Modelling environmental effects on fracture damage of natural fibre-reinforced polymer composites

The behaviour of sodium hydroxide treated sisal fibre-reinforced polyester composite subjected to environmental ageing conditions has been studied with the aim of modelling its effect on the composite fracture damage for the purpose of long-term performance prediction.
The composite was accelerated aged by hydrothermal ageing by immersing test specimens in distilled water at respective temperatures of 23oC, 40oC and 60oC for a maximum period of 6 months. At regular intervals tests were conducted to assess the retention of the mechanical properties. Tests conducted included tensile tests on single sisal fibres, neat cured polyester resin, and sisal fibre reinforced polyester composite test specimens. Retention of fracture strength of the composite was further assessed both at macro and micro levels. The assessment of the fracture toughness properties was conducted on eccentrically loaded in tension single-edge-notch (ESE(T)) specimens; whereas fibre/matrix interfacial shear strength was assessed through the single fibre microbond method. Viscoelastic properties of matrix material which include storage modulus, loss modulus, and glass transition temperature were further determined and monitored using the Dynamic Mechanical Analysis (DMA) method. All tests conducted included test of non-hydrothermally treated test specimens for the purpose of setting a comparative baseline and determining the effect the ageing process poses on the composite properties.
Results from the single fibre microbond tests revealed significant reductions in the interfacial microbond strengths as the absorbed moisture levels intensified.
Results from fractographic differences in the Scanned Electron Microscopic (SEM) images of the fractured microbond test specimens indicate little to no traces of the residual matrix material present on the sisal fibre at the original location of the polyester resin droplet along the fibre. This brings out a possible conclusion that fibre debonding is an established fracture mechanism in hydrothermally aged sisal fibre reinforced composite systems. The presence of this residual matrix appears to be dependent on the duration of the applied hydrothermal ageing.