Mechanical behavior of FRP composite/steel double strap adhesive joints exposed to short-term tropical environments

Fibre reinforced composites have been employed in numerous industrial applications, including strengthening or rehabilitating damaged pipelines, buildings, bridges, or offshore structures. The composite bonded system has proven to be an alternative repair method to traditional process of welded joining or even substituting a new metal pipe. However, composite/metal joints experience adhesive failure owing to adverse environmental conditions. This study aims to analyze the short-term durability of carbon fibre composite/steel double strap joints (DSJ) under tropical laboratory, outdoor, and aggressive environment conditions. First, the shear properties of aged epoxy adhesive were determined through the modified Arcan test. Secondly, several set of DSJ structure was prepared and tested to determine the bonded joint performance and damage modes under tensile load subjected to various exposure conditions. Additionally, pre-stressed DSJ configuration was tested to investigate the integrity of the CFRP plate, the CFRP plate-adhesive interface, and the adhesive-steel interfaces, subjected to various exposure conditions. Results showed that the shear strength of aged epoxy reduced by 20% though the shear modulus increased by 6%. Migration of water in molecular chains was believed to have caused a reduction in strength. Meanwhile, the bond strength of the DSJ unstressed joint increased by 25%, 50%, and 40% in indoor, outdoor, and aggressive environmental conditions, respectively. Whereas the bond strength for the DSJ sample increased by 20%, 96%, and 109% for pre-stressed indoor, outdoor, and aggressive environment conditions, respectively. Post-curing of the matrix occurred after prolonged exposure, which multiplied the cross-link density and enhanced the strength. As a result, significant changes occurred in the properties that are primarily impacted by the matrix in CFRPs. The finding has significance for establishing standards and quality control of composite adhesive rehabilitation systems for reinforcing deteriorated buildings and pipelines, particularly in Malaysi and countries with tropical condition.