In this work, a glass/epoxy material system applied in wind turbine blades was used to evaluate degradation processes induced by water ingression. Composite and neat epoxy specimens were conditioned in demineralised water at 50 • C for 4800h and tested quasi-statically and in fatigue. Comparing results from mechanical tests in composite specimens, significant degradation was found, with up to 36% lower static shear strength and three orders of magnitude shorter fatigue life. For neat epoxy specimens, a lower degree of degradation was observed, with up to 17% lower tensile and bending moduli and strength. Specimens dried after having been immersed were also tested. For composite samples, recovery of shear stiffness and strength was incomplete. For neat resin, stiffness and bending strength were completely recovered but a decrease in the strain at failure was observed. It is hypothesised from differences in magnitude and reversibility of degradation between composite and neat resin that matrix degradation is accompanied by high differential swelling stresses and damage to the fibre/matrix interface in composites. The damage due to moisture ingression and the subsequent changes in failure behaviour are further investigated through thermal analysis (DSC, DMA) and optical microscopy.

A combined experimental and numerical investigation is conducted on the anisotropic water diffusion behaviour of unidirectional glass/epoxy composites. Experimental diffusivity values are obtained by immersing thin material slices for each of its planes of orthotropy extracted from a thick composite panel and interphase measurements are performed using thermal analysis. In order to elucidate the observed anisotropy, the diffusion process is modelled at the microscale using a representative volume element (RVE) of the material with random fibre distribution. Water concentration gradients are applied to the micromodel and a homogenisation procedure is used to retrieve the macroscopic diffusivity coefficients. The influence of the interphase around the fibres on the diffusion process is modelled by making the matrix diffusivity a function of the distance to the nearest fibre using a level set field. The models are used to fit the experimental data and test a number of hypotheses that may explain the observed anisotropy. The effect of fibres acting as barriers for water movement is found to partially explain the observed transverse diffusivity. However, a fit is only obtained by allowing faster diffusivity at the interphase. In the longitudinal direction, a fit can only be found by allowing for orthotropic interphase diffusivity.

This paper investigates the viscoelastic/viscoplastic/fracture behavior of an epoxy resin. A state-of-the-art pressure-dependent elastoplastic constitutive model (Melro et al. (2013)) is expanded to include viscoelasticity, viscoplasticity and a modified damage formulation with linear softening and shrinking pressure-dependent fracture surface. A water plasticization model with a single degradation factor is proposed. A set of new quasi-static and fatigue experiments is used to calibrate the model and assess its predictive capabilities. The model correctly represents the rate dependent plasticity and fracture initiation behavior of the studied epoxy. The stiffness and strength degradations after plasticization are also accurately captured. The model is found to be less suitable in reproducing the measured loading-unloading behavior, which displayed strong nonlinearity in combination with limited permanent deformation. Nevertheless, reasonably accurate fatigue life predictions in the low-cycle regime are obtained.

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