Crack Front Shape Evolution in Structural Components subjected to Fatigue Loading

Abstract

Fatigue analysis is one of the most important and challenging aspects in the design and evaluation of engineering structures working under fluctuating mechanical or thermal loading. Although extensive research has been undertaken over the past two centuries to improve fatigue life prediction methods, there are still many issues and problems remaining, which warrant further study. One such issue is adequate modelling of the evolution of the shape of structural defects (cracks) in structural components subjected to fatigue loading. Procedures and methods that are currently employed for fracture and fatigue failure forecasting are largely based on two-dimensional (2D) stress or strain field assumptions, which simplify the actual geometry of the structural components and defect shapes. As documented in many previous studies, these simplifications can lead to significant errors and to non-conservative predictions. There is also much experimental evidence indicating the significant influence of three-dimensional (3D) effects on fatigue crack growth, as well as on brittle fracture initiation. The 3D effects include, but are not limited to, the variation of stresses and stress intensity factors along the crack front, the presence of the 3D corner (vertex) singularities and the existence of coupled fracture modes, in addition to the classic fracture modes (modes I, II and III). In addition, there is the strong effect of the out-of-plane constraints on fatigue crack closure and crack growth rates in plate and shell components. Therefore, an account of more realistic (3D) shapes of structural defects and the 3D effects associated with these geometries is of a great importance in order to gain more confidence in fatigue life predictions, decrease the cost of inspections and maintenance, and allow structures to operate beyond design service life predictions. In addition, the implementation of 3D fatigue models can help to reduce various uncertainties and assumptions associated with the current 2D modelling. Direct numerical simulations of 3D fracture and fatigue problems remain difficult. Therefore, this thesis aims to develop new, simplified, semi-analytical methods for the evaluation of front shapes of fatigue cracks and fatigue life in typical structural components, such as plates and round bars, which utilise more realistic geometries of structural defects. The thesis further elucidates the role of plasticity-induced crack closure and the 3D corner singularity on the crack front shape and its evolution. It is expected that the new methods, which are developed in this thesis, may provide more accurate predictions of crack growth and fatigue life expectancy for typical structural components. This hypothesis is supported by extensive validation studies and comparisons against previous theoretical results and experimental data. The main body of the thesis (Chapters 4 - 7) is presented in the form of a collection of published journal and conference articles authored by the candidate, who made a significant contribution to the conceptualisation, data analysis, calculations and drafting involved. A compilation of the candidate’s publications relating to the main topic of the thesis but with less significant involvement is also provided in the Appendix. In addition, several Chapters (Chapters 1 - 3 and 8) are included to communicate the context, significance of this work and cohesive presentation, as well as to summarise the main outcomes of this thesis.