RU2344407C1 - Method of testing biaxial stretching of sheet material - Google Patents

Abstract

FIELD: physics; test engineering.

SUBSTANCE: present invention pertains to test engineering and can be used for determining mechanical properties of sheet materials under biaxial stretching conditions in mechanical engineering, automobile manufacturing aircraft engineering and other industries. A disc shaped specimen is put onto a matrix, loaded by a flexible die until destruction, on the forming surface of which an antifriction coating is deposited before testing. The value of limit of elasticity ε_lim is calculated using the formula ε_lim =ln(t₀/t), where t₀, t is the thickness of the working part of the specimen before and after testing, respectively.

EFFECT: increased accuracy of testing, reduced labour input and material usage.

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Description

The invention relates to testing equipment and can be used to determine the characteristics of the mechanical properties of sheet materials under biaxial tension in mechanical engineering, automotive, aircraft and other industries.

A known method of testing sheet materials for biaxial tension, which consists in the fact that the sample in the form of a plate is mounted on the matrix, loaded with a punch with a spherical working surface until it breaks, the fracture moment is determined by cracks in the bulging part of the sample, and the technology of the test material is judged from the data obtained [ one]. The disadvantage of this method is the impossibility of testing under uniform biaxial tension due to the uneven loading conditions of the sample.

Closest to the proposed technical solution is a method of testing sheet materials for biaxial tension, presented in the copyright certificate [2].

In this method, several samples of different diameters are successively tested. In this case, the disk-shaped sample is placed on the matrix and loaded with a rigid punch located coaxially with the matrix. Loading is carried out along the working part of the sample and, when cracks appear on it, the ultimate ductility is determined.

Known technical solution has the following disadvantages:

- low accuracy due to a significant influence on the test results of the friction forces between the sample and the punch, as a result of which the working part of the sample is deformed under conditions of inhomogeneous biaxial tension;

- great complexity and significant consumption of material associated with the need to test several samples.

The claimed technical solution is aimed at improving the accuracy of the test, reducing its complexity and reducing material consumption.

This is achieved by the fact that in the method for testing biaxial sheet materials according to the invention, to ensure uniform biaxial tension, the specimen is loaded with an elastic punch to fracture, on the forming surface of which a fluoroplastic-based antifriction coating is applied before testing, which serves as a lubricant and is used repeatedly .

The drawing shows a diagram of the method: to the left of the axis of symmetry - the initial position of the sample before the test, to the right - the position of the sample after the test.

The method is carried out in the following way. A disk-shaped sample is cut from a sheet of the material under study and its thickness t ₀ is measured in the central zone of the working part. Then the sample 1 is placed on the mirror of the rigid matrix 2, to the container 3, in which the elastic punch 4 is placed, located coaxially with the matrix, a press force is applied and the sample is immersed until fracture.

In order to create a uniform deformation of the working part of the test specimen throughout its deformation up to failure, an antifriction coating based on fluoroplastic is applied before testing to be used as a lubricant and used repeatedly. The use of an elastic punch with an anti-friction coating allows you to create hydrostatic (uniform) pressure from the side of the elastic medium on the sample, significantly reduce the magnitude of the friction forces between the sample and the punch, and minimize their effect on the distribution of deformations in the working area of the sample.

As a result, the working part of the sample is deformed under loading under conditions of uniform biaxial tension. In addition, the use of anti-friction coating allows to increase the wear resistance of the elastomer and thereby significantly increase its service life.

After the test, the sample 1 is removed from the matrix 2, the thickness t of its working part is measured near the formed crack, and the ultimate ductility ε _{pr is} calculated by the formula:

ε _ol = ln (t ₀ / t).

The implementation of the proposed method will allow, in comparison with the known technical solution, to increase the accuracy and reliability of determining the characteristics of the mechanical properties of sheet materials under biaxial tension, reduce the complexity of the test and reduce the consumption of material.

An example of a specific implementation of the method

Biaxial tension was carried out on a standard test machine R-20. Three round samples with a diameter of 210 mm and a thickness of t ₀ = 1.5 mm made of aluminum alloy D16AM were tested. Loading of the samples to failure was carried out in an experimental draft stamp with an elastic punch into a circular rigid matrix with a diameter of 70 mm and a radius of the exhaust rib of 10 mm. As the elastomer used technological rubber brand 3826 with an initial hardness of 70 HSD. Before testing, an antifriction coating based on fluoroplastic 4 was applied to the forming surface of the elastic punch. To reduce the influence of friction forces, a 0.1 mm thick polyethylene film was placed between the sample and the matrix.

To verify the implementation of the conditions of uniform biaxial tension on the working part of the sample after the test, the strains were determined using a dividing grid preliminarily applied to the sample by a photocontact method from a system of intersecting circles with a diameter d ₀ = 2.6 mm. For this purpose, along the radius located near the crack, but not intersected by it, the cell sizes of the distorted dividing grid were measured using a BMI-1 instrumental microscope with an accuracy of ± 0.005 mm.

In the process of drawing an elastic punch into a rigid round matrix, the flat workpiece is converted into a spherical shell. The calculation of the circumferential ε _t and meridional ε _m strains was performed using the relations

where a, b are the cell sizes of the deformed dividing grid, respectively, in the circumferential and meridional directions.

Based on the established values of the strains, a parameter of the form of the deformed state α = ε _m / ε _{t was} determined. For almost the entire length of the working part of each of the tested samples, the parameter of the form of the deformed state remained constant, and its value was close to unity, which indicated that the working part of the sample was deformed under loading under uniform biaxial tension.

The destruction of each of the three tested samples occurred through the formation of cracks in the center of the working part, which also confirmed that the conditions of uniform biaxial tensile deformation are realized during the test. The ultimate plasticity of the material ε _pr is equal to the strain intensity ε ₀ accumulated at the time of fracture. In case of biaxial tension

From the condition of plastic incompressibility of the material it follows that

where ε _z = ln (t / t ₀ ) is the deformation in the direction normal to the surface of the sample.

From a comparison of relations (2) and (3) it follows that the ultimate ductility

To establish the ultimate ductility on the working part of each of the tested samples, the thickness t was measured near the rupture site.

An independent determination of the ultimate ductility ε _pr by the method of dividing grids (calculation by formula (2)) and by measuring the thickness of the working part of the sample before and after the test (calculation by formula (4)) gave almost the same values of ultimate ductility, and its value averaged over the test results of three samples turned out to be equal to 0.69.

Thus, the presented experimental data allow us to conclude that it is possible to implement, with a sufficient degree of accuracy, the proposed method for testing biaxial sheet materials.

The proposed method allows to determine with high accuracy and reliability the characteristics of the mechanical properties of sheet materials under conditions of uniform biaxial tension deformation. The proposed method can be used, in particular, to build a hardening curve, to establish the ultimate stable deformation and ultimate ductility of sheet material with biaxial uniform tension, which are necessary for constructing diagrams of the maximum formability of the material and used in the design of technological processes for metal forming. Using the proposed test method will determine the necessary characteristics of the mechanical properties of sheet materials used in various industries, by conducting tests in the mechanical laboratories of industrial enterprises and research institutes.

Information sources

1. Avdeev B.A. Technique for determining the mechanical properties of materials. M: Engineering. 1965. S.391-397.

2. AS of the USSR 688859, cl. G01N 3/08, 09/30/79. BI No. 36.

Claims (1)

A method of testing sheet materials for biaxial tension, namely, that a disk-shaped sample is placed on a matrix, loaded with a punch located coaxially with the matrix, and ultimate plasticity is determined, characterized in that the sample is loaded with an elastic punch until it breaks, on the forming surface of which the beginning of the test is applied anti-friction coating.