Experimental and numerical investigations of tensile specimens containing multiple flaws in the ductile-to-brittle transition region

Abstract

In thick-walled ferritic steel forgings, a large number of flaws can arise due to an unfavorable chemical composition or an erroneous heat treatment. Thus, a comprehensive understanding of the interaction of multiple flaws and their failure mechanisms is necessary.In this paper, experimental and numerical results from tensile specimens containing multiple flaws nearly parallel to the loading direction are presented. The material used for these investigations is the ferritic pressure vessel steel 22NiMoCr3-7, which is similar to an SA 508 Cl.2 steel. The experiments were conducted in the upper and in the lower shelf of the ductile-to-brittle transition region of fracture toughness. In addition, several specimens were tested to determine the material characteristics and to examine the fracture mechanisms for different levels of stress triaxiality.For numerical evaluations, damage mechanics models were used to investigate flaw interaction and the fracture mechanisms. In the upper shelf, an extended non-local Rousselier model was used. The numerical investigations in the lower ductile-to-brittle transition region were carried out with a coupled damage mechanics model. This coupled model consists of the extended non-local Rousselier model and an extended Beremin model, which was newly developed. The modifications of the models are accounting for low stress triaxiality and mixed fracture (dimple and cleavage fracture).A comparison between the numerical und experimental results shows that the defined damage mechanics models are able to describe the deformation and failure behavior of the tested specimens with a high accuracy. The investigations show that the flaw interaction behavior is strongly dependent on the temperature and the arrangement of the cracks. In the upper shelf, the crack growth is shear-stress dominated, whereas in the lower ductile-to-brittle transition region the crack growth is preferably normal to the loading direction.