Measurements of the effects of pure and salt water absorption on the rate-dependent response of an epoxy matrix

Abstract

The study reports the measured effects of water absorption on epoxy resin. Epoxy samples were exposed to wet conditioning environments including pure water, NaCl-water solution, and pure water at boiling temperature, measuring absorption as a function of time. Vickers hardness and indentation creep tests were performed and the mechanical response of the material to uniaxial stress was also measured in both compression and tension, at imposed strain rates in the range 0.001-2500 s -1 . It was found that the absorption of both pure and salt water caused decrease of stiffness, yield stress and hardness, but only mildly affected the sensitivity of the response to the imposed strain rate and the tensile ductility. Mechanical testing after re-drying of the samples revealed the permanent effects of water absorption.

Effects of water absorption on the mechanical properties of GFRPs

“…Water exposure reduced the Young's modulus by 80%, and the strength by 50% on QS90 specimens, while reductions were of 80% and 58% (on E and σu, respectively) for QS45 specimens. QS90-WS and QS45-WS specimens (Figures 6,7) showed a higher strain to failure than fully dry specimens, in line with the slight increase in ductility of the matrix which we measured in [28].…”

“…Now focus on Figure 8b and Figure 8d: the strength of the QS90 specimens is, as expected, lower than the strength of the neat resin, due to the stress concentration induced by the presence of the fibres. While the measurements in [28] suggested that the strength of the matrix could be highly recoverable upon re-drying, for the composite this is not the case. This reinforces the notion that the fibre-matrix interface strength is permanently weakened by water absorption.…”

“…In a similar study [28] on the same epoxy resin, absent the fibres, we measured the percent water content at resin saturation, %Δws r = 2.81%. Upon assuming that the glass fibres do not absorb any water, it is possible to estimate the percent water content in the composite %Δ , using simple rule-of-mixtures.…”

“…A cubic unit cell Ω of side length 1 and fibre volume fraction vf = 0.56 was modelled as in Figure 4, consistent with cubic packing of the fibres in the matrix. The properties of resin and E-glass fibres, obtained from reference [28] and preliminary experiments, were used to calibrate the constitutive models for fibres and matrix. For the resin, the elastic moduli used were = 2.6 GPa, = 2.0 GPa, = 2.4 GPa; and the corresponding Poisson's ratios were = = = 0.4.…”

Strain-rate and off-axis loading effects on the fibre compression strength of CFRP laminates: Experiments and constitutive modelling

“…In addition, the 10° specimens are noticeably lacking any significant nonlinear behaviour in the dynamic tests, Figure 4 d). Both of these differences can be attributed to the viscous stiffening and hardening of the matrix at higher strain-rates, which has been observed in [16].…”