On the fracture locus of ductile materials

Abstract

Predicting the limits for safe use of a material under combined stresses requires the application of a fracture criteria. The stress triaxiality and the equivalent strain to fracture is found to be the most important parameters governing ductile fracture. Work by Rice and Tracey lead to the well-established opinion that the relationship between these two parameters are exponential. However, Bao and Wierzbicki found that the exponential relationship does not t for the low triaxiality and negative triaxiality regimes.

This thesis presents the experimental work done to investigate the results obtained by Bao and Wierzbicki on the high strength aluminium alloy AA7075-T651 that is widely used for industrial applications. Material tests were performed i three different direction on a rolled plate of this material; namely the 0º, 45º and 90° directions. It is found that the three regimes suggested by Bao and Wierzbicki are better suited to predict fracture. It is also found that the anisotropy of the material greatly affects the shape of the fracure locus.

Full-field measurement methods were used. It is concluded that the use of digital image correlation for the measurements of in-plane deformations are a good measurement method for material testing since it enables the tracking of strain localizations, and thus clearly capture the material behavior. This is a good tool to use when calibrating material models in the finite element method, and is likely to be an indispensable tool in the future as more and more advanced models are developed and the accuracy requirements are increasing.