Influence of Loading Rate on the Fracture Toughness of High Strength Structural Steel

Abstract

It is known that rates of loading influence the fracture behaviour of most ferritic steels. High loading rates could change a stable ductile tearing behaviour to an unstable brittle fracture by altering the ductile-to-brittle transition curve. This is predicted to be material dependent, with lower strength structural steels showing a larger tensile property loading rate sensitivity compared to high strength structural steels. A programme of mechanical testing was carried out on S690QL and S960QL to determine the influence of loading rate on the fracture behaviour of high strength structural steels with yield strength > 690 MPa and yield-to-tensile ratio above 0.90. The loading rates considered are those anticipated in offshore in-service conditions, with K-rates up to the order of magnitude of 106 MPa√m/s. Results from tensile tests show that the strengths of these grade of steels are relatively unaffected by the effect of loading rate. However, brittle fracture, which is controlled by material strengthening as a result of principal stress in front of the crack, is both loading rate and temperature dependent. Results from tests at quasi-static and elevated loading rates show changes in the fracture behaviour in terms of transition temperature. A shift to a higher ductile-to-brittle transition temperature was observed as the loading rate increases. This was associated with a reduction in the fracture toughness value on the lower transition region. The reference temperature, T0, at a K-rate of 1 MPa √m/s using Master Curve concepts is estimated to be around -116 °C and -108 °C for Charpy-sized pre-cracked and standard (25x25 mm) SENB specimens respectively, under quasi-static conditions for S690QL. The dynamic T0,d is -70.4 °C in the same steel for Charpy-sized pre-cracked specimens at K-rates up to 106 MPa √m/s.