CN109870367B - Determination method and test device for bending springback parameter of high-strength aluminum alloy plate - Google Patents
Determination method and test device for bending springback parameter of high-strength aluminum alloy plate Download PDFInfo
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Abstract
The invention discloses a method for determining bending resilience parameters of a high-strength aluminum alloy plate and a test device. The determination method comprises the following steps: acquiring parameters of an elastoplastic hardening material model of the high-strength aluminum alloy plate by adopting a unidirectional tensile test; carrying out a three-point bending test on a strip-shaped high-strength aluminum alloy plate sample to obtain the circumferential strain parameter of the sample; according to parameters of the elastoplastic hardening material model, specification parameters of the sample, test parameters of the three-point bending test and circumferential strain parameters of the sample, and by combining a bending moment balance equation of the sample, a geometric relation equation of the sample and a volume equation of the sample, solving to obtain a bending angle, a curvature radius of a stress neutral layer and a wall thickness of the sample; obtaining the hoop stress of the outermost layer according to the bending angle, the curvature radius and the wall thickness of the stress neutral layer of the sample; and determining the elastic modulus of the high-strength aluminum alloy plate according to the hoop stress of the outermost layer and the hoop strain parameter of the sample. The invention can improve the accuracy of the calculation of the bending resilience parameter.
Description
Technical Field
The invention relates to the field of plate bending analysis, in particular to a method for determining bending resilience parameters of a high-strength aluminum alloy plate and a test device.
Background
The light weight is one of effective ways for realizing the aims of energy conservation, emission reduction and environmental protection, and is concerned by various fields in recent years. The design of the machine body structure and parts thereof is completed mainly from two aspects of structure lightening and material lightening. Under the background of light weight, the high-strength aluminum alloy plate has wide and profound application value and is widely applied to the fields of automobiles, aerospace and the like. Because the high-strength aluminum-clad plate has a large yield ratio, the rebound phenomenon in the stamping forming process is more obvious, and the rebound problem becomes one of the main problems influencing the quality of the plate forming parts. Springback is a common factor affecting the quality of the bending process, and springback control is also a technical difficulty in the bending process. The existence of the springback problem causes poor part forming precision, increases the workload of die testing and repairing and the workload of shape correction after forming, and therefore, effective measures are urgently needed to be taken in production practice. Therefore, the rebound analysis of the materials is particularly important in the forming process analysis process.
The elastic modulus is one of the important factors influencing the resilience, and in general engineering problem analysis, the elastic modulus is often treated as a constant, and actually, the elastic modulus is continuously changed along with the plastic deformation of the metal. In order to improve the precision of the sheet forming numerical simulation, the influence of the plastic deformation on the elastic modulus needs to be considered in the analysis of the springback problem. In the rebound analysis, how to obtain the rebound parameters through proper tests is a necessary premise of the rebound analysis and is also a source factor influencing the accuracy of the analysis result. The method for testing the elastic constant of the material mainly comprises a dynamic test method and a static test method. The conventional plate resilience parameter testing method mainly comprises a loading-unloading-loading … cyclic loading method of a plate sample, a stretching-unloading method of different pre-strained plate samples and the like. The loading-unloading-loading … of the strip sample is usually carried out on a servo-driven tensile testing machine, and the bending and instability phenomena of the strip test are easy to occur in the unloading process by the scheme, so that the test accuracy is influenced; although the problem of buckling instability can be avoided by the stretching-unloading method of the pre-strain batten sample, the test results of the two methods are resilience parameters in a unidirectional tensile stress state, are greatly different from the real stress state of a bending process, and seriously affect the accuracy of resilience analysis.
Disclosure of Invention
The invention aims to provide a method for determining bending springback parameters of a high-strength aluminum alloy plate and a test device, so as to improve the accuracy of springback analysis.
In order to achieve the purpose, the invention provides the following scheme:
a method for determining bending springback parameters of a high-strength aluminum alloy plate comprises the following steps:
acquiring parameters of an elastoplastic hardening material model of the high-strength aluminum alloy plate by adopting a unidirectional tensile test;
carrying out a three-point bending test on a strip-shaped high-strength aluminum alloy plate sample to obtain the annular strain parameter of the high-strength aluminum alloy plate sample; when the three-point bending test is carried out, the upper surface of the high-strength aluminum alloy plate sample is subjected to a vertically downward acting force, two supporting rollers for supporting the high-strength aluminum alloy plate sample exert a supporting force on the high-strength aluminum alloy plate sample, and the direction of the acting force applied to the upper surface of the high-strength aluminum alloy plate sample is superposed with a perpendicular bisector of a connecting line of the two supporting rollers;
according to the parameters of the elastoplastic hardening material model, the specification parameters of the high-strength aluminum alloy plate sample, the test parameters of the three-point bending test and the circumferential strain parameters of the high-strength aluminum alloy plate sample, combining a bending moment balance equation of the high-strength aluminum alloy plate sample, a geometric relation equation of the high-strength aluminum alloy plate sample and a volume equation of the high-strength aluminum alloy plate sample, and solving to obtain the bending angle, the stress neutral layer curvature radius and the wall thickness of the high-strength aluminum alloy plate sample;
obtaining the hoop stress of the outermost layer according to the bending angle, the curvature radius of the stress neutral layer and the wall thickness of the high-strength aluminum alloy plate sample;
and determining the elastic modulus of the high-strength aluminum alloy plate according to the outermost layer hoop stress and the hoop strain parameters of the high-strength aluminum alloy plate sample.
Optionally, the high-strength aluminum alloy plate sample that adopts rectangular shape carries out three point bending test, still includes before: preparing a plurality of strip-shaped high-strength aluminum alloy plate samples, wherein the thickness of each high-strength aluminum alloy plate sample is less than 3mm, and black and white speckles are sprayed on the bottom surfaces of the high-strength aluminum alloy plate samples.
Optionally, the obtaining of the parameters of the elastoplastic hardening material model of the high-strength aluminum alloy plate by using the uniaxial tensile test specifically includes:
carrying out a unidirectional tensile test on the high-strength aluminum alloy plate to obtain a real stress-strain curve of the high-strength aluminum alloy plate;
and fitting the elastoplastic hardened material model according to the real stress-strain curve to obtain the parameters of the elastoplastic hardened material model.
Optionally, the high-strength aluminum alloy plate sample that adopts rectangular shape carries out three point bending test, specifically includes:
adjusting the distance between the two supporting rollers to ensure that the perpendicular bisector of the horizontal connecting line of the two supporting rollers is superposed with the force application direction of the bending male die of the universal material testing machine;
placing the high-strength aluminum alloy sheet sample on two supporting rollers to enable the lower surface of the high-strength aluminum alloy sheet sample to be in contact with the top ends of the two supporting rollers;
adjusting the bottom end of a bending male die of a universal material testing machine to be in contact with the upper surface of the high-strength aluminum alloy plate sample;
setting the loading mode of the universal material testing machine to be a loading-unloading mode to carry out loading test on the high-strength aluminum alloy plate sample;
and collecting the annular strain parameters of the high-strength aluminum alloy plate sample in the unloading process by using a DIC (digital computer) online strain measurement system.
Optionally, the solving, according to the parameters of the elasto-plastic hardened material model, the specification parameters of the high-strength aluminum alloy sheet sample, the test parameters of the three-point bending test and the hoop strain parameters of the high-strength aluminum alloy sheet sample, in combination with the bending moment equilibrium equation of the high-strength aluminum alloy sheet sample, the geometric relation equation of the high-strength aluminum alloy sheet sample and the volume equation of the high-strength aluminum alloy sheet sample, of the bending angle, the curvature radius of the stress neutral layer and the wall thickness of the high-strength aluminum alloy sheet sample specifically includes:
according to the bending moment balance equation M of the high-strength aluminum alloy plate sampleExternal force-MInternal forceThe bending angle α and the radius of curvature ρ of the stress neutral layer were obtained for the high-strength aluminum alloy sheet sample at 00And a first expression f of the wall thickness t1(α,ρ0T) is 0; wherein,
Mexternal forceF is three-point bendingPressure at the end of loading in the bending test, h is the rolling reduction at the end of loading in the three-point bending test, W is the horizontal spacing between the two support rollers, RsFor each support roller radius;
Minternal forceB is the width of the high-strength aluminum alloy sheet sample, rho is the curvature radius of any layer, and sigma is the internal force bending moment of the high-strength aluminum alloy sheet sampleTheta interiorIn order to provide the hoop stress of the inner layer,
σtheta outerThe stress is the hoop stress of the outer layer,
r is the radius of curvature of the outermost layer,
r is the radius of curvature of the innermost layer,
k is the strength coefficient of the elastoplastic hardened material model, and n is the hardening index of the elastoplastic hardened material model;
according to the geometric relation equation of the high-strength aluminum alloy plate sample
Obtaining a bending angle α and a curvature radius rho of a stress neutral layer of the high-strength aluminum alloy sheet sample0Second expression f2(α,ρ0)＝0；
According to the volume equation of the high-strength aluminum alloy plate sample
Obtaining a bending angle α and a curvature radius rho of a stress neutral layer of the high-strength aluminum alloy sheet sample0And a third expression f of the wall thickness t3(α,ρ0T) is 0; wherein, t0Is the high-strength aluminum alloy sheetThe initial thickness of the sample;
according to the first expression, the second expression and the third expression, an equation system solving mode is adopted to obtain the bending angle α and the curvature radius rho of the stress neutral layer of the high-strength aluminum alloy sheet sample through solving0And the value of the wall thickness t.
Optionally, according to the bending angle, the curvature radius of the stress neutral layer and the wall thickness of the high-strength aluminum alloy sheet sample, obtaining the outermost hoop stress, specifically including:
by using
Obtaining the outmost hoop stress sigmaθWherein K is the strength coefficient of the elastoplastic hardened material model, n is the hardening index of the elastoplastic hardened material model, and rho0Is the radius of curvature of the stress neutral layer and t is the wall thickness.
Optionally, the determining the elastic modulus of the high-strength aluminum alloy plate according to the outermost layer hoop stress and the hoop strain parameter of the high-strength aluminum alloy plate sample specifically includes:
using a formula
Determining the elastic modulus E of the high-strength aluminum alloy plate, wherein the sigma isθIs the outermost hoop stress, epsilonθSIn order to unload the circumferential strain at the center of the high-strength aluminum alloy plate sample at the beginningθEThe hoop strain at the center of the high-strength aluminum alloy plate sample at the end of unloading is shown.
The utility model provides a test device of high-strength aluminum alloy plate bending springback parameter, includes: the device comprises a universal material testing machine, a movable cross beam, two supporting rollers and a DIC online strain measurement system;
the bending male die of the universal material testing machine is connected to the universal material testing machine through the movable cross beam; the two supporting rollers are fixed below a bending male die of the universal material testing machine and are positioned on two sides of the bending male die; the two supporting rollers are used for supporting a high-strength aluminum alloy plate sample to be tested; the universal material testing machine adjusts the downward moving distance of the bending male die by adjusting the upper position and the lower position of the movable cross beam, and further adjusts the pressure applied to the high-strength aluminum alloy plate sample;
the DIC online strain measurement system comprises a computer and a CCD camera, wherein the CCD camera is fixed at the bottom of the universal material testing machine and used for collecting strain images of the bottom surface of the high-strength aluminum alloy plate sample in the testing process; and the computer receives the acquired strain image sent by the CCD camera and is used for determining the strain parameters of the high-strength aluminum alloy plate sample according to the strain image.
Optionally, the method further includes: a slide rail and a mold base; the die base is fixed at the bottom of the universal material testing machine, the slide rail is fixed on the die base, and the two supporting rollers are arranged on the slide rail; the testing device adjusts the horizontal distance between the two supporting rollers by adjusting the positions of the two supporting rollers on the sliding rail.
According to the specific embodiment provided by the invention, the invention discloses the following technical effects:
the conventional method for testing the resilience parameters of the plate is mainly used for measuring the resilience parameters in a unidirectional tensile stress state, and is greatly different from the real stress state of a bending process, so that great errors can be generated, and the accuracy of resilience analysis is seriously influenced. The invention adopts a three-point bending test to measure the elastic modulus under the bending condition, is more approximate to the stress condition of the plate in the stamping forming process, is the same as the real stress state of the bending process, and obtains more accurate rebound parameters.
Compared with the traditional scheme of measuring strain by using the displacement parameters or the strain gauge output by the testing machine, the method and the device have the advantages that the strain in the bending process of the plate is measured by using the DIC online measurement technology, 3D data in the whole field range can be obtained, the measurement result is more accurate, and more accurate resilience parameters can be obtained.
Common plate resilience parameter testing methods such as a strip sample 'loading-unloading-loading …' cyclic loading method, a different pre-strain strip sample 'stretching-unloading' method and the like are complex to operate, the buckling phenomenon of a plate is easy to occur in the unloading process, and great errors are brought to test results. The invention adopts the conventional unidirectional tensile test and the three-point bending test, can control different strain degrees by regulating and controlling different rolling reduction, has simple and easily controlled operation, and effectively avoids the instability phenomenon that the sheet is easy to bend.
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In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without inventive exercise.
FIG. 1 is a schematic flow chart of a method for determining a bending springback parameter of a high-strength aluminum alloy sheet according to the present invention;
FIG. 2 is a schematic structural diagram of the test apparatus for the bending springback parameter of the high-strength aluminum alloy sheet according to the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in further detail below.
FIG. 1 is a schematic flow chart of a method for determining bending springback parameters of a high-strength aluminum alloy plate. As shown in fig. 1, the method comprises the following steps:
step 100: and (3) acquiring parameters of the elastoplastic hardening material model of the high-strength aluminum alloy plate by adopting a unidirectional tensile test.After the unidirectional tensile test is carried out on the high-strength aluminum alloy plate, the true stress-strain curve of the high-strength aluminum alloy plate can be obtained, and then the elastoplastic hardened material model is fitted according to the true stress-strain curve, so that the elastoplastic hardened material model (sigma-K epsilon) can be obtainedn) The parameters include strength factor K and hardening index n.
Step 200: and carrying out a three-point bending test by adopting a strip-shaped high-strength aluminum alloy plate sample to obtain the annular strain parameter of the high-strength aluminum alloy plate sample.
Before a three-point bending test is carried out, a plurality of strip-shaped high-strength aluminum alloy plate samples need to be prepared, wherein the thickness of each strip-shaped high-strength aluminum alloy plate sample is less than 3mm, the length of each strip-shaped high-strength aluminum alloy plate sample can be 150mm, and the width of each strip-shaped high-strength aluminum alloy plate sample can be 20 mm. The specific specification of the sample can be designed according to actual requirements. The bottom surface of the high-strength aluminum alloy plate sample is sprayed with black and white speckles.
And during the three-point bending test, the upper surface of the high-strength aluminum alloy plate sample is supported by a vertically downward acting force, the two supporting rollers of the high-strength aluminum alloy plate sample apply a supporting force to the high-strength aluminum alloy plate sample, and the acting force direction of the upper surface of the high-strength aluminum alloy plate sample coincides with the perpendicular bisector of the connecting line of the two supporting rollers.
The test process is as follows:
adjusting the distance between the two supporting rollers to ensure that the perpendicular bisector of the horizontal connecting line of the two supporting rollers is superposed with the force application direction of the bending male die of the universal material testing machine;
placing the high-strength aluminum alloy sheet sample on two supporting rollers to enable the lower surface of the high-strength aluminum alloy sheet sample to be in contact with the top ends of the two supporting rollers;
adjusting the bottom end of a bending male die of a universal material testing machine to be in contact with the upper surface of the high-strength aluminum alloy plate sample;
setting the loading mode of the universal material testing machine to be a loading-unloading mode to carry out loading test on the high-strength aluminum alloy plate sample;
and collecting the annular strain parameters of the high-strength aluminum alloy plate sample in the unloading process by using a DIC (digital computer) online strain measurement system.
The DIC online strain measurement system can calculate a plurality of information of the sample in the deformation process by using a digital image correlation algorithm through pictures acquired by a CCD camera, wherein the information comprises position, displacement, strain and the like. Digital Image Correlation (DIC): the principle is that a computer finds out an image related area through gray scale by comparing images before and after deformation of an object to be measured, so as to calculate the surface displacement and strain distribution of the object. In the whole measuring process, one or two image collectors are used for collecting images of the object to be measured before and after deformation, and 3D data in the whole field range can be obtained through calculation. DIC technology does not need special environmental requirements, can be used in indoor and outdoor common environments, and can perform strain measurement as long as images can be obtained in principle.
Step 300: according to the parameters of the elastoplastic hardening material model, the specification parameters of the high-strength aluminum alloy plate sample, the test parameters of the three-point bending test and the circumferential strain parameters of the high-strength aluminum alloy plate sample, and by combining a bending moment balance equation of the high-strength aluminum alloy plate sample, a geometric relation equation of the high-strength aluminum alloy plate sample and a volume equation of the high-strength aluminum alloy plate sample, the bending angle, the curvature radius of the stress neutral layer and the wall thickness of the high-strength aluminum alloy plate sample are obtained through solving. The specific solving process is as follows:
according to the bending moment balance equation M of the high-strength aluminum alloy plate sampleExternal force-MInternal forceThe bending angle α and the radius of curvature ρ of the stress neutral layer were obtained for the high-strength aluminum alloy sheet sample at 00And a first expression f of the wall thickness t1(α,ρ0T) is 0; wherein,
Mexternal forceThe external force bending moment of the high-strength aluminum alloy plate sample is F, the pressure at the end of loading in the three-point bending test, h, the rolling reduction at the end of loading in the three-point bending test, W, the horizontal distance between two supporting rollers and RsFor each support roller radius;
Minternal forceB is the width of the high-strength aluminum alloy sheet sample, rho is the curvature radius of any layer, and sigma is the internal force bending moment of the high-strength aluminum alloy sheet sampleTheta interiorIn order to provide the hoop stress of the inner layer,
σtheta outerThe stress is the hoop stress of the outer layer,
r is the radius of curvature of the outermost layer,
r is the radius of curvature of the innermost layer,
k is the strength coefficient of the elastoplastic hardened material model, and n is the hardening index of the elastoplastic hardened material model;
according to the geometric relation equation of the high-strength aluminum alloy plate sample
Obtaining a bending angle α and a curvature radius rho of a stress neutral layer of the high-strength aluminum alloy sheet sample0Second expression f2(α,ρ0)＝0；
According to the volume equation of the high-strength aluminum alloy plate sample
Obtaining a bending angle α and a curvature radius rho of a stress neutral layer of the high-strength aluminum alloy sheet sample0And a third expression f of the wall thickness t3(α,ρ0T) is 0; wherein, t0The initial thickness of the high-strength aluminum alloy plate sample is obtained;
solving to obtain the height by adopting an equation system solving mode according to the first expression, the second expression and the third expressionBending angle α and curvature radius rho of stress neutral layer of strong aluminum alloy plate sample0And the value of the wall thickness t.
Step 400: and obtaining the outmost hoop stress according to the bending angle, the curvature radius of the stress neutral layer and the wall thickness of the high-strength aluminum alloy plate sample. In particular, using formulae
Obtaining the outmost hoop stress sigmaθWherein K is the strength coefficient of the elastoplastic hardened material model, n is the hardening index of the elastoplastic hardened material model, and rho0Is the radius of curvature of the stress neutral layer and t is the wall thickness.
Step 500: and determining the elastic modulus of the high-strength aluminum alloy plate according to the hoop stress of the outermost layer and the hoop strain parameters of the high-strength aluminum alloy plate sample. Specifically, using a formula
Determining the elastic modulus E of the high-strength aluminum alloy plate, wherein the sigma isθIs the outermost hoop stress, epsilonθSIn order to unload the circumferential strain at the center of the high-strength aluminum alloy plate sample at the beginningθEThe hoop strain at the center of the high-strength aluminum alloy plate sample at the end of unloading is shown.
The invention also provides a test device for the bending resilience parameters of the high-strength aluminum alloy plate, which is used for carrying out a three-point bending test so as to obtain all the parameters in the determination method. FIG. 2 is a schematic structural diagram of the test apparatus for the bending springback parameter of the high-strength aluminum alloy sheet according to the present invention. As shown in fig. 2, the test apparatus includes the following structure: the device comprises a universal material testing machine 1, a movable cross beam 2, two supporting rollers 5 and a DIC online strain measurement system.
The universal material testing machine 1 comprises a bending male die 3 for applying pressure to a sample to be tested, and the bending male die 3 is connected to the universal material testing machine 1 through the movable cross beam 2; the two supporting rollers 5 are fixed below the bending male die 3 and are positioned on two sides of the bending male die 3; and the two supporting rollers 5 are used for supporting the high-strength aluminum alloy plate sample to be tested. Before the test, the sample 4 is placed on the supporting roller 5, the center of the sample 4 is opposite to the bending male die 3, the position of the movable cross beam 2 is adjusted to enable the bending male die 3 to be just contacted with the sample 4, and the pressure received by the sample 4 is zero. During testing, the universal material testing machine 1 adjusts the downward movement distance of the bending male die 3 by adjusting the upper and lower positions of the movable cross beam 2, and further adjusts the pressure applied to the test sample 4.
The DIC online strain measurement system comprises a computer 6 and a CCD camera 8, wherein the CCD camera 8 is fixed at the bottom of the universal material testing machine 1 and used for collecting strain images of the bottom surface of a high-strength aluminum alloy plate sample in the testing process; and the computer 6 receives the acquired strain image sent by the CCD camera 8 and is used for determining the strain parameters of the high-strength aluminum alloy plate sample according to the strain image. In order to overcome the influence of light on the quality of images collected by the CCD camera 8, a light supplement lamp 7 can be additionally arranged during testing, and the light supplement lamp 7 can be fixed at the bottom of a universal material testing machine or on the bottom of the testing machine.
Preferably, but not limited to, the test device may further comprise a slide and a mold base 9. The die base 9 is fixed at the bottom of the universal material testing machine 1, the slide rail is fixed on the die base 9, and the two support rollers 5 are arranged on the slide rail; the testing device adjusts the horizontal distance between the two supporting rollers 5 by adjusting the positions of the two supporting rollers 5 on the sliding rail so as to adapt to tests of samples of various specifications.
According to the invention, a three-point bending test is used for realizing the deformation process of the material, and the rebound parameters are obtained by combining deformation measurement by a DIC strain online measurement technology and stress calculation by a plastic bending theory, so that more accurate rebound parameters can be obtained in a real bending stress state. The conventional method for testing the resilience parameters of the plate is mainly used for measuring the resilience parameters in a unidirectional tensile stress state, and is greatly different from the real stress state of a bending process, so that larger errors can be generated, and the accuracy of resilience analysis is seriously influenced. According to the invention, a three-point bending test device is used for realizing the deformation process of the material, and the rebound parameters are obtained by combining deformation measurement by a DIC strain online measurement technology and stress calculation by a plastic bending theory, so that more accurate rebound parameters can be obtained in a real bending stress state.
The following further illustrates the embodiments of the present invention in conjunction with a specific embodiment thereof.
The specific implementation case is as follows: determination process of bending resilience parameter of high-strength aluminum alloy AA7075 (thickness of 2mm)
1. Uniaxial tensile test
Designing a plate tensile sample according to a room temperature test method of a national standard GB/T228.1-2010 metal material tensile test, carrying out a tensile test, obtaining a real stress-strain curve of the high-strength aluminum alloy plate, and fitting an elastoplastic hardening material model (sigma-K epsilon)n) Strength factor K and hardening index n.
2. Preparation of test specimens
A plurality of strip AA7075 aluminum alloy plate samples (150 x 20 x 2mm) are manufactured, black and white speckles are sprayed on one surface of each sample by using matte spray paint, and the specific operation method comprises the following steps: a layer of primer is sprayed by white spray paint, and then black spray paint is intermittently sprayed at a certain distance from the board surface, so that the black paint falls on the white primer to form black and white speckles.
3. Installation and debugging test device
Installing a die base with a slide rail on a machine base table surface of a compression space below a universal material testing machine, installing a bending male die on the lower part of a movable cross beam, adjusting a supporting roller to a certain distance and locking, placing a bending sample coated with speckles on the supporting roller to ensure that the center of the sample is opposite to the bending male die and one side without the speckles faces the bending male die, and adjusting the position of the movable cross beam to ensure that the movable cross beam is just contacted with the sample; adjusting the DIC on-line strain measurement system, adjusting the position and angle of the CCD camera, fully exposing the center and the nearby area of the bent sample in the whole visual field range, and reserving a certain sample movement allowance according to the test. And adjusting the position and the light intensity of the light supplement lamp, and adjusting the focal length and the exposure time of the CCD camera to enable the strain measurement system to obtain the optimal analysis result.
4. Three point bend test procedure
The loading mode of the universal material testing machine is set to be a loading-unloading mode to load the plate sample, the loading speed and the unloading speed are set according to the test requirements, the loading displacement is set to be a certain value, and the unloading displacement and the loading displacement are set to be the same value. In the loading-unloading process, the data change of pressure and reduction in the loading process is recorded by a universal material testing machine, and the strain change of the plate in the whole process is recorded by a strain measurement system.
5. Data processing
The width b and initial thickness t of the sample are extracted from the test process0Strength coefficient K, hardening index n, and support roller radius RsThe distance W between the supporting rollers, the pressure F and the rolling reduction h after the loading is finished, and the circumferential strain epsilon at the center of the sample when the unloading is startedθSAnd the hoop strain epsilon at the centre of the specimen at the end of unloadingθE. Wherein, the width b and initial thickness t of the sample0Is the value obtained when the sample is prepared in the step 2; the strength coefficient K and the hardening index n are values obtained by the step 1 unidirectional tensile test; radius R of the support rollersThe distance W between the supporting rollers is a numerical value which can be measured on a testing device after the installation and debugging in the step 3 are finished; the pressure F and the reduction h are values collected by the testing machine in the step 4 loading process; two hoop strains epsilonθS、εθEIs a numerical value acquired by a DIC online strain measurement system in the unloading process. The modulus of elasticity E during unloading was then calculated by the following procedure:
the first step is as follows:
r is to besW, F, h, substituting the values into the external force bending moment equation:
respectively substituting the numerical value of K, n, the outermost layer curvature radius R and the innermost layer curvature radius R into an inner layer hoop stress expression and an outer layer hoop stress expression:
wherein,
and the inner layer annular stress expression and the outer layer annular stress expression are introduced into an inner force bending moment equation:
substituting the external force bending moment and the internal force bending moment into a bending moment balance equation: mExternal force-MInternal force0, one obtains a stress neutral layer curvature radius ρ containing only the unknown amount of bend angle α0And a first expression for wall thickness t: f. of1(α,ρ0,t)＝0。
The second step is that:
h, W and RsThe values of (a) will be substituted into the geometric equation:
obtaining a curve angle α containing unknown quantities, the radius of curvature rho0The second expression of (1): f. of2(α,ρ0)＝0。
The third step:
b, t0And R, r into the volume invariant equation:
obtaining another bending angle α containing unknown quantities, the radius of curvature p0And a third expression for wall thickness t: f. of3(α,ρ0,t)＝0。
The fourth step:
from f1(α,ρ0,t)＝0，f2(α,ρ0)＝0，f3(α,ρ0T) is 0, the bending angle α and the curvature radius rho can be obtained0And the value of the wall thickness t.
The fifth step:
α ρ0Substituting t into the outermost layer hoop stress formula
Obtaining the outmost hoop stress sigmaθThe numerical value of (c).
And a sixth step:
during unloading, the outermost hoop stress is represented byθBecomes zero, and therefore the modulus of elasticity during unloading is calculated using the following formula:
the elastic modulus E of the high-strength aluminum alloy AA7075 plate is obtained.
6. Increasing the numerical value of the rolling reduction h, carrying out a plurality of groups of loading-unloading test processes, calculating according to the physical quantity measured in the test to obtain more springback parameters under different plastic deformation conditions, and further obtaining the change rule of the springback parameters along with the plastic deformation.
The embodiments in the present description are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other.
The principles and embodiments of the present invention have been described herein using specific examples, which are provided only to help understand the method and the core concept of the present invention; meanwhile, for a person skilled in the art, according to the idea of the present invention, the specific embodiments and the application range may be changed. In view of the above, the present disclosure should not be construed as limiting the invention.
Claims (7)
1. A method for determining bending springback parameters of a high-strength aluminum alloy plate is characterized by comprising the following steps:
acquiring parameters of an elastoplastic hardening material model of the high-strength aluminum alloy plate by adopting a unidirectional tensile test;
carrying out a three-point bending test on a strip-shaped high-strength aluminum alloy plate sample to obtain the annular strain parameter of the high-strength aluminum alloy plate sample; when the three-point bending test is carried out, the upper surface of the high-strength aluminum alloy plate sample is subjected to a vertically downward acting force, two supporting rollers for supporting the high-strength aluminum alloy plate sample exert a supporting force on the high-strength aluminum alloy plate sample, and the direction of the acting force applied to the upper surface of the high-strength aluminum alloy plate sample is superposed with a perpendicular bisector of a connecting line of the two supporting rollers;
according to the parameters of the elastoplastic hardening material model, the specification parameters of the high-strength aluminum alloy plate sample, the test parameters of the three-point bending test and the circumferential strain parameters of the high-strength aluminum alloy plate sample, combining a bending moment balance equation of the high-strength aluminum alloy plate sample, a geometric relation equation of the high-strength aluminum alloy plate sample and a volume equation of the high-strength aluminum alloy plate sample, and solving to obtain the bending angle, the stress neutral layer curvature radius and the wall thickness of the high-strength aluminum alloy plate sample;
obtaining the hoop stress of the outermost layer according to the bending angle, the curvature radius of the stress neutral layer and the wall thickness of the high-strength aluminum alloy plate sample;
and determining the elastic modulus of the high-strength aluminum alloy plate according to the outermost layer hoop stress and the hoop strain parameters of the high-strength aluminum alloy plate sample.
2. The method for determining the bending springback parameter of the high-strength aluminum alloy sheet according to claim 1, wherein the three-point bending test is performed by using a strip-shaped high-strength aluminum alloy sheet sample, and the method further comprises the following steps: preparing a plurality of strip-shaped high-strength aluminum alloy plate samples, wherein the thickness of each high-strength aluminum alloy plate sample is less than 3mm, and black and white speckles are sprayed on the bottom surfaces of the high-strength aluminum alloy plate samples.
3. The method for determining the bending springback parameter of the high-strength aluminum alloy plate as claimed in claim 1, wherein the obtaining of the parameter of the elastoplastic hardening material model of the high-strength aluminum alloy plate by using the uniaxial tensile test specifically comprises:
carrying out a unidirectional tensile test on the high-strength aluminum alloy plate to obtain a real stress-strain curve of the high-strength aluminum alloy plate;
and fitting the elastoplastic hardened material model according to the real stress-strain curve to obtain the parameters of the elastoplastic hardened material model.
4. The method for determining the bending springback parameter of the high-strength aluminum alloy sheet according to claim 1, wherein the three-point bending test is performed by using a strip-shaped high-strength aluminum alloy sheet sample, and specifically comprises the following steps:
adjusting the distance between the two supporting rollers to ensure that the perpendicular bisector of the horizontal connecting line of the two supporting rollers is superposed with the force application direction of the bending male die of the universal material testing machine;
placing the high-strength aluminum alloy sheet sample on two supporting rollers to enable the lower surface of the high-strength aluminum alloy sheet sample to be in contact with the top ends of the two supporting rollers;
adjusting the bottom end of a bending male die of a universal material testing machine to be in contact with the upper surface of the high-strength aluminum alloy plate sample;
setting the loading mode of the universal material testing machine to be a loading-unloading mode to carry out loading test on the high-strength aluminum alloy plate sample;
and collecting the annular strain parameters of the high-strength aluminum alloy plate sample in the unloading process by using a DIC (digital computer) online strain measurement system.
5. The method for determining the bending springback parameter of the high-strength aluminum alloy sheet according to claim 1, wherein the step of solving the bending angle, the curvature radius of the stress neutral layer and the wall thickness of the high-strength aluminum alloy sheet sample according to the parameters of the elastoplastic hardening material model, the specification parameters of the high-strength aluminum alloy sheet sample, the test parameters of the three-point bending test and the hoop strain parameters of the high-strength aluminum alloy sheet sample in combination with the bending moment balance equation of the high-strength aluminum alloy sheet sample, the geometric relation equation of the high-strength aluminum alloy sheet sample and the volume equation of the high-strength aluminum alloy sheet sample specifically comprises:
according to the bending moment balance equation M of the high-strength aluminum alloy plate sampleExternal force-MInternal forceThe bending angle α and the radius of curvature ρ of the stress neutral layer were obtained for the high-strength aluminum alloy sheet sample at 00And a first expression f of the wall thickness t1(α,ρ0T) is 0; wherein,
Mexternal forceThe external force bending moment of the high-strength aluminum alloy plate sample is F, the pressure at the end of loading in the three-point bending test, h, the rolling reduction at the end of loading in the three-point bending test, W, the horizontal distance between two supporting rollers and RsFor each support roller radius;
Minternal forceB is the width of the high-strength aluminum alloy sheet sample, rho is the curvature radius of any layer, and sigma is the internal force bending moment of the high-strength aluminum alloy sheet sampleTheta interiorIn order to provide the hoop stress of the inner layer,
σtheta outerThe stress is the hoop stress of the outer layer,
r is the radius of curvature of the outermost layer,
r is the radius of curvature of the innermost layer,
k is the strength coefficient of the elastoplastic hardened material model, and n is the hardening index of the elastoplastic hardened material model;
according to the geometry of the high-strength aluminum alloy plate sampleEquation of relation
Obtaining a bending angle α and a curvature radius rho of a stress neutral layer of the high-strength aluminum alloy sheet sample0Second expression f2(α,ρ0)＝0；
According to the volume equation of the high-strength aluminum alloy plate sample
Obtaining a bending angle α and a curvature radius rho of a stress neutral layer of the high-strength aluminum alloy sheet sample0And a third expression f of the wall thickness t3(α,ρ0T) is 0; wherein, t0The initial thickness of the high-strength aluminum alloy plate sample is obtained;
according to the first expression, the second expression and the third expression, an equation system solving mode is adopted to obtain the bending angle α and the curvature radius rho of the stress neutral layer of the high-strength aluminum alloy sheet sample through solving0And the value of the wall thickness t.
6. The method for determining the bending springback parameter of the high-strength aluminum alloy sheet according to claim 1, wherein the obtaining of the outermost hoop stress according to the bending angle, the curvature radius of the stress neutral layer and the wall thickness of the high-strength aluminum alloy sheet sample specifically comprises:
by using
Obtaining the outmost hoop stress sigmaθWherein K is the strength coefficient of the elastoplastic hardened material model, n is the hardening index of the elastoplastic hardened material model, and rho0Is the radius of curvature of the stress neutral layer and t is the wall thickness.
7. The method for determining the bending springback parameter of the high-strength aluminum alloy plate according to claim 1, wherein the determining the elastic modulus of the high-strength aluminum alloy plate according to the hoop stress of the outermost layer and the hoop strain parameter of the sample of the high-strength aluminum alloy plate specifically comprises:
using a formula
Determining the elastic modulus E of the high-strength aluminum alloy plate, wherein the sigma isθIs the outermost hoop stress, epsilonθSIn order to unload the circumferential strain at the center of the high-strength aluminum alloy plate sample at the beginningθEThe hoop strain at the center of the high-strength aluminum alloy plate sample at the end of unloading is shown.
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