Tensile behaviour of unaged and hygrothermally aged discontinuous Bouligand structured CFRP composites

The aim of this article is to determine the effects of hygrothermal ageing on the tensile behaviour of asymmetric discontinuous helicoidally stacked (Bouligand structured) carbon fibre reinforced plastic (CFRP) composites. Eight different discontinuous Bouligand stacking sequences were manufactured using both major and minor pitch angles. The major pitch angles used were 90° and 120° while minor pitch angles at 5°, 10°, 15° and 25° were stacked from each of the major pitch angles. The composites were tested in tension as either dry unaged specimens or following hygrothermal ageing in seawater at the constant temperatures of 40°C and 60°C for over 2000 h. Both tensile modulus and tensile strength are found to be detrimentally affected by hygrothermal ageing and the extent to which ageing affects these properties is shown to be a function of the inter-ply pitch angle. All discontinuous Bouligand structured composites that were hygrothermally aged at the higher temperature of 60°C were less stiff and weaker than those aged at 40°C and those that were unaged. This is a result of increased heat exacerbating the ingress of water and consequently damaging fibre-matrix interfaces and plasticizing the matrix. The results showed that the minor pitch angles had clear effects on the strength and stiffness of the composites. Composites with 120° major pitch angles exhibited superior elastic modulus and strength values compared with composites with 90° major pitch angles. The Hashin damage model is shown to be accurate in predicting failure in discontinuous Bouligand structured CFRP composites, as evidenced by comparison to fracture paths observed after mechanical testing.