Experimental and numerical studies on ductile fracture behavior of X80 pipeline steel: Phase ratio and grain size

The fracture characteristics of X80 pipeline steel, particularly the dependence of fracture mechanism on micro-component and micro-structure, represent an important and intricate issue. This study investigates the fracture performance of X80 steel through experimental analysis and multi-scale numerical simulation. The critical J-integral and critical crack-tip opening displacement (CTOD) are determined via the fracture toughness test, and the fracture morphology showed that the fracture characteristic is a ductile fracture. A mechanical model is proposed to describe the elastic–plastic and fracture behavior of X80 steel under different micro-component ratios, including the related changes caused by the increased bainite content: the change in stress–strain relationship, the transformation in hardening behavior from three to two stages, and the change in propagation and evolution of macro-cracks. The simulation results effectively reproduce the crack propagation process in X80 steel with varying micro-component ratios, indicating that the fracture resistance of X80 steel does not continuously increase with an increase in bainite content. A balance must be struck between the exceptional plasticity of ferrite and the high strength of bainite. Additionally, a representative volume element (RVE) utilizing Voronoi tessellation is used to examine the effect of grain size on intergranular fractures. Grain refinement leads to an increased number of turns and deflections in the crack trajectory within the RVE model, enhancing the obstruction to fracture and thereby improving fracture resistance. These research findings contribute to a deeper understanding of the fracture characteristics of X80 pipeline steel and provide a theoretical foundation for optimizing its fracture performance. • The fracture toughness test of X80 high-strength pipeline steel is carried out. • A model is proposed to describe the deformation and fracture of X80 steel. • The model can describe elastoplastic behavior at various bainite/ferrite ratios. • The cohesive element is combined in the model to achieve the fracture simulation. • The relationship between fracture and micro-component ratio is investigated. • The dependence of intergranular microfracture on grain size is investigated.