Simulations of Crack Propagation in Porous Materials

Failure propagation behavior of thermally sprayed coatings containing many random pores is investigated. The porous coatings are subjected to either external mechanical loads or residual stresses generated by temperature changes. The failure growth criterion is governed by the critical energy release rate. In our finite element analysis, the cohesive model is used to separate element boundaries during crack propagation in the inhomogeneous materials. The accuracy of the cohesive elements for the quasi-static crack growth is closely evaluated by an error analysis. We have observed that the cohesive elements may artificially increase the model compliance and introduce numerical errors. In order to minimize such errors, the parameters for cohesive model must be chosen carefully. Their numerical convergence and stability conditions with an implicit time integration scheme are also examined. In the porous material analysis, crack propagation is simulated to characterize its unique failure process. It appears a crack tends to propagate along the shortest path between neighboring pores. In addition, crack/pore coalescence mechanism causes the apparent crack length to increase discontinuously. Under thermally loaded conditions, the residual stresses generated by material mismatch in multilayered coatings drive cracks to grow. Using the present crack propagation model, the critical temperature leading to the complete porous coating failure can be approximated.