Analysis of ductile fracture by extended unified strength theory

Ductile fracture initiation models for predicting cracks or cavities nucleation in crack-free metals are vital to meet the rising requirements of lightweight and safety for engineering applications. In order to improve accuracy of the ductile fracture initiation models such as the Extended Mohr Coulomb (EMC) type model and calibrate them easily, the Extended Unified Strength Theory (EUST) is proposed which can investigate the effects of hydrostatic pressure, the Lode angle, the intermediate principal stress, the independent principal shear stresses, and the corresponding normal stresses acting on the same planes on ductile fracture. By using the plasticity model, the EUST is transformed from stress space to the space of equivalent strain to fracture (epsilon) over bar (f), the stress triaxiality eta, and the normalized Lode angle parameter (theta) over bar. The parameters of the EUST fracture locus related to uniaxial tensile strength, uniaxial compressive strength, and shear strength possess intuitive physical meanings. Many other fracture loci including the EMC fracture locus are special cases of the EUST and the relations of the parameters between the present model and the existed models are established. The test data on 2024-T351 aluminum alloy and TRIP 780 steel sheets are used to calibrate and verify the EUST model and several typical extended models are selected for comparison purpose, and then a detailed parametric study is performed to understand how and why the transformed fracture locus evolves with the parameters. This study paves the way to predict the fracture locus with the minimum number of simple tests for ductile fracture in engineering applications. Furthermore, the linear UST fracture locus can be extended for more complex conditions.