Examination of eroded surfaces of ductile materials under a scanning electron microscope typically shows thin slivers or platelets; w Ε½ .
x these subsequently break off to provide wear debris. Cousens and Hutchings A.K. Cousens, I.M. Hutchings, Wear 88 1983 335-348 have studied this phenomenon in careful experiments, in which commercially pure aluminium and aluminium alloys were eroded by using glass beads and iron shot in an air blast erosion tester. It was observed that a thin surface layer hardened both due to embedding of the fragments of erodent and due to work hardening. The repeated impacts by erodents, then, caused the softer material to extrude out backwards, from discontinuities in the hardened layer. It is proposed that this 'backward extrusion' is due to 'plastic ratchetting' of the material below the hardened layer, which is progressively compressed and extruded out. The proposed mechanism has been modelled by using kinematical shakedown theorem of the theory of plasticity and the impact pressure which needs to be exceeded for the process to continue has been evaluated. By expressing the impact pressure in terms of impact velocity and material properties of the shot and the eroding surface a non-dimensional velocity is defined and related to the shakedown behaviour of the eroding system. It is shown that for the backward extrusion process to continue the non-dimensional impact velocity must be above a critical value.