Ductile tearing of aluminium plates: experiments and modelling

This paper presents an experimental and numerical study on quasi-static ductile tearing of thin plates of the aluminium alloy AA6016 in three tempers. Depending on the temper, the main fracture mechanism in the plate tearing tests changes from grain boundary failure to coalescence of voids nucleated at the constituent particles. The experiments are complemented by nonlinear finite element simulations using an enriched Gurson-TvergaardNeedleman (GTN) model to describe the material response. The onset of accelerated void growth is initiated either by incipient material softening (named the softening model) or by the occurrence of strain localization (named the localization model). It was found that strain localization takes place at a critical porosity f(c), which depends on the current hydrostatic and deviatoric stress states. While the failure strain depends on the stress path, the critical porosity appears to be path independent. A third method is proposed (named the f(c)( T, L) model), where a critical porosity surface f(c) = f(c) (T, L) is used to determine when accelerated void growth starts. The surface is generated beforehand by solving for strain localization under proportional stress states defined by the stress triaxiality T and the Lode parameter L. By comparing the simulations to the experiments, it was found that the localization model performed well for a wide range of stress states. The softening model does not portray dependence on the Lode parameter and is therefore less versatile. The localizationmodel and the f(c)(T, L) model gave similar predictions, but some minor differences were observed for two of the three tempers.
NTNU; Research Council of Norway [250553]
The authors gratefully appreciate the financial support from NTNU and the Research Council of Norway through the FRINATEK Program, Project No. 250553 (FractAl). The authors would like to acknowledge Mr Sander N. Sundt, Mr Aslak M. Blytt and Mr Trond Auestad for assisting with the experimental programmes. The authors would also like to thank Dr Lars E. B. Daehli for user subroutines on imposing stress states, Dr Susanne Thomesen for assisting with the SEM images and Dr Olaf Engler for providing the materials.