Finite element analysis of vickers indentation cracking processes in brittle solids using elements exhibiting cohesive post-failure behaviour

The cracking processes during the indentation test of brittle solids is simulated by means of the finite element method (FEM) using elements exhibiting cohesive post-failure behaviour and alumina as the model material. The results show that at low indentation loads, median cracks could nucleate at full loading but Palmqvist cracks only nucleate in the unloading stage and that they may not join up even after full unloading. Such cracks are stable as they are embedded in a region of high hydrostatic compression throughout the indentation test. At high indentation loads, both median and Palmqvist cracks nucleate early during the loading stage and coalesce to form a half-penny crack on further loading. Although the cracks are embedded in a region of high hydrostatic compression during loading, an annular tensile region eventually develops in between the cracked material beneath the indenter and the surrounding uncracked material during the unloading stage of the macro-indentation. This not only provides the driving force for existing cracks to grow but also new crack systems to form. The present work shows that for brittle solids with negligible plastic deformation, the mismatch in elastic recovery between the cracked and uncracked bodies on unloading plays an important role in indentation fracture processes.