Yuezhong Feng

Date of Award

5-2012

Level of Access Assigned by Author

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Advisor

Zhihe Jin

Second Committee Member

Michael Peterson

Third Committee Member

James Sucec

Abstract

Monolithic and functionally graded ceramics (FGCs) have promising engineering applications because of their unique properties, such as excellent hardness, and corrosion, oxidation and high temperature resistances. But their inherent brittleness may cause premature failure under severe thermal loading conditions. When subjected to thermal shocks, ceramics and FGCs suffer multiple surface cracking. These surface cracks greatly influence the thermal fracture behavior of the materials. Knowledge of thermal fracture behavior is essential to their applications in severe thermal gradient environment. Thermally induced stresses may cause initiation of pre-existing surface cracks. The crack propagation in thermally shocked ceramics may be arrested depending on the severity of thermal shock, thermal stress field characteristics and material properties. Consequently the materials will suffer strength degradation resulting from the growth of cracks. This work develops thermo-fracture mechanics models investigating the thermal fracture behavior of homogeneous and FGC plates with initial surface cracks subjected to thermal shocks. Four models are considered: (1) an FGC plate with an array of periodic edge cracks with equal crack length, (2) an FGC coated ceramic plate with an array of periodic edge cracks with equal crack length, (3) a homogeneous ceramic plate with an array of periodic edge cracks with alternating crack lengths, (4) an FGC plate with an array of periodic edge cracks with alternating crack lengths. In this study, we examine the thermally induced stress intensity factors, the critical thermal shock to initiate the cracks, the crack propagation and the residual strength of the damaged plates after thermal shock, which characterize the thermal fracture behaviors of plates with surface cracks. The singular integral equation method is used to analyze the thermal shock crack problem. For the plates with multiple surface cracks of equal length subjected to a thermal shock, a Cauchy type singular integral equation is developed. For the plates with multiple surface cracks of alternating lengths, both Cauchy type integral equations and hyper singular integral equations are formulated. In deriving the singular integral equations, a Fourier transform/superposition method is employed. The singular integral equations are solved numerically according to the singular integral equation theory. The stress intensity factors, critical thermal shock, and residual strength then can be determined based on the obtained solutions of the integral equations. Numerical results for homogeneous and functionally graded ceramics are presented to examine the effects of initial crack lengths, initial crack length ratios, crack spacing, and material gradation on the thermal fracture behaviors, such as thermal stress intensity factor, critical thermal shock, thermal shock damage and residual strength. It is found that surface crack morphology in combination with thermal property gradation significantly influences the thermal shock fracture and residual strength behavior of the materials.

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