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Environmental durability of bond behaviour between steel and carbon fibre reinforced polymer
thesisposted on 06.02.2017, 03:20 by Nguyen, Tien Cuong
Since the last three decades, many studies have proven the effective use of CFRP composites in strengthening and retrofitting steel structures. However, one of the main restrictions to more widespread use of CFRP in strengthening and rehabilitating steel structures has been the uncertainty in the durability of steel /CFRP adhesively-bonded systems. This is especially true when such systems are exposed to outdoor environments. The data reporting this issue in the literature is currently limited and thus a systematic investigation of the effects of outdoor environments are required. The outdoor environments mainly include temperature, moisture and ultraviolet. Temperature is a detrimental factor to the adhesive layer between steel and CFRP because it may reach the glass transition temperature (Tg) of the adhesive, leading to a rapid reduction in the bond strength. Moisture, as another aspect of the durability, by diffusion through the adhesive or transport along the oxide/adhesive interface or absorption through the porous adherend, can penetrate to the interfaces between the adhesive and the steel adherend. Once moisture has penetrated into the steel /CFRP systems, degradation can occur due to two types of degradation: the degradation of the adhesive itself, and the degradation of the adhesive/adherend interface. In terms of ultraviolet, although the Earth’s atmosphere filters out most of the total solar radiation, 6% of the total solar radiant flux is ultraviolet radiation and is able to reach the Earth’s surface. Ultraviolet radiation has a wavelength between 290 and 400 nm, and the energy created by these wavelengths can dissociate the molecule bonds in iv most polymers, leading to the degradation of polymeric materials. The degradation may range from mere surface discoloration to extensive loss of mechanical properties. It should be also mentioned that combining these degrading factors together with loading may cause a significant degradation in mechanical properties of steel /CFRP systems. Double strap joints were chosen to investigate the performance of steel/CFRP composite systems subjected to environmental effects. When a double strap joint is tensioned, the strain in the carbon fibres must develop directly from the bond between the metal and the fibre. In this way, the fundamental behaviour of the metal-fibre bond, including the maximum strength and the effective bond length, can be investigated under static load. In the present research, the first aim was to investigate the effects of elevated temperatures on the mechanical characteristics of steel /CFRP double strap joints. It was found that due to elevated temperatures, the joint failure mode changed from cohesion to CFRP and adhesive interface debonding. Effective bond length was found to rapidly increase. Finally, joint strength and stiffness significantly decreased with temperature especially when temperature reached above the glass transition temperature (Tg) of the adhesive. Based on the ultimate load prediction model developed by Hart-Smith for double lap joints and kinetic modeling of the mechanical degradation of the adhesive, a mechanism-based model is proposed to describe the change of effective bond length, stiffness and strength degradation for steel/CFRP double strap joints at elevated temperatures. In the next part of the research, time factor was taken into account. By applying different load levels, the time-to-failure of the joints was monitored and analyzed. It was found that when temperature was close to or higher than Tg, the joint failure was time dependent. Based on the proposed temperature and time-dependent material property models, the time dependence failure time of steel /CFRP double strap joints was well described. Next, the effects of long term exposure to UV and associated temperatures were investigated. It was shown that UV exposure does not influence the tensile strength of CFRP composites. The tensile strength of the adhesive reduced slightly while modulus showed significant increase after long term exposure. Although the tensile modulus of adhesive exposed to only thermal environment also increased, it is considerably less than that induced by UV exposure. The UV exposure also led to a decrease in joint strength but an increase in stiffness, caused by the temperature effect rather than the UV rays. The next stage of the research investigated the effects of sea water and moisture. It was shown that sea water appeared to reduce the joint strength and stiffness both by 20% after 1 year of exposure, and the joints exposed to combinations of temperature and humidity in the environmental chamber showed little decrease in joint strength and stiffness after 1000 h of exposure. A model was developed to predict the strength and stiffness degradation of the joints in sea-water and the results compared well with the experimental results. Finally, the effects of sustained load and curing were examined. Itwas found that the effects of sustained load were significant which could catastrophically cause joint failure when it was combined with cyclic temperature and humidity. Elevated curing of the steel /CFRP bonded joint helped to improve the mechanical performance of the joints at elevated temperatures and increased the time-to-failure of the joints. Partial safety factors proposed in design guidelines for FRP composites under environmental effects were used to evaluate the experimental results obtained, suggesting a very critical degradation of joint mechanical properties due to the environmental effects investigated in this study.