Fracture failure of mild steel and its application in steel pipeline

The introduction of defects, such as scratches, pits and holes, are inevitably involved in engineering structures. Among the threats of defects to structural failures, cracks have been well recognised as a major cause of potential failure in structural components. As engineering structures are prone to mixed-mode fracture due to the oblique orientation of cracks or multi-axial loading, there is a well justified need to investigate the fracture behaviour and mechanism of engineering materials under mixed-mode loading. This research investigates the fracture characteristics of mild steels under the full range of in-plane loading. A novel experimental method was developed to determine the onset of crack extension and the critical values of fracture parameters. Comprehensive experimental work was undertaken to analyse the trends of fracture resistance of mild steels under various in-plane loading from the perspective of the material properties (ductility, strain hardening level, mode I fracture toughness), crack tip deformation characteristics and fracture mechanisms. In addition, the fracture behaviour of naturally corroded mild steel was fully investigated. With the test and analysis results, the fracture criteria based on elastoplastic fracture mechanics were applied in reliability analysis with the time-dependent reliability method. This research highlights the mechanisms of the mode mixity effect on steel fracture resistance and argues that the practical applications of fracture toughness tested under mixed-mode loading are necessary. This research contributes to the body of knowledge of fracture behaviour of elastoplastic materials and improves the application of ductile fracture criteria and the fracture resistance with crack extension to structural integrity assessments.