Strain avalanches in plasticity

Abstract

Plastic deformation at the mechanism level in all solids occurs in the form of discrete thermally activated individual stress relaxation events. While there are clear differences in mechanisms between dislocation mediated events in crystalline solids and by individual shear transformations in amorphous metals and semiconductors, such relaxation events interact strongly to form avalanches of strain bursts. In all cases the attendant distributions of released energy as amplitudes of acoustic emissions, or in serration amplitudes in flow stress, the levels of strain bursts are of fractal character with fractal exponents in the range from −1.5 to −2.0, having the character of phenomena of self-organized criticality, SOC. Here we examine strain avalanches in single crystals of ice, hcp metals, the jerky plastic deformations of nano-pillars of fcc and bcc metals deforming in compression, those in the plastic flow of bulk metallic glasses, all demonstrating the remarkable universality of character of plastic relaxation events.

Keywords:

• plasticity
• strain bursts
• avalanches

Acknowledgements

I gratefully acknowledge many discussions with Julia Greer on the plastic response of micro and nano pillars, a field to which she added much. I also thank Dr Jerome Weiss for furnishing an electronic file of Figure 1 and similarly express my thanks to Dr David Klaumuenzer for furnishing an electronic file of Figure 2.

Notes

1. The term STZ as introduced by Falk and Langer [37] and as adopted often by many others indiscriminately, are pre-existing in the structure as site imperfections often possessing principal directions and as in transitions in “two level activated systems”, flip from one type into another, under stress. In distinction STs, as originally introduced by Argon [4] do not pre-exist, but are consequences of local stress-activated structural transformations centered around a fertile site containing LL environments.