Carbon Fibre Reinforced Polymer Patched Steel under Fatigue Loading and Pre-exposure to Extreme Environmental Conditioning

Abstract

Civil infrastructure is witnessing an increasing number of degraded and deteriorated steel structures. With current retrofitting and rehabilitation techniques becoming outdated the implementation of carbon fibre reinforced polymer (CFRP) materials have excelled with their impressive strength to weight ratios and chemical resistance. Unfortunately the longevity of bonded CFRP systems is largely unproven under industrial conditions, in particular under combined environmental exposure and fatigue loading.    Environmental exposure for such applications often involves elevated temperature and moisture. Moisture ingress can be detrimental to adhesive integrity and can provide the necessary environment for galvanic corrosion between steel and CFRP materials. Temperature changes, particularly elevated temperatures, can soften epoxy adhesives as they reach their glass transition temperature (Tg), in turn, significantly reducing their strength. Additionally, this rubberisation can further intensify the level of moisture absorption, compounding and exacerbating the degradation. Finally these deteriorated materials are likely to be more detrimentally affected by the application of strenuous loading which can significantly reduce their strength even further.    Thus, the broad aim of this research is to better understand the durability and fatigue performance of CFRP/steel systems. The first stage of research into the bond performance of CFRP/steel was to quantify the amount of localised corrosion (pitting) created between CFRP and steel during submergence in simulated seawater solutions at elevated temperature. Pitting creates high stress concentrations and can become the site of premature cracking and fracture. Specimen submergence in seawater solutions showed that isolated pitting was insignificant, with general chloride corrosion being more substantial than localised galvanic damage. Consequently, the potential degradation of the adhered joint under such conditions appeared to be more influential on the durability and fatigue performance of CFRP/steel systems.    The next stage focused on an investigation of the bond strength of CFRP patched steel double lap specimens after fatigue loading and environmental exposure. To investigate the most damaging and destructive scenarios to durability and strength of the joints, the service loading, fatigue loading, exposure temperature and submergence duration were altered. Normal modulus materials were unable to survive the application of high stress fatigue cycles after environmental exposure. On the other hand, high modulus sheeting specimens survived all loading and environmental conditioning, experiencing strength losses of only 10% on average.    The final stage was to investigate methods to improve the bond integrity of double-lap joints to reduce the amount of degradation during submergence and loading. Firstly, a high Tg adhesive was implemented to maintain joint integrity and strength under elevated temperature. Furthermore, carbon nanotubes (CNT) were dispersed into common structural epoxies to increase the physical and mechanical properties of the adhesives, however their addition increased adhesive viscosity and decreased their workability. Also a chemical bond promoter, silane, was introduced to enhance the chemical bond of CFRP/steel joints to ease the strength losses resulting from submergence and fatigue. However, levels of strength reduction remained comparable between un-treated and silane pre-treated samples, implying that degradation is not necessarily due to an issue between the interface of steel and epoxy, but more likely within the bulk adhesive. Thus, silane pre-treatment is perhaps more effective when combined with multi-layered patches as they commonly witness more steel and adhesive interfacial failures.    After bond performance was explored in detail, investigations progressed to examine the fatigue performance of CFRP repaired damaged steel after seawater submergence. Tri-layered CFRP patches were applied to pre-cracked steel plates prior to exposure. Configurations consisted of either single or double sided repair, with or without silane pre-treatment. Several double sided repaired specimens managed to survive in excess of 6 million cycles without any visible damage after exposure, while single sided specimens maintained at least an 80% increase in fatigue life over bare steel.    Finally, a numerical model was developed to predict the fatigue life of pre-exposed CFRP repaired steel, which was validated via the experimental investigations. The influence of environmental conditioning was incorporated into linear elastic fracture mechanics theory to accurately predict the degradation and fatigue performance of exposed repairs.    This research provides advanced understanding into the durability of adhered CFRP to steel. Investigations focused on the combination of fatigue loading and environmental exposure, which is likely for structural elements expected to utilise CFRP strengthening. Studies showed that CFRP systems were capable of surviving conditions that are more severe than those expected during their industrial life cycle. It can be concluded that CFRP can potentially provide revolutionary rehabilitation performance for steel structures, even under extreme environmental and loading scenarios.