A metal sheet bulging test device and method capable of realizing continuous rotation of the stress principal axis

Abstract

The present invention belongs to the field of performance testing of metal sheets under complex non-linear loading conditions, and provides a bulging test device and method for metal sheets that can achieve continuous rotation of the stress principal axis. By changing the aspect ratio value of the ellipse of the upper die cross-section of the rotating female die and the directions of the major and minor axes, the present invention changes the constraint boundary conditions received by the poles of the metal sheet during the bulging process, thereby realizing the changes in the in-plane principal and secondary stress directions and magnitudes at the poles of the metal sheet during bulging. Through the device and method of the present invention, an accurate constitutive model of the metal sheet can be established under conditions closer to the actual forming process; the rotation path of the stress principal axis direction, the change path of the magnitude, and the initial angle between the metal sheet material direction and the stress principal axis direction can all be adjusted according to actual requirements.

Description

Technical Field

[0001] The present invention relates to the field of performance testing of metal sheets under complex non-linear loading conditions, and particularly to a bulging test device and method for metal sheets capable of realizing continuous rotation of the stress principal axis. Background Art

[0002] With the significant improvement of requirements for lightweight, reliability and long life in the fields of aerospace and automotive industries, the demand for using integral shaped thin-shell components to replace traditional block-by-block welded and assembled structures is becoming increasingly urgent. To achieve the forming of complex shaped thin-walled integral components, it is often necessary to undergo complex deformation processes. During this process, changes in the shape, size and boundary conditions of the blank result in a complex non-linearity of the loading path: while the stress path continuously changes, the direction of the stress principal axis also undergoes continuous and complex rotations. In addition, due to the directionality of the manufacturing process, actual thin sheet materials have a certain degree of anisotropy. Therefore, different material directions exhibit different deformation characteristics and forming performances when matched with different loading paths. Therefore, to accurately describe and characterize the deformation characteristics of the sheet during complex forming processes and to guide actual forming, it is necessary to obtain corresponding stress, strain and other data through tests that are as close as possible to the conditions of the blank and die constraints during actual forming.

[0003] For the testing of the deformation performance of thin sheets, the currently mainly used methods include simple proportional loading, such as unidirectional tension / compression method, circular / elliptical cross-section die bulging method, etc., and non-proportional loading developed on this basis, such as bi-axial loading cross-tension method and stepped die bulging method (Patent No. 201611240021.6), etc. The above simple proportional loading methods obtain the performance of the thin sheet under a certain specific and unchanging plane stress state, where neither the stress path nor the direction of the stress principal axis changes, which is very different from the complex loading process experienced by the sheet during actual deformation. In non-proportional loading, the bi-axial loading cross-tension method changes the tensile force and tensile speed in two mutually perpendicular directions on the cruciform specimen, so that the ratio of the two principal in-plane stresses changes during the deformation process. However, when the equivalent strain exceeds 10% or even a smaller value, the specimen has seriously deviated from its original shape, the deformation is extremely unstable and uncontrollable, and the subsequent stress-strain data obtained from the deformation is severely distorted and unusable, and the stress and strain information in the non-stable deformation and final failure and destruction stages of the material cannot be obtained. However, the determination of the deformation behavior of the sheet in this stage is crucial for the evaluation of its forming ability and the determination of the forming process.

[0004] As an improved new non-proportional loading method, the stepped female die bulging method (patent number 201611240021.6) uses a stepped female die with a continuously changing cross-sectional shape along the bulging height direction for bulging, so that the stress ratio in the middle deformation zone of the metal sheet blank continuously changes during the whole bulging process to obtain non-proportional loading deformation. Due to the continuous stability of the bulging loading process and the intuitive visibility of CCD camera data acquisition, this method can obtain sheet metal deformation data within a larger equivalent strain range, and the stress and strain information in the non-stable deformation stage in the later period of the test and the final failure stage can also be collected in real time. Therefore, this method provides the possibility for a more comprehensive evaluation of the performance of metal sheets. However, this method can only change the stress ratio by changing the ratio of the major and minor axes of each stepped female die, and cannot realize the rotation of the stress principal axis in the sheet metal bulging area, so it cannot obtain the sheet metal deformation performance under the condition of continuous rotation of the stress principal axis, and cannot accurately characterize the influence of the continuous and complex changes of the material direction and stress direction during actual forming on the material forming performance, and its application prospect is restricted.

[0005] In addition, the original thin sheet used for forming complex-shaped thin-walled parts usually has initial anisotropy. After undergoing deformation with continuously changing boundary conditions during the forming process, both the degree and direction of anisotropy will change. In the aforementioned existing test methods, only the initial included angle between the material direction before deformation and the stress principal axis direction is set, and the included angle during subsequent deformation cannot be actively and accurately controlled, which is quite different from the actual deformation situation during forming. Therefore, the aforementioned test methods can only obtain the test data of the yield and flow behavior of the sheet metal under a fixed principal stress ratio or a simple variable stress ratio path, and cannot be used to guide the complex forming process in which the principal stress ratio and the stress principal axis in the plane change continuously and non-linearly at the same time.

[0006] In the existing liquid pressure bulging device for metal sheets, the lower surface of the lower die of the bulging female die is often directly in contact with the upper surface of the force-applying piston of the sealing force-applying device. When the parallelism of the two surfaces is low, it will directly lead to sealing failure during the bulging process and the experiment fails. When the wall thickness of the metal sheet blank is relatively thick, a large blank-holding force is required for sealing, and at this time, a very high-tonnage force-applying piston is required. In addition, the lower die is prone to tipping when the piston descends, and the safety and stability of the bulging device are relatively low.

[0007] To sum up, in order to accurately describe the deformation behavior of metal sheets under the non-linear loading condition of stress principal axis rotation, measure the stress and strain data during the whole process, be used to evaluate the forming performance of metal sheet materials under complex loading conditions, guide the determination of actual forming process parameters, and improve the safety, stability and sealing ability of the bulging device, it is necessary to establish a metal sheet bulging test device and method that can realize continuous rotation of the stress principal axis. Summary of the Invention

[0008] The present invention is proposed to solve the problems that the existing performance testing methods for metal sheets can only change the stress ratio of the in-plane measurement points by changing the simple loading force or boundary conditions, but cannot achieve the rotation of the stress principal axis, thus unable to obtain non-proportional loading material performance parameters that are closer to the actual forming process and can be used to accurately guide the determination of forming process parameters, and the low safety stability and sealing ability of the existing bulging devices. Furthermore, a metal sheet bulging test device and method capable of realizing continuous rotation of the stress principal axis are proposed.

[0009] The technical principle of the present invention is described as follows:

[0010] By changing the axis length ratio value of the ellipse of the cross-section of the rotating female die upper die and the directions of the major and minor axes, the present invention changes the constrained boundary conditions of the pole of the metal sheet during the bulging process, thereby realizing the change of the in-plane principal and secondary stress directions and magnitudes at the pole of the metal sheet during bulging. According to the geometric characteristics of the cavity of the rotating female die upper die, when the pole of the metal sheet bulges to any height in the rotating female die, the boundary constraint shape it receives is an ellipse, and at this time, the geometric shape of the bulging area can be approximated as a rotating ellipsoid. To illustrate the technical principle of the rotating female die, the following will be explained through the force analysis diagram and the outer contour diagram of the bulging area when the vertex of the metal sheet bulging area shown in FIGS. 5(a) and 5(b) bulges to a height of h in the rotating female die and the height difference between the constraint boundary of the rotating female die upper die and the cavity entrance is H.

[0011] Approximating the geometric shape of the metal sheet during the bulging process as a rotating ellipsoidal surface, with the rotation axis of the ellipsoidal surface parallel to the major axis direction of the elliptical constraint boundary of the rotating female die upper die, a force analysis is performed on the pole P of the metal sheet surface to obtain the force balance equation:

[0012]

[0013] Among them, p is the bulging pressure at this moment, and t is the pole wall thickness. ρ θ and are respectively the radii of curvature at the pole along the major axis and minor axis directions of the cavity of the rotating female die upper die, l θ and are respectively the lengths at the pole along the x and y directions, and σ θ and are respectively the stress components at the pole along the major axis and minor axis directions.

[0014] Simplifying Equation (1), the Laplace equation is obtained:

[0015]

[0016] Then, the in-plane principal stress σ1 and in-plane secondary stress σ2 at the pole of the metal sheet bulging area can be calculated by the following formula:

[0017]

[0018] As can be seen from the above formula (2), when the metal thin plate is bulged in the rotating female die, the directions of the principal and secondary in-plane stresses at the pole depend on the directions of the major and minor axes of the boundary ellipse at that instant. Therefore, by rotating the major and minor axes of the rotating boundary ellipse in the bulging height direction, the rotation of the directions of the principal and secondary in-plane stresses at the pole can be achieved; by changing the axis length ratio of the ellipse, the ratio of the principal and secondary in-plane stresses at the pole can be changed.

[0019] In addition, according to the volume invariance assumption, the wall thickness at point P can be calculated by the following formula:

[0020]

[0021] where t0 is the initial wall thickness of the metal thin plate. and are the strains in the directions of the major and minor axes of the cross-sectional ellipse boundary of the model cavity on the rotating female die at each instant, which can be obtained by recording with a data acquisition system.

[0022] The technical solution of the present invention is as follows: A metal thin plate bulging test device capable of realizing continuous rotation of the stress principal axis, comprising a rotating female die upper die 5, a rotating female die lower die 6, a sealing force application system 18, a pressure medium output system 15, a data acquisition system 1, a control system 13 and a support frame;

[0023] The control system 13 is respectively connected to the data acquisition system 1, the pressure medium output system 15 and the sealing force application system 18;

[0024] The data acquisition system 1 is located above the support frame and is used for collecting images;

[0025] Inside the support frame, the rotating female die upper die 5, the rotating female die lower die 6, the hydraulic pressure plate 7, and the hydraulic pressure plate backing plate 8 are arranged in sequence from top to bottom; the hydraulic pressure plate 7 and the hydraulic pressure plate backing plate 8 are moved up and down through the pressure plate ejection mechanism 11 and the backing plate ejection mechanism 10 respectively; one end of the pressure medium output system 15 is connected to the hydraulic pressure plate 7 through the pressure medium output high-pressure hose 14 for transmitting the high-pressure medium 19 for bulging, and the other end is connected to the control system 13 through the pressure sensor 16 for transmitting the pressure value; the sealing force application system 18 is connected to the pressure plate ejection mechanism 11 and the backing plate ejection mechanism 10 respectively through the sealing force application liquid medium high-pressure hose 17 for transmitting the sealing force application liquid medium 21; a cavity is opened inside the rotating female die upper die 5, and the cavity is a through elliptical hole that gradually shrinks from bottom to top in the height direction and gradually rotates in the horizontal plane in the long and short axis directions. The axis length ratio of the elliptical hole remains unchanged or continuously changes simultaneously according to the material performance test requirements; one end of the rotating female die upper die 5 is connected to the support frame, and it is fixedly connected to the upper backing plate of the support frame with bolts; on the outer edge of the other end, a circular semi-circular groove and a serrated concave-convex groove are arranged in sequence; the rotating female die lower die 6 is installed on the hydraulic pressure plate backing plate 8, and a semi-circular protruding rib and a serrated concave-convex groove are arranged on its outer edge, which are used in cooperation with the rotating female die upper die 5. The metal thin plate 23 is placed between the rotating female die upper die 5 and the rotating female die lower die 6 and pressed tightly to achieve blank holding on the metal thin plate 23 during the bulging deformation.

[0026] The hydraulic pressure plate 7 is a stepped cylinder with a through hole opened inside, and the through hole is communicated with the pressure medium output high-pressure hose 14; the hydraulic pressure plate 7 includes an upper large-diameter cylinder and a lower small-diameter cylinder; an O-ring groove 20 is opened on the upper surface of the upper large-diameter cylinder for placing an O-ring; after the metal thin plate is clamped and blank held, when the hydraulic pressure plate 7 moves upward and contacts the metal thin plate 23 and continues to apply pressure, the O-ring deforms for sealing; the lower small-diameter cylinder is nested in the stepped hole provided on the hydraulic pressure plate backing plate 8 to prevent the hydraulic pressure plate 7 from slipping out and tipping over when the piston of the pressure plate ejection mechanism 11 descends too low, so as to improve the stability of the overall device.

[0027] The design purpose of the hydraulic pressure plate backing plate 8 is to fix the rotating female die lower die 6, and it can still provide a reliable blank holding force when the wall thickness of the plate to be tested is relatively thick or circumferentially uneven, and improve the sealing ability of the device.

[0028] The metal thin plate bulging test device capable of realizing continuous rotation of the stress principal axis further includes a heating system 22, which is arranged on the outer edge of the rotating female die lower die 6; after the induction coil of the heating system 22 is wound with heat insulation material, it is placed on the hydraulic pressure plate backing plate 8 through the insulating material; when high-temperature bulging is adopted, the high-pressure medium 19 for bulging is a high-pressure gas medium.

[0029] The support frame includes an upper backing plate 4, a lower backing plate 12, support tie rods 9, an acrylic plate 2, and a spacer block 3. A trapezoidal through-hole window larger than the maximum cross-section of the cavity of the rotary female die upper die 5 is opened in the center of the upper backing plate 4 for the CCD camera of the data acquisition system 1 to capture the speckles in the bulging area on the surface of the metal sheet. The acrylic plate 2 is placed above the upper backing plate 4 through the spacer block 3 to prevent the liquid from splashing and contaminating and damaging the lens of the CCD camera when the metal sheet bursts. The upper backing plate 4 and the lower backing plate 12 are connected by the support tie rods 9. The upper backing plate 4 and the lower backing plate 12 have the same outer dimensions and are provided with countersunk threaded holes around, and are locked with the support tie rods 9 through hexagon socket head cap screws.

[0030] The cavity of the rotary female die upper die 5 faces the CCD camera lens of the data acquisition system 1 and is used to collect the deformation data of the vertex of the bulging area of the metal sheet 23 in real time during the bulging process. The control system 13 is connected to the data acquisition system 1. The control system 13 issues start and end commands for data acquisition to the data acquisition system 1, and stores and post-processes the data.

[0031] The pressure medium output system 15 is started under the command issued by the control system 13, and conveys the high-pressure medium 19 for bulging at a specific pressure to the hydraulic filling plate 7 through the pressure medium output high-pressure hose 14 according to the preset pressure increase curve to conduct a bulging test.

[0032] A metal sheet bulging test method capable of realizing continuous rotation of the stress principal axis is as follows:

[0033] Step 1: Design the rotary female die upper die 5 and the rotary female die lower die 6 according to the change requirements of the two principal stress directions and numerical values at the vertex of the bulging area of the metal sheet. Use the theoretical model and the numerical simulation model to calculate the principal stresses at the vertex during the bulging test, and determine the cavity shape and size of the rotary female die upper die 5 and the rotary female die lower die 6, the pressure change path of the bulging, and the blank holding force.

[0034] Step 2: Manufacture the rotary female die upper die 5 and the rotary female die lower die 6, and apply a lubricant inside the cavity of the rotary female die upper die 5 to reduce the influence of friction on the stress in the bulging area. Cut the metal sheet according to the outer diameter dimensions of the rotary female die upper die 5 and the rotary female die lower die 6, and spray speckles on one side of the metal sheet surface.

[0035] Step 3: Connect the control system 13 to the sealing force application system 18, the data acquisition system 1, and the pressure medium output system 15; connect the pressure medium output system 15 and the hydraulic pressure application plate 7 through the high-pressure hose 14 for pressure medium output; set the hydraulic change path and the blank holding force during the test in the control system 13; place the metal sheet 23 between the upper die 5 of the rotating die and the lower die 6 of the rotating die according to the initial direction in which the material direction of the metal sheet 23 forms a set angle with the major axis at the cavity entrance of the upper die 5 of the rotating die, with the speckle side facing the upper die 5 of the rotating die, and start the sealing force application system 18; the hydraulic pressure application plate spacer 8 moves upward, and the metal sheet deforms under the action of the semi-circular protruding ribs of the lower die 6 of the rotating die and the semi-circular grooves of the upper die 5 of the rotating die to complete blank holding; the hydraulic pressure application plate 7 moves upward, continues to apply pressure after its upper surface contacts the metal sheet 23, and the O-ring deforms to achieve sealing; at the same time, start the spacer ejecting mechanism 10 and the pressure plate ejecting mechanism 11 of the sealing force application system 18. The sealing force application system 18 collects the blank holding force and transmits it to the control system 13. After the blank holding force value displayed by the control system 13 reaches the preset value, the spacer ejecting mechanism 10 and the pressure plate ejecting mechanism 11 stop operating; arrange the data acquisition system 1 directly above the window of the upper backing plate 4 for data acquisition;

[0036] Step 4: Operate the control system 13 and start the pressure medium output system 15 to enable the metal sheet 23 to stably undergo stress principal axis continuous rotation bulging deformation under the action of the high-pressure medium 19 for bulging. The bulging pressure is recorded and saved in real time in the control system 13; the data acquisition system 1 synchronously records the displacement, strain, wall thickness, and contour curvature of the vertex of the bulging area during the entire test process;

[0037] Step 5: When the deformation degree of the vertex of the bulging area and the high-pressure liquid pressure reach the preset maximum value, or after the metal sheet bursts, operate the control system 13 to end the test and relieve the pressure, turn off the CCD camera, and save the test data; the hydraulic pressure application plate 7 moves downward, and the bulged metal sheet 23 is disassembled and taken out;

[0038] Step 6: Substitute the strain and curvature radius of the vertex of the metal sheet bulging area obtained during the bulging test using the rotating die, as well as the corresponding bulging pressure, into the theoretical model to calculate and obtain the stress-strain curve and the change curve of the Lode coefficient, etc. of the vertex of the metal sheet bulging area under the rotation path of the specific principal stress direction, and establish the constitutive equation.

[0039] The beneficial effects of the present invention are:

[0040] 1. The metal sheet bulging test device and method capable of realizing continuous rotation of the stress principal axis proposed by the present invention can establish an accurate constitutive model of the metal sheet under conditions closer to the actual forming process. During actual forming, the boundary constraint conditions received by the metal sheet change in real time, and the material direction and stress principal axis direction within the sheet surface also change complexly simultaneously. In the current forming of complex-shaped thin-walled components, constitutive models established through tests under fixed principal stress ratios or simple variable stress ratio paths are widely used to guide parameter determination. However, the new bulging test method proposed by the present invention can realize continuous rotation of the stress principal axis based on variable stress ratios, thus being closer to the actual forming process. The constructed constitutive model will significantly improve the guiding accuracy for actual forming.

[0041] 2. For the metal sheet bulging test device and method capable of realizing continuous rotation of the stress principal axis proposed by the present invention, the rotation path of the stress principal axis direction, the change path of the magnitude, and the initial angle between the metal sheet material direction and the stress principal axis direction can all be adjusted according to actual needs, can be arbitrarily selected, with high flexibility and a wide range of achievable parameters. Only through the design of the shape and size of the elliptical cross-section of the mold cavity on the rotating female die and the setting of the metal sheet material direction, the in-plane principal and secondary stress directions and magnitudes at the vertex of the sheet bulging area can be flexibly changed. In addition, in terms of the design of the mold cavity on the rotating female die, the major and minor axis directions and the axis length ratio of the cross-sectional ellipse can be flexibly selected within a large range of 0 - 60° and 0.4 - 1 respectively; in terms of the metal sheet material direction, based on the initial rolling direction of the sheet, the angle between the material direction at any angle to it and the major axis direction of the upper mold cavity inlet can be arbitrarily selected within 0 - 90°. The above parameter setting ranges cover the stress ratio, stress direction, and the change range of the angle between the material and the mold direction experienced by the metal sheet in the forming of most complex-shaped thin-walled components. Therefore, the present invention can meet the parameter setting requirements in a wide range.

[0042] 3. The metal sheet bulging test device and method capable of realizing continuous rotation of the stress principal axis proposed by the present invention have strong adaptability, can be applied to a wide variety of metal materials, and have good prospects for popularization and application. For metal billets with good plasticity and thin wall thickness, normal temperature hydraulic bulging can be directly carried out; for metal billets with poor plasticity, thick wall thickness, or the need to test at different temperatures, based on the normal temperature bulging device, a heating device is added outside the mold, and the liquid pressure medium is changed to gas to carry out hot gas pressure bulging, and the non-linear loading deformation behavior data of the stress principal axis rotation of the metal sheet at different temperatures can be obtained.

[0043] IV. The combination form of the rotating female die lower die, the blank holding pressure plate, the blank holding pressure plate backing plate, the pressure plate ejection mechanism and the backing plate ejection mechanism proposed by the present invention can prevent the blank holding pressure plate from slipping out and tipping over when the piston of the pressure plate ejection mechanism descends too low, and can still provide a large blank holding force when the wall thickness of the plate to be tested is relatively thick or circumferentially uneven, which can significantly improve the safety stability and sealing ability of the overall device. In addition, the addition of the backing plate ejection mechanism can apply the blank holding force together with the pressure plate ejection mechanism during blank holding and sealing, reducing the tonnage requirement of the pressure plate ejection mechanism during the sealing of thicker plates.

[0044] V. The metal sheet bulging test device and method proposed by the present invention that can realize the continuous rotation of the stress principal axis can obtain the transfer situation of the hardening zone and the main deformation zone in the plate plane when the material direction and the stress principal axis direction change continuously and complexly by collecting the change information of the in-plane strain nephogram during the test, and reasonably set the process preforming steps accordingly to improve the wall thickness uniformity and comprehensive performance of the formed part. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Fig. 1(a) is a schematic diagram of the room temperature bulging principle of the metal sheet bulging test device that can realize the continuous rotation of the stress principal axis;

[0046] Fig. 1(b) is a sectional view of area A in Fig. 1(a);

[0047] Fig. 2(a) is a schematic diagram of the high temperature bulging principle of the metal sheet bulging test device that can realize the continuous rotation of the stress principal axis;

[0048] Fig. 2(b) is a sectional view of area B in Fig. 2(a);

[0049] Fig. 3(a) is a schematic diagram of the rotating male die upper die with an equal axis length ratio;

[0050] Fig. 3(b) is a perspective view of the rotating male die upper die with an equal axis length ratio at different bulging heights;

[0051] Fig. 3(c) is a schematic diagram of the elliptical cross-section at the cavity entrance of the rotating male die upper die with an equal axis length ratio, the height h0 from the cavity entrance is 0, and the rotation angle θ0 of the major axis of the ellipse is 0;

[0052] Fig. 3(d) is a schematic diagram of the elliptical cross-section at the cavity height h1 of the rotating male die upper die with an equal axis length ratio, the height h1 from the cavity entrance is h1>0, and the rotation angle θ1 of the major axis of the ellipse is θ1>0;

[0053] Fig. 3(e) is a schematic diagram of the elliptical cross-section at the cavity height h2 of the rotating male die upper die with an equal axis length ratio, the height h2 from the cavity entrance is h2>h1, and the rotation angle θ2 of the major axis of the ellipse is θ2>θ1;

[0054] Fig. 3(f) is a schematic diagram of the elliptical cross-section at the cavity outlet of the rotating male die upper die with an equal axis length ratio, the height h from the cavity entrance f>h2, the rotation angle θ of the major axis of the ellipse f >θ2;

[0055] Figure 4(a) is a schematic diagram of the upper die of the rotating die with variable axis length ratio;

[0056] Figure 4(b) is a perspective view of the upper die of the rotating die with variable axis length ratio at different bulging heights;

[0057] Figure 4(c) is a schematic diagram of the elliptical cross-section at the entrance of the cavity of the upper die of the rotating die with variable axis length ratio. The height h0 from the cavity entrance is 0, and the rotation angle θ0 of the major axis of the ellipse is 0;

[0058] Figure 4(d) is a schematic diagram of the elliptical cross-section at the height h1 of the cavity of the upper die of the rotating die with variable axis length ratio. The height h1 from the cavity entrance is h1>0, and the rotation angle θ1 of the major axis of the ellipse is θ1>0;

[0059] Figure 4(e) is a schematic diagram of the elliptical cross-section at the height h2 of the cavity of the upper die of the rotating die with variable axis length ratio. The height h2 from the cavity entrance is h2>h1, and the rotation angle θ2 of the major axis of the ellipse is θ2>θ1;

[0060] Figure 4(f) is a schematic diagram of the elliptical cross-section at the outlet of the cavity of the upper die of the rotating die with variable axis length ratio. The height h from the cavity entrance is f >h2, the rotation angle θ of the major axis of the ellipse f >θ2;

[0061] Figure 5(a) is a force analysis diagram of the vertex of the bulging area of the metal thin plate when the bulging height is h in the rotating die;

[0062] Figure 5(b) is an outline diagram of the bulging area of the vertex of the bulging area of the metal thin plate when the bulging height is h in the rotating die.

[0063] Among them, 1 is the data acquisition system, 2 is the acrylic plate, 3 is the spacer block, 4 is the upper backing plate, 5 is the upper die of the rotating die, 6 is the lower die of the rotating die, 7 is the hydraulic filling plate, 8 is the backing plate of the hydraulic filling plate, 9 is the support pull rod, 10 is the ejecting mechanism of the backing plate, 11 is the ejecting mechanism of the pressing plate, 12 is the lower backing plate, 13 is the control system, 14 is the high-pressure hose for outputting the pressure medium, 15 is the pressure medium output system, 16 is the pressure sensor, 17 is the high-pressure hose for sealing and applying the force of the liquid medium, 18 is the sealing and applying force system, 19 is the high-pressure medium for bulging, 20 is the O-ring groove, 21 is the sealing and applying force liquid medium, 22 is the heating system, 23 is the metal thin plate. Specific implementation manners

[0064] The following further describes the specific implementation manners of the present invention in conjunction with the accompanying drawings and technical solutions.

[0065] Embodiment 1: As shown in FIGS. 1 and 3, the metal sheet bulging test device and method capable of realizing continuous rotation of the stress principal axis are implemented according to the following steps:

[0066] Step 1: Design the upper rotary die 5 and the lower rotary die 6 according to the path change requirements of the two principal stress directions at the vertex of the metal sheet bulging area. Use a theoretical model to calculate the principal stresses at the vertex during the bulging test process, and establish a numerical model for simulation calculation. Thus, determine variables such as the included angle between the initial material direction and the cavity inlet of the upper rotary die 5, the equal ratio reduction ratio of the cavity cross-section of the upper rotary die 5 in the bulging height direction, the pressure change path of the high-pressure medium 19 for bulging, and the sealing blank-holding force jointly provided by the upper rotary die 5, the lower rotary die 6, and the hydraulic filling plate spacer 8. Check whether the strengths of the upper backing plate 4, the support tie rods 9, and the lower backing plate 12 meet the test requirements.

[0067] Step 2: Fabricate the upper rotary die 5 and the lower rotary die 6, and apply a lubricant inside the cavity of the upper rotary die 5 to reduce the influence of friction on the stress in the bulging area. Cut the test metal sheet 23 according to the outer diameter dimensions of the upper rotary die 5 and the lower rotary die 6. After cleaning the surface of the blank, spray speckles on one side thereof.

[0068] Step 3: Place the metal sheet 23 between the upper rotary die 5 and the lower rotary die 6 at a specific initial angle between the material direction of the metal sheet 23 and the major axis at the cavity inlet of the upper rotary die 5, and keep the speckle side facing the upper rotary die 5. Set the hydraulic change path and the blank-holding force during the test process in the control system 13, and connect the control system 13 to the data acquisition system 1, the pressure medium output system 15, and the sealing force application system 18. Operate the control system 13, start the spacer ejecting mechanism 10, the hydraulic filling plate spacer 8 moves upward, driving the lower rotary die 6 to move upward, and the metal sheet 23 deforms between the circular groove of the upper rotary die 5 and the circular rib of the lower rotary die 6 to achieve blank-holding. The hydraulic filling plate 7 moves upward, and after its upper surface contacts the metal sheet 23, continue to apply pressure, and the O-ring placed in the O-ring groove 20 deforms until sealing is achieved. At the same time, start the ejecting mechanism 10 of the sealing force application system 18 and the ejecting mechanism 11 of the pressing plate to apply the blank-holding force of the sealing force application system 18 and transmit it to the control system 13. After the blank-holding force value displayed by the control system 13 reaches the preset value, the spacer ejecting mechanism 10 and the pressing plate ejecting mechanism 11 stop operating; arrange the data acquisition system 1 directly above the window of the upper backing plate 4, and use the high-pressure hose 17 of the pressure medium output to connect the pressure medium output system 15 and the hydraulic filling plate 7.

[0069] Step 4: Operate the control system 13 and start the pressure medium output system 15, so that the metal sheet 23 undergoes stable stress principal axis continuous rotation bulging deformation under the action of the high-pressure medium 19 for bulging according to the preset pressure path, and the bulging pressure is recorded and saved in real time in the control system 13. The high-pressure medium 19 for bulging used in the normal temperature test is usually a liquid medium such as high-pressure water or oil. During the whole process of the test, data such as the displacement, strain, wall thickness, and contour curvature of the vertex of the bulging area are synchronously recorded using the data acquisition system 1.

[0070] Step 5: After the deformation degree of the vertex of the bulging area of the metal sheet 23 and the high-pressure liquid pressure reach the preset maximum value, or after bursting, operate the control system 13 to end the test, remove the high-pressure medium 19 for bulging in the pressure medium output system 15, turn off the data acquisition system 1, and save the test data. Start the seal application force system 18 to perform the unloading action, and the charging hydraulic plate 7 and the charging hydraulic plate backing plate 8 move downward simultaneously, driving the rotating female die 6 to move downward, and disassemble and take out the bulged metal sheet 23.

[0071] Step 6: Substitute the data parameters such as the strain and curvature radius of the vertex of the bulging area of the metal sheet 23 obtained during the bulging test using the rotating female die, and the corresponding bulging pressure, etc. into the theoretical model, calculate and obtain the stress-strain curve and the change curve of the Lode coefficient, etc. of the vertex of the bulging area of the metal sheet under the rotation path of the specific principal stress direction, and establish the constitutive equation. Save the strain nephogram of the bulging area and analyze the transfer situation of the hardening area and the main deformation area in the plane of the sheet during the bulging process.

[0072] The beneficial effects of this embodiment are as follows: The bulging test method and device for a metal sheet with continuous rotation of the stress principal axis can establish an accurate constitutive model of the metal sheet under conditions closer to the actual forming process, which will significantly improve the guiding accuracy for actual forming. The long and short axis directions of the cross-sectional ellipse can be flexibly selected within a large range of 0 to 60°, and the angle between the initial material direction and the long axis direction of the upper mold cavity entrance can be arbitrarily selected within 0 to 90°, which can meet the parameter setting requirements in a large range. This embodiment can collect the change information of the in-plane strain nephogram during the test, obtain the transfer situation of the hardening area and the main deformation area in the plane of the sheet when the material direction and the stress principal axis direction change continuously and complexly, and reasonably set the process pre-forming steps accordingly to improve the wall thickness uniformity and comprehensive performance of the formed part. The sealing form adopted in this embodiment can significantly improve the safety stability and sealing ability of the overall device, and significantly reduce the tonnage requirement for the platen ejection mechanism.

[0073] Embodiment 2: As shown in FIGS. 1 and 4, for the metal sheet bulging test device and method capable of realizing continuous rotation of the stress principal axis in this embodiment, on the basis of the equiaxial ratio rotating female die upper die in Embodiment 1, when designing the rotating female die upper die 5 in Step 1, the changes in the principal and secondary stress directions and magnitudes at the vertex of the bulging area of the metal sheet 23 are considered simultaneously, and the rotating female die upper die 5 is designed as a variable axis ratio rotating female die. Other steps and parameters are the same as those in Embodiment 1.

[0074] The beneficial effect of this embodiment is that: compared with the equiaxial ratio rotating female die upper die that can only realize the rotation of the stress principal axis at the pole of the bulging area proposed in Embodiment 1, the variable axis ratio rotating female die upper die proposed in this embodiment can also realize the synchronous change of the principal and secondary stress ratios within the range of 0.4 - 1 in the plane of the pole of the bulging area during the bulging process, broadening the application range of the rotating female die bulging test and being closer to the in-plane principal and secondary stress change path during the forming process of actual complex-shaped thin-walled components.

[0075] Embodiment 3: As shown in FIGS. 2 and 3, for the metal sheet bulging test device and method capable of realizing continuous rotation of the stress principal axis in this embodiment, on the basis of the test device set in Embodiment 1, a heating system 22 is additionally provided at the outer edge of the rotating female die lower die 6 in Step 2 of Embodiment 1. The induction coil of the heating system 22 is wound with heat insulation material and placed on the hydraulic filling plate spacer 8, and the two are separated by insulating material. Turn on the heating system 22, heat the die to the set temperature and keep it warm in the closed die state without placing the metal sheet. After the temperature is stable, open the die, place the metal sheet and seal it according to Step 3 of Embodiment 1. After the temperature displayed in the heating system 22 is stable at the target test temperature again, start the bulging test according to Step 4 of Embodiment 1. In this embodiment, the high-temperature bulging method is adopted, so the high-pressure medium 19 used for bulging in the test is a high-pressure gas medium. After the test, first turn off the heating system 22 and cut off the power supply, and then perform unloading and data processing according to other steps and parameters in Embodiment 1. Other steps and parameters are the same as those in Embodiment 1.

[0076] The beneficial effect of this embodiment is that: compared with the normal-temperature rotating female die bulging test proposed in Embodiment 1, an induction heating device is added in this embodiment, expanding the implementation temperature range of the rotating female die bulging test from normal temperature to high temperature. A high-temperature rotating female die bulging test method and device suitable for metal materials that are less plastic at normal temperature and usually require hot gas pressure bulging to produce parts are proposed, making the rotating female die bulging test method and device highly adaptable, applicable to a wide variety of metal materials, and having good prospects for popularization and application.

Claims

1. A metal sheet bulging test device capable of realizing continuous rotation of the stress principal axis, characterized in that The metal sheet bulging test device includes a rotating female die upper die (5), a rotating female die lower die (6), a sealing force application system (18), a pressure medium output system (15), a data acquisition system (1), a control system (13), and a support frame; The control system (13) is respectively connected to the data acquisition system (1), the pressure medium output system (15), and the sealing force application system (18); The data acquisition system (1) is located above the support frame and is used for acquiring images; Inside the support frame, the rotating female die upper die (5), the rotating female die lower die (6), a hydraulic filling plate (7), and a hydraulic filling plate backing plate (8) are arranged in sequence from top to bottom; the hydraulic filling plate (7) and the hydraulic filling plate backing plate (8) are respectively moved up and down through a press plate ejection mechanism (11) and a backing plate ejection mechanism (10); one end of the pressure medium output system (15) is connected to the hydraulic filling plate (7) through a pressure medium output high-pressure hose (14) for transmitting the high-pressure medium (19) for bulging, and the other end is connected to the control system (13) through a pressure sensor (16) for transmitting the pressure value; the sealing force application system (18) is respectively connected to the press plate ejection mechanism (11) and the backing plate ejection mechanism (10) through a sealing force application liquid medium high-pressure hose (17) for transmitting the sealing force application liquid medium (21); a cavity is opened inside the rotating female die upper die (5), and the cavity is a through elliptical hole that gradually shrinks from bottom to top in the height direction and gradually rotates in the horizontal plane in the long and short axis directions, and the axis length ratio of the elliptical hole remains unchanged or continuously changes simultaneously; one end of the rotating female die upper die (5) is connected to the support frame, and a circular semi-circular groove and a serrated concave-convex groove are sequentially arranged on the outer edge of the other end; the rotating female die lower die (6) is installed on the hydraulic filling plate backing plate (8), and semi-circular raised ribs and serrated concave-convex grooves are arranged on its outer edge and are used in cooperation with the rotating female die upper die (5), and the metal sheet (23) is placed between the rotating female die upper die (5) and the rotating female die lower die (6) and is clamped, and blank-holding of the metal sheet (23) is realized during the bulging deformation; The hydraulic filling plate (7) is a stepped cylinder with a through hole opened inside, and the through hole is communicated with the pressure medium output high-pressure hose (14); the hydraulic filling plate (7) includes an upper large-diameter cylinder and a lower small-diameter cylinder; an O-ring groove (20) is opened on the upper surface of the upper large-diameter cylinder for placing an O-ring; after the metal sheet is clamped and blank-held, the hydraulic filling plate (7) moves upward to contact the metal sheet (23) and then continues to apply pressure, and the O-ring deforms for sealing; the lower small-diameter cylinder is nested in a stepped hole provided on the hydraulic filling plate backing plate (8).

2. The sheet metal bulging test device capable of realizing continuous rotation of the stress principal axis according to claim 1, wherein It further includes a heating system (22), which is arranged on the outer edge of the rotating female die lower die (6); after the induction coil of the heating system (22) is wound with a heat-insulating material, it is placed on the hydraulic filling plate backing plate (8) through an insulating material; when high-temperature bulging is adopted, the high-pressure medium (19) for bulging is a high-pressure gas medium.

3. The metal sheet bulging test device capable of realizing continuous rotation of the stress principal axis according to claim 1 or 2, characterized in that, The support frame includes an upper backing plate (4), a lower backing plate (12), support tie rods (9), an acrylic plate (2), and spacer blocks (3); a trapezoidal through-hole window larger than the maximum cross-section of the cavity of the rotary female die upper die (5) is opened in the center of the upper backing plate (4) for the CCD camera of the data acquisition system (1) to capture the speckles in the bulging area on the surface of the metal sheet; the acrylic plate (2) is placed above the upper backing plate (4) through the spacer blocks (3) to prevent the liquid from splashing and contaminating and damaging the lens of the CCD camera when the metal sheet bursts; the upper backing plate (4) and the lower backing plate (12) are connected by the support tie rods (9).

4. The metal sheet bulging test device capable of realizing continuous rotation of the stress principal axis according to claim 3, characterized in that, The cavity of the rotary female die upper die (5) faces the CCD camera lens of the data acquisition system (1) for real-time acquisition of the deformation data of the vertex of the bulging area of the metal sheet (23) during the bulging process; the control system (13) is connected to the data acquisition system (1), and the control system (13) issues start and end commands for data acquisition to the data acquisition system (1), and stores and post-processes the data; The pressure medium output system (15) is started under the command issued by the control system (13), and conveys the high-pressure medium (19) for bulging at a specific pressure to the hydraulic filling plate (7) through the pressure medium output high-pressure hose (14) according to the preset pressure increase curve to conduct the bulging test.

5. A sheet metal bulging test method capable of realizing continuous rotation of the stress principal axis by using the sheet metal bulging test device described in claim 4, characterized in that, The specific steps are as follows: Step 1: Design the rotary female die upper die (5) and the rotary female die lower die (6) according to the change requirements of the two principal stress directions and numerical magnitudes at the vertex of the bulging area of the metal sheet, use the theoretical model and the numerical simulation model to calculate the principal stresses at the vertex during the bulging test process, and determine the cavity shape and size of the rotary female die upper die (5) and the rotary female die lower die (6), the bulging pressure change path, and the blank holding force; Step 2: Manufacture the rotary female die upper die (5) and the rotary female die lower die (6), apply a lubricant inside the cavity of the rotary female die upper die (5) to reduce the influence of friction on the stress in the bulging area; cut the metal sheet according to the outer diameter dimensions of the rotary female die upper die (5) and the rotary female die lower die (6), and spray speckles on one side of the surface of the metal sheet; Step 3: Connect the control system (13) to the sealing force application system (18), the data acquisition system (1), and the pressure medium output system (15); connect the pressure medium output system (15) and the hydraulic filling plate (7) through the high-pressure hose (14) for pressure medium output; set the hydraulic change path and blank holding force during the test in the control system (13); place the metal sheet (23) between the rotating female die upper die (5) and the rotating female die lower die (6) with the speckle side facing the rotating female die upper die (5) according to the initial direction that the material direction of the metal sheet (23) forms a set angle with the long axis at the cavity entrance of the rotating female die upper die (5), and start the sealing force application system (18); the hydraulic filling plate backing plate (8) moves upward, and the metal sheet deforms under the action of the semi-circular protruding ribs of the rotating female die lower die (6) and the semi-circular grooves of the rotating female die upper die (5) to complete blank holding; the hydraulic filling plate (7) moves upward, continues to apply pressure after its upper surface contacts the metal sheet (23), and the O-ring deforms to achieve sealing; at the same time, start the backing plate ejection mechanism (10) and the pressing plate ejection mechanism (11) of the sealing force application system (18), the sealing force application system (18) collects the blank holding force and transmits it to the control system (13), and the backing plate ejection mechanism (10) and the pressing plate ejection mechanism (11) stop operating after the blank holding force value displayed by the control system (13) reaches the preset value; arrange the data acquisition system (1) directly above the window of the upper backing plate (4) for data acquisition; Step 4: Operate the control system (13) and start the pressure medium output system (15) to enable the metal sheet (23) to stably undergo stress principal axis continuous rotation bulging deformation under the action of the high-pressure medium (19) for bulging, and the bulging pressure is recorded and saved in real time in the control system (13); the data acquisition system (1) synchronously records the displacement, strain, wall thickness, and contour curvature of the bulging zone vertex during the entire test process; Step 5: When the deformation degree of the bulging zone vertex and the high-pressure liquid pressure reach the preset maximum value, or after the metal sheet bursts, operate the control system (13) to end the test and relieve the pressure, turn off the CCD camera, and save the test data; the hydraulic filling plate (7) moves downward, and the bulged metal sheet (23) is disassembled and taken out; Step 6: Substitute the strain and radius of curvature of the bulging zone vertex of the metal sheet obtained during the rotating female die bulging test, as well as the corresponding bulging pressure, into the theoretical model to calculate and obtain the stress-strain curve and the change curve of the Lode coefficient of the bulging zone vertex of the metal sheet under the rotation path of the specific principal stress direction, and establish the constitutive equation.

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