Hydrothermal ageing of glass/epoxy composites for wind turbine blades

In this work, a glass/epoxy material system commonly applied in wind turbine design was used to evaluate damage processes brought by water ingression during service life. Composite short-beams and neat epoxy beams and dog-bones were conditioned by water immersion at 50º until saturation and tested both statically and in fatigue. By comparing results from mechanical tests with those from reference specimens, significant fibre-matrix interface damage was identified, with reductions of up to 34% on the static strength and fatigue life reduction of up to three orders of magnitude. On the other hand, neat epoxy specimens showed a lower degree of damage, with up to 20% lower moduli and strength properties. Microscopic observations of conditioned specimens point out to extensive debonding on composite specimens after immersion, while neat epoxy specimens show signs of colour changes possibly due to hydrolytic attack. Lastly, changes in the glass transition temperature of the resin due to water ingression are assessed through the DMA technique. From the results, apart from a Tg reduction brought by material degradation, additional relaxation peaks brought by phase changes in the absorbed water molecules were identified.Structural EngineeringCivil Engineering and Geoscience

Experimental/numerical study of anisotropic water diusion in glass/epoxy composites

In this work a glass/epoxy composite commonly used in wind turbine blades is exposed to a humid environment at an elevated temperature. To research the anisotropic di_usion behaviour observed in unidirectional composite specimens, experimental results of slices cut along the three directional planes of the laminate immersed in demineralised water at 50_C are coupled with numerical modelling. The weight of the slices was measured at regular intervals, from which the uptake behaviour could be deduced. The process was modelled using a 3-dimensional RVE of the material, where di_usion is modelled as steady-state and the di_usivity in each direction was measured by applying concentration gradients to the model. The experimental data shows similar water uptake behaviour for samples in both transverse directions, while the water uptake in the _bre direction was signi_cantly faster. A proper _t according to Fick's law was obtained for the transverse direction, while this was not possible for the samples in _bre direction, suggesting a strong dependency of the di_usion behaviour on the _bre orientation. Results from the proposed numerical models show that the geometric e_ect of _bres acting as barriers for the water movement is indeed responsible for part of the observed anisotropy.Applied Mechanic

Hygrothermal ageing behaviour of a glass/epoxy composite used in wind turbine blades

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 4800 h 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.Applied Mechanic

Numerical/experimental study of the monotonic and cyclic viscoelastic/viscoplastic/fracture behavior of an epoxy resin

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.Accepted Author ManuscriptApplied MechanicsMaterials- Mechanics- Management & Desig

A combined experimental/numerical investigation on hygrothermal aging of fiber-reinforced composites

This work investigates hygrothermal aging degradation of unidirectional glass/epoxy composite specimens through a combination of experiments and numerical modeling. Aging is performed through immersion in demineralized water. Interlaminar shear testes are performed after multiple conditioning times and after single immersion/redrying cycles. Degradation of the fiber-matrix interface is estimated using single-fiber fragmentation tests and reverse modeling combining analytical and numerical models. A fractographic analysis of specimens aged at 50°C and 65°C is performed through X-ray computed tomography. The aging process is modeled using a numerical framework combining a diffusion analysis with a concurrent multiscale model with embedded hyper-reduced micromodels. At the microscale, a pressure-dependent viscoelastic/viscoplastic model with damage is used for the resin and fiber-matrix debonding is modeled with a cohesive-zone model including friction. A comparison between numerical and experimental results is performed.Applied MechanicsMaterials- Mechanics- Management & Desig

Combined experimental/numerical investigation of directional moisture diffusion in glass/epoxy composites

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.Applied MechanicsMaterials- Mechanics- Management & Desig