Abstract:
Fibre reinforced polymer materials are regularly employed as reinforcing materials for structural elements. Carbon fibre reinforced polymers (CFRP) are particularly suited for strengthening of steel structural elements because of their lightweight nature and superior mechanical properties. Exposure to environmental stresses can weaken connections between bonded materials. This study was undertaken to further knowledge in relation to the performance of adhesively bonded CFRP-to-steel joints subjected to peel loading. Two epoxy adhesives were assessed with regard to their aging behaviour at different elevated temperature and hygrothermal conditioning scenarios and for various exposure periods. The performance of unaged specimens was also tested as a control. Adhesive coupon specimens and a modified mode I fracture test arrangement for use with dissimilar materials was employed.
A total of 48 adhesive coupons were evaluated including 6 unaged adhesive coupons, 24 adhesive coupons subjected to elevated temperature and 18 adhesive coupons subjected to hygrothermal conditioning.
In total, 54 CFRP-to-steel bonded specimens were evaluated including 6 unaged specimens, 24 specimens subjected to elevated temperature, 18 specimens subjected to hygrothermal conditioning and 6 specimens with different bond-line thickness.
Exposure of coupons of both adhesives to elevated temperatures resulted in a considerable reduction in elastic moduli. Maximum strain of adhesive coupons fabricated with Adhesive A and B experienced an overall average increase of 30% and 39%, respectively, after exposure to elevated temperatures. The maximum failure load of fracture specimens manufactured with Adhesive A and B reduced up to 9% and 12%, respectively, after exposure to elevated temperature. Despite possessing higher strain energy release rate and higher load carrying capacity of fracture specimens manufactured with Adhesive B compared to Adhesive A, the bonding performance of Adhesive B is more affected by elevated temperature and this needs to be noted when using Adhesive B in areas with hot climates.
Humid and wet conditioning caused up to 44% and 63% drop in elastic modulus, 31% and 55% reduction in tensile strength, and 32% and 44% increase in average in maximum strain of adhesive materials, respectively.
Hygrothermal conditioning may cause the adhesion failure near to the edges of the adhesive joints. The load-carrying capacity of adhesive joints and strain energy release rate depend on the adhesive and hygrothermal environment, and can be degraded up to 15%. Reducing the bond-line thickness to less than 1 mm can result in a major reduction in load-carrying capacity and strain energy release rate of CFRP-to-steel bonded specimens.
A nonlinear finite element analysis which employed a cohesive zone crack model was conducted and showed that the proposed modified experimental arrangement was dominated by mode I behaviour. Finite element analysis was also performed to simulate moisture diffusion through the bonding of CFRP-to-steel joint.