Analytical modeling and analysis of fracture of functionally graded coatings

功能梯度涂层（FGCs）是一种隔热、防磨损、防腐蚀的涂层材料，广泛应用于核能、航空航天、工程等领域，需要承受不同的热载荷和机械载荷，并具有耐高温、耐磨损和耐腐蚀的性能。然而，裂纹可以从初始缺陷或在制造或服务期间出现的微裂纹开始。因此，研究梯度涂层结构的断裂对于更好地理解梯度涂层的抗断裂性能具有重要意义。该项目的主要目标是开发一个计算半分析模型的断裂分析的FG热障涂层的均匀基板进行热机械负荷与塑性纳入模型。FGC/均质结构（半无限各向同性介质）的断裂问题将在热载荷和/或机械载荷（例如，热通量、由冷却-加热引起的残余热应力、张力）。这些问题反映了FGC结构开发过程中出现的最重要的情况。具有预先存在的裂纹系统（边缘裂纹、涂层和基底之间的界面裂纹或FGC中的内部裂纹）的FGC结构的问题通常将通过积分方程和考虑裂纹处的塑性区（在Dugdale模型的框架中）来表述。然后，典型的裂纹图案FGC从文献中的实验结果将详细研究的基础上导出的模型。FGMs的热性能和弹性性能将从其适用性的角度来模拟FGCs在热载荷和机械载荷下的裂纹问题。为了简化问题，相关的函数形式，如指数或幂律函数，将被用来模拟FGC的非均匀特性。在此半分析模型的基础上，将针对FGC中不同的真实的材料组合（陶瓷/陶瓷、陶瓷/金属）和材料性能变化进行FGC断裂过程的模拟。该模拟将为选择合适的材料组合和FGC的级配分布提供知识，以优化FGC的抗断裂性能。

Functionally graded coatings (FGCs) are applied in thermal, wear and corrosion barriers which are used in different fields, such as, nuclear energy, aerospace, engineering, etc. They are subjected to different thermal and mechanical loading and have to resist high temperature, wear and aggressive environments. However, cracks can initiate from initial defects or micro-cracks appear during manufacturing or service. Therefore, the study of fracture of FGC structures is important for a better understanding of the fracture resistance of graded coatings. The primary objective of the project is to develop a computational semi-analytical model for the fracture analysis of FG thermal barrier coatings on a homogeneous substrate subjected to thermo-mechanical loading with plasticity incorporated in the model. The problem of fracture of FGC/homogeneous structures (semi-infinite isotropic medium) will be investigated under the influence of thermal and/or mechanical loads (e.g., a heat flux, residual thermal stresses caused by cooling-heating, tension). These problems reflect the most important cases which arise during the exploitation of FGC structures. The problem for FGC structures with pre-existing systems of cracks (edge cracks, interface cracks between the coating and substrate or internal cracks in the FGC) will be formulated in general by means of integral equations and with accounting for plastic zones (in the frame of the Dugdale model) at the cracks. Then, typical crack patterns for FGCs resulting from experiments available in the literature will be studied in detail on the basis of the derived model. The thermal and elastic properties of FGMs will be modeled from the point of view of its applicability to problems of cracks in FGCs under thermal and mechanical loadings. In order to simplify the problem relevant functional forms, such as, exponential or power law functions, will be used to simulate the non-homogeneous properties of FGCs. On the basis of this semi-analytical model the simulation of fracture processes in FGCs will be performed for different real material combinations (ceramic/ceramic, ceramic/metal) and material property variations in the FGC. This simulation will provide knowledge for choosing appropriate material combinations and the gradation profile of FGCs in order to optimize the fracture resistance of the FGCs.