Ductile fracture prediction for sheet metal under non-proportional loading paths

Researcher: Rugang Chai

Aim:

This study mainly involves establishing a new approach to forecast the onset of ductile fracture for sheet metals under non-proportional loading cases.

Background:

Ductile fracture models under linear strain paths have been studied for several decades. Therefore, a proportional loading path should be assumed to obtain the reasonable ductile fracture prediction. However, only except for the few cases such as parts with axisymmetric and uniform sections, non-linear strain paths during which the ratio between the major and minor strains varies occur during almost all of the forming processes. The ductile fracture prediction under non-proportional loading cases should be studied.

Methodology:

The incremental method and dual potential rules are applied to map the currently often used strain-based or stress-based ductile fracture models to the principal strain space. Then the principal strains for pre-loading and sub-loading are plotted in the fracture loci to predict whether fracture onset occurs. A novel nonlinear cumulative damage definition is also proposed to consider the influence of nonlinear loading paths.

Key Findings to date:

Several often used fracture models such as DF2012, DF2014, DF2016 and Hosford-Mean stress are mapped into the principal strain space with the incremental and dual potential methods. The experimental data for AA2024-T351 under non-proportional loading conditions are applied to verify the proposed approach, which indicates that the proposed method provides reasonable fracture prediction under non-proportional loading conditions.

Fig. 1 Fracture loci mapped from (a) DF2012, (b) DF2014, (c) DF2016, and (d) Hosford mean stress based and prediction results under plane strain compression followed by shear

Future Work:

• Pre-compression followed by bending tests will be verified.
• Pre-strain experiments based on Marciniak test will be designed.

Contact:

Rugang Chai

Institute for Frontier Materials (IFM)

Deakin University, Geelong

Email: [email protected]