Numerical and Experimental Residual Stress Analysis on Laser Hardfacing Layer

• Main Author: 魏中聖
• Other Authors: 敖仲寧
• Format: Others
• Language: en_US
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/79230260231513079005

博士 === 國立中正大學 === 機械系 === 91 === The hole-drilling method is applied extensively in the residual stress measurement. The integral hole-drilling method is further developed for improving the accuracy of the hole-drilling method especially for measuring residual stresses varying abruptly with depth. One of the important factors affecting the accuracy of stress values obtained from hole-drilling method is the calibration coefficients. A three-dimensional finite element model was established to determine and to improve the calibration coefficients for integral hole-drilling method. The calibration coefficients could be determined within one computation procedure with this model. The relationship between the calibration coefficients and plate thickness was investigated and the calibration coefficients determined in this work can be extended to measure residual stresses of either thin or thick plates. Experimental validation of the calibration coefficients for integral hole-drilling method based on the improved 3-D FE model was achieved using bending test of a cantilever beam. Experimental results of the bending test show a significant improvement of the accuracy of relieved stress calculation. Experimental results also show that significant error in the residual stress measurement could be induced if calibration coefficients were not chosen correctly for corresponding plate thickness. A transitional dimensionless thickness was proposed by examining the calculated relieved stresses obtained from the calibration coefficients for different plate thicknesses. The probability bounds of relieved stress corresponding to both cases were also calculated to reveal the improvement of the calibration coefficients obtained from 3-D model. Numerical simulation on the bending test was performed using a FEM model. The results show that the calculated residual stress value was strongly affected by the hole diameter used to evaluate the residual stress, and the error was attributed to the deviation of real hole geometry from an idealized model. Correction of the diameter was proposed to effectively reduce the error in the residual stress calculation. Integral hole-drilling method with improved calibration coefficients was applied to determine residual stresses at the interface of the clad layer for better accuracy. The transverse and longitudinal residual stress distributions near the interface between the clad layer and substrate is less uniform as that in the clad layer owing the phase transformation of the substrate and dissimilar properties between clad layer and substrate. The measured results at each hole depth increment revealed a good reproducibility of the integral hole-drilling method. Numerical analysis on the residual stress distribution of the laser clad layer was conducted considering the phase transformation of the substrate. A modified Goldak’s model was used to simulate the heat source and a reasonable shallow fusion zone was achieved. Thermo-metallurgical computation enables the quantitatively determination of phase proportion of the substrate after laser cladding. The resulted temperature history and the phase proportion were used to interpret the residual stress distribution obtained from integral hole-drilling method. The predicted residual stress distribution was consistent with the compared with the measured residual stresses in trends. With the help of numerical analysis, we had a better understanding on the development of the residual stresses in the laser clad layer.