J-R curve determination of G20Mn5QT cast steel using CT specimen with varying in-plane and out-of-plane constraints based on normalization method and GTN model

Abstract

In this paper, the fracture toughness tests of compact tensile (CT) specimens with varying in-plane constraints were carried out. J-R curves of G20Mn5QT cast steel were firstly measured by normalization method. The results show that with the increase of crack lengths, the in-plane constraints strengthen and J-R curves drop. The fracture morphology was analyzed by Scanning Electron Microscope (SEM). The fracture mechanism is revealed from ductile fracture through mixed ductile and brittle fracture to brittle fracture with the enhancement of in-plane constraints. Then GTN model was used to predict J-R curves of G20Mn5QT using CT specimens with different in-plane and out-of-plane constraints. The effects of f0, fN, fc and fF in GTN model on load–displacement curves, J-R curves and crack evolution process were studied in detail for calibrating GTN model parameters precisely. It is demonstrated that the numerical results using the calibrated GTN model are in good agreement with the experimental measurements, which proves that GTN model is a convenient and effective tool for evaluating the fracture toughness of materials. The increase of in-plane or out-of-plane constraints makes J-R curves descent. There exists interaction between the in-plane and out-of-plane constraints. The lower in-plane constraints strengthen the out-of-plane constraints effects, while the higher in-plane constraints are not sensitive to the out-of-plane constraints.