Influence of a zero-load rest period in elevated-temperature fatigue

Abstract

A preliminary investigation of the influence of rest periods on Stage I crack growth has been performed on a 2½ per cent Ni Cr Mo steel by testing cylindrical specimens under high-strain reversed torsion conditions at 20, 100, 200, 300 and 400°C. Increases in lifetime of up to 100 per cent are observed, and their magnitude is a function of temperature and the rest-period duration, with maximum lifetime occurring at 300°C and a total rest period of 20 hours. Strain ageing is cited as the dominant process, which explains the dependence of optimum lifetime conditions on both temperature and total rest period duration. Since components are frequently rested and/or subjected to variable strain-rate conditions over a range of temperatures, it is suggested that results obtained from uninterrupted laboratory tests may be conservative unless overageing occurs leading to cyclic softening.