3D Interlock Composites Multi-Scale Viscoelastic Model Development, Characterization and Modeling

Composites with three-dimensional (3D) reinforcements are increasingly used in the aerospace industry to overcome the 2D laminates limitations (low impact and delamination resistance). Structural parts are now made of composites reinforced with 3D fabrics due to their high specific mechanical properties and delamination resistance. However, residual stresses development is inherent to the manufacturing process and can induce geometrical distortions, leading to performance losses. Numerical tools have been developed to predict the residual stresses but coherent input material models are required to improve the tools accuracy. The aim of this thesis was to develop and validate a thermodynamically consistent linearly viscoelastic model to predict the mechanical properties of a composite made of an epoxy resin and a 3D interlock woven carbon fibers fabric, as a function of temperature and degree of cure (DoC). The model was developed for the neat epoxy resin first. Then, the warp and weft tows' properties were computed by homogenization and the composite's properties were finally obtained through a second homogenization procedure considering the resin, warp and weft tows behaviors, as functions of temperature and degree of cure. The model has been implemented by another Ph.D student into a numerical tool to predict the residual stresses development during 3D woven composite parts manufacturing and succeeded to model the viscoelastic behavior that occurred during the post-curing. First, a characterization campaign on the epoxy resin was carried out. Temperature- and degree of cure-dependent linearly viscoelastic behavior was assessed through multi-temperature tests using a three point bending fixture in a Dynamic Mechanical Analysis (DMA) apparatus. Multi-temperature relaxation tests were performed in the resin's linear domain strain range on fully and partially cured specimens, at temperatures ranging from 30 �C to their respective glass transition temperatures. The viscoelastic constitutive model development was based on the Thermodynamics of irreversible processes to ensure a thermodynamically admissible model. The Time Temperature Superposition Principle (TTSP) was applied and master curves were constructed for each performed test to compute the shift factors. An Arrhenius law was identified to relate temperature and degree of cure into the shift factors, with the glass transition temperature Tg as reference temperature. The degree of cure-dependent glass transition temperature was added through the DiBenedetto relationship. The relaxation stiffness tensors were associated with the relaxation times through a continuous spectra function.