Constitutive law and fracture criteria of X90 ultrahigh-strength gas-transmission steel pipe material

For trial-produced X90 gas-transmission pipe, quasi-static tensile tests and stress triaxiality calculation were carried out on five kinds of different notched round bars. It has been found that because of sample notch, when stress triaxiality is increased by 2. 43 times, fracture strain and damage strain energy will be reduced by 29% and 71% respectively. Universal material testing machine and Hopkinson bar test unit were used for tensile experiments at different strain rates. It has been discovered that strain rate has less effect on failure strain, and in a case of quasi-static state and high-rate state, the largest difference is about 10%. Based on Johnson-Cook constitutive and failure model, X90 steel pipe constitutive model with consideration of stress rate effect was established as well as the failure model considering strain rate and stress triaxiality. Meanwhile, according to damage mechanics theory, X90 pipe steel fracture criterion was obtained on a basis of plastic uniform elongation and damage strain energy. Based on the assumption of constant density critical value of material damage strain energy and test data, the relation expression was obtained between X90 pipe steel fracture characteristic length and stress triaxiality as well as specimen diameter.