Dynamic Effects on Fracture Toughness for Ferritic Steel in the Ductile-to-Brittle Transition

Abstract

Dynamic loading effects on fracture toughness of ferritic steel have been evaluated in the ductile-to-brittle transition, considering loading rates representative of object drops. To verify that the design fracture toughness curve, initially defined from static tests, remains conservative for the integrity assessment of object submitted at low temperature to a dynamic impact due to a drop, experiments on 16MND5 ferritic steel have been performed. A three-point bending set-up and a thermal chamber have been designed in order to perform dynamic fracture tests on large Single Edge-notched Bending SE(B) specimens, at very low temperature. Considering that the reference temperature of the material is −122°C (defined from quasi-static tests), dynamic drop tests have been performed at −120°C, −80°C and 0°C in order to cover the ductile-to-brittle transition of the material. A shift of +80°C of the reference temperature has been observed with the increase in the stress intensity rate, from less than 1 MPa.m0.5/s in quasi-static tests to values up to 105 MPa.m0.5/s for the dynamic SE(B) tests. Numerical simulations of the tests, compared to classical static analysis, have confirmed that the effects of inertia and viscosity on fracture toughness are negligible at these temperatures (<0°C). Equivalent static analysis appears sufficient to study such dynamic tests.