Anisotropic viscoplasticity and fracture of fine grained metallic aluminum foil used in Li-ion batteries

Aluminum 1235-H18 foils with sub-micron grain dimensions are often used as current collectors in Li-ion batteries. Due to their contribution to the structural integrity of batteries under impact loading, their plastic and fracture response is investigated in detail. Using a novel micro-tensile testing device with a piezoelectric actuator, dogbone specimens with a 1.25mm wide and 5.7mm long gage section are tested for three different in-plane material orientations and for strain rates ranging from 10−5/s to 10−2/s. It was found that the stress at a proof strain of 2% increased by about 25% from 160 MPa to 200 MPa within this range of strain rates. Furthermore, pronounced in-plane anisotropy is observed as reflected by Lankford ratios variations from 0.2 to 1.5. A material model is proposed which borrows elements of the anisotropic Yld2000-2d plasticity model and integrates these into a basic viscoplasticity framework that assumes the multiplicative decomposition of the equivalent stress into a strain and strain rate dependent contributions. The anisotropic fracture response is characterized for a strain rate of 10−4/s using notched tension and Hasek punch experiments. It is found that a simple stress-state independent version of the anisotropic MMC fracture initiation model provides a reasonable approximation of the observed experimental results.