Effect of Hydrostatic Pressure on the Direct-Stress Fatigue Strength of an Alloy Steel

Abstract

This paper describes tests in which specimens are subjected to cyclically varying direct stress, that is, in ‘push-pull’ with additional tensile and compressive mean stress, both at atmospheric pressure and also while the material is subjected to ambient fluid pressure of the order 3000 atm (20 ton/in 2 ); for this work a novel testing machine was developed. The results of tests at atmospheric pressure are in good agreement with earlier tests and show that the range of direct stress leading to fatigue failure is a continuous function over the range of mean stress used. When fluid pressure of 20 ton/in 2 is superimposed on the stress system, the range of reversed stress required to cause fatigue failure is increased by a small, but definite, amount. For the tensile mean stress values investigated the fatigue limit appears to have been raised until it coincides with the range of stress required to cause ‘gross yielding’ with a reduction in area greater than that which occurs in a static tension test at atmospheric pressure. For the compressive mean stress values investigated, fatigue failure has been entirely eliminated.