Temperature and Loading-Rate Dependence on a Local Criterion for Cleavage Fracture in the Transition Region of the Tempered Martensitic Steel Eurofer97.

Abstract

Finite element calculations of stationary cracks were performed in the ductile to brittle transition region of the reduced activation tempered martensitic steel Eurofer97, for which a large fracture toughness database was already available. Two models were run in this work: i) compact tension specimens simulated in 3D, and ii) small scale yielding boundary layer model in 2D plane strain conditions. The analysis was focused on the unusual low fracture toughness of Eurofer97 on the lower shelf, with respect to other "ferritic" steels, as well as in the lower transition. The analysis of the near tip stress field suggested that the minimum stress intensity factor on the lower shelf represents the minimum loading condition to have the local fracture stress act over a distance of the order of the prior austenite grain size. The effect of loading rate on the tip stress fields were also investigated by considering a strain rate dependence of the flow stress explicitly in the finite element calculations. These effects were shown to be non-negligible and to depend on the ratio between the peak stress in the fracture process zone and the local fracture stress.