Methodology for Characterizing Complex Strain of Polymeric Thin Films*

Traditional methods for analyzing microscale mechanics of polymeric thin films are typically limited to uniaxial deformation. The increasing expansion of research in flexible electronics and soft robotics make it imperative to improve our understanding of how these films behave under a complex strain  field. With the advancement in metamaterials, non-linear microscale mechanics can be investigated to improve our understanding of how soft materials will behave in real-life conditions. This work will discuss the methodology of characterizing crazes under complex strainusing image analysis. Using a 3D printed auxetic elastomeric lattice substrate with a tailored Poission’s ratio from -0.8 to +0.8, a complex strain field is induced within the supported thin films allowing for nonlinear deformation to occur within each lattice cell. The crazing phenomena was evaluated with image analysis to qualitatively and quantitatively describe the craze propagation and density under different strain fields. The experimental results were compared to a linear elastic finite element model for predicting local stress state in the film/lattice structure. Further development of complex strain techniques can lead to more realistic evaluations of thin films in operating conditions.