CN113804537B

CN113804537B - Clamp capable of applying pull/press cyclic load to CT test piece

Info

Publication number: CN113804537B
Application number: CN202110895673.8A
Authority: CN
Inventors: 刘丰睿; 董齐; 时新红; 王林娟; 骈瑢
Original assignee: Beihang University
Current assignee: Beihang University
Priority date: 2021-08-05
Filing date: 2021-08-05
Publication date: 2022-08-16
Anticipated expiration: 2041-08-05
Also published as: CN113804537A

Abstract

The invention relates to a clamp capable of applying a tensile/compressive cyclic load to a CT test piece. The clamp is connected with the testing machine through the clamping part of the working unit, and in order to prevent the clamp from sliding when loading, the roughness of the clamping surface needs to be increased; the CT specimen is placed in a groove of the working unit, and when the jig is used to provide a press-press cycle load, the upper and lower surfaces of the groove are in direct planar contact with the top and bottom ends of the CT specimen, whereas when the jig is used to provide a pull-pull cycle load, a pin unit is used to transfer the load. The invention can apply two different types of loads to the CT test piece, thereby improving the utilization rate of the clamp; meanwhile, the structural form of the clamp is optimized, a pin unit is not required to be placed when compression loading is carried out, and the clamping time of the CT test piece is saved; increase for the CT test piece and prevent the slip table, when preventing that anchor clamps from carrying out plane compression loading, the CT test piece is removed and is removed, has guaranteed load centering nature.

Description

Clamp capable of applying pull/press cyclic load to CT test piece

Technical Field

The invention belongs to the field of research on metal material fracture mechanics, and particularly relates to a clamp capable of applying a tensile/compressive cyclic load to a CT test piece. The clamp can provide a compression-compression cyclic load at the crack prefabrication stage and can also provide a pulling-pulling cyclic load at the crack propagation rate measuring stage, so that the related tests such as the metal crack propagation rate, the crack propagation threshold value, the fracture toughness measurement and the like can be quickly and efficiently completed.

Background

The linear elastic fracture mechanics studies are sharp cracks, so that the cracks need to be preformed before starting the fracture toughness and Fatigue crack propagation rate test tests, and the cracks are preformed mainly by using a pull-pull cyclic load, but the researches show that the method can cause higher crack closing load and remote crack surface closing phenomenon, and the cracks are preformed by using a press-press cyclic load.

At present, the test fixture has no unified form because the test standard of the prefabricated crack of the CT test piece under the pressure-pressure cyclic load does not exist. In the literature (Compression pre-cracking to generation near hole thickness rates in two aluminum alloys [ J ]. International Journal of failure, 2005.), the authors proposed adding small aluminum blocks between the conventional U-shaped fixture and the CT specimen to apply the pressure-pressure cycle load required for pre-cracking, so that after the pre-cracking was over, the U-shaped fixture can still be used as the crack propagation test, and the fixture schematic diagrams are shown in fig. 1 and 2.

Firstly, when pressure-pressure cyclic loading is carried out, the pin unit is always placed in the pin hole to prevent a test piece from splashing possibly when compression overload occurs, so that a small aluminum block and the pin unit can apply pressure-pressure cyclic loading to a CT test piece at the same time, microcracks and plastic deformation occur at the edge of the pin hole, and the measurement precision of a subsequent crack expansion rate test is influenced; meanwhile, the small aluminum block is simply placed between the CT test piece and the clamp without a fixing device, so that the small aluminum block is likely to move in the cyclic loading process, the load centering property is poor, and the design of the novel clamp is significant.

Disclosure of Invention

The technical problem of the invention is solved: the defects of the existing clamp are found, when the U-shaped clamp applies pressure-pressure cyclic load, the pin unit may contact with the pin hole, so that microcracks and plastic deformation are generated at the pin hole; in the ballast prefabrication crack process, the small aluminum block is likely to move, so that the load is poor in centering performance, and therefore, the novel clamp is capable of simplifying the structural form and improving reasonably aiming at the problems existing in the existing clamp.

The technical scheme of the invention is as follows: a clamp capable of applying tension/compression cyclic load to a CT test piece consists of a working unit 2 and a pin unit 3 shown in figure 3.

Firstly, as seen from the front view of the working unit 2, the upper end of the working unit 2 is a clamping part in a cuboid shape, the working unit 2 of the clamp is required to be connected with a testing machine through the clamping part, the length and the width of the clamping part are designed according to the size of a chuck of the testing machine, the thickness of the clamping part is usually 2 times of that of a CT test piece 4, in order to prevent the clamp from sliding in the loading process, the clamping part is subjected to anti-slip treatment during processing, and the roughness of the clamping surface is increased; the lower end of the working unit 2 is also a cuboid, and the volume of the working unit is larger than that of the clamping part because a pin hole for bearing load needs to be machined, so that the strength requirement of the clamp is ensured; the middle part of the working unit 2 is mainly a transition part connecting the upper end and the lower end, so that the stress concentration is avoided, and the working unit is rounded at the position.

Viewed from the side, the working unit 2 has a recess with a rectangular cross section on its side, which serves to receive the CT specimen 4, so that the width of the rectangular cross section is adapted to the thickness of the CT specimen 4. The cylindrical pin unit 3 is matched with the pin hole of the working unit 2 and the pin hole of the CT specimen 4, and has relatively high processing precision, because the load applied to the CT specimen 4 is directly provided by the pin unit. The test piece for testing is characterized in that four anti-sliding tables 5 are additionally arranged on the basis of a CT standard test piece, the size of the CT standard test piece can refer to the ASTM E647 standard, the height of each anti-sliding table 5 is 2-4 mm, the width of each anti-sliding table is determined by the size of the test piece, the determination principle is that the anti-sliding tables 5 can prevent the CT test piece 4 from moving left and right in the loading process, and the centering performance of the load is guaranteed to be unchanged.

Next, the states of the respective portions of the jig in the compression loading and the tension loading, respectively, will be described.

The upper surface direct contact CT test piece 4's of the 2 recess of operating unit top during compression loading, the centre does not leave the space, this replacement little aluminium pig that can be fine, carry out the plane contact loading, also need not worry about little aluminium pig and remove about the influence load centering nature, simultaneously because CT test piece 4 has increased antiskid platform 5, consequently the installation pin unit again prevents that the test piece from having removed during compression loading, so not only can improve the efficiency of clamping CT test piece 4, can also avoid the damage that the pin unit probably caused the pinhole.

During tensile loading, the working unit 2 and the pin unit 3 are matched for clamping the CT test piece 4, the pin unit simultaneously penetrates through pin holes of the working unit 2 and the CT test piece 4, and a space is reserved between the upper surface of a groove of the working unit 2 and the top end of the CT test piece 4. The overall view of the fixture and the various parts are shown in figures 3-8. And 4, a CT specimen, as shown in fig. 9.

The invention has the advantages that:

the method has the advantages that 1, a set of clamp is used, so that not only can a pressure-pressure circulating load be applied to the CT test piece, but also a tension-tension circulating load can be applied, and the utilization rate of the clamp is fully improved;

the clamp has the advantages that 2, the structural form of the clamp is optimized, and pins do not need to be placed during compression loading, so that damage to pin holes caused by the pins can be avoided, and meanwhile, the clamping efficiency of the test piece is improved;

advantage 3, no longer use little aluminium pig to carry out the plane contact loading, solved the problem that little aluminium pig probably appears moving in the loading process, guaranteed load centering nature.

Drawings

FIG. 1 is a schematic diagram of a two-dimensional structure of a conventional U-clamp for tensile loading;

FIG. 2 is a schematic view of a compression loading two-dimensional structure of a conventional U-clamp;

FIG. 3 is a schematic diagram of the three-dimensional structure of the clamp for tensile loading according to the present invention;

FIG. 4 is a schematic view of a compression loaded three-dimensional structure of the clamp of the present invention;

FIG. 5 is a schematic view of a two-dimensional structure for tensile loading of the clamp of the present invention;

FIG. 6 is a schematic view of a clamp compression loading two-dimensional structure according to the present invention;

FIG. 7 is a schematic diagram of a two-dimensional structure of a working unit of the jig of the present invention;

FIG. 8 is a schematic diagram of a two-dimensional structure of a jig pin unit according to the present invention;

FIG. 9 is a schematic view of a CT specimen according to the present invention.

The numbers in the figures illustrate the following:

1-small aluminum blocks; 2-a working unit; 3-a pin unit; 4-CT test piece; 5-anti-skid platform

Detailed Description

The technical solution of the present invention is further described below with reference to fig. 1 to 9 and implementation steps.

1. The CT test piece 4 is made of 7075 aluminum alloy, a small CT test piece is used in the test, the detailed structure size of the test piece can refer to the ASTM-E647 standard, and then a clamp is designed according to the size of the CT test piece 4;

2. and (4) carrying out pressure-pressure cyclic load to prefabricate cracks. The test is carried out on an MTS-880 fatigue testing machine, firstly, the clamping parts of the clamp working units 2 are respectively fixed by the upper and lower chucks of the testing machine, enough space is reserved between the upper and lower clamp working units 2 to place the CT test piece 4 while the centering property is ensured, then the CT test piece 4 is installed in the groove of the clamp working unit 2, the working unit 2 is placed between the two anti-sliding tables 5, then the upper chuck is gradually lowered to ensure that the top end and the bottom end of the CT test piece 4 just contact with the upper and lower surfaces of the groove, finally, the contact condition and the centering property of the clamp working unit 2 and the test piece are checked once again, and the test is started after the correctness is ensured. The maximum compressive load used in the test is such that the absolute value of the stress intensity factor at the crack tip

Wherein E is the elastic modulus of the material and the stress intensity factor is given in units of

And (5) observing the crack length of the cut of the CT test piece 4 by using a microscope, recording, and stopping loading when the crack is expanded to 0.5mm to finish the prefabricated crack. Taking down the CT test piece 4, and still remaining the clamp working unit 2 on the testing machine;

3. tensile-tensile cyclic load crack propagation test. The distance between the upper clamp working unit 2 and the lower clamp working unit 2 is adjusted, the CT test piece 4 with a prefabricated crack is installed on the lower clamp working unit 2, then the upper chuck of the testing machine is slowly adjusted, the pin unit 3 can just penetrate through pin holes in the clamp and the CT test piece 4 at the same time, the upper clamp and the lower clamp and the CT test piece 4 are in the same vertical line, the load centering performance during loading is guaranteed, the detailed inspection is performed at last, and the test is started after the test is guaranteed to be correct. Determining the corresponding load size according to the test requirements, and recording the load cycle number and the crack length in the test process, thereby calculating the crack propagation rate;

4. after the crack propagation test is finished, the CT test piece 4 is taken down, and then the clamp working unit 2 is taken down from the clamp head of the tester.

Claims

1. A clamp capable of applying a tensile/compressive cyclic load to a CT test piece comprises a working unit and a pin unit; the method is characterized in that: the upper end of the working unit is a cuboid clamping part, the working unit of the clamp is connected with the testing machine through the clamping part, and the length and the width of the clamping part are designed according to the size of a chuck of the testing machine; the lower end of the working unit is also a cuboid, a pin hole bearing load needs to be machined at the lower end, the volume of the pin hole is larger than that of the clamping part, and the strength requirement of the clamp is guaranteed; the middle part of the working unit is connected with the transition parts at the upper end and the lower end, so that stress concentration is avoided, and rounding treatment is performed; the side surface of the working unit is provided with a groove with a rectangular cross section, a CT test piece is placed in the groove, and the width of the rectangular cross section is matched with the thickness of the CT test piece; the cylindrical pin unit is matched with a pin hole of the working unit and a pin hole of the CT test piece, and the load borne by the CT test piece is directly provided by the pin unit.

2. The clamp for applying the pull/press cyclic load to the CT test piece as recited in claim 1, wherein: the thickness of the clamping part is 2 times of the thickness of the CT specimen.

3. A clamp for applying a pull/press cyclic load to a CT specimen according to claim 1 or 2, wherein: the clamp is prevented from sliding in the loading process, the clamping part needs to be subjected to anti-sliding treatment, and the roughness of the clamping surface is increased.

4. The clamp for applying the pull/press cycle load to the CT specimen as recited in claim 3, wherein: the test piece for testing is characterized in that four anti-slip tables are added on the basis of a CT standard test piece, the size of the CT standard test piece refers to the ASTM E647 standard, the height of each anti-slip table is 2-4 mm, and the width of each anti-slip table is determined by the size of the test piece.

5. The clamp for applying the pull/press cycle load to the CT specimen as recited in claim 4, wherein: the principle of determination is that the anti-slip table prevents the CT test piece from moving left and right in the loading process from the edge of the test piece to the edge of the pin hole, and the centering performance of the load is guaranteed to be unchanged.

6. The clamp for applying the pull/press cyclic load to the CT test piece as recited in claim 1, wherein: the upper surface of the working unit groove directly contacts the top end of the CT test piece during compression loading, no gap is left in the middle, and a small aluminum block is replaced to perform plane contact loading.

7. The clamp for applying the pull/press cyclic load to the CT test piece as claimed in claim 1, wherein: the working unit and the pin unit are matched for clamping the CT test piece during tensile loading, the pin unit simultaneously penetrates through pin holes of the working unit and the CT test piece, and a space is reserved between the upper surface of the groove of the working unit and the top end of the CT test piece.

8. The working method of the clamp capable of applying the pull/press cyclic load to the CT test piece based on the claim 1 is characterized in that: when the cracks are prefabricated under the pressure-pressure cyclic load; the test is carried out on an MTS-880 fatigue testing machine, firstly, the upper and lower chucks of the testing machine are used for respectively fixing the clamping parts of the clamp working units, enough space is reserved between the upper and lower clamp working units to place a CT test piece while the centering property is ensured, then the CT test piece is installed in the groove of the clamp working units, the working units are placed between two anti-sliding tables, then the upper chucks are gradually lowered, the top end and the bottom end of the CT test piece are just in contact with the upper and lower surfaces of the groove, finally, the contact condition and the centering property of the clamp working units and the test piece are checked once again, and the test is started after the correctness is ensured.

9. The working method of the clamp capable of applying the tensile/compressive cyclic load to the CT test piece as recited in claim 8, wherein: the maximum compressive load used in the test is such that the absolute value of the stress intensity factor at the crack tip

Observing the crack length of the cut of the CT test piece by using a microscope and recording the crack length, stopping loading when the crack is expanded to 0.5mm, and finishing the crack prefabrication; and taking down the CT test piece, and still keeping the clamp working unit on the testing machine.

10. The working method of the clamp capable of applying the tensile/compressive cyclic load to the CT test piece according to claim 1 is characterized in that: in a pull-pull cyclic load crack propagation test; adjusting the distance between the upper clamp working unit and the lower clamp working unit, mounting the CT test piece with the prefabricated crack on the lower clamp working unit, then slowly adjusting an upper chuck of the testing machine to enable the pin unit to just pass through pin holes on the clamp and the CT test piece at the same time, enabling the upper clamp and the lower clamp and the CT test piece to be on the same vertical line to guarantee load centering during loading, finally checking once again in detail, and starting the test after no error is guaranteed; the crack propagation rate was calculated by determining the corresponding load magnitude according to the test requirements and recording the number of load cycles and the crack length during the test.