Strain assessment in cracked sheet metals by optical grid method

The present work is an extension of the optical grid method for the experimental investigation of deformation in stamped sheet metals. The classical method presents difficulty or is inapplicable where the deformation has resulted in the tearing of the sheet. In these cases, the measurement result is not available right where the most interesting data are expected. In this work, authors try to overcome this limitation by using an xFem-like approach, in which the classical polynomial functions, that approximate the displacement field of an element, are supported by additional “enrichment” functions that allow to describe discontinuities or singularities of the displacement field. The proposed method extends the measure up to the free edge of the crack. The capabilities of this technique were assessed by two virtual experiments where the deformed shapes were assumed as samples; the former consists in a FEM simulated bulge test and permitted to evaluate the method error in presence of through crack; the latter reproduces a plausible crack opening and was used to evaluate the method behavior in presence of a crack tip.