RU2659458C1 - Method for testing sheet materials to axisymmetrical drawing - Google Patents

Abstract

FIELD: technological processes.

SUBSTANCE: invention relates to sheet stamping, in particular to the sheet materials mechanical properties study for their formability evaluation, as well as to the use of shaping sheet-forming operations in CAD/CAE-systems for computer modeling and design. Essence: cutting the circular workpiece from the tested sheet material. Applying the dividing grid onto the workpiece. Placing the workpiece into the testing machine tooling. Before stretching between the workpiece and the punch and the clamp, directly above the punch end, placing the antifriction gasket from film with such overall dimensions that during the testing workpiece touches the punch and clamp surfaces only through this antifriction gasket. On the double-acting testing machine with bottom drive and two outer and inner sliders workpiece flange pressing is performed by the tooling clamping with the outer slider upward stroke with minimum pressing force. Axisymmetric part drawing from the workpiece is performed with the bottom upwards using the antifriction gasket on the punch stroke, by its end, made flat with the undercut, with a radially rounded edge in the tooling matrix with the inner slider upward stroke. When carrying out tests, the overall dimensions of the blank are gradually increased to the maximum dimensions, at exceeding of which the drawing by the passage of the punch in the matrix leads to the detachment of the bottom of the part from the wall. Producing the machine diagram for the drawing force and the pressing force dependence along the drawing stroke. Calculating the limiting coefficient of drawing, elongated part relative height along the largest cavity on the part wall, maximum drawing force and minimum pressing force, near the dangerous section of the part with the minimum thickness from the change in the dividing grid, deformations are determined and the values of these strains are plotted on the limit deformation diagram.

EFFECT: technical result: the ability to construct additional points in the limit deformation diagram (LDD), and thereby increase the LDD accuracy and the LDD application accuracy in CAD/CAE-systems, reduces the time and improves the quality of the technological processes and tooling design for sheet punching, saves the sheet material by reducing the percentage of rejects during the technological processes adjustment, and also simplifies the sheet material and equipment selection for sheet stamping parts, for example, of cars and other equipment sheet-stamping parts.

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Description

The invention relates to the field of sheet stamping, in particular, to the study of the mechanical properties of sheet materials to assess their formability as the possibility of obtaining plastic deformations without destroying the sheet billet obtained from sheet material in form-forming operations of sheet stamping, as well as for plotting points on the diagram of ultimate strains ( DPD) and use in CAD / CAE-systems (Computer-Aided-Design / Computer-Aided-Engineering-systems) in computer modeling and designing sheet-forming operations forming before their introduction in the automotive and other industries.

A known method of testing sheet materials for exhaustion by laying a workpiece from the test sheet material in the device, pressing the workpiece flange between the die and the clamp and drawing the cylindrical cap with a punch in the matrix (Romanovsky V.P. Handbook of cold stamping. - L .: Publishing House "Mechanical Engineering" , 1979, p. 500, Fig. 397, c). The disadvantages of this method: modern anti-friction linings between the punch and the workpiece are not used, which provide large deformations of the workpiece and more points on the DPD, a dividing grid is not applied to the workpiece in order to determine deformations near the point of separation of the bottom of the stretched part from the wall.

A known method of constructing a diagram of ultimate strains on the basis of relative uniform elongation δ _p according to GOST 11701-84 "Metals. Test methods for tensile thin sheets and tapes" (Zharkov VA Modeling in the Marc system of material processing in mechanical engineering. Part 7. Testing and stretching editing. - Vestnik mashinostroeniya, 2013, No. 3, pp. 43-48). However, this method does not take into account the fact that δ _p at the beginning of neck formation on the sample is much less than the relative elongation at the end of neck formation and near the point of rupture of the sample.

Known tests for the molding of samples cut from sheets and blanks (Zharkov V.A. Testing of materials. Forming of sheets, tapes and strips. - Vestnik mashinostroeniya, 2017, No. 1). The drawback of these tests of samples and blanks cut from sheets is that the nature of the deformations near the fracture site of the workpiece when it is stretched, molded, or pulled by a punch into the matrix, when the rigidly clamped edge of the workpiece does not move to the die hole with only shallow and thinner blanks low parts, does not correspond to the nature of deformations near the place of destruction of the workpiece during drawing, for example, sheet-stamped axisymmetric parts widely used in industry, when paradise preform is moved towards the opening of the matrix to obtain deep and high detail. Therefore, to construct diagrams of ultimate strains (DPD), thinning (DPF), strain intensities (DPID) and stress intensities (DPIN) and stresses (DPN), it is necessary to additionally test sheets for axisymmetric drawing of parts with a limited number of sheets known from the certificate of the supplier of sheets (without DPD, DPU, DPID, DPIN, DPN) of mechanical properties according to GOST 11701-84.

The objective of the invention is to determine the rational parameters of the drawing of axisymmetric parts in production, supplementing the DPD of sheet materials with points based on tests for axisymmetric drawing, more accurate use of DPD in CAD / CAE systems, reducing time and improving the quality of design of technological processes and equipment for sheet stamping, obtaining savings in sheet material by reducing the percentage of rejects during the debugging of technological processes, as well as in simplifying the choice of sheet material and equipment sheet metal stamping of parts, for example, sheet-stamped car parts and other equipment.

The problem is solved as follows. Two criteria are used to determine the likelihood of a sheet blank being destroyed in form-forming sheet stamping operations, such as drawing or molding complex parts such as axisymmetric, box or body, stretching or tightening of sheets:

1) destruction due to deformations: at each stage of deformation of the sheet stock, the points with the coordinates of the smallest principal strain ε ₂ and the largest principal strain ε ₁ for all elements of the sheet stock should be located below the DPD of the sheet material ε ₁ = ƒ (ε ₂ ) with a certain margin P _d deformation ductility; for a given abscissa ε ₂ take the ordinate ε ₁ to DPD for 1;

2) failure due to stresses: points with coordinates of the principal stresses σ ₁ and σ ₂ must be located inside the sheet metal sheet σ ₁ = ƒ (σ ₂ ) with a certain margin of stress plasticity P _s ; DPN is built using DPD according to the equations of connection between strains and stresses; DPN corresponds to the ultimate plasticity ellipse σ ₁ ² -σ ₁ σ ₂ + σ ₂ ² = σ _S ² .

The method of testing sheet materials for axisymmetric drawing consists in laying the workpiece from the test sheet material into a snap on a testing machine, pressing the workpiece flange between the die and the clamp, drawing out the workpiece with a punch in the matrix, and differs from that of the test sheet material a round billet is cut out, a dividing grid is applied to the billet, before the hood between the billet and the punch and the clamp, directly against the end of the punch, lay antifriction A gasket such as a film of polyethylene or Teflon without an aperture and of such overall dimensions that, during the test, the workpiece touches the surfaces of the punch and clamp only through this antifriction gasket, on a double-acting testing machine with a lower drive and two external and internal sliders, the workpiece flange is pressed by pressing snap during the stroke of the outer slider with the minimum, only to eliminate folding of the flange during the hood, by pressing force, drawing an axisymmetric part from the back The lugs are carried out upside down using an antifriction gasket for the passage with a punch, its end face made flat with an undercut, with a radius rounded along the radius, into the tooling matrix during the upward slide of the slide, during testing, the overall dimensions of the workpiece are gradually increased to the maximum dimensions, exceeding which the exhaust to the passage by the punch into the matrix leads to the separation of the bottom of the part from the wall, a machine diagram of the dependence of the drawing force and the pressing force along the drawing is obtained, the limit is calculated the first draw ratio, the relative height of the elongated part by the maximum depression parts on the wall, the maximum drawing force and the minimum clamping force near the dangerous section of a detail from a minimum thickness to change the pitch and deformation of the mesh is determined values of these deformations are applied to the diagram limiting deformations.

In a particular case, an axisymmetric part with a wide flange is drawn to the limit depth with a displacement of the edge of the workpiece from an out-of-workpiece blank with an out-of-draw ratio, after which the bottom is separated from the wall of the draw-out part; near the separation point, deformations are determined by the change in the dividing grid and the values of these deformations are applied to the diagram of ultimate strains; at the same time, the dependence of the maximum drawing depth to the destruction of the workpiece on the prohibitive coefficient of drawing is built.

An anti-friction gasket with a hole above the punch end is also used to improve the tension of the workpiece opposite this hole.

The yield stress σ _S depending on the strain intensity ε _i = ln (1 + δ _p ) is calculated taking into account the strengthening of the workpiece according to the formula V. Zharkov (Zharkov VA Modeling in the Marc system of material processing in mechanical engineering. Part 7. Testing and editing by stretching. - Bulletin of mechanical engineering, 2013, No. 3, p. 43-48):

, n=ln{1-σ_0,2/[σ_B(1+δ_p)]}/ln[ln(1+δ_p)],

, n = ln {1-σ _0.2 / [σ _B (1 + δ _p )]} / ln [ln (1 + δ _p )],

where the yield strength σ _0.2 or σ _T , the tensile strength σ _B and δ _p to start the formation of the neck on the sample is determined according to GOST 11701-84.

The DPD of the sheet material in the form of a functional dependence ε ₁ = ƒ (ε ₂ ) is built on the points, the base points are obtained, including using this method of testing for an axisymmetric hood. For different test parameters, different points on the DPD are obtained.

To build a DPD, a blank mesh cells are applied to a workpiece of thickness s ₀ , usually in the form of circles with a diameter of l ₀ . The diameter of the cells is selected so that after testing near the break point of the workpiece, the circles turn into ovals or ellipses with a small axis of symmetry of length l _min and a large axis of symmetry of length l _max , and the thickness s _{ƒ of the} workpiece gradually increases in the direction normal to the break of the workpiece contour to the break line. In this case, the shear deformations and tangential stresses in the direction of the minor and major axes of the oval are equal to zero, as a result of which the linear strains ε ₁ and ε ₂ and stresses σ ₁ and σ _2, respectively, in the direction of the major and minor axes of the oval are the main ones. The third principal stress σ ₃ in the thickness direction of the sheet material is zero. The axis of the ovals l _min and l _max measure and calculate ε ₁ = ln (l _max / l ₀ ) and ε ₂ = ln (l _min / l ₀ ) in the center of the cell. The third principal strain ε ₃ = ln (s _ƒ / s ₀ ) is calculated either from the results of measuring the thickness s _ƒ in the center of the cell or from the condition ε ₁ + ε ₂ + ε ₃ = 0 of the incompressibility of the sheet material: ε ₃ = -ε ₁ -ε ₂ . If all three strains ε ₁ , ε ₂ and ε ₃ are measured, then the incompressibility condition is used to evaluate the accuracy of the measurements.

To build a DPD on a grid of a rectangular coordinate system, they postpone: in the positive and negative directions of the horizontal abscissa axis, the smallest strain ε ₂ = ln (l _min / l ₀ ); in the positive direction of the vertical ordinate axis, the greatest strain ε ₁ = ln (l _max / l ₀ ), and from the condition ε ₁ + ε ₂ + ε ₃ = 0 of the incompressibility of the sheet material, it follows that the three strains ε ₁ , ε ₂ and ε _3, at least one deformation during plastic deformation of the sheet material has a positive value. Since the destruction of the workpiece during the test or the workpiece made of sheet material during the stamping process of the part can occur only as a result of thinning, always s _ƒ <s ₀ , and deformation δ _{s, ƒ} = (s _ƒ -s ₀ ) / s ₀ , ε ₃ = ln (s _ƒ / s ₀ ) = ln (1 + δ _{s, ƒ} ) cells near the break point of the workpiece or workpiece will always have negative values.

The left half of the DPD at ε ₂ <0 corresponds to uniaxial stretching of samples from sheet material and the compression of the workpieces by this method for axially symmetric drawing, the axis ε ₂ = 0 to plane deformation, the right half of DPD at ε ₂ > 0 to biaxial stretching during molding sheet material.

In production, in order to improve accuracy and quality, as well as to assess the formability of the part, a dividing grid is applied to the workpiece, after stamping in hazardous places, the parts are calculated on the nets by deformations, compared with DPD, determining the plasticity margin before failure and, if necessary, measures are prescribed to reduce deformations in hazardous places and reduce the percentage of defects during the debugging of technological processes. Often, the calculation of deformations of the workpiece by grids is replaced or combined with CAD / CAE modeling, for example, in the Marc CAD / CAE system of MSC Software Corporation (USA) or in the AutoForm program of AutoForm Engineering GmbH (Switzerland); CAD / CAE modeling also requires DPD.

The essence of this method of testing for an axisymmetric hood using equipment for a testing machine is shown in FIG. 1-4, to the left of the vertical axis - before the test, to the right - after drawing the axisymmetric part: 1 - a punch with a flat end face with a diameter of D _p and a rounding of the radius of radius r _p , 2 - a matrix with a hole with a diameter of D _m and with a rounding of radius r _m , 3 - clamp, 4 - workpiece, 5 - anti-friction gasket without a hole (Fig. 1 and Fig. 2) and with a hole with a diameter of D _h (Fig. 3 and Fig. 4), 6 - video camera.

On a standard double-acting test machine with a lower drive and two external and internal sliders, the method of testing sheet materials for axisymmetric drawing of a workpiece upside down in a device with a punch with a flat bottom, die and clamp is implemented as follows. A round blank 4 of diameter D = 2R is cut from the test sheet material. Alternatively, cut a rectangular, square or other shape blank, the contour of which is described around a circle of diameter D. A dividing grid is applied to the blank to measure it before the test and after testing and calculating the maximum deformations of the blank before breaking.

To reduce friction and due to this, scratches and scuffs on the workpiece when moving it along the stamp tools during drawing, to improve biaxial tension and increase deformations of the central part of the workpiece above the end of the punch 1 between the surfaces of the punch 1, matrix 2 and clamp 3 contacting the workpiece, lay anti-friction a gasket 5 in the form of a thin film, for example, of polyethylene or teflon, of such overall dimensions that, during the test, the surface of the workpiece 4 touches the surface of the punch 1 and clamp 3 only black of the anti-friction gasket 5. The gasket 5 antifriction used as without holes (FIG. 1 and FIG. 2) and with a hole diameter D _h of the end of the punch 1 (FIG. 3 and FIG. 4) to further stretch the blank opposite this opening .

On the test machine, pressing the workpiece flange is performed by pressing the tool during the external slide upward with a minimum, only to eliminate the folding of the flange during the hood, by pressing force Q. The workpiece is drawn upside down through the antifriction gasket to the passage of the punch into the tool matrix during the internal slide upward movement.

The test process is observed from above through the hole of the matrix 2 visually or using video cameras 6 and related computers and at the beginning of the destruction, which is characterized by the appearance of first small cracks that are invisible to the lumen in a dangerous place with a minimum thickness, and then one crack is visible on the lumen the entire thickness of the workpiece and the drop in the forming force fixed by the devices, the test is stopped.

During the tests, the overall dimensions of the workpiece are gradually increased to the maximum size, when exceeded, the exhaust to the passage by the punch into the matrix leads to the separation of the bottom of the part from the wall. A machine diagram of the dependence of the drawing force and the pressing force along the drawing line is obtained. The limiting drawing coefficient, the relative height of the elongated part along the largest depression on the part wall, the maximum drawing force and the minimum pressing force are calculated. Near a dangerous section of a part with a minimum thickness, deformations are determined by a change in the dividing grid and the values of these deformations are applied to the diagram of ultimate strains.

The end face of the punch is flat with undercut and with a rounded radius on the edge.

Camcorders 6 record the shape change of the workpiece and transmit information to a computer to build those points of the DPD that allows you to get this test method.

From the limit blank, tests are carried out for an axisymmetric hood according to the first paragraph of the formula (Fig. 1. Fig. 3) with a diameter of D_limwhen reliable drawing of the part with the maximum height H is still possible_lim ^''= H_lim/ [(d_one+ d₂) / 2] = H_lim/ d₁₂= H_lim/ [(D_m+ D_p) / 2] a punch to the passage into the matrix with a limiting drawing coefficient K_lim= D_lim/ [(d_one+ d₂) / 2] = D_lim/ d₁₂= D_lim/ [(D_m+ D_p) / 2] and the ultimate drawing force F_lim= F_one-Q, where d_one - maximum (for cylindrical, conical and stepped parts) the outer diameter of the wall of the extruded part between the roundings at the bottom and flange (if there is a flange), equal to the diameter of the matrix D_m, d₂ - the minimum internal diameter of the wall of the part, equal to the diameter of the punch D_p, average diameter of the part d₁₂= (d_one+ d₂) / 2, F_one - stamping force F_one= F_lim+ Q for press and stamp instruments or for the calculations below in engineering theory and in the CAD / CAE system. If at K_lim to perform an incomplete (not in the passage) hood, you get a part with a narrow flange.

In the particular case (Fig. 2, Fig. 4), from a prohibitive workpiece with a prohibitive drawing coefficient, drawing a part with a wide flange is possible only to the maximum depth h _lim ^' with a shift of the workpiece edge from D to D _ƒ at K _ƒ = D / D _ƒ , after which there is a separation of the bottom from the wall of the drawn part with the force F _ƒ = F _{1, ƒ} -Q _ƒ ; Q or Q _ƒ - the minimum pressing force from folding of the workpiece flange, previously determined from the reference books and specified during testing. Near the separation point, the coordinates of the points ε ₁ , ε ₂ are determined by changing the dividing grid and these points are applied to the DPD. At the same time, the dependence h = ƒ (K) is constructed for K≥K _lim , and if K = K _lim , then h = H _lim .

In addition to or instead of the DPD (if there is no DPD), the depth of the part to be drawn with the flange h ^' = h / d ₁₂ = h / [(D _p + D _m ) / 2] or without the flange H ^' = H / d ₁₂ = H / [(D _p + D _m ) / 2], taking into account the trimming of the uneven edge of the part after drawing in production, is compared with h _lim ^' or H _lim ^' obtained as a result of full-scale tests using this method or CAD / CAE modeling. Moreover, H _lim ^' is determined by the largest depression on the part wall, H _lim ^'' by the largest protrusion, and the degree of unevenness of the height, called festoon formation, is χ = (H _lim ^'' -H _lim ^' ) / H _lim ^' ⋅ 100% at anisotropy coefficient V. Zharkov Z _θ = (ε ₃ -ε ₂ ) / (0.5ε ₁ ) 100%, where θ is the angle of inclination of the major axis of the oval of the mesh cell after testing to the direction of sheet rolling (Zharkov V.A. Material tests. Classification and theory // Herald of mechanical engineering. 2016. No. 6. S. 51-58). If P = (1-h ^' / h _lim ^' ) 100%> P _lim or P = (1-H ^' / H _lim ^' ) 100%> P _lim , then the part is recommended for manufacturing at the factory, where P _lim is the stock or reserve plasticity of the workpiece, approximately (depending on the differences in test and production conditions) equal to the relative uniform elongation of the workpiece δ _p according to GOST 11701-84. The dimensions of the workpiece are first determined by reference from the condition of equality of the median surface of the part and the workpiece, then the possibility of drawing the part without destroying the workpiece is evaluated based on the test results of this method, and then it is specified during CAD / CAE modeling and debugging of the die for drawing.

In all tests, changes in the dividing grid previously applied to the workpiece are recorded in order to build the DPD points ε ₁ = ƒ (ε ₂ ), as well as parameters: color and surface roughness of the workpiece before and after failure, type of crack, h _lim ^' , H _lim ^' , K _lim , K _ƒ , α, K _lim '= [(D _lim -d ₁₂ ) / D _lim ] 100% = {[D _lim - (D _m + D _p ) / 2] / D _lim } 100%, minimum thickness s _{min of the} workpiece near the fracture site, forces F _lim , F _ƒ , F ₂ , Q, F _{2, ƒ} , Q _ƒ , their ratios F _lim / F _ƒ , (F _ƒ -F _lim ) / F _ƒ , folding of the workpiece and the danger of folds from the flange to the walls of the extruded part; Ali after removal from the die.

If an expensive stamp is required for cutting round blanks, then a cut of rectangular blanks can be performed on guillotine shears, moreover, such a cut provides sheet saving by eliminating jumpers compared to cutting. If the corners of the square billet are placed in the direction in which the angles of the square billet are laid in the direction where the maximum depression is formed on the wall of the elongated part due to anisotropy of the sheet, then the limiting height of the part can be increased by N percent after trimming the uneven edge of the part wall; determination of N is one of the objectives of this test. As the parameters of the hood change, various points on the DPD are obtained.

The method is also used to study the effect of various lubricants and coatings in combination with various anti-friction linings on the maximum depth of the part. Comparing the drawing coefficient K = D / [(d ₁ + d ₂ ) / 2] = D / [(D _p + D _m ) / 2] and the depth h ^' = h / [{d ₁ + d ₂ ) / 2] = h / [(D _p + D _m ) / 2] the shape changes, the deformation field and stresses of the stamped part, on which the dividing grid was previously applied, with the limiting values K _lim , h _lim ^' = h _lim / [(d ₁ + d ₂ ) / 2], the fields and the DPD after testing the preforms by this method for failure, determine the plasticity margin in the hazardous areas of the stamped part. If the supply is insufficient, design and technological measures are being developed to improve the formability of the part. The dimensions of the working parts of the equipment depend on the characteristics of the testing machine, the thickness s and the dimensions of the parts stamped during testing. The one-way gap between the punch and the matrix is c _pm ≈s√ K _lim , but if anti-friction linings are used, then c _pm increases by the thickness of these linings; r _p and r _m are equal to several s and are chosen so that the maximum drawing depth is maximum, ceteris paribus. The method is also intended to establish rational drawing parameters K _lim , D _p , D _m , r _p , r _m , c _pm , Q for groups of parts and sheets.

This test method allows you to build additional points on the DPD and thereby increase the accuracy of the DPD and the accuracy of the use of DPD in CAD / CAE systems, reduces time and improves the quality of the design of technological processes and equipment for sheet stamping, saves sheet material by reducing the percentage of rejects during the debugging of technological processes, and also simplifies the choice of sheet material and equipment for sheet stamping of parts, for example sheet-stamped parts of cars and other equipment.

Claims (15)

1. The method of testing sheet materials for axisymmetric drawing by stacking the workpiece from the test sheet material in a snap on a testing machine, pressing the workpiece flange between the die and the clamp, drawing the workpiece with a punch in a matrix, characterized in that a round blank is cut from the test sheet material, a dividing grid is applied to the workpiece, the workpiece is placed in the rig of the testing machine, before the hood between the workpiece and the punch and the clamp, directly above the end of the punch, lay the antifriction gasket from the film with such overall dimensions that during the test the workpiece touches the surfaces of the punch and clamp only through this antifriction gasket, on a double-acting test machine with a lower drive and two external and internal sliders, the pressing of the workpiece flange is performed by pressing the tool during the course of the external slide up with minimum clamping force, the axisymmetric part is pulled out of the workpiece upside down using an antifriction gasket on the passage with a punch, its end face made flat with undercut, with a radius rounded off in the tool matrix during the inner slide upward stroke, during the tests, the overall dimensions of the workpiece are gradually increased to the maximum size, when exceeded, the exhaust to the passage by the punch into the matrix leads to the separation of the bottom of the part from the wall, get a machine diagram of the dependence of the drawing force and the pressing force along the drawing, calculate the maximum drawing coefficient, the relative height of the elongated part along the largest depression on the part wall, the maximum drawing force and minimum clamping force, near a dangerous section of a part with a minimum thickness, deformations are determined by a change in the dividing grid and the values of these deformations are applied to the diagram of ultimate strains. 2. The method according to p. 1, characterized in that the axially symmetric part with a wide flange is drawn to the maximum depth with a displacement of the edge of the workpiece from the translucent preform with an exorbitant drawing coefficient, after which the bottom separates from the wall of the extruded part; near the separation point, deformations are determined by the change in the dividing grid and the values of these deformations are applied to the diagram of ultimate strains; at the same time, the dependence of the maximum drawing depth to the destruction of the workpiece on the prohibitive coefficient of drawing is built. 3. The method according to p. 1, characterized in that the antifriction gasket is performed with a hole above the end of the punch. 4. The method according to p. 1, characterized in that an antifriction gasket made of polyethylene is placed over the end of the punch. 5. The method according to p. 1, characterized in that an antifriction gasket made of Teflon is placed over the end of the punch.

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