Investigation of the effects of material parameters on the relationship between crack tip constraint and CTOD fracture toughness

In elasto-plastic fracture mechanics, the fracture toughness is not a material intrinsic parameter but depends on the constraint level ahead of the crack tip. The accurate determination of the effect of the crack tip constraint on the fracture toughness is very important for the safety assessment of crack-containing structures. In this work, the pipeline steels are selected as the research materials. The single edge notched bending (SENB) specimens and the complete Gurson model are used to investigate the effects of material parameters on the relationship between the crack tip constraint and the fracture toughness. The research results show that the main factor affecting the crack tip constraint is strain hardening capacity. The main factors affecting the critical crack tip opening displacement (CTODc) are strain hardening capacity and initial void volume fraction. There is a logarithmic relationship between the constraint and the fracture toughness. Higher strain hardening capacity and higher yield strength can result in higher decreasing degree of the fracture toughness in the increasing process of the constraint. Higher initial void volume fraction can result in lower decreasing degree of the fracture toughness. A constraint-based fracture failure criterion considering material parameters is proposed and validated. The validation results show that the proposed criterion has high accuracy and great potential to be applied to the research of the ductile fracture of pipelines.