The determination of static and dynamic yield stresses using a steel ball

Abstract

The static and dynamical yield stress of the material of a thick steel plate may be estimated by pressing and by dropping a hard steel ball on a plane surface of the plate which has been ground and then polished. Under these conditions, the first appearance of an indentation on the polished surface can be detected with good accuracy, either by an optical interference method, or by an optical reflexion method. The statical experiment consists in finding the least force which must be applied to the steel ball to produce a permanent indentation, whilst the dynamical experiment consists in finding the least normal velocity of impact which gives similarly a permanent indentation. Using either the Guest-Mohr principal-stress difference or the von Mises shear strain energy hypotheses as criteria of failure, combined with an analysis of the stresses in the plate, it is shown how the appropriate yield stress can be calculated from the experimental data. Tests were made on a specimen of mild steel, two specimens of homogeneous armour plate and a very hard nickel-chrome steel of the type used for ball and roller bearings. The ratio of the dynamic value of the yield stress to the static value was found to increase as the hardness number decreases; the ratio was practically unity for the nickel-chrome steel, about 1⋅1 for the armour plate and about 2 for the mild steel. The values of the static yield stress found by the ball method and by an ordinary tensile or compression test are different; this is probably due partly to the inaccuracy of the criteria of plastic flow, partly to the difference in work-hardening in the two experiments, and partly to changes in the structure of the surface due to polishing. This discrepancy is without effect on the ratio of the dynamic to static yield stress as determined by the ball method, since the stress distributions in the static and dynamic ball experiments are identical.