An experimentally driven computational analysis of thin laminates

The present study is motivated by the spreading application of thin metal laminates in different technological fields, for the production of flexible electronics, nano or micro-devices and beverages packaging. The actual material coupling is usually designed in order to meet different functional requirements, including the bearing of mechanical actions. Specific features of these composites are the small thickness of the layers, which may behave differently from the corresponding bulk materials. Hence, current research topic concerns material systems made of the coupling of thin metal foils, polymer plies and, possibly, with paperboard which, the role of each layer is substantial to enhance the functionality of above systems. Therefore, according to complexity of considered materials, current study is composed of three investigations: micro-mechanical behavior of freestanding aluminum foil (Al foil) and simulation of crack propagation, mechanical behavior of thin laminated structure made of Al/polymer coupling and an inverse analysis procedure for the material parameter identification of composite material (including paperboard composite and Al laminate). The numerical models were performed in small linear elastic strain and large plastic strain regime. Al and polymer were considered as an isotropic material, and their behavior was described by isotropic linear elasticity and Huber-Hencky-Mises plasticity model. The homogenized material response of the paperboard composite has been expressed by the orthotropic linear elasticity and Hill’s plasticity model. The selected Al foil has 9 micron thickness. Its mechanical response was evaluated on the basis of uniaxial tensile tests performed on plain and initially notched material samples. Displacements were measured during the experiment using three-dimensional digital image correlation (3D DIC) technique. Different numerical models were developed to study the failure mode of Al foil and its mechanical behavior using plane stress, shell and membrane elements. The problem exhibited high sensitivity to the modeling details. Simulations based on shell elements reproduced the wrinkling phenomena which also occurred in the experimental test but could not capture the observed failure mode. On the other hand, membrane elements simulated a realistic failure mode but without any wrinkles. Crack propagation was simulated introducing an interface layer characterized by a bilinear traction-separation law and discretized by cohesive elements. The interface properties calibrated based on the engineering stress-strain curve of plain sample could capture the response of both center and side cracked Al foils accurately. The adhesion properties of the interfaces can usually be characterized only indirectly. The effect of different interface and material properties on the overall response and on the failure mode of Al/polymer laminate has been investigated numerically in an extensive parametric study. The influencing parameters consist of thickness and Young’s modulus of the polymer layer, different interfacial properties and a type of imperfection (Al and polymer with the wavy-shape surface). The necking phenomena due to thinning of the Al foil was defined as the overall load carrying capacity of the Al laminate which is relevant to Al constitutive properties. Despite significant effect of polymer thickness on the nominal stress-strain curve of the laminate, its behavior is unaffected by the change in polymer Young’s modulus. The interface characteristics influenced mainly the softening regime. The mechanical response of paperboard laminates, which are used in beverage packaging, has been finally investigated. In this case, realistic biaxial stress states were induced by inflation test. 3D DIC returned the whole displacements field in the heterogeneous sample. These measurements were exploited to characterize the materials by inverse analysis and utilizing trust-region-reflective algorithm. The numerical model results based on the identified material properties showed excellent agreement with the experimental output. In the end, a sensitivity analysis performed on the anisotropic behavior of the paperboard composite emphasized that the anisotropic behavior become more distinguishable by embedding a laminate inclusion or by making a hole in the paperboard composite.