Fracture toughness assessment of the X80 steel by nanoindentation technique and a modified constitutive model

Abstract Fracture toughness of X80 steel is investigated and characterized by the nanoindentation technique and a modified constitutive model. Firstly the load-displacement of the X80 steel is tested by nanoindentation technique. Based on the load-displacement, the relationship of the nanoindentation load-depth is established. Secondly, the elastic modulus calculation equation was modified by introducing the radial displacement correction factor γ considering the pile-up phenomenon. At last, based on the strain energy density and indentation fracture energy theory, the nanoindentation fracture toughness constitutive model of X80 is proposed combined with the modified elastic modulus and the critical plastic depth. The fracture toughness calculated by the proposed method locates in the dispersion band range tested by the standard three point bending specimen, while the result calculated by the original method is far away from the dispersion band range. The comparison results illustrate that the proposed method can well characterize the fracture toughness of the X80 steel. The present work is of great significant for ensuring the long term and safety operation of the petroleum and natural gas transporting pipeline.