A novel hysteresis energy based fatigue failure criterion for an epoxy based polymer using a viscoplastic material model

Kurzfassung

Epoxy based composite materials are increasingly used for a wide range of applications. Both Airbus (A350) and Boeing (787) have reached a composite share of over 50 % in the high performance long range aircraft segment. Currently, overly conservative strain based design criteria prevent the utilization of the full lightweight capabilities of such materials. Thorough research in damage, reliability, and fatigue can help overcome these limitations. However, the effort for a systematic characterization of the damage behavior is enormous. Micromechanical evaluations have proven to be a valuable approach to understanding the damage and failure of heterogeneous materials like composites. For this, an accurate material model for the epoxy is needed as many failure mechanisms are matrix-governed or at least matrix-originated. In this work, the phenomenology of the material behavior of the epoxy based resin LY564/Aradur 22962 under static and cyclic loading conditions has been studied. From the experimental observations in static tension tests under different strain rates, relaxation tests, DMA tests, as well as intermediate holding loading/unloading tests, a viscoplastic material behavior can be concluded. A viscoplastic material model originally introduced for polypropylene by [1] was modified and used to model the behavior of the resin in 3D loading conditions. A tool based on Python/Qt (PySide) has been developed to simulate and sequentially calibrate the material parameters from the experimental data. A validation with cyclic tests shows good correlation of the material model and the cyclic response of the material. Furthermore, the fatigue behavior of the material under tension-tension loading has been studied. Load controlled tension-tension experiments at different load levels have been performed. From the load-displacement history of each specimen, potential fatigue life estimators were investigated including the hysteresis energy per cycle as well as the accumulated hysteresis energy. Using the calibrated viscoplastic material model, a multiaxial fatigue failure model based on the accumulated hysteresis energy is proposed and is shown to predict the failure of neat resin under uniaxial loading. Further work includes the application of the fatigue failure model to multiaxial loading conditions in a micromechanical analysis using representative volume elements of composite materials. [1] M. Kästner et al.: Inelastic material behavior of polymers – Experimental characterization, formulation and implementation of a material model. In: Mechanics of Materials 52, pp. 40-57, 2012.