Experimental and theoretical investigation on the role of friction in Nakazima testing

Forming limit strains measured for the same specimen geometry under low and high friction conditions are typically incompatible, suggesting incorrectly that the FLD might be a function of friction. It is demonstrated in this paper that friction does not change the strain path at material points within a given specimen geometry, but strongly affects the relative strain rates across the specimen. It is the friction effects on relative strain rates that causes different material points to reach different critical conditions based on the local stress state. Furthermore, by accounting for the nonlinear strain path, i.e., by computing the stress state for each critical condition, it is found that a unique stress-limit is found for both low and high friction conditions, and from this observation it is possible to obtain a single strain-based forming limit curve that is independent of friction condition. 105