A Critical Examination of Sachs’ Material-Removal Method for Determination of Residual Stress

Abstract

Sachs’ method is an experimental procedure used primarily in the determination of residual stresses in autofrettaged thick cylinders. In its usual form it involves fixing axial and hoop direction strain gauges to the OD of a tube; strain readings are then obtained after each incremental removal of material from the bore. Sachs’ analysis assumes that the remaining tube unloads in linear-elastic fashion throughout the process and that superposition may therefore be employed to quantify the residual stresses within the original tube. By numerical simulation of two complete Sachs’ experimental sequences with “open end” conditions it is demonstrated that the assumption of elastic unloading is invalidated by the Bauschinger effect. Sachs’ method thereby overestimates compressive residual bore hoop stresses in a typical tube by between 24% and 43%. If used as the basis for cyclic pressurization fatigue lifetime predictions with pre-existing cracks, such discrepancies will cause overestimates in fatigue lifetime of an order of magnitude. Sachs’ experimental procedure is therefore not recommended as a reliable or conservative method for determination of residual stress.