Characterization of bonding behavior between wet lay-up carbon fibre reinforced polymer and steel plates in double-strap joints under extreme environmental temperatures

Extreme environmental temperatures are considered amongst many potential sources of infrastructural disfunctioning. This phenomenon emerges severely when it comes to the adoption of fibre reinforced polymer (FRP) in strengthening civil infrastructures due to the vulnerability of one FRP constituent, viz. polymer matrix, and most critically, the interfacial adhesive layer to extreme exposures. Accordingly, the main concern of the research program described in this thesis is in characterizing the bonding aspects between steel adherends and carbon fibre reinforced polymer (CFRP) at predefined non-ambient temperatures from both extremes, viz. subzero and elevated thermal exposures. In this research, the basic material and geometrical configuration of the experimental specimens is wet lay-up CF130RP and CF530RP composite reinforcement bonded to steel plates in double-strap joint configuration. Extensive literature reviews on the effect of extreme thermal exposures on different FRP strengthened real-life and small scale structural members were conducted. However, it is shown that most of published research is highlighting the synergistic detrimental effects within the durability context, in general. In addition, much more insight is paid in studying FRP strengthened concrete structures compared to steel substrata. Another distinguished feature in past studies is the scatter and divergence in behavioural trends of the non-ambient exposure FRP joints and the tendency to generalize those trends on all CFRP-steel joints without displaying adequate emphasis on the key roles played by the mechanical and thermal attributes of CFRP components, individually. In the experimental program, a new fabricating procedure of the composite double-strap joint was devised in an attempt to optimize the quality control and thus maximize the reliability of the results. Three different commercially available epoxide resins and two different-moduli carbon fibres were involved in the production of composite joints. All specimens were sUbjected to direct tensile testing at their predetermined thermal exposures after concluding their corresponding thermal stabilization procedure. Mixed-mode failure (i.e. either two or three patterns) was the prominent experimental failure pattern within almost all specimens. The outcome of the experimental research has emphasized the key effect of CFRP constituents, viz. the adhesive (i.e. epoxy) and CF reinforcing fibres, on the joints capacities, failure patterns, and CFRP strain and stress distributions, at all experimented temperatures. The theoretical and numerical validations of the experimental results were reasonably good in terms of all of the aforementioned parameters and at both ambient and extreme thermal exposures. Some experimental strain-capturing practices were highlighted as possible sources of divergence from theoretical and numerical models which was confined only to the vicinity of both bondlength lap ends, and at load levels much higher than normal service loads of the joints. The well-established stress-based method for predicting joint capacities in conjunction with triggering failure pattern from FE models was implemented by applying the suitable failure criteria. This method which was investigated previously, mainly in terms of ambient thermal exposures, has proved its effectiveness in predicting failure loads for the current extreme-exposures composite joints.