Simulation of Strain Gauge Thermal Effects During Residual Stress Hole Drilling Measurements

Abstract

The hole drilling residual stress measuring method is based on drilling a small hole in a material sample and measuring the strain relieved in the hole vicinity by a special strain gauge rosette. The temperature and thermal strain induced by the drilling process can cause significant errors in the relieved strain measurement. The paper deals with computer simulation of the thermomechanical process in the sample during drilling. The first part is devoted to the evaluation of heat flux from the drilling tool to the drilled material using the sample surface temperature measured by thermography. The second part deals with determination of real strain and strain gauge thermal output (apparent strain) at the strain gauge location during and after drilling. The paper describes the computer modelling technique for solving an indirect thermal problem of drilling heat flux determination and a direct thermomechanical problem for a set of the process alternatives. Comparisons of simulated and experimentally determined temperatures and strains are presented.

Error and Uncertainty Analysis of the Residual Stresses Computed by Using the Hole Drilling Method

“…Successively, by using the same calculation code, the uncertainty of the computed RS as well as the single uncertainty contribution related to each influence parameter, have been computed by considering the following typical uncertainty mean values of the main influence parameters (taken from literature [1–18, 21–23]):…”

“…In the following, a procedure to correct the main errors affecting the computed RS, is proposed. It considers all the error sources described in the previous chapter, and it is based on the above mentioned studies reported in literature [2][3][4][5][6][7][8][9][10][11].…”

“…By applying Equation (17) to Equation (11), the following formula that allows the user to evaluate the uncertainty of the P stress component versus the uncertainty of the four influence parameters (E, v, p i , a i ), is obtained: Similar relationships are obtained for the other stress components Q and T, by applying Equation (17) to Equations (12) and (13). In such relationships, the uncertainties u c (a i ) and u c (b i ) of the influence coefficients (a i , b i ) provided by Equations (9,10) are obtained by adding the typical uncertainty of the coefficients provided directly from the ASTM standard (due to the accuracy of the numerical simulations used to their calculation), the uncertainty of the hole depths h i , the hole diameter D 0 , the rosette diameter D and the polynomial coefficients a r,s and b r,s .…”

“…Obviously, to correct the main errors and to estimate the RS uncertainty, the code demands that all the influence parameters (eccentricity, hole diameter etc.) and the corresponding uncertainty values have to be determined previously by direct measurements, accurate estimations or reasonably assumption made also by considering the results reported in literature [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][21][22][23].…”

Averaging effects of a strain gage

“…Zubin [15] carried out a theoretical error investigation of calibration beams used for the determination of the gage factor. In 2005, a computer simulation of strain gage thermal effect errors during residual stress hole drilling measurements was carried out and is presented in reference [16]. In 2006, Robinson discussed the fact that stress analysis engineers are probably no more aware of the background regarding some aspects of the gages than is the average person [17].…”