-shaped specimen for quasi-biaxial tensile tests based on uniaxial tensile testing machines; the second is the plane cross-shaped specimen for biaxial tensile tests based on biaxial tensile testing machines. Since the strip-shaped quasi-biaxial tensile test specimen can only meet the biaxial tensile test with a maximum loading ratio of 1:2, it is not universally applicable; the plane cross-shaped specimen can not only meet the conditions of different loading ratios, but also has a simple manufacturing process. Therefore, the plane cross-shaped specimen is currently used as the mainstream specimen for biaxial tensile tests at home and abroad.

[0004] In order to make the stress and strain distribution in the center area of ​​the specimen uniform during the test, many scholars have made improvements to the biaxial specimen, such as Figure 1 As shown in the figure, the stress concentration phenomenon in the center area of ​​the cross is improved by setting an arc chamfer 4 at the cross intersection. Although this improvement makes the stress and strain distribution in the center area of ​​the specimen uniform to a certain extent, there are still certain problems. For example, the specimen will tear at the arc in the actual test, which ultimately leads to the actual test results being contrary to the expected test results. Summary of the Invention

[0005] In order to solve the above technical problems, the present invention provides a novel biaxial tensile test specimen and a simulation test method.

[0006] In the first aspect, the present invention provides a new type of specimen for a biaxial tensile test. The new type of specimen for a biaxial tensile test is cross-shaped as a whole and includes a central connecting portion and four loading arms. The thickness of the central connecting portion gradually decreases from the outer edge to the center position; the two side surfaces of each loading arm in the width direction are connecting side surfaces, and the connecting side surfaces include a first arc surface, a second arc surface and a vertical surface in sequence from the side close to the central connecting portion to the side away from the central connecting portion. The first arc surface is concave on the side where the central connecting portion is located, and the second arc surface is concave on the side away from the central connecting portion.

[0007] Optionally, curved grooves are formed on both the upper surface and the lower surface of the central connecting portion.

[0008] Optionally, grooves are provided on both the upper surface and the lower surface of the loading arm along the length direction of the loading arm, and each groove is connected to a corresponding curved groove.

[0009] In a second aspect, the present invention provides a novel simulation test method for a specimen for a biaxial tensile test, comprising: Set up the constitutive model of the specimen for the new biaxial tensile test; The constitutive model was imported into the finite element software ABAQUS to establish a finite element model of the specimen for the new biaxial tensile test; Simulate biaxial tensile tests under different loading ratios and obtain the stress value in the central area of ​​the specimen through finite element simulation; Analyze the calculation results of stress and load under different loading ratios, and fit the relationship between stress and load: , where is stress, P For load.

[0010] Optionally, the constitutive model is a time-based integral constitutive model, and the integral constitutive model expression is: Where: is strain, t is time, Relaxation modulus equation, τ is the shear force, and ε0 is the initial strain.

[0011] Optionally, after establishing the finite element model of the new biaxial tensile test specimen, a 1 / 8 model is established using the symmetry of the new biaxial tensile test specimen, and the 1 / 8 model is meshed using eight-node three-dimensional solid elements C3D8H.

[0012] Optionally, the load is a displacement load.

[0013] The technical solution provided by the embodiment of the present invention has the following advantages compared with the existing technology: 1. By gradually reducing the thickness of the central connecting portion from the outer edge to the center, the center of the specimen for the new biaxial tensile test can be significantly deformed during the test, thus facilitating observation by the experimenter.

[0014] 2. By providing a first arc surface and a second arc surface on each loading arm, that is, forming four arc chamfers on each loading arm, the shear stress at the chamfer position can be greatly reduced compared with two arc chamfers. This design can not only reduce the calculation error of the stress at the center position, but also avoid the tearing phenomenon at the arc chamfer during the test. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a structural schematic diagram of an existing sample in the background technology of the present invention.

[0016] Figure 2 This is a schematic structural diagram of a novel biaxial tensile test specimen provided in Example 1 of the present invention.

[0017] Figure 3 The Mises stress cloud diagram provided in Example 2 of the present invention when the strain is 20%.

[0018] Figure 4 This is a picture of a sample that has not been tested provided in Example 2 of the present invention.

[0019] Figure 5 This is a picture of the sample during the destruction process during the test provided in Example 2 of the present invention.

[0020] Figure 6 This is a picture of the sample provided in Example 2 of the present invention after being completely destroyed.

[0021] Figure 7 This is a stress-load relationship curve diagram under four loading ratios provided in Example 2 of the present invention.

[0022] Explanation of the reference numerals: 1. Central connecting portion; 2. Loading arm; 21. First arc surface; 22. Second arc surface; 23. Vertical surface; 3. Groove; 4. Arc chamfer. DETAILED DESCRIPTION

[0023] A specific embodiment of the present invention is described in detail below with reference to the accompanying drawings, but it should be understood that the protection scope of the present invention is not limited by the specific embodiment.

[0024] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "axial", "radial", "circumferential" and the like to indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the technical solutions of the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0025] Example 1: like Figure 2 As shown, this embodiment provides a new type of specimen for biaxial tensile test. The new type of specimen for biaxial tensile test is cross-shaped as a whole and includes a central connecting part 1 and four loading arms 2. The thickness of the central connecting part 1 gradually decreases from the outer edge to the center position; the two side surfaces of each loading arm 2 in the width direction are connecting side surfaces, and the connecting side surfaces include a first arc surface 21, a second arc surface 22 and a vertical surface 23 in sequence from the side close to the central connecting part 1 to the side away from the central connecting part 1. The first arc surface 21 is concave toward the side where the central connecting part 1 is located, and the second arc surface 22 is concave toward the side away from the central connecting part 1.

[0026] Curved grooves are formed on both the upper and lower surfaces of the central connecting portion 1 .

[0027] In this embodiment, both the upper surface and the lower surface of the central connecting portion 1 are curved surfaces, and the space surrounded by the curved surfaces is a curved groove.

[0028] Grooves 3 are provided on both the upper and lower surfaces of the loading arm 2 along the length direction of the loading arm 2 , and each groove 3 is connected to a corresponding curved groove.

[0029] Example 2: This embodiment provides a novel simulation test method for a specimen for a biaxial tensile test, comprising: Step 1: Write the UMAT subroutine and set the constitutive model of the specimen for the new biaxial tensile test through the UMAT subroutine; In this embodiment, the constitutive model is a time-based integral constitutive model, and the integral constitutive model expression is: Where: is strain, t is time (s), is the relaxation modulus equation (MPa), τ is the shear force (kN), and ε0 is the initial strain.

[0030] Step 2: Import the UMAT subroutine into the finite element software ABAQUS to establish a finite element model of the specimen for the new biaxial tensile test; In this embodiment, after establishing the finite element model of the novel biaxial tensile test specimen, a 1 / 8 model is established using the symmetry of the novel biaxial tensile test specimen, and the 1 / 8 model is meshed using eight-node three-dimensional solid elements C3D8H.

[0031] In this embodiment, the 1 / 8 model is as follows: first, the sample model is divided into two parts by a plane perpendicular to the thickness direction of the sample model, and then the two parts of the model are divided into eight parts by two mutually perpendicular vertical planes. Each part is 1 / 8 of the original model. Due to the symmetry of the model, the force conditions of each 1 / 8 model are the same, which can simplify the model and improve the calculation efficiency.

[0032] Step 3: simulate biaxial tensile tests under different loading ratios and obtain the stress value in the central area of ​​the specimen through finite element simulation; In this embodiment, the load is a displacement load, and the loading ratio refers to the proportional relationship between the loads applied to the specimen in two mutually perpendicular directions (usually referred to as principal directions, such as the X-axis and the Y-axis).

[0033] Step 4: Analyze the calculation results of stress and load under different loading ratios, and fit the relationship between stress and load: , where is the stress (MPa), P is the load (kN).

[0034] In this embodiment, the UMAT subroutine includes: Step 1: Obtain the material constants, historical stresses, historical strains, and current incremental strain increments from the main program; Step 2: Determine whether the damage criterion is met. If so, calculate the damage. If not, proceed to the next step. Step 3: Use the incremental constitutive equation to solve the stress increment and Jacob matrix of the current incremental step; Step 4: Update the stress matrix and Jacob matrix and return to the main program.

[0035] In order to verify the accuracy and effectiveness of the new biaxial tensile test specimen designed in this scheme, the finite element simulation results of the new biaxial tensile test specimen at a loading rate of 2 mm / min and a loading ratio of 1:1 are compared with the corresponding actual test results.

[0036] The biaxial tensile test was carried out on an MTS biaxial tensile testing machine. The load acting on each arm of the cross-shaped specimen was recorded by a load sensor in each direction. The deformation of the central area of ​​the cross-shaped specimen was recorded by a CCD camera. The strain values ​​in each direction in the central area of ​​the biaxial specimen were measured by optical measurement methods.

[0037] like Figure 3 As shown, from Figure 3 As can be seen from the figure, during the simulation test, the maximum stress during loading was mainly concentrated in the center of the biaxial specimen. At the same time, during the test, it was observed that the initial location of specimen failure began in the center of the specimen. During the 2mm / min displacement loading process, the camera captured each frame at a speed of 5 seconds, and failure was a very fast process, so no corresponding photos were recorded during the actual test. Both the simulation and test results met the test objectives. During the isotropic biaxial loading process, the equivalent uniaxial loading load was a uniaxial tension at a 45° angle to the biaxial loading direction, which would cause the isotropic biaxial loaded specimen to eventually break with a crack direction at a 45° angle to the loading direction.

[0038] like Figures 4-6 As shown, Figure 4 The picture of the sample that has not been tested is shown in Figure 2. Figure 5 This is a picture of the sample during the destruction process during the test. Figure 6 This is a picture of the sample after it is completely destroyed. The sample changes during the real biaxial tensile test are shown in the figure below. Figures 4-6 As shown in the figure, it can be seen that the crack angle is approximately 45° to the loading direction, which is completely consistent with the expected result. Comparing the simulation results with the actual test results, it can be concluded that the biaxial tensile specimen designed in this scheme is reasonable and effective.

[0039] At the same time, other loading ratio conditions are simulated to extract the load values ​​on the loading arm and the average stress results in the central area, such as Figure 7 shown. Figure 7 The stress-load relationship curves under four loading ratios.

[0040] In the analysis Figure 7 After calculating the stress and load results shown in the figure, it is found that the stress in the center of the specimen is mainly affected by the load applied to the loading arm, and has nothing to do with the load ratio. Therefore, when the biaxial tensile test is carried out on the biaxial tensile specimen designed based on this scheme, as long as the load change value on the loading arm is known, the stress change value in the center of the specimen can be accurately determined, that is, the stress value to be obtained in the biaxial tensile test of this scheme. Figure 7 By fitting the curve, the corresponding stress value can be obtained. The fitting results are as follows:

[0041] , where is stress, P For load.

[0042] The above embodiments of the invention are only a few specific embodiments of the present invention. However, the embodiments of the present invention are not limited thereto. Any changes that can be conceived by those skilled in the art should fall within the scope of protection of the present invention.

Claims

1. A novel biaxial tensile test specimen, characterized in that: The novel biaxial tensile test specimen is cross-shaped as a whole and includes a central connecting portion (1) and four loading arms (2). The thickness of the central connecting portion (1) gradually decreases from the outer edge to the center position. Both side surfaces of each loading arm (2) in the width direction thereof are connecting side surfaces, and the connecting side surfaces sequentially comprise a first arc surface (21), a second arc surface (22), and a vertical surface (23) from a side close to the central connecting portion (1) to a side away from the central connecting portion (1), the first arc surface (21) being recessed toward the side where the central connecting portion (1) is located, and the second arc surface (22) being recessed toward a side away from the central connecting portion (1).

2. The novel biaxial tensile test specimen according to claim 1, characterized in that: The upper surface and the lower surface of the central connecting portion (1) are both formed with curved grooves.

3. The novel biaxial tensile test specimen according to claim 2, characterized in that: The upper surface and the lower surface of the loading arm (2) are both provided with grooves (3) along the length direction of the loading arm (2), and each groove (3) is connected to a corresponding curved groove.

4. The novel biaxial tensile test specimen simulation test method according to any one of claims 1 to 3, characterized in that: include: Set up the constitutive model of the specimen for the new biaxial tensile test; The constitutive model was imported into the finite element software ABAQUS to establish a finite element model of the specimen for the new biaxial tensile test; Simulate biaxial tensile tests under different loading ratios and obtain the stress value in the center area of ​​the specimen for the new biaxial tensile test through finite element simulation; Analyze the calculation results of stress and load under different loading ratios, and fit the relationship between stress and load: , where is stress, P For load.

5. The novel simulation test method for a biaxial tensile test specimen according to claim 4, characterized in that: The constitutive model is a time-based integral constitutive model, and the integral constitutive model expression is: Where: is strain, t is time, is the relaxation modulus equation, τ is the shear force, and ε0 is the initial strain.

6. The novel biaxial tensile test specimen simulation test method according to claim 4, characterized in that: After establishing the finite element model of the new biaxial tensile test specimen, the symmetry of the new biaxial tensile test specimen was used to establish a 1 / 8 model, and the eight-node three-dimensional solid element C3D8H was used to divide the mesh of the 1 / 8 model.

7. The novel simulation test method for a biaxial tensile test specimen according to claim 4, characterized in that: The load is a displacement load.

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