Measurement Of Full-range Hardening Curve In Compression Test Of Metals

With the rapid development of numerical simulation technology,the accuracy and reliability of finite element simulation have gradually improved,and it has become the main tool for simulating the forming process of metal products and formulating forming process specifications,which has strongly promoted the development of modern metal plastic precision forming technology.The constitutive model of metals is an important prerequisite for finite element simulation,and the accuracy of the material constitutive model directly determines the accuracy of the finite element simulation for product forming.The hardening curve is generally obtained by uniaxial tensile tests.However,during the metal forming process like forging,extrusion,rolling,pendulum rolling,roll forging,etc.,it is mainly subjected to compression stress and local deformation can reach a very large strain,so the true deformation behavior of the material can be better reflected by obtaining a large strain range hardening curve of the material under compression stress.The metal compression hardening curve is usually measured by uniaxial cylindrical specimen compression test,whose premise is that the friction at the end face of the specimen can be ignored,the specimen is uniformly stressed and it is subjected to uniaxial compression stress.In fact,even with good lubrication,the friction between the end face of the metal cylindrical specimen and the platen can't be completely eliminated,resulting in a bulging in compression specimen,uneven stress distribution within the specimen and it is no longer a uniaxial compression stress state,so the calculated compression stress-strain curve has a significant error.The hardening curve is only accurate enough before the compression specimen shows a obvious bulging,and its corresponding strain range is relatively small.In view of the fact that traditional cylindrical compression specimen cannot accurately obtain a large strain range hardening curve,this paper designs hyperbolic contour compressionspecimen and dumbbell compression specimen for ductile metals that can avoid the influence of friction and proposes corresponding stress and strain calculation methods to obtain a large strain range hardening curve.The research is to use hypothetical materials with different hardening models to simulate the compression test of the designed specimens,output the simulated compression test curve data and summarize the calculation method of compression stress correction by regression method.Based on that,compression tests are performed on hyperbolic contour specimens and dumbbell specimens made from actual materials to obtain the corresponding large strain range hardening curves and which are compared with the corresponding hardening curve obtained by the inverse method to verify the accuracy and reliability of the measurement method proposed in this article.The main research results are as follows:(1)Hyperbolic contour compression specimen and dumbbell-shaped compression specimen are designed,both specimens consist of a contact end with a larger diameter at two ends and a thinner gauge section in the middle.Such a structure can focus the deformation in middle section of the specimen.It is also confirmed in the finite element simulation result that the friction's effect on the measurement values of the specimen can be neglected.(2)The method for measuring the hardening curve of a hyperbolic profile specimen is suitable for metal materials with a strain hardening exponent within 0.2.Within the strain range of 0-0.7,the theoretical error of the stress correction is within3.5%.Hyperbolic contour specimens of materials with a higher strain hardening exponent show a significant error in fitting function,so accurate stress correction cannot be achieved.The compression test results of the hyperbolic contour specimens of Q420 low carbon steel and AA1060 pure aluminum show that due to the influence of comprehensive factors such as test error,when the measured hardening curve is used as the input material model for simulation,the error between simulation load-displacement curve and experiment curve is within 3%,the measured accurately hardening curves between strain range 0-0.7 can meet the engineering requirements.(3)The compression-hardening curve measurement method fordumbbell-shaped specimens is suitable to ductile metal materials with any hardening exponent.This is because the dumbbell-shaped specimens continue to develop bulging during the entire compression process and the stress is corrected based on the bulging.Within the strain range of 0-0.7,the theoretical error of stress correction is within 2%.It is also found in the finite element simulation that this type of specimen can meet the material of any hardening exponent and the corresponding stress calculation method can obtain the material's accurate hardening curve.Compared with the actual test curve,the simulated load-displacement curve obtained from the hardening curve measured by the compression test of dumbbell specimens of Q235 low carbon steel and H62 brass has an error within 2%,the measured accurately hardening curves between strain range 0-0.7 can meet the engineering requirements.(4)Two inverse measurement methods for measuring hardening curve are proposed for the designed compression specimen.The comparison results of the conventional tensile test,the the conventional cylindrical specimen compression test,the designed specimen compression test and the corresponding hardening curve measured by the corresponding inverse method of four different metal materials show that the tensile hardening curves and the compression hardening curves are different from each other and the maximum tensile strain only reach 0.36.The hardening curves of traditional cylindrical compression specimens are in the strain range about0-0.5,but due to factors such as end face friction and measurement errors,the hardening curve has some errors.The designed hyperbolic contour specimen and dumbbell specimen's maximum compression strain range can reach about 0.7 and it can better match the hardening curve obtained by the inverse method,which confirms the reliability of the proposed stress correction method.