The thesis reviews the historical development of stress analysis for the solution of stress functionsin the neighbourhood of cracks or flaws. The analysis assumes the material is a brittle continuum obeying the laws of linear elasticity. The influence of material structure on such analysis is considered both from the experimental and the theoretical point of view. For the experimental investigation, clay ceramics based on Kaolinite are used to provide a range of different coarse polyphase structures in the form of test pieces suitable for fracture testing in a three point bend rig. The values of the fracture stress OF, the critical stress intensity Kic and the modulus of elasticity E obtained in this way, are found to follow similar trends and to vary with the clay formulation. Scanning electron microscopy has been employed to study the intrinsic structure of the material. It is found that the measured values, not only of Kic and E, but also OF can be related to the structure. Thus in these materials, OF is not random flaw controlled and OF, Kic and E are all material influenced parameters. The validity of applying the stress equations to structured materials is shown to depend on the stress field at the crack tip being of the same form for both OF and Kic determinations. It is argued that in the case of quasi brittle materials, this is ensured by the plastic zone at the crack tip. In the case of the brittle ceramic materials investigated here, it is shown that similarity of stress at the crack tip is produced by subcritical crack growth. With subcritical crack growth, a physical significance can be attributed to derived variables such as the equivalent elastic crack, and the material dependence of OF can be explained.
|Date of Award||1977|