Residual Stresses in Machined Surfaces. Elaboration of Linear Equation and Design of Annular Test Specimen

Abstract

One of the most important methods of measuring residual stresses (R.S.) is the relaxation method. It is principally based upon the geometrical change of the specimen form, from one state of internal equilibrium (in stress distribution) to a new one, due to removal of a layer from one, side of the specimen. Stablein equation (1931) and those developed by Frisch and Thomsen (1950) are used even to-day in computation of surface R.S. However, the use of these equations is quite, laborious and calls for some treatment of the test results before implementation. Furthermore, these equations were developed primarily for the computation of R.S. introduced in the whole volume of the component by some non uniform plastic flow (cold or hot plastic process). In the current work a new equation was derived for machined components by approaching the part as formed from two separated wholenesses:- the surface layer as a Machined Affected Zone (M.A.Z.) bounded to a stress-free body. Superposition of the two results-in linear relations between a constant (based upon component's parameters) and the measured test values, i.e. the thickness of the etched film and the change in the curvature radius after each step. Results obtained from test performed by Metcut were implemented in the computation and compared with those acquired while using Stablein equation. The correlation was more than satisfactory.