A Method of Discerning Frictional Stress and Internal Stress by the Stress Relaxation Test

Applicability of the stress relaxation test as a method of discerning frictional stress and internal stress is examined theoretically. It is found that the strain rate should change continuously on the onset of stress relaxation when an appreciable amount of frictional stress exists, while the strain rate should change discontinuously at the starting point of the stress relaxation when the frictional stress is negligibly small or the internal stress is the dominant component of the flow stress. Whether the contribution of the frictional stress to the flow stress is appreciable or not, can therefore be determined by comparing the strain rates immediately before and after the start of the stress relaxation. By applying this method to the experiment of high-temperature deformation of metals and alloys, the following results are obtained : In pure aluminum and vanadium, the frictional stress is negligibly small, that is, the flow stress is almost equal to the internal stress. Meanwhile, in solutionhardened alloys, Al-5.7 at %Mg and V-5.0 at %Fe, the frictional stress appreciably contributes to the flow stress. Merits and demerits of the proposed method are discussed comparing with the usual methods for measuring the internal stress. (Received January 31, 1977)