On and Off-Axis Cyclic Behaviour of 3D-Woven Composites: Experimental Testing and Macroscale Modelling

  • Oddy, Carolyn (GKN Aerospace)
  • Song, Meng yi (Bristol University)
  • Stewart, Christian (Bristol University)
  • El Said, Bassam (Bristol University)
  • Ekh, Magnus (Chalmers University of Technology)
  • Hallett, Stephen (Bristol University)
  • Fagerström, Martin (Chalmers University of Technology)

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Composites with 3D-woven reinforcement have shown a number of promising characteristics. In particular, their through-thickness reinforcements create a material with high out-of-plane properties, fracture toughness and energy absorption capabilities. However, industrially applicable models describing their mechanical performance are still needed. In terms of computational efficiency, macroscale models, which describe the material as a homogeneous and anisotropic solid, sit at the forefront. Developing, calibrating and validating such a model though, is not a trivial task. At minimum, testing is required in the main orthotropic directions, i.e. tension and compression in the three reinforcement directions and in the three shear planes. Further, these materials are known to show varying levels of non-linearity depending on loading mode. It is important to understand and model the mechanisms that cause this non-linearity. Finally, it is also essential to understand and account for how multiaxial loading impacts the material behaviour. This joint work takes steps towards addressing these challenges associated with calibrating, further developing and validating a macroscale model for 3D-woven composites originally developed by Oddy et al. The main focus is given to the in-plane tensile and shear behaviour of a carbon fibre reinforced epoxy layer-to-layer angle interlock. In order to calibrate the model, tensile specimens aligned in the warp and weft direction as well as at 45 degree off-axis are tested. More notably, these specimens are loaded and unloaded cyclically until failure. By loading them cyclically, it is possible to directly determine what types of subscale behaviours are leading to the non-linearity seen in the test. Tracking how the stiffness degrades at each unloading cycle indicates how damage is developing, which gives the basis for calibrating a standard continuum damage model. The other source of non-linearity that can be tracked is the development of permanent strain. This is captured using an isotropic hardening plasticity model. Finally 15 degree and 30 degree to the warp direction off-axis tensile specimens are tested cyclically. This not only provides validation testing for the macroscale model but also greater understanding of how 3D-woven composites behave under mixed normal and shear loading.