Effects of temperature and moisture on the fracture behaviour of composite adhesive joints

Abstract Mode I interlaminar fracture toughness of CFRP joints co-cured and co-bonded was experimentally characterised under several environmental conditions. Two test campaigns were carried out: one with as-received specimens tested at −54 °C and 25 °C, and another with hygrothermally aged samples tested at 25 °C and 80 °C. Dynamic mechanical analysis and scanning electron microscopy were used to explain distinct results. For co-cure, the propagation onset for as-received samples started at the interlayer region between the two adherends before migrating deeper into the adherend, while for as-received co-bonded samples, propagation onset was mainly cohesive. After aging, both bonding techniques failed directly in the adherend, in a fibre-tear fashion. Thus, the aging process presented a higher influence on failure mechanisms than the testing temperature. Regarding performance, mode I fracture toughness did not present a great variation for co-cured samples, since failure locus was always in the adherend. However, the initiation mode I fracture toughness of co-bonding decreased between as-received and aged samples. This difference is attributed to the presence of the adhesive in co-bonded systems, which showed to be more affected by environmental conditions than systems formed by fibre and matrix only, such as co-cured ones.