EDGE CRACKING BEHAVIOR OF A COATED HOLLOW CYLINDER DUE TO THERMAL CONVECTION

Edge cracking is one of major damage modes for coatings subjected to thermal transients. After penetrating across coating thickness, edge cracks usually cause interfacial decohesion and hence result in the detachment of coating from substrate, which leads to the ultimate loss of the protective effect on the substrate. The edge cracking behavior due to thermal convection is studied in this paper for a coated hollow cylinder, where the thermal stress intensity factor is used to characterize the crack driving force. Firstly, by using the Laplace transform technique, closed-form solutions are obtained for the transient temperature as well as thermal stresses. Secondly, the weight function for an edge crack in a coated hollow cylinder is determined by using the three-parameter method proposed by Fett et al. Finally, the thermal stress intensity factor at the edge crack tip is evaluated based on the principle of superposition and the derived weight function. The dependence of the normalized thermal stress intensity factor is examined on the normalized time, edge crack depth, substrate/coating thickness ratio as well as thermal convection severity. It is shown that the peak thermal stress intensity factor occurs neither at the very beginning nor at the thermal steady state of a thermal transient, but at an intermediate instant. The severer thermal convection generates a peak thermal stress intensity factor not only higher in magnitude but also earlier in time. Should other conditions remain invariant, the thermal stress intensity factor is a decreasing function of the edge crack depth; a thicker coating or a thinner substrate may enhance the thermal transient resistance of a coating.