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The technique of strengthening steel structures with carbon fibre-reinforced polymer (CFRP) has attracted growing attention in research field and in practice, and thus, its environmental durability is of high importance. This paper describes an investigation on the bond characteristics between high modulus CFRP sheets and steel plates under elevated temperature exposures. Tensile tests were carried out on CFRP/steel plate double strap joints at different temperatures (20, 40 and 50 °C) that represent the usually encountered conditions for civil infrastructure. High modulus (640 GPa) unidirectional carbon fibre sheets were applied by wet lay-up fabrication method. The fracture surfaces of the failed specimens were studied using a scanning electron microscope and the failure mechanisms were discussed. It was found that the ultimate load decreased significantly when the test temperature was above the glass transition temperature of the adhesive. Larger effective bond lengths were found for the joints tested at the glass transition temperature. It is obvious that the short-term thermal exposure has little effect on their failure patterns. All the specimens failed by fibre breakage at the joint and decohesion accompanied some joints with very short bond length. [ABSTRACT FROM AUTHOR]

Recent studies on the application of carbon fibre reinforced polymer (CFRP) materials to defected steel structures have demonstrated the potential for significant reduction of stress intensity factor (SIF) values at crack tips leading to extended fatigue lives. However, most of the previous research relied on experimental and numerical methods, which were either expensive or time-consuming. In this paper, the SIF values at crack tips of steel plates strengthened with bonded composite materials were evaluated using linear elastic fracture mechanics. The analysis was based on the classical solution of SIF values of plain steel plates, considering load share effect and geometry correction factor change resulted from the overlay patch. The effect of different parameters were demonstrated and compared with experimental results, including initial damage degrees of specimens, geometric and mechanical properties of retrofitting materials and bond locations. Good agreement with the experimental data indicated that this approach could conservatively predict the SIF values with reasonable accuracy. A parametric study on variables including the CFRP modulus, the bond width and bond length was conducted based on this method to further investigate their effect on the SIF values. [ABSTRACT FROM AUTHOR]

CFRP sheets are extensively used to strengthen defected steel members through a wet layup process. Different techniques were used to control the quality of bonding, such as vacuum bags and rollers. Recently strand CFRP sheets were applied in the strengthening of steel elements with manufacturer suggested epoxy resin and a primary resin. This study investigated the fatigue behaviour of steel beams and double strap steel joints bonded with strand CFRP sheets. Eight beams were tested under four point bending and eight double strap steel joints were tested under axial tension. Comparisons were made on the failure modes and the fatigue life for specimens bonded with and without the primer epoxy resin. Results showed that the fatigue life of defected steel beams and double strap steel joints bonded with strand CFRP sheets are comparable to that of specimens strengthened with CFRP plates and other high modulus CFRP sheets, whereas a linear reduction in fatigue life was observed when the primer resin was applied on the steel surface of the samples. [ABSTRACT FROM AUTHOR]

This paper summarizes the environmentally assisted degradation of the bond between steel and carbon fiber-reinforced polymer (CFRP) composites through analysis of the mechanical performance of loaded and unloaded steel/CFRP double strap joints exposed to various harsh environments such as elevated temperatures, seawater, cyclic temperature and humidity, and ultraviolet (UV) radiation. It was found that both joint stiffness and strength significantly decreased by up to 80% when tested at elevated temperatures near to or greater than of the adhesive used. The mechanical properties reduced by about 20% when exposed to seawater for up to 1 year. Although no significant reduction in stiffness and strength were found when the unloaded joints were exposed to cyclic temperature between 20 and 50°C with the added presence of humidity, the loaded joints catastrophically failed during the exposure, showing that the combination of load, temperature, and moisture was the most critical scenario. In addition, UV exposure also led to a decrease in strength but an increase in stiffness for the unloaded joints, caused by the postcuring with the heat supplied by the UV rays. [ABSTRACT FROM AUTHOR]

ABSTRACT Welded steel connections of infrastructures are susceptible to fatigue failure. Advanced carbon fibre reinforced polymer (CFRP) has been demonstrated promising for fatigue strengthening of steel structures. Limited research was conducted on CFRP strengthening of welded connections. This paper focuses on the application of ultra high modulus (UHM) CFRP plates with a modulus of 460 GPa to strengthen steel plates with longitudinal fillet weld attachment using five CFRP strengthening configurations. A series of fatigue tension tests were carried out with constant amplitude fatigue loading. Beach marking technique was adopted to record the crack propagation process. Effects of CFRP bond length, bond width and bond locations on fatigue performance of welded steel joints were investigated. The experimental results showed that UHM CFRP plates could generally increase the fatigue life of the welded steel joints. It seems better to apply CFRP on the welding side of the specimen to achieve longer fatigue life. Then, the effects of weld and weld attachment on the CFRP strengthening efficiency was further studied by comparing experimental results of non-welded steel plates with single side UHM CFRP plate strengthening. Finally, the classification method was adopted to assess the strengthening efficiency of the UHM CFRP plate to the steel plates with longitudinal weld attachment. [ABSTRACT FROM AUTHOR]

ABSTRACT The fatigue life of cracked steel members can be greatly extended by externally attached carbon fibre reinforced plastics (CFRP), which reduces the stress intensity factors (SIFs) at the crack tip. Access to cracks is sometimes limited and the CFRP has to be attached away from the cracks. There is a lack of knowledge on SIFs for such strengthening scheme. This paper presents the effects of CFRP bond locations on the Mode I SIF of centre-cracked tensile (CCT) steel plate. The Mode I SIF at the crack tip is calculated using the finite element (FE) models. A correction factor is introduced as a function of CFRP bond location and crack length. The FE results are compared and agree well with experimental tests conducted by the authors. By combining with another two factors (one considering CFRP mechanical properties and the other considering CFRP bond width) derived previously by the authors, SIF formulae are proposed for CFRP reinforced CCT steel plates. [ABSTRACT FROM AUTHOR]

Web crippling is the major failure mode of thin-walled members when they are subjected to concentrated loading. Carbon fiber-reinforced polymer (CFRP) is found to be promising for strengthening metallic structural members. This paper reports improved web-crippling capacity of sharp-corner aluminum tubular sections: rectangular hollow section (RHS) and square hollow section (SHS), by attaching CFRP to their webs. Twenty four specimens were tested with four CFRP strengthening configurations applied on each of six different aluminum RHS and SHS sections. Significant increase in load-carrying capacity was obtained. Further comparison is made between CFRP strengthened aluminum tubular sections and cold-formed steel counterparts in respect of strengthening efficiency. Underlying mechanism of different failure modes and strengthening efficiencies of various strengthening configurations are discussed with the assistance of FEM simulation. [ABSTRACT FROM AUTHOR]