RU2620781C1 - Sheet material test method - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: separating grid is applied to the workpiece from the tested sheet material, the workpiece is stacked into the device, the workpiece edge is clamped between the die and the clamp, the workpiece is mould by punch to destruction, and the points are constructed on the diagram of limiting deformations from the grid separating measurement results after testing. From the tested sheet material, a workpiece is cut with dimensions exceeding the dimensions available on the rift holder in plan. Under the workpiece, a temporary spacer is put of the same size as the blank from the material which plasticity parameters are not lower than the ductility of the tested workpiece and the hole in the temporary spacer is made the diameter smaller than the punch diameter. The contacting surfaces of the temporary spacer and the workpiece are degreased, therebetween, powder of rosin type is poured. Before forming, between the punch and the temporary spacer, directly above the punch end inside the rift in plan, anti-friction gasket is placed of such dimensions that, during the test process, the temporary spacer is touching the punch surface only after the anti-friction gasket. The punch end is made flat and undercut and with a radiused edge. On the testing double-acting machine with lower drive and two outer and inner sliders, the hard workpiece clamping edge is made as a rift in plan over the circle, concentric circular contour of the punch in plan, during the outer slider moving up. The workopiece is mould with the bottom up through the anti-friction gasket and temporary spacer by the punch during the internal slider moving up.

EFFECT: reduction of labour, time and cost of building the forming limit diagram of sheet materials, reduced time and improved quality of process design and tooling for sheet stamping, receive savings of sheet material by reducing scrap rates when debugging processes, as well as simplification of the sheet material selection and equipment for parts sheet stamping.

6 cl, 1 dwg

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 stampability, as the possibility of obtaining plastic deformations without destroying a sheet billet obtained from a sheet material, on forming operations of sheet stamping, and also for use in CAD / CAE- systems (Computer-Aided-Design / Computer-Aided-Engineering-systems) for computer modeling and design of form-changing sheet metal stamping operations before their implementation in the automotive and others aslyah industry.

Known methods for testing sheet materials by applying a dividing grid to the workpiece from the test sheet material, laying the workpiece in the device, clamping the edge of the workpiece between the die and the clamp, forming the workpiece with a punch before breaking and plotting points on the ultimate strain diagram (DPD) from the results of measuring the dividing grid after tests (Romanovsky V.P. Handbook of cold stamping. - L .: Publishing house "Mashinostroenie", 1979, p. 500, Fig. 397). The disadvantages of the known methods: the lack of rigid clamping of the edge of the workpiece with the help of rifts before molding, which requires increased dimensions of the workpiece for testing; do not use modern anti-friction pads, providing more points on the DPD.

A known method of constructing a diagram of ultimate strains and a device for its implementation according to patent RU No. 2134872, priority from 08/20/1999, in which a block assembled from a workpiece with a clamp and a matrix is installed in the container and the workpiece is deformed with a steel shot with a diameter of 0.5-1, 5 mm with a punch in the power plant. A disadvantage of the known patent is that it requires the use of special expensive equipment and a long period of testing and building a DPD.

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.

The objectives of the invention are to reduce the complexity, timing and cost of building DPD sheet materials, reducing time and improving the design quality of technological processes and tooling for sheet stamping, saving sheet material by reducing the percentage of scrap during debugging of technological processes, as well as simplifying the choice of sheet material and equipment for sheet stamping parts, such as car body parts and other equipment.

The problems are solved as follows. In the method of testing sheet materials, which consists in applying a dividing grid to the workpiece from the test sheet material, laying the workpiece in the device, clamping the edge of the workpiece between the die and the clamp, forming the workpiece with a punch before breaking, and plotting points on the limit strain diagram based on the results of measuring the dividing grid after tests, from the test sheet material, the workpiece is cut with overall dimensions exceeding the overall dimensions of the rift available on the clamp in plan, under the blank they lay a process gasket of the same size as the workpiece, of a material whose ductility is not lower than the ductility of the test workpiece, and the hole in the process gasket is made with a diameter smaller than the diameter of the punch, the contact surfaces of the process gasket and the workpiece are degreased and a powder of type rosin, before molding between the punch and the process gasket directly above the end of the punch inside the rift in the plan lay anti-friction a gasket of such overall dimensions that, during the test, the technological gasket touches the surface of the punch only through this anti-friction gasket, while the end face of the punch is flat with undercut and with a radius rounded off on a double-acting testing machine with a lower drive and two external and internal sliders rigid clamping of the edge of the workpiece is performed by a rift, in the plan around the circumference, concentric to the round contour of the punch in the plan, while the outer slider moves upward, the workpiece is molded to pulp is carried out upside down through the antifriction gasket and the technological gasket by the punch during the course of the internal slide up.

The anti-friction liner, in particular, can be used as a type of film made of polyethylene or Teflon. The blank may have, in particular, a round, rectangular, square or other shape.

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 box-shaped or body parts, stretching or tightening sheets:

с определенным запасом P_d пластичности по деформациям; при заданной абсциссе ε₂ принимают ординату ε₁ до ДПД за 1;1) destruction as a result of 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

with a certain margin of P _d deformation ductility; for a given abscissa ε ₂ take the ordinate ε ₁ to DPD for 1;

с определенным запасом P_s пластичности по напряжениям; ДПН строят с помощью ДПД по уравнениям связи между деформациями и напряжениями; ДПН соответствует предельному эллипсу пластичности σ₁ ²-σ₁σ₂+σ₂ ²=σ_s ².2) failure due to stresses: points with the coordinates of the principal stresses σ ₁ and σ ₂ should be located below the diagram of ultimate stresses (DP) of the sheet material

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 yield stress σ _s depending on the strain rate ε _i = ln (1 + δ _p ) is calculated taking into account the strengthening of the workpiece by the formula (Zharkov VA Modeling in the Marc system of material processing in mechanical engineering. Part 7. Tensile testing and editing. - Engineering Bulletin, 2013, No. 3, pp. 43-48):

σ _s = σ _0.2 + σ _in (1 + δ _p ) ε _i ⁿ , n = ln {1-σ _t / [σ _in (1 + δ _p )]} / ln [ln (1 + δ _p ) ],

where the yield strength σ _0.2 , the tensile strength σ _in and the relative uniform elongation δ _p to start the formation of the neck on the sample is determined according to GOST 11701-84 "Metals. Tensile test methods for thin sheets and ribbons".

The DPD of the sheet material in the form of a functional dependence ε ₁ = f (ε ₂ ) is built on the points, the base points are obtained by this method of testing for uniaxial and biaxial deformation of round, rectangular, square or other shapes of blanks cut from the same sheet material using the device containing a punch, a matrix and a clamp, and the overall dimensions of the workpieces exceed the overall dimensions of the rift located on the clamp, and a hard clamp on the edge of the workpiece is performed along the entire length of the rift. For different test parameters, different points on the DPD are obtained.

. Диаметр ячеек подбирают таким образом, чтобы после испытания вблизи места разрыва заготовки окружности превращались в овалы или эллипсы с малой осью симметрии длиной

и большой осью симметрии длиной

, а толщина s_f заготовки плавно увеличивалась в направлении от места разрыва контуру заготовки по нормали к линии разрыва. При этом сдвиговые деформации и касательные напряжения в направлении малой и большой осей овала равны нулю, вследствие чего линейные деформации ε₁ и ε₂ и напряжения σ₁ и σ₂ соответственно в направлении большой и малой осей овала являются главными. Третье главное напряжение σ₃ в направлении толщины листового материала равно нулю. Оси овалов

и

измеряют и рассчитывают

и

в центре ячейки. Третью главную деформацию ε₃=ln(s_f/s₀) рассчитывают или по результатам измерений толщины s_f в центре ячейки, или из условия ε₁+ε₂+ε₃=0 несжимаемости листового материала: ε₃=-ε₁-ε₂. Если измеряют все три деформации ε₁, ε₂ и ε₃, то условие несжимаемости используют для оценки точности измерений.On a workpiece of thickness s ₀ , cells of a dividing grid are applied, usually in the form of circles with a diameter

. The diameter of the cells is selected so that, after testing near the rupture point, the workpieces of the circle turn into ovals or ellipses with a small axis of symmetry of length

and a large axis of symmetry of length

and the thickness s _{f of the} workpiece gradually increased in the direction from the point of tearing, the contour of the workpiece normal to the line of tear. 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 major. The third principal stress σ ₃ in the thickness direction of the sheet material is zero. Axis ovals

and

measure and calculate

and

in the center of the cell. The third main strain ε ₃ = ln (s _f / s ₀ ) is calculated either from the results of measurements of the thickness s _f 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.

; в положительном направлении вертикальной оси ординат - наибольшую деформацию

, причем из условия ε₁+ε₂+ε₃=0 несжимаемости листового материала следует, что из трех деформаций ε₁, ε₂ и ε₃ как минимум одна деформация во время пластического деформирования листового материала имеет положительное значение. Так как разрушение заготовки в процессе испытания или заготовки из листового материала в процессе штамповки детали может происходить только вследствие утонения, то всегда s_f<s₀, и деформации δ_s,f=(s_f-s₀)/s₀, ε₃=ln(s_f/s₀)=ln(1+δ_s,f) ячейки вблизи места разрыва заготовки или заготовки всегда будут иметь отрицательные значения.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 deformation

; in the positive direction of the vertical ordinate axis - the greatest strain

moreover, from the condition ε ₁ + ε ₂ + ε ₃ = 0 of the incompressibility of the sheet material, it follows that of 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 of the part can occur only due to thinning, then always s _f <s ₀ , and deformation δ _{s, f} = (s _f -s ₀ ) / s ₀ , ε ₃ = ln (s _f / s ₀ ) = ln (1 + δ _{s, f} ) cells near the break point of the workpiece or workpiece will always have negative values.

The left half of the DPD for ε ₂ <0 corresponds to uniaxial tension with compression of the sheet material elements, the axis ε ₂ = 0 to plane deformation, the right half of the DPD for ε ₂ > 0 to biaxial tension of the sheet material elements.

In production, 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 from the nets by deformations, compared with DPD, determining the ductility 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 workpieces by grids is replaced or combined with CAD / CAE modeling, for example, in the Marc system of MSC Software Corporation (USA) or in the AutoForm program of AutoForm Engineering GmbH (Switzerland), which also requires DPS.

The essence of the test tooling for a testing machine is shown in FIG. 1, to the left of the vertical axis - before the test, to the right - after breaking the workpiece in the form of a through-through crack: 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 rounding of the edge radius r _m , 3 - clamp, 4 - rift, 5 - billet, 6 - antifriction film, 7 - process gasket with a hole with a diameter of D _h .

On a double-acting test machine with a lower drive and two external and internal sliders, a method for testing sheet materials by molding a workpiece upside down in a device with a punch with a flat bottom, die and clamp is implemented as follows. From the test sheet material, a round, rectangular, square or other shape blank 5 is cut with minimum overall dimensions in plan view from above, exceeding the overall dimensions of rift 4 in plan. On the workpiece on its central part with a diameter of D = 2R, which after rigid clamping by rift 4 is changed by a punch and a matrix, a dividing grid is applied to measure it before the test and after testing and calculating the maximum deformations of the workpiece before breaking.

To improve biaxial tension before breaking the central part of the workpiece, a technological gasket 7 is placed under the workpiece of the same size as the workpiece, from a material whose ductility is in the form of relative uniform elongation and elongation after breaking according to GOST 11701-84 "Metals. Test methods for tensile thin sheets and tapes "not lower than the ductility of the test piece. If the preform is made of low carbon steel, the process gasket 7 is also made of low carbon steel of the same or more ductile grade. The hole in the process gasket 7 with a diameter D _h smaller than the diameter D _{p of the} punch 1 is deburred and polished so that during testing the edges of the gasket openings do not break, and only the central part of the workpiece breaks above this hole. The contacting surfaces of the process gasket 7 and the workpiece 5 are degreased and rosin-type powder is poured between them in order to increase friction, so that the gasket 7 due to friction contributes to the greatest possible deformation of the central part of the workpiece 6 to failure.

To reduce friction between the technological gasket 7 and the punch 1, an anti-friction gasket 6 is laid in the form of a thin film of polyethylene or Teflon of such dimensions that during the test the technological gasket 7 touches the surface of the punch 1 only through this antifriction gasket 6. On the upper and lower surfaces of the antifriction gaskets 6 apply lubricant.

If the forces of the testing machine on which the tooling with the workpiece is installed are insufficient for forming the grooves with the rift 4 simultaneously on the workpiece 5 and on the process gasket 7, then molding is performed on the same test machine before the test or alternately on the process gasket 7 and workpiece 5 or on a separate press in the same equipment. Camcorders 8 record the shape of the workpiece and transmit information to a computer to build a DPD.

Rigid clamping of the edge of the workpiece 5 is performed by a rift 4 on the clip 3 in the plan in a circle concentric with the round contour of the punch 1 in the plan. The preform 5 is formed up to rupture into the hole of the matrix 2 with a diameter of D _m upside down through the antifriction film 6 and the process gasket 7 by the punch 1, the end face of which is flat with a rounded radius r _p and an undercut in the center to reduce friction between the antifriction film 6 and the end the punch 1 and due to this ensure biaxial tension of the Central part of the workpiece 5 to fracture. After clamping the edge of the workpiece during testing, only the central part of the workpiece with a diameter of D = 2R is deformed, while outside this diameter the workpiece is not deformed. Therefore, the shape of the workpiece contour can be any one determined from the conditions of saving sheet material and the simplicity of cutting a workpiece from this sheet material, if only the workpiece contour everywhere goes beyond the rift contour 4 in plan.

The punch 1 is fixed on the inner slider, and the clamp 3 on the outer slider of the testing machine. During the upward movement of the outer slider with the clamp 3, this clamp 3 with the rift 4 rigidly clamps the edge of the workpiece, after which the outer slider stops. In the subsequent upward stroke of the inner slider with the punch 1, this punch 1 performs the molding of the central part of the workpiece to failure. The test process is observed from above through the opening of the matrix 2 visually or using video cameras 8 and a computer connected with them, and at the beginning of fracture, which is characterized by the appearance of a crack visible on the lumen across the entire thickness of the workpiece and the formation force fixed by the instruments, the test is stopped.

This test method reduces the complexity, timing and cost of building the DPD of sheet materials, reduces time and improves the quality of the design of technological processes and tooling for sheet stamping, saves sheet material by reducing the percentage of scrap during debugging of technological processes, and also greatly simplifies the choice of sheet material and equipment for sheet stamping of parts, for example, car body parts and other equipment.

Claims (13)

1. The method of testing sheet materials by applying a dividing grid to the workpiece from the test sheet material, laying the workpiece in the device, clamping the edge of the workpiece between the die and the clamp, forming the workpiece with a punch before breaking, and plotting points on the ultimate strain diagram based on the results of measuring the dividing grid after the test, characterized in that from the test sheet material, a workpiece is cut with overall dimensions exceeding the overall dimensions of the rift available on the clip, in plan, under the workpiece, lay a technological gasket of the same size as the workpiece, from a material whose ductility is not lower than the ductility of the tested workpiece, and the hole in the technological gasket is made with a diameter smaller than the diameter of the punch, the contacting surfaces of the process gasket and the workpiece are degreased and a rosin-type powder is poured between them, Before molding, between the punch and the process gasket directly above the end of the punch inside the rift, an antifriction gasket of such overall dimensions is laid in a plan that, during the test, the technology gasket touches the surface of the punch only through this antifriction gasket, while the end face of the punch is flat with an undercut and with a rounded radius edge on a double-acting test machine with a lower drive and two external and internal sliders, the hard clamp of the workpiece edge is performed with a rift in a plan around a circle concentric with the round contour of the punch in plan, with the outer slider moving upwards, forming the workpiece before breaking is carried out upside down through the antifriction gasket and the technological gasket by the punch during the course of the internal slide up. 2. The method according to p. 1, characterized in that a round billet is cut from the test sheet material. 3. The method according to p. 1, characterized in that the rectangular blank is cut from the test sheet material. 4. The method according to p. 1, characterized in that a square-shaped blank is cut from the test sheet material. 5. The method according to p. 1, characterized in that they lay the anti-friction liner made of polyethylene. 6. The method according to p. 1, characterized in that they lay the anti-friction pad of Teflon.

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