PLASTIC RATCHETTING AS A MECHANISM OF EROSIVE WEAR

Many researchers have observed plate-like wear debris in tests on erosion of ductile metals at near normal impingement. We propose that this is due to 'plastic ratchetting' of a thin surface layer, which is progressively extruded out in the form of thin slivers which subsequently break off to provide wear debris. The process is driven by the repeated random impacts of the erodent. During the impact high contact stresses are generated. If these exceed the elastic limit then localised plastic flow takes place. As a result the material strain hardens and protective residual stresses are developed promoting shakedown, i.e. steady-state behaviour which is entirely elastic. However, if the impact stresses are high enough it is shown that shakedown is impossible and plastic ratchetting would be expected, i.e. small increments of plastic strain are accumulated leading to progressive extrusion of a thin layer of plastically deformed material. The proposed mechanism has been modelled by using the kinematical shakedown theorem of the theory of plasticity. A similar mechanism leading to wear has been shown to take place in lubricated sliding, where slivers of a softer surface are extruded by the pummelling action of the asperities on a harder counterface (A. Kapoor and K.L. Johnson, Proc. R. Sec. London, Ser. A, 445 (1994) 367-381).