The aging effects on composite stiffness are important when structures are monitored under long-term mechanical loads and harsh chemical environment. This work aims to reveal the role of fibre-to-matrix interfacial aging on elastic constants due to a long-term sulfuric acid immersion at a high temperature and pressure. The aging effects of glass fibre matrix interfaces are rather impossible to measure directly and they cannot be easily subtracted from macro scale test results. Here, extensive numerical simulations are automatically run to show the sensitivity of all the essential elastic constants on the experimentally observed output of mechanical tests in reality. Material models are formulated for two different length-scales to understand (1) the sensitivity of the layer’s length scale related elastic constants and (2) micro scale material properties. The well-established Halpin-Tsai parameters are used to include interfacial effects. The experimental results show that the aging significantly affects tensile and flexural behavior of glass fibre vinylester epoxy composites: tensile and flexural stiffness decreased 6–49% and ultimate strength values 13–34%. The simulations presented that the degradation of fibre matrix interfaces cannot be excluded and they verified the degradation of glass-fibres’ modulus due the conditioning.
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