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

Abstract

Dynamic loading effects on ferritic steel toughness have been evaluated in the brittle-to-ductile transition, considering loading rates representative of object drops. To verify that the brittle-to-ductile transition curve, initially defined from static tests, tends to shift to higher temperatures due to dynamic effects even in the case of object drops, experiments on 16MND5 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) specimen, at very low temperature using a drop-shock machine. In a first step, considering that the reference temperature of the material (according to the master curve concept) is −122 °C, dynamic tests at −120 °C have been performed. These tests have confirmed that the fracture mode is still brittle at this temperature, when an impact speed of 4.85 m/s is used. Elastic-plastic or viscoplastic dynamic simulations of the tests, compared to classical static analysis, have demonstrated that the effects of inertia and viscosity on fracture toughness are negligible considering the very low values obtained on these tests at −120 °C. These results also confirm the decrease of fracture toughness due to dynamic loading compared to experimental data from static tests. A further step will be to complete this demonstration with dynamic tests at higher temperatures in the brittle-to-ductile transition.