Finite strain elasticity based cohesive zone model for mechanoluminescent composite interface: I. Stiffness of the undamaged interface

Light emission from inorganic crystals due to elastic loading, termed piezoluminescence or EML, is of great practical interest due to repeatability of emission. Two architectures demonstrated for utilization of EML are thin films and particulate composites, with the majority of recent literature focusing on the latter. Specifically, EML of doped zinc sulfide phosphor particles dispersed in polymer matrices has been recently demonstrated for a growing number of applications. This necessitates the need to understand the micromechanics of stress transfer within these composites for building accurate EML models as well as fatigue models to predict functional life estimates of the EML devices. Macroscale stress measurements can be combined with material properties through micromechanical models to estimate stress transfer to the particulate phase. Apart from the material properties of the matrix and particles, the bonding between the two phases also influences stress transfer significantly. The bonding at the interface is modeled through a CZM which provides a deterministic way to track interfacial damage due to fatigue. A bilinear CZM defines the undamaged stiffness of the interface ( kσint) and its resistance to damage ( k˜σint) in two piecewise linear stages. In this paper, we utilize established micromechanical models to determine the stiffness of the undamaged interface kσint between EML ZnS:Cu particles and PDMS matrix in the normal and tangential directions ( kσn and kσt) with the help of experimentally determined mechanical and morphological properties of EML-PDMS composites. kσn and kσt determined are then validated by comparing stress strain behavior of a finite element representative volume with experimental monotonic tensile test results to find good correlation.