| Residual stresses,which can considerably influence the engineering properties of materials and structural components,is an important problem concerned but not completely solved till now.The hole-drilling method for residual stress is widely used and theoretically elaborated.The Integral Method,which is considered to be the most important and fundamental theory,is based on a calibration carried out by Finite-Element-Analyses.After introducing the existing models,a new type of finite element model using general axsiymetric element with more efficiency and versatile is considered in the paper.With the method development,the calibration coefficients for residual stress determined within a large thickness range are studied and analyzed.Residual stresses are frequently of interest in crack growth and fracture evaluations.Data on both surface and interior stresses are often needed for such evaluations.However,characterization of interior stress fields by experimental methods becomes challenging,especially for thin plate applications.The strain gauge hole drilling method using incremental drilling needs further development and evaluation for estimation of interior stress which is the subject of this study.The calibration coefficients determined from ASTM Standard Test Method E837 are generally valid only for thick plates.The dimensions of the plates have to satisfy specimen size criterion proposed in the prior research.Few study has been conducted on residual stress measurement of relative thin plates by using the incremental hole drilling method combined with optical measuring method.The thickness range for thin plates,which can be applied in residual stress measurement,has not yet been defined clearly.The thickness of the workpiece,which affects the release strain around the hole,has a significant effect on the measurement.Therefore,a relatively large error will occur if calibration coefficients that do not match the plate thickness are applied to calculate the released stress.To reduce experimental error,it is essential to correct calibration coefficients for thin plates.Compared with the conventional hole drilling method combined with strain gauge rosette measurement,optical measurement methods have two obvious advantages: one is that the measuring point can be selected after the hole drilling process due to full-field measurement of the fringe pattern;for a strain gauge rosette measurement,the measurement sites are fixed and in variable which may lead to an incorrect strain value.The other one is that the measured values from strain gauge rosette are average making the computation of residual stress an average.Due to the difference between optical method and electric-measurement method,the method of determing calibration coefficietns is also different.Based on both uniform stress field and nonuniform stress field,a comparative study of calibration coefficients calculated by area element method,segment method and single point method is carried out.Also,the fitting calibration formulas for incremental hole drilling method combined with optical methods have been proposed,which could not only greatly improve the overall performance and flexibility of the residual stress computation but also the working efficiency.An effective phase extracting technique by using discrete Fourier series is proposed.A detailed description of the reconstruction method is given.The suggested method is verified by simulation with software MATLAB.Meanwhile,the experimental results show that this method is effective for suppressing the high order phase error.A detailed mathematical description of ripple phase error due to vibration is presented.Based on the Fourier spectrum the effect of vibration on fringe gray distribution and phase is studied.In grating moiré interferometry,stress calculation is conducted with the calibraion coefficients that match the thickness of specimen,which significantly increases the accuracy of the calculated value.In the stress measurement of thin specimen,the deviation of calculated value due to mismatched coefficients is over 10%.In this paper,the stress deviation is reduced by half through coefficient correction.Finally,based on the least square method both the releasing strain curve and the releasing stress curve of specimen under axial tensile stress field caused by drilling a hole are given.The experiment results are in good agreement with those of finite element analysis. |
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