Brittle Fracture in Heterogeneous Materials — Theory, Experiments, and Numerical Modelling

Brittle fracture in the ductile-to-brittle transition regime raises complex issues for structural integrity assessments, particularly when fracture toughness is influenced by heterogeneity. This thesis investigates brittle failure through theoretical, numerical, experimental, and probabilistic approaches, focusing on understanding and modelling fracture behaviour in multipass welds. A probabilistic framework is developed to evaluate how spatial heterogeneity affects failure probability curves obtained from fracture toughness testing. The distinction between small-scale and large-scale heterogeneity is formalised based on the relative size of the heterogeneity compared to the fracture process zone. Numerical simulations show how heterogeneity shapes failure probability curves, and scaling methods are introduced to relate results from miniature specimens to full-scale conditions. Experimental work includes fracture toughness testing and fractography of agedand unaged weld metals. Thermal ageing introduces heterogeneity in toughness through zone-specific embrittlement caused by phosphorus segregation. The role of grain orientation in brittle failure is investigated, and limitations of standard testing methods under heterogeneous conditions are addressed. The developed methods form a foundation for fracture assessment instructurally heterogeneous materials and support more reliable interpretation of fracture toughness data in safety-critical applications.