Estimating ductile crack initiation in structural steel under combined tension and shear stress using a modified void growth model

Ductile fracture initiation is one of the important issues in recent research on structural steels. Many models have been established to simulate the mechanism of ductile fracture and predict the ductile crack initiation in different loading conditions. The void growth model is one of the most successful micro-models that is originally developed to estimate ductile crack initiation in the first mode of fracture under monotonic loading. However, one of the main shortcomings of these types of uncoupled void growth models is the incapability of them in capturing the behavior of steels under shear stress and the effect of it on the resulting voids and the process of ductile fracture in low and moderate-stress triaxialities. A newly proposed modified void growth model represented in this article is able to overcome this issue by introducing normalized maximum shear stress. A series of scanning electron microscopy examinations and experimental and numerical monotonic tensile examinations have been carried out on the notched round bar and flat groove plate specimens with different notch radii to calibrate the represented fracture model. Steel-holed plates with symmetric notches around the hole and notched shear plates have been tested and analyzed to demonstrate the application and validation of the represented void growth model in predicting the initiation of ductile fracture in various combinations of tension and shear under monotonic loading. Different angles of notches were used in the specimens to achieve diverse stress states to examine the application of the represented micro-model in non-uniform stress state fields. Finally, the role of the characteristic length, as the representative of the density of the inclusions in the material, on the accuracy of the model has been discussed in this article.