Crack bridging in functionally graded coatings

A crack bridging analysis is carried out to predict crack propagation in coatings made of functionally graded materials (FGM). The FGM coating is taken to be a ceramic/metal composite with its gradation characterized by the local volume fractions of metal and ceramic phases. Fracture in the FGM coating is resisted by the plastic deformation of metal ligaments in the crack wake that bridge the crack; the crack bridging, however, is not uniform. A position-dependent crack bridging model is developed taking into account the coating gradation and metal plasticity. The model is subsequently used ina finite element analysis to predict the reduced fracture driving force. It is found that crack bridging in the FGM coating can significantly reduce the crack tip stress intensity. It is also found that coating gradation has a strong influence on the fracture driving force and the crack length at arrest. The present finite element model can be extended readily to study the effect of large-scale plastic deformation on-crack growth in a FGM coating. (C) 1997 Elsevier Science Ltd.