Micromechanics investigation of defects in solid materials

• Main Author: Yang, Jing
• Other Authors: Zhou Kun
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Mechanical engineering::Mechanics and dynamics
• Online Access: http://hdl.handle.net/10356/75225
• Institution: Nanyang Technological University

Microdefects such as cracks, vacancies, voids and inclusions are often formed in materials during their manufacturing processes or under working conditions. These defects have significant influences on the mechanical and physical properties of components, especially those for aerospace, automotive and offshore engineering applications, and may eventually result in the damages and failures of these components. Therefore, it is of great importance to investigate the mechanics of materials with these microdefects for the minimization of potential damage and failure of the components. Firstly, a semi-analytic solution is proposed to solve the elastic-plastic fracture behaviors of an infinite space with multiple cracks and inhomogeneous inclusions under the remote tensile stress in this thesis. Based on the Equivalent Inclusion Method, each inhomogeneous inclusion can be modeled as a homogenous inclusion with the initial eigenstrains plus unknown equivalent eigenstrains. The cracks are regarded as a distribution of edge dislocations with unknown densities based on the Distributed Dislocation Technique. By using a modified conjugate gradient method, all the unknown equivalent eigenstrains and dislocation densities are obtained iteratively. The fast Fourier transform and discrete convolution are adopted to improve the computational efficiency. According to the Dugdale model, the plastic zone sizes of cracks can be obtained by canceling the stress intensity factor due to the closure stress and that due to the applied external loading. The effect of the Young’s moduli and positions of inhomogeneous inclusions on the plastic zone sizes is investigated. Secondly, two semi-analytic solutions for the elastic-plastic fracture behaviors of a half-space with cracks subjected to the prescribed loading and contact loading are also developed in this thesis. For the inhomogeneous contact problem, the unknown contact area and pressure can be obtained iteratively when the surface displacement induced by the subsurface cracks and contact loading converges with a numerical algorithm. According to the modified Dugdale crack model, it is found that the plastic zone sizes of crack tips are significantly influenced by the yield strength of the matrix material, the crack depth, the original crack length, and the external loading. Thirdly, the elastic-plastic behaviors of a film-substrate with inhomogeneous inclusions under contact loading are studied by modeling the coating material as an inhomogeneous inclusion with respect to the substrate. A plasticity loop and an incremental loading process are used to obtain the accumulative plastic strain iteratively. This model considers not only the interaction among the contact loading body, embedded inhomogeneous inclusions and film materials, but also the elastic-plastic behaviors of the film-substrate system. Finally, the elastic-plastic behaviors of a half-space with inhomogeneous inclusions and cracks subjected to contact loading are studied. This model considers not only the horizontal cracks but also the vertical cracks. The plastic zones in the half-space can be determined based on the substrate stress distribution and the von Mises yield criterion. The research work in this thesis not only provides knowledge of the damage behaviors of materials containing microdefects, but also is of guiding significance for the improvements of functionality and reliability of engineering components.