Understanding the effect of residual stresses on surface integrity and how to measure them by a non-destructive method

Abstract

In teaching the theory of solid mechanics of metallic materials there are basically two kinds of stresses that a component can be subjected to. The first are the applied stresses generated from a loading condition that the component experiences in service. This load can be either a static or dynamic where the stresses are easily determined by traditional strength of materials equations, continuum mechanics or by finite element analysis. The second type of mechanical stress that occurs in materials is classified as residual stresses. These are the stresses that remain in the material after all the applied loads are removed. Mechanical engineering and engineering technology students have a difficult time understanding the generation of residual stresses, measuring them and their overall effect on design life. Residual stresses typically come from non-uniform plastic flow due to some previous loading or manufacturing process. Some of these processes are but not limited to casting, machining, welding, grinding, shot peening, quenching, nonuniform cold working such as twisting, bending, forging and drawing. Engineering students must learn that residual stresses will have an effect on the surface integrity. However, the literature shows that depending on the magnitude and direction that these stresses hold they may be harmful or beneficial to the overall design life. Applied stresses and residual stresses add algebraically, as long as their sum does not exceed the elastic limit of the material. Compressive residual stresses prolong fatigue life while tensile residual stresses will cause premature brittle failure. It is important to teach that residual stresses must form an equilibrium system within the component. This means that tensile residual stress in a part must be balanced by compressive residual stresses in another. There are two teaching methods for measuring residual stresses. The first is the mechanical relaxation dissection method which is very time consuming and inherently destructive. This method utilizes strain gauges to measure the strain relaxation after the part is sectioned away from the main body. The second method that is taught in this paper is the technique of residual stress measurement by x-ray diffraction. This method depends on the fact that the spacing d of the atomic (hkl) planes in a metal material is altered by stress and that d can be determined by measuring the angular position of a diffracted x-ray beam. The fractional change in d is the strain from which the stress can be calculated. © American Society for Engineering Education, 2008.