Strain analysis in sandwich structures using digital image correlation

Sandwich structures are composed by different thin-layered materials. One example is a sandwich made of metal - polymer - metal layers, where the internal polymer layer is given by a polypropylene-polyethylene (PP/PE) foil. In experimental observations it turns out that an emerging strain localization requires the entire field information. The application of Digital Image Correlation (DIC) is common in measuring the surface strains of mechanically loaded specimens. However, the literature – both in scientific publications as well as the theory manuals of the providers – leave the question open how the detailed formulation of the strains in the curvilinear setting has to be performed. In this short essay, the fundamentals are discussed, where we draw on surface descriptions developed in the context of finite elements. A second topic treats the strain determination on the surface of finite elements, since for the case of material parameter identification the displacements or even the strains are required for the solution of the entire boundary-value problem. Even here, the strains on the surface of three-dimensional element formulations are not provided by most commercial finite element programs. Thus, the concept of strain determination is transferred to the output of finite element programs as well.