Numerical investigations of internal and residual stresses in aluminium-polymer laminate foils during stretch formingMittwoch (05.07.2017) 14:20 - 14:40 Uhr Bestandteil von:
Pharmaceutical tablets are often stored in blister packages. If it is necessary to guarantee high protection against gases, aluminum-polymer laminate foils are used. These high barrier foils consist of a layer of PA, a core layer of Al and a layer of PVC. To manufacture these so called cold-formed blister packs, cavities are formed in the aluminium-polymer laminate foil by a stretch forming process at room temperature. As due to the complex material structure the forming process is not yet well understood, the developers of cold-formed blister packaging machines design the punches for forming the cavities bigger than needed for the tablets being packed. Therefore, the final goal of our research project is to decrease the cavity size and consequently save material. The aim of the present work is to analyze the stress distribution in the layered structure.
To gain insight into the mechanical behavior of the composite material, we performed micromechanical simulations with the finite element method. Due to the large differences in elastic and plastic properties of the metal and the polymer layers even uniaxial loading leads to a strong variation of stresses from one layer to the other and a multiaxial stress state builds up. To study the behavior on the microscale a representative volume element model with periodic boundary conditions is presented in which the three layers are mapped and described individually with J2-plasticity. This first model is applied to simulate various simple load cases. The simulations reveal the internal stresses and strains in the single layers and show that a multiaxial stress state builds up because of the interactions of the different layers.
Furthermore, a stretch forming model with simplified punch geometry was designed. Again the foil is modeled with a multilayer description. With this model the stretch forming of the composite foil to a cavity is simulated and the stresses and strains in each material layer during the stretch forming process are studied. Additionally, the residual stresses which are observed in the layers after unloading are analyzed. Thereby, it is checked whether the residual stresses could be the cause for a typical failure, which occurs under certain conditions during the blister package production.