Test device and test method for realizing strain path mutation

Abstract

The invention relates to a test device and a test method for realizing strain path abrupt change. The test device comprises an x-direction insert (1) used as an active loading shaft; the y-direction insert (2) is used as a driven shaft; positioning pins are arranged on the x-direction insert (1) and the y-direction insert (2); a positioning hole is formed in a long arm of the cross-shaped sample (3); a positioning hole of an x-direction long arm of the cross-shaped sample (3) is circular, a positioning hole of a y-direction long arm of the cross-shaped sample (3) is long circular, the cross-shaped sample (3) can be mounted on a positioning pin through the positioning holes, and the y-direction insert (2) can be driven by the x-direction insert (1) to move. Compared with the prior art, the invention provides the method and the device for realizing strain path sudden change, the continuous sudden change of the biaxial strain path can be realized only by installing the device on the uniaxial tensile testing machine, and the device can be freely assembled and disassembled, is easy to maintain and is simple to operate.

Description

Technical Field

[0001] The present invention relates to the technical field of material performance testing, and in particular to a testing device and a testing method for realizing a sudden change in strain path. Background Art

[0002] In the field of material performance testing, proportional loading tests such as uniaxial tension, uniaxial compression, simple shear, Nakajima test, Marciniak test, etc. have been widely used to measure the yield, hardening, necking, fracture and other behaviors of materials. However, in modern industry, the forming of complex parts usually undergoes complex strain path changes rather than simple proportional deformation. The formability of metal sheets under strain path changes is completely different from that under proportional loading conditions. Therefore, a full understanding of the mechanical behavior of metal sheets under strain path mutations is crucial to optimizing manufacturing processes and improving formability.

[0003] At present, the two-stage loading method is the most commonly used test method to achieve strain path change. In this test method, the pre-strain path is obtained by a uniaxial tensile test (or Marciniak test) of an oversized specimen. A small specimen for the next step of loading is cut from the pre-strained large specimen. Subsequently, the small specimen is subjected to secondary deformation, and its mechanical properties such as yield, hardening, necking, and fracture are measured. In the above method, there is elastic unloading between the two-stage loading, which simplifies the operation of the test. However, in some complex forming processes (such as progressive forming, deep drawing, etc.), the material point will experience a sudden change in the strain path without unloading, and this loading condition is beyond the test range of the existing method. On the other hand, a biaxial tensile testing machine with bidirectional independent servo control can easily achieve continuous strain path mutation conditions, but this type of device has very high requirements for the control system and is expensive. Summary of the invention

[0004] The purpose of the present invention is to provide a test device and a test method for realizing sudden change of strain path, which can realize continuous sudden change of biaxial strain path by only being installed on a uniaxial tensile testing machine.

[0005] The object of the present invention can be achieved by the following technical solution: A test device for realizing a sudden change in strain path, comprising:

[0006] An x-direction insert used as an active loading axis;

[0007] A y-direction insert used as a driven axis;

[0008] The x-direction insert and the y-direction insert are both provided with positioning pins, and the long arm of the cross-shaped specimen is provided with positioning holes;

[0009] The positioning hole of the x-direction long arm of the cross-shaped specimen is circular, and the positioning hole of the y-direction long arm is oblong. The cross-shaped specimen can be installed on the positioning pin through the positioning hole, and the y-direction insert can move under the drive of the x-direction insert.

[0010] Preferably, the x-direction insert and the y-direction insert are provided with an inclination angle on the contacting sides. The inclination angle can be defined as a.

[0011] Further preferably, the x-direction insert includes an x-direction main body and sliding blocks symmetrically arranged on both sides of the x-direction main body, and the y-direction insert includes a y-direction main body and sliding grooves symmetrically arranged on both sides of the y-direction main body;

[0012] The sliding block portion and the sliding groove portion are in contact and are provided with mutually matching inclination angles.

[0013] Further preferably, assuming that the maximum static friction coefficient of the contact is μ, the preferred range of the inclination angle a is 0−arctan(1 / μ).

[0014] More preferably, both the x-direction main body and the y-direction main body are provided with a sample receiving groove, and the positioning pin is arranged in the sample receiving groove along the z-direction.

[0015] Preferably, the positioning pin is a cylindrical positioning pin.

[0016] Further preferably, the outer diameter of the positioning pin is the same as the inner diameter of the circular positioning hole.

[0017] In the present invention, the oblong positioning hole comprises a straight portion and semicircular arc portions located at both ends of the straight portion.

[0018] Further preferably, the outer diameter of the positioning pin is the same as the inner diameter of the semicircular arc portions at both ends of the oblong positioning hole, and is smaller than the distance between the two ends of the oblong positioning hole.

[0019] Preferably, the length direction of the oblong positioning hole is the y-axis direction. The length of the straight portion of the oblong positioning hole can be defined as l0.

[0020] Preferably, the y-direction insert is slidably arranged on the guide column and can move along the guide column driven by the x-direction insert.

[0021] Preferably, a plurality of slits are provided on one side of the x-direction long arm and the y-direction long arm of the cross-shaped specimen close to the central area.

[0022] Preferably, the speckle pattern is sprayed on the central area of ​​the cross-shaped sample. The side length of the central area can be defined as w0.

[0023] Preferably, the cross-shaped test specimen is cut from a metal sheet blank.

[0024] Preferably, the test device for achieving a sudden change in strain path comprises a pair of x-direction inserts and a pair of y-direction inserts symmetrically arranged in a cross shape.

[0025] A test method for achieving a sudden change in strain path, using the above device, comprises the following steps:

[0026] S1: Install the cross-shaped specimen on the positioning pins of the x-axis insert and the y-axis insert through the positioning holes;

[0027] S2: stretch the x-direction insert along the x-axis and drive the y-direction insert to move along the y-axis through friction contact;

[0028] S3: In the first stage of stretching, the locating pin on the y-axis insert can move freely in the oblong locating hole without loading the y-axis of the cross-shaped specimen. At this time, the cross-shaped specimen is only subjected to uniaxial tensile deformation in the x-axis.

[0029] S4: In the second stage of stretching, the locating pin on the y-axis insert reaches the end of the oblong locating hole, the y-axis of the cross-shaped specimen is suddenly loaded, and the cross-shaped specimen undergoes biaxial deformation in the subsequent loading.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1. The present invention provides a method and device for achieving sudden change in strain path, which can achieve continuous and sudden change in biaxial strain path by simply being installed on a uniaxial tensile testing machine.

[0032] 2. The present invention can be freely loaded and unloaded, easy to maintain and simple to operate.

[0033] 3. Compared with the existing biaxial tensile testing device, the device of the present invention has lower manufacturing difficulty and manufacturing cost.

[0034] 4. The present invention can flexibly design different sizes of pre-strain by changing the size of the oblong hole of the sample; and can flexibly design different degrees of strain path mutation and biaxial stretching ratio in the second loading stage by changing the inclination angle of the insert contact.

[0035] 5. The present invention has no specific requirements on the type of test material and is applicable to a variety of metal and non-metal materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a structural schematic diagram of the test device of the present invention;

[0037] Figure 2 It is a schematic diagram of the structure of the cross-shaped specimen of the present invention;

[0038] Figure 3 Schematic diagram of strain path with different pre-strain sizes;

[0039] Figure 4 Schematic diagram of strain paths with different mutation degrees;

[0040] In the figure: 1-x-direction insert, 2-y-direction insert, 3-cross-shaped specimen, 31-circular positioning hole, 32-oblong positioning hole, 33-slit, 4-guide column, a-contact inclination angle. DETAILED DESCRIPTION

[0041] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0042] Example 1

[0043] A test device for realizing a sudden change in strain path includes an x-direction insert 1 and a y-direction insert 2, wherein the x-direction insert 1 serves as an active loading axis, the y-direction insert 2 serves as a driven axis, and the y-direction insert 2 can move under the drive of the x-direction insert 1. In addition, positioning pins are provided on both the x-direction insert 1 and the y-direction insert 2, and positioning holes are provided on the long arms of a cross-shaped specimen 3, and the cross-shaped specimen 3 is detachably mounted on the positioning pins through the positioning holes.

[0044] Furthermore, in this embodiment, the positioning hole of the long arm in the x direction of the cross-shaped specimen 3 is circular, and the positioning hole of the long arm in the y direction is oblong.

[0045] A test method for realizing a sudden change in strain path using the above device comprises the following steps:

[0046] S1: Install the cross-shaped specimen 3 on the positioning pins of the x-direction insert 1 and the y-direction insert 2 through the positioning holes;

[0047] S2: stretch the x-direction insert 1 along the x-axis, and drive the y-direction insert to move along the y-axis through friction contact;

[0048] S3: Due to the design of the oblong positioning hole on the long arm in the y direction, in the first stage of stretching, the positioning pin on the y-direction insert 2 can move freely in the oblong positioning hole without loading the y-axis of the cross-shaped specimen 3. At this time, the cross-shaped specimen 3 is only subjected to uniaxial tensile deformation in the x direction;

[0049] S4: In the second stage of stretching, the locating pin on the y-axis insert 2 reaches the end of the oblong locating hole, the y-axis of the cross-shaped specimen 3 is suddenly loaded, and the cross-shaped specimen 3 undergoes biaxial deformation in subsequent loading.

[0050] Example 2

[0051] A test device for realizing a sudden change in strain path, wherein an x-direction insert 1 and a y-direction insert 2 are provided with a contact inclination angle a on the contacting sides, and the y-direction insert 2 is slidably arranged on a guide column 4 arranged along the y direction, and the y-direction insert 2 can move along the y-axis of the guide column 4 driven by the x-direction insert 1. The rest is the same as that of embodiment 1.

[0052] Example 3

[0053] A test device for realizing a sudden change in strain path, such as Figure 1 As shown, it consists of an x-axis insert 1, a y-axis insert 2 and a guide column 4. The specimen is cross-shaped, the positioning hole of the x-axis long arm is circular, just matching the positioning pin on the x-axis insert 1, and the positioning hole of the y-axis long arm is an oblong hole. Figure 2 As shown, multiple slits are machined on the four arms of the specimen to improve the uniformity of deformation in the central area. Two guide pillars 4 are inserted in two y-direction inserts 2 to ensure the symmetry of the deformation of the cross-shaped specimen. The x-direction is the active loading axis, and the y-direction is the driven axis.

[0054] When the x-direction insert 1 is stretched along the x-axis, the y-direction insert 2 is driven to move along the y-axis through frictional contact. Since the positioning hole of the cross-shaped specimen in the y-direction is an oblong hole, in the first stage of stretching, the positioning pin on the y-direction insert 2 can move freely in the oblong hole without loading the y-axis of the cross-shaped specimen. At this time, the specimen is only subjected to uniaxial tensile deformation in the x-direction. In the second stage of stretching, the positioning pin reaches the end of the oblong hole, the y-axis of the cross-shaped specimen is suddenly loaded, and the specimen undergoes biaxial deformation in subsequent loading.

[0055] like Figure 3 As shown in the figure, by changing the length l0 of the straight part of the oblong hole of the specimen, different sizes of pre-strain can be flexibly designed, that is, the specimen can undergo a sudden change at the strain point p1, p2 or p3. Figure 4 As shown, by changing the contact inclination angle a of the x-axis insert 1 and the y-axis insert 2, different degrees of strain path mutation and the biaxial stretching ratio in the second loading stage can be flexibly designed. Specifically, the nominal value of the prestrain can be estimated as E1=2l0cot(a) / w0, and its logarithmic value can be estimated as ε1=ln(1+E1); the biaxial stretching ratio in the second loading stage can be estimated as dε xx :dε yy =cot(a).

[0056] In terms of data acquisition, the biaxial tensile load can be measured by the force sensor embedded in the insert. Before the test, the speckle pattern is sprayed on the central area of ​​the specimen, and the strain field in the deformation area is measured by digital image correlation (DIC).

[0057] Example 4

[0058] A test device and test method for realizing a sudden change in strain path, wherein a cross-shaped specimen 3 is cut from a metal sheet blank, a slit 33 is machined around the central area, a circular positioning hole 31 is machined on the long arm in the x-direction, and an oblong positioning hole 32 is machined on the long arm in the y-direction. A speckle pattern is sprayed on the central area of ​​the specimen before the test begins. The x-direction insert 1 of the device of this embodiment is connected to the loading shaft of a uniaxial tensile testing machine, and the specimen is installed in the center of the device. After the stretching begins, the x-direction insert 1 drives the y-direction insert 2 to move along the y-axis through friction contact. In the first loading stage, since the positioning hole of the cross-shaped specimen 3 in the y-direction is an oblong hole, the positioning pin on the y-direction insert can move freely in the oblong hole without loading the y-axis of the cross-shaped specimen 3. At this time, the specimen is only subjected to uniaxial tensile deformation in the x-direction. In the second loading stage, the positioning pin reaches the end of the oblong hole, the y-axis of the cross-shaped specimen 3 is suddenly loaded, and the specimen undergoes biaxial deformation in subsequent loading. The metal sheet achieves a sudden change from uniaxial tension to biaxial tension in the above operation, and the two-stage loading is a continuous process without unloading. During the test, the biaxial tensile load is measured by the force sensor embedded in the insert, and the strain field in the central deformation zone is measured by digital image correlation technology (DIC).

[0059] The above description of the embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A test device for realizing a sudden change in strain path, characterized in that: include: An x-direction insert (1) used as an active loading axis; A y-direction insert (2) used as a driven shaft; The x-direction insert (1) and the y-direction insert (2) are both provided with positioning pins, and the long arm of the cross-shaped specimen (3) is provided with positioning holes; The positioning hole of the x-direction long arm of the cross-shaped specimen (3) is circular, and the positioning hole of the y-direction long arm is oblong. The cross-shaped specimen (3) can be mounted on the positioning pin through the positioning hole, and the y-direction insert (2) can move under the drive of the x-direction insert (1).

2. The test device for achieving a sudden change in strain path according to claim 1, characterized in that: The x-direction insert (1) and the y-direction insert (2) are provided with an inclination angle on the sides in contact with each other.

3. The test device for achieving a sudden change in strain path according to claim 2, characterized in that: The x-direction insert (1) comprises an x-direction main body and sliding blocks symmetrically arranged on both sides of the x-direction main body, and the y-direction insert (2) comprises a y-direction main body and sliding grooves symmetrically arranged on both sides of the y-direction main body; The sliding block portion and the sliding groove portion are in contact and are provided with mutually matching inclination angles.

4. The test device for achieving a sudden change in strain path according to claim 3, characterized in that: The x-direction main body and the y-direction main body are both provided with a sample containing groove, and the positioning pin is arranged in the sample containing groove along the z-direction.

5. The test device for achieving a sudden change in strain path according to claim 1, characterized in that: The positioning pin is a cylindrical positioning pin, the outer diameter of which is the same as the inner diameter of the circular positioning hole, and the outer diameter of the positioning pin is the same as the inner diameters of both ends of the oblong positioning hole.

6. The test device for achieving a sudden change in strain path according to claim 1, characterized in that: The y-direction insert (2) is slidably arranged on the guide column (4) and can move along the guide column (4) driven by the x-direction insert (1).

7. The test device for achieving a sudden change in strain path according to claim 1, characterized in that: A plurality of slits (33) are provided on one side of the long arms in the x direction and the long arms in the y direction of the cross-shaped sample (3) close to the central area.

8. The test device for achieving a sudden change in strain path according to claim 1, characterized in that: The central area of ​​the cross-shaped sample (3) is sprayed with a speckle pattern.

9. The test device for achieving a sudden change in strain path according to claim 1, characterized in that: It comprises a pair of x-direction inserts (1) and a pair of y-direction inserts (2) which are symmetrically arranged in a cross shape.

10. A test method for achieving strain path mutation, characterized in that: The method is carried out using the device according to any one of claims 1 to 9, comprising the following steps: S1: Install the cross-shaped specimen (3) on the positioning pins of the x-direction insert (1) and the y-direction insert (2) through the positioning holes; S2: stretching the x-direction insert (1) along the x-axis and driving the y-direction insert to move along the y-axis through friction contact; S3: In the first stage of stretching, the positioning pin on the y-axis insert (2) can move freely in the oblong positioning hole without loading the y-axis of the cross-shaped specimen (3). At this time, the cross-shaped specimen (3) is only subjected to uniaxial tensile deformation in the x-axis; S4: In the second stage of stretching, the locating pin on the y-axis insert (2) reaches the end of the oblong locating hole, the y-axis of the cross-shaped specimen (3) is suddenly loaded, and the cross-shaped specimen (3) undergoes biaxial deformation in subsequent loading.

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