Cyclic strain rate effects in fatigued face-centred and body-centred cubic metals

Abstract

The present work deals mainly with the effect and the use of strain rate and temperature changes during cyclic deformation as a means to obtain valuable information on the thermally activated dislocation glide processes, based on the assessment of reversible changes of the thermal effective stress and of transient changes of the athermal stress. The importance of closed-loop testing in true plastic strain control with constant cyclic plastic strain rate throughout the cycle is explained and emphasized, especially with respect to the case of strain rate sensitive materials. Stress responses of face-centred cubic and body-centred cubic (bcc) metals to cyclic strain rate changes are presented to illustrate that the deformation modes of these two classes of materials differ characteristically at temperatures below that the so-called knee temperature of bcc metals. When such tests are performed in cyclic saturation, the temperature and strain rate dependence of bcc metals can be measured very accurately on one and the same specimen, permitting a thorough analysis of thermal activation.

Keywords:

• cyclic strain rate changes
• cyclic temperature changes
• thermally activated dislocation glide
• closed-loop tests
• true plastic strain control

Acknowledgements

Some of the work presented here dates back to (unpublished) studies done at the Max-Planck-Institut für Metallforschung in Stuttgart in the eightíes. The author gratefully acknowledges the interest of the director of the institute, Professor Alfred Seeger, in these studies and the support provided.

Notes

Dedicated to the memory of Sir Alan Cottrell.