RU2537341C2 - Method of determining properties of deformation - Google Patents

Abstract

FIELD: measurement technology.

SUBSTANCE: flat sample of round shape is loaded with elastic punch in round rigid matrix in several stages, at each of which the intensity of the stress and the intensity of deformations is determines, and the diagram of actual stresses is made.

EFFECT: accuracy and reliability of the test is increased by determining the deformations and stresses in a wide range of plastic deformations directly based on the experimental data.

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Description

The invention relates to the processing of metals by pressure, in particular to the determination of technological parameters of processes (stresses, strains).

A known method of determining the diagrams of true stresses of metallic materials by mechanical tensile testing [1]. In this case, a so-called machine diagram in the force-elongation coordinates is obtained, on the basis of which, by means of a special graphic construction and calculation, a true stress diagram (hardening diagram) is obtained in coordinates of the true stress-strain.

The determination of such diagrams encounters great difficulties associated with calculating the true cross-sectional area of the test sample, corresponding to the tensile force at a given time, especially from the moment of deformation localization (neck formation).

Closest to the proposed technical solution is a method for determining the properties of deformation, presented in the patent [2].

In this method, a plate-shaped sample is subjected to free bending on two supports by a cylindrical punch and the resulting machine diagram is compared with similar calculation diagrams obtained by selecting various models of piecewise-linear approximation of true stress diagrams.

The disadvantage of this method is that it allows you to build a diagram of true stresses only with relatively small plastic deformations.

The claimed technical solution is aimed at improving the accuracy and reliability of the test, as well as expanding its technological capabilities.

This is achieved by the fact that in the method for determining the deformation properties, a true stress diagram is built in the region of both small and large plastic strains established directly from experimental data.

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

The method is as follows. A sample 1 in the form of a disk is cut from a sheet of the material to be studied, a dividing grid is applied to one of the working surfaces of it. Then the sample is placed on a mirror of a rigid round matrix 2. To the container 3, in which the elastic punch 4 is placed, a press force is applied and the sample is drawn in stages to a predetermined depth. Due to the use of an elastic medium as a punch material during testing, a high uniformity of pressure distribution on the working part of the sample is ensured, and thus biaxial uniform tension is realized within it. In this case, the sample is clamped to the matrix mirror along its entire contour, and the clamping force increases during deformation.

After each stage of the test, the sample is removed from the matrix and its dimensions are measured. In the process of drawing an elastic punch into a rigid round matrix, the flat blank is transformed into a spherical segment for which the main radii of curvature in the circumferential and meridional directions are equal

$$R=\frac{h^2+{(c-r)}^2}{2h},(\mathrm{one})$$

where h is the height of the spherical segment;

r is the radius of the fillet of the sample (radius of the exhaust rib of the matrix);

c is the sum of the hemisphere of the spherical segment and the fillet radius.

The pressure intensity q on the sample from the side of the elastomer and the size of the sample calculate the stress intensity

$${\sigma }_0=q\frac{R}{2t}.(2)$$

Here t is the thickness of the sample at the pole of the spherical segment;

$$q=\frac{\mathrm{four}P}{\pi D_k^2},(3)$$

where P is the deforming force;

D _k is the diameter of the container with an elastic medium.

The deformed state of the formed spherical segments is established by the method of dividing grids. According to the initial and final sizes of the cells of the dividing grid, at each drawing step, the circumferential ε _t and meridional ε _m strains are determined, and the strain intensity is calculated

$${\epsilon }_0=\frac{2}{\sqrt{3}}\sqrt{{\epsilon }_t^2{\epsilon }_m^2{\epsilon }_t{\epsilon }_m}(\mathrm{four})$$

and build a diagram of true stresses in the coordinates of the stress intensity σ ₀ - strain intensity ε ₀ .

The implementation of the proposed method will allow, in comparison with the known technical solution, to increase the accuracy and reliability of determining the properties of deformation and expand its technological capabilities.

An example of a specific implementation of the proposed method.

The tests were carried out on flat circular samples with a diameter of 210 mm and a thickness of 1.5 mm from aluminum alloys D16AM and D16AT. The extract was carried out on a test machine ЦД40 in a special exhaust stamp with an elastic punch in a circular rigid matrix with a diameter of 70 mm with a radius of the exhaust rib of 10 mm. As the elastomer used technological rubber brand 3826 with an initial hardness of 70HSD. To reduce the effect of friction forces on the test results and ensure uniform biaxial tension between the sample and the elastomer, a 0.4 mm thick fluoroplastic film was placed with good antifriction properties. For the same purpose, a 0.1 mm thick polyethylene film was placed between the sample and the matrix.

To determine the strain intensity before the test, a dividing grid was applied to the surface of the samples by a photocontact method from a system of intersecting circles with a diameter of d ₀ = 2.6 mm. Since the stress-strain state of the spherical segments obtained by stretching the sheet blanks with an elastic medium into a round rigid matrix is close to biaxial uniform stretching, when establishing a deformed state they were limited to determining the strains of three cells located near the pole of the sphere. For this, after drawing using a BMI-1 instrumental microscope, with an accuracy of 0.005 mm, the dimensions a, b of the cells of the deformed dividing grid were measured in the circumferential and meridional directions, respectively.

The circumferential ε _t and meridional ε _m strains were determined using the relations $${\epsilon }_t=\ln (a/d_0);{\epsilon }_m=\ln (b/d_0).(5)$$

Then, according to the formula (4), the strain intensity ε _{0 was} calculated.

The pressure q from the side of the elastic medium on the sample was determined by the value of the deforming force P for each stage of deformation by the formula (3). Having determined the sizes t, R obtained by drawing a spherical segment, using the formula (2), we determined the stress intensity value hundred and built true stress diagrams for the tested materials in the region of both small and large plastic deformations. The limiting values of the strain intensity in the proposed method for the studied materials (ε _{spec ≈} 0.8) significantly exceeds their level achieved when tested under uniaxial tension (ε _spec ≈ 0.2). The use of such diagrams will improve the accuracy of designing sheet metal stamping operations and forecasting technological failures of the deformation type.

The proposed method allows to increase the accuracy and reliability of determining the properties of deformation. Using the proposed method provides the ability to determine the true stress diagrams in a wide range of plastic deformations and allows you to replace the known method of determining true stress diagrams using tensile tests of flat samples. The proposed method does not require the use of special equipment and, due to the simplicity of its implementation, can be used in mechanical laboratories of industrial enterprises.

Information sources

1. GOST 1497-84. Metals Tensile test methods. M., Standartinform, 2005.

2. RF patent 2020013, cl. B21D 5/00. 09/30/94, BI No. 18.

Claims (1)

A method for determining stresses during deformation of parts made of sheet metal materials, including constructing a diagram of true stresses under elastic-plastic impact on a flat sample with a cylindrical punch, characterized in that a dividing grid is applied to one of the working surfaces of the flat sample in the form of a disk and it is stretched by an elastic punch in a round rigid matrix with an exhaust rib with the formation of a spherical segment in several stages, ensuring the sample is pressed against the matrix mirror along the entire circuit, and at each stage, the intensity of the stresses is determined by the magnitude of the deforming force, and the strain rate, which is used to build a true stress diagram, is determined by the method of dividing grids.