Research On Residual Stress Measurement And Vibration Stress Relief Of Sheet Metal

Plastic effect would lead to measurement error when high residual stress was measured using hole-drilling method(HDM).To solve the problem,the correction of calibration coefficients in the ASTM E837-13a standard was studied in this paper.At the same time,the system of electromagnetic vibration exciter for vibration stress relief(VSR)of metal sheet was formed and the VSR of sheet metal was studied.Research contents of this paper were listed as follows.Firstly,a method correcting plastic effect was studied when high residual stress of aluminum alloy specimen was measured by HDM.The calibration coefficients in the ASTM E837-13a standard were corrected based on the distortional strain energy density theory;Finite element simulated calibration by ANSYS software considering double linear elastic-plastic theory was carried out;Verification experiment by stepwise tensile loading using electronic universal testing machine was carried out,before and after the correction,the ranges of average value of high residual stress measurement error were 0～80%and 0～12%;Verification experiment by simulated biaxial stress was done using ANSYS.Results showed that in the residual stress measurement by HDM,firstly,a and b in ASTM E837-13a should be used in calculating residual stress σ,If the σ obtained was smaller than 0.5 σs,the σ was the true residual stress;Otherwise,a and b should be corrected following the method in this study,then the corrected a and b should be used to calculate the true residual stress.Secondly,the system of electromagnetic vibration exciter for VSR of metal sheet was formed.According to the requirement,overall scheme of electromagnetic vibrator exciter was designed,including structure of movable coil and 8 spring pieces for supporting moving coil;Modal analysis of movable coil was carried out using ANSYS.The movable coil size,structure and material were changed to make the first resonance frequency of the movable coil be greater than the maximum operating frequency of the exciter.Magnetic circuit of the vibrator exciter was simulated using ANSYS and the magnetic flux density in its working air gap was obtained.Thirdly,the result of VSR of 7075 aluminum alloy was studied.In the experiment,a large residual stress in specimen was generated by quenching;According to ANSYS finite element modal analysis,the clamping conditions and excitation frequency of 7075 aluminum alloy specimen were determined and the VSR experiments to the specimen were carried out;Effect of VSR was evaluated by HDM.After low frequency,medium frequency and high frequency VSR,the average value of the first principal stress were reduced from 118 MPa to 57 MPa,80 MPa and 13 MPa.Finally,the vibration warp relief of copper sheet was studied by experimental research.Using ANSYS,modal analysis of copper sheet was carried out;The excitation schemes were used with low frequency 50 Hz,intermediate frequency 300 Hz,high frequency 5 kHz,the excitation time of each scheme were 25 min;The laser sensor was used to measure the warp of copper sheet,so as to determine the residual stress change of copper sheet before and after VSR.The effect of exciting frequency of VSR of copper sheet was investigated by experiments.Via VSR,the average values of warp elimination rate of copper sheet in station one,two and three were about 80%,35%and 30%,respectively.