Bauschinger yield in the range 400–1025°C during cyclic deformation of high temperature alloys

Abstract

Reversed-load (low-cycle-fatigue type) experiments were carried out on 13 different high temperature alloys (ferritic, austenitic and superalloys, single crystal materials, a directionally solidified alloy and two titanium alloys) in the range 400–1025°C. By means of interrupted tests in the steady state, the asymmetrical nature of the Bauschinger effect was examined, i.e., having regard to the magnitude and direction of the (prior) stress reversal during the course of the hysteresis loop. The cyclic stress–strain curve was characterised using the Ramberg–Osgood power deformation relation. The yield stress in a given direction during the course of a cycle could be made to vary according to the reversal point in the interrupted tests. A simple interpretation is provided in terms of the prevailing difference between the back stress and a friction stress. Comparison of the back-stress behaviour is made with that calculated from the widely used Chaboche relation. Predictions of yield stress behaviour are compared with values derived from (i) kinematic hardening, (ii) stored energy arguments, (iii) friction stress values and (iv) a Masing model based on a continuous yield distribution. Using an overall yield–strain criterion of 0.05%, a formula to predict yield under interrupted cycling conditions was hence derived for engineering use.

Keywords:

• cyclic stress–strain
• back stress
• friction stress
• model material
• yield strain criteria
• hysteresis loop
• yield map