Application guidelines for the parting out step in a through thickness residual stress measurement procedure

The first step in an often used destructive procedure for determining through thickness residual stresses in pipes and plates is a parting out step in which a coupon is removed from the parent material while strain gauges monitor the coupon's deformations. The changes in stress that occur at the gauged surfaces when the coupon is parted out, together with information about the stresses in the parted out coupon are used in a back computation method to estimate the stress distributions in the coupon before it was parted out. Either of two back computation methods can be used. One method assumes that the through thickness stress distribution changes that occur when the coupon is parted out lie on a straight line. This method is computationally quick and easy to use, but can be inaccurate if the lateral dimensions of the coupon are too small. The other method uses influence coefficients and greatly extends the range of coupon dimensions for which accurate results can be obtained; but, it is more elaborate than the straight line model and involves two rather time consuming steps. This paper presents guidelines for determining when the more elaborate influence coefficient method must be used in place of the straight line model in terms of the error that the simpler model would produce in the estimated residual stress distribution. This paper also simplifies the application of the influence coefficient method by tabulating the results of the two time consuming steps over the range of coupon dimensions for which this method is typically required. Example problems using the two back computation methods are presented.