Effect of 3D stress states at crack front on deformation, fracture and fatigue phenomena

Abstract

Theoretical, numerical and experimental studies involving elastic plate components, weakened by through-the-thickness cracks and subjected to loading parallel to the plane of the plate, are often based on plane stress or plane strain simplifications. These simplifications essentially reduce the dimensionality of the physical three-dimensional problem and enable the achievement of effective analytical and numerical solutions for many important practical problems. The influence of various three-dimensional effects, such as the variation of stresses across the plate thickness, effects of the three-dimensional corner (vertex) singularities and coupling of fracture modes II and III, on the deformation and stresses near the crack front are at present largely ignored or viewed as negligible for all practical purposes. As a result of this view, the outcomes of experimental studies and fracture tests are also commonly analysed within the framework of the plane theories of elasticity. Nevertheless, a number of theoretical and experimental studies over the past two decades have demonstrated that the predictions made within these theories can be unsatisfactory and the effect of three-dimensional stress states at the crack front on deformation, fatigue and fracture of plate components can be significant. This thesis aims to elucidate the role of three-dimensional stress states in the deformation, fracture and fatigue phenomena further. The main outcomes of this thesis are: (1) the development and validation of a simplified method for the evaluation of the fatigue crack front shapes and their effect on the steady-state fatigue crack growth rates in plate components; (2) investigation of the effect of three-dimensional corner (vertex) singularities on the stress intensities and displacement field near the crack front; and (3) development and validation of a new experimental approach for the evaluation of mode I and mode II stress intensity factors from the measurement of the out-of-plane displacements in the near crack tip region, which are affected by three-dimensional effects, and, in particular, by the 3D corner (vertex) singularity. This new research is important in many engineering contexts. For example, the new theoretical model, which takes into account the actual shape of the crack front, can be utilised in advanced fatigue life calculations, as well as in failure investigations. The latter is possible as the shape of the fatigue crack front can now be related to the parameters of fatigue loading. The new experimental approach developed in this thesis can be useful in fracture characterisation of thick plate components with through-cracks. This approach specifically addresses the situation when the Kdominance zone, or William’s solution convergence domain, are relatively small. In this case, the data extraction region can be affected by the three-dimensional stress states leading to significant errors in the evaluation of the stress intensity factors when using traditional approaches. This thesis is presented in the form of a compendium of published papers that are the summation of the research undertaken by the author. The five articles which form the main body of the thesis are united by a common theme, which is the investigation of three-dimensional effects near the crack front on stresses and displacements, fracture and fatigue phenomena. Two appendices are also included; they represent a compilation of the candidate’s publications related to the main topic of the thesis.