A unique multi-scale damage model is developed by means of a “loose coupling” of a damage percolation model with a damage-based finite element (FE) model, allowing ductile fracture in sheet forming operations to be modelled. The coupled approach utilizes a Gurson Tvergaard-Needleman (GTN)-based yield surface to account for the global softening effects of void damage, while the local damage development and void coalescence events Within the microstructure are modelled using the damage percolation code.

Stretch flange forming, a typical sheet forming process, is modelled using the coupled approach. Two automotive aluminum alloys are considered, AA5182 and AA5754. A void coalescence-suppressed GTN-based FE simulation is performed first and a mesh area of interest (A01) is determined based on the calculated damage distribution. Microstructural features are characterized through a matrix erosion tessellation technique. Particle size and spatial distribution are mapped onto the A01 by the damage percolation code, which predicts damage development within the measured microstructures. The formability of stretch flanges is predicted, based upon the onset of catastrophic failure triggered by profuse void coalescence across the measured second phase particle field. Damage development is quantified in terms of predicted crack and void areal fractions.

Stretch flange forming experiments and quantitative stereology are conducted to assess the formability and damage development. Two primary failure modes are observed in the fractured stretch flange-samples, radial necking at the cutout edge and circumferential cracking at the punch nose. An increase in sheet thickness from 1.0 to 1.6 mm promotes a transition from radial necking to circumferential cracking. Within both materials, the damage rate is higher in the 1.6 mm sheet due to the more severe bend/unbend within the drawbead region and at the punch radius, favouring damage-induced circumferential cracking.

This research demonstrates that void nucleation at second phase particles dominates the damage development during the stretch flange forming process. The contribution of void growth to the overall damage rate is small. During the majority of the stretch flange forming operation, void coalescence is confined to individual particle clusters. Inter-cluster coalescence encompassing several particle clusters triggers catastrophic failure across the entire particle field. The void nucleation strain exerts a strong impact on the prediction of formability and damage rate. Higher nucleation strains tend to retard damage development and lead to higher predicted formability. A parametric study on void nucleation strain suggests nucleation strains of 0.2-0.5 for both alloys. The predicted formability and damage development Within the A01 are similar to the experimental results.