Consistency of Strain Fields and Thickness Distributions in Thermoforming Experiments Through Stereo DIC

Abstract

This paper proposes a methodology to determine wall thickness distributions in thermoformed products derived from in-situ surface strain measurements obtained with stereo digital image correlation (DIC), under the assumption of material incompressibility. Wall thickness equations are derived for the Green-Lagrange, Hencky, Biot, logarithmic EulerAlmansi and Euler-Almansi strain definitions and validated with an analytic example. The equations are then used to calculate the wall thickness from digital image correlations with both a commercial software (VIC-3D, version 2012) and an academic software (MatchID3D). Obviously, the choice of the strain definition in the correlation software should not influence the resulting wall thickness values. The comparison reveals that the wall thickness values prove to be identical, regardless of the strain definition that was used, and manual measurements show the validity of the DIC based results. It was also found that, when using VIC-3D version 2009, an earlier release in the software series of Correlated Solutions, all but the Green-Lagrange strain definition bring forth unrealistic minimum principal strain values, resulting in different values for the wall thickness.