BAUSCHINGER AND YIELD EFFECTS DURING CYCLIC LOADING OF HIGH-TEMPERATURE ALLOYS AT 550-DEGREES-C

High strain fatigue experiments were carried out on thirteen different high temperature alloys at 550 degrees C. By means of multiple step t ests and interrupted tests in the steady state, the asymmetrical natur e of the Bauschinger effect was examined i.e. having regard to the mag nitude and direction of the stress reversal point during the course of the hysteresis loop. The cyclic stress-strain curve was characterised using the well known Ramberg-Osgood deformation relationship, and dis tinction was made between actual curve shape and the locus approximati on. The yield stress during the course of a cycle could be made to var y according to the reversal point in the interrupted tests, but even i n steady state cycling, the cyclic yield point was found to decrease a s the total strain range was ina eased. Limited investigations were al so performed during evolutionary (hardening or softening) cycling and all results were interpreted in terms of the prevailing back stress an d friction str ess. A formula to predict yield was hence derived for e ngineering use.