Low cycle fatigue specimen design, hardening, softening and strain concentration effects at elevated temperatures

Abstract

This paper considers the likely strain concentration effects generated in the shoulder/gauge intersection region of typical specimens employed in low cycle fatigue tests at high temperatures. The starting point is taken as the well known elastic stress concentration factor curves of Peterson which are extrapolated using data in the literature and other supporting (finite-element) calculations. Elastic–plastic values of strain concentration factors are then derived using the relations due to (a) Neuber, (b) Stowell–Hardrath–Ohman and (c) an energy method. It is shown in each case that these factors decrease with evolutionary cyclic hardening and increase with cyclic softening in a systematic way, as the material behaves according to the Ramberg–Osgood deformation law. A sensitivity study is carried out for each method, both for a ‘model material’ and real alloys and the implications of the shoulder radius/gauge diameter ratio is considered against the recommendations of various Testing Standards and Round-Robin undertakings.