热对流作用下筒壁涂层的边裂行为

边裂(边缘开裂)是涂层热致损伤的主要模式之一. 边缘裂纹穿透涂层后,常导致界面脱粘从而驱使涂层与基体剥离,最终丧失对基体的保护作用. 本文以热应力强度因子表征边缘裂纹的扩展驱动力,研究筒壁涂层在热对流作用下的边裂行为. 首先,利用拉普拉斯变换法,得到了瞬态温度场及热应力场的封闭解. 其次,运用Fett等的三参数法确定了筒壁涂层边缘裂纹的权函数. 最后,基于叠加原理和权函数方法计算了边缘裂纹的热应力强度因子. 探讨了无量纲时间、边缘裂纹深度、基体/涂层厚度比、热对流强度等参数对热应力强度因子的影响规律. 结果表明:热应力强度因子的峰值既非发生在热载荷初始时刻,也非发生在热稳态时刻,而出现在时间历程的中间时刻;增大热对流强度不仅可提高热应力强度因子的峰值,而且使峰值提前出现;其他条件相同时,热应力强度因子随着边缘裂纹长度的增大而降低;增大涂层厚度或减小基体厚度可增强涂层抵抗瞬态热载荷的能力. 

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.