Three-Dimensional Fracture Of Macro-Structures And Failure Of Micro-Materials

Materials and structures are two main issues in aerospace and mechanical sciences. Under the complex loadings, the surface flaws of large complex structures and the original defects of materials would initiate the propagation of micro three-dimensional (3D) cracks, and result in catastrophic accidents. For the limitations of the traditional theories of two-dimensional cracks, the safety of actual structures containing 3D cracks cannot be guaranteed completely without the out-of-plane stress constraint. As it is difficult to obtain the accurate analytical solutions for the stress constraints of general 3D cracks, we have conducted systemically detailed researches on the out-of-plane stress constraint by finite elements (FE) and proposed a two-parameter description of non-through thickness 3D crack fields. Several problems in damage tolerant design were also investigated based on continuum mechanics and probability theory.For the severe service-environments, such as high-pressures etc, the requirements for the properties of materials have reached the nano scales, and carbon nano-materials are used as reinforcements of the advanced composites increasingly. As the allotropes of carbon are abundant, new carbon phases are attracting the sights of researchers.High-pressure is either an important method or a condition to generate new phenomenon and synthesize new phases of materials. Therefore, the physical mechanics behaviors of carbon structures, such as nanographite, carbon nanotubes and diamond, under high pressures are investigated by use of molecular dynamics combining with quantum mechanics and FE.The main results and academic contributions of the dissertation can be summarized as following:1) For the difficulty to obtain the accurate analytical solution of the stress constraints around 3D cracks, detailed 3D FE analyses have been conducted to study the out-of-plane constraint factor Tz around a straight through-thickness crack, a quarter-elliptical corner crack and a semi-elliptical crack embedded in an isotropic elastic plate subjected to uniform remote tension loading. The distributions of Tz are minutely studied in the forward section of the cracks. Strong 3D zones (Tz>0) along the crack front were investigated despite the far field stress-free boundary conditions. A unified formula to describe the radial distribution of Tz in the normal plane of the crack...