The Relationship between Residual Stress and Hardness and the Onset of Plastic Deformation.

Abstract

Abstract : The stress distribution in the wall of a hollow steel cylinder that had been autofrettaged varies from compressive at the inside diameter to tensile at the outside diameter. The question of how the Rockwell-C (Rc) hardness varies with residual stress was treated previously. In order to generalize the previously developed concepts, in this report the hardness in the wall was measured as a function of radial position using various hardness testers. Each of the hardness testers used a different applied load to indent the sample surface to measure its hardness. The residual stress of the sample was measured using ultrasonic techniques. From a model proposed by Frankel, Abbate, and Scholz, the relationship between the residual stress and the onset of plastic deformation was derived, and the experimental dependence of Rc on residual stress was shown. From previous work we saw that the effect of residual stress on measured hardness stems from the effect of stress on the onset of plastic deformation: plastic deformation is delayed and hardness is increased for a compressive residual stress, and plastic deformation is enhanced for a tensile residual stress, therefore the measured hardness is decreased. From this work, we see that the effect is detectable for the tests using larger loads, Rc and Rockwell-D (RD) and becomes washed out for the tests using lower loads (i.e., less total plastic deformation). The Rockwell-A (RA) and the Microdur testers using Vickers indenters did not show the hardness dependence on residual stress. We point out that Rockwell-C and Rockwell-D hardness tests on gun steel should be done with an awareness that residual stress can affect the results, and carefull Rockwell-C and Rockwell-D tests can be used to obtain residual stress distribution.