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Abstract: Fiber reinforced polymer (FRP) materials were used to repair cracked non-load-carrying cruciform welded joints; and both the finite element method and the boundary element method were employed to study the fatigue crack propagation life of specimens repaired with FRP materials. The stress intensity factors (SIFs) were calculated for cracked welded joints that were unrepaired or were repaired with various FRP materials. The reduction of the SIFs for the repaired specimens was evident with the comparison of the control specimens. The fatigue life was then estimated with the NASGRO equation, which is modified from the Paris law. It was observed that the resulting fatigue improvement agreed with existing experimental data, indicating that fatigue strength can be upgraded with composite material repairs. The effects of various repair parameters on fatigue life were also examined. Single-sided repairs on cracked surfaces were more effective than double-sided repairs, due to the shift of the neutral axis. An increased modulus of FRP materials and adhesive achieved better fatigue performance. The use of more patch layers also had a positive effect, while increased adhesive thickness had a negative effect on fatigue life. [Copyright &y& Elsevier]

Abstract: Extensive research has been conducted on static bond behaviour between CFRP and steel. However, very limited research is available on the effect of fatigue loading on the bond behaviour between CFRP and steel. This paper attempts to fill the knowledge gap in this area. A series of static and fatigue tests on UHM (ultra high modulus) CFRP plate and steel plate double strap joints were conducted. Five specimens were tensioned to failure under static loading as control specimens. The other 12 specimens were tested under fatigue loading with load ratios ranging from 0.2 to 0.6 (defined as the ratio of the maximum fatigue load to the average static bond strength of control specimens). After going through pre-set number of fatigue cycles, the specimens were tensioned to failure under static loading. The failure modes, residual bond strength and residual bond stiffness of such specimens were compared with those of control specimens, to facilitate the investigation of the effect of fatigue loading on the bond behaviour. Microscopic investigation was also performed to reveal the underlying failure mechanism. [Copyright &y& Elsevier]

Abstract: The present work investigates an innovated methodology for manufacturing wet lay-up CFRP/steel double-strap joints and characterises their interfacial bond experimentally under infrastructural subzero exposures. A series of tensile tests is carried out on these joints after being conditioned at subzero temperatures ranging from −40°C up to 20°C. Three rheologically different epoxide resins and NM (normal modulus) unidirectional CF (carbon fibre) plies are used for the joints’ fabrication. Failure patterns, joint strength, and strain and LSS (lap-shear stress) distributions for the tested joints are obtained. Useful discussions and conclusions are provided, particularly those related to the influence of subzero temperatures and rheological properties of the current adhesives on the investigated parameters. [Copyright &y& Elsevier]

Using fibre reinforced polymer (FRP) composites together with seawater and sea sand concrete (SWSSC) in coastal areas will overcome the environmental issues of using ordinary concrete and corrosion problems of conventional steel reinforcements. The present research investigates the mechanical properties of different SWSSC filled FRP tubes after exposure to seawater. Glass, carbon, and basalt filament wound FRP tubes were filled with SWSSC and exposed to seawater for different exposure durations at different temperatures. A total number of 567 Hoop tension and compression tests were carried out after conditioning to investigate the mechanical properties degradation of the tubes. In addition, scanning electron microscopy (SEM) and micro computed tomography (micro-CT) analyses were conducted on representative samples to study the degradation mechanisms and damage progression. Finally, the long-term mechanical performance of SWSSC filled tubes under seawater was predicted based on Arrhenius theory and using the experimental data. According to the test data, generally, the samples with multiple fibres direction showed better durability compared to the tubes with fibres oriented in hoop directions. Moreover, carbon tubes experienced the smallest degradation while glass and basalt tubes showed almost the same range of degradation. [ABSTRACT FROM AUTHOR]

Abstract This study investigates the flexural and impact behaviour of GFRP laminates after exposure to elevated temperatures. The effect of fibre's length and orientation, laminate's thickness, and exposure time is studied. A total number of 540 tests in terms of three-point bending and Charpy impact tests were conducted to obtain the mechanical properties. In addition, SEM analyses were carried out to investigate the degradation mechanisms. Finally, statistical study was conducted to investigate the contribution of each variable and develop probabilistic models using ANOVA and linear Bayesian regression method. The results showed that generally the flexural and impact properties of GFRP laminates decrease by increasing the temperature and time of exposure as well as decreasing the laminates' thickness. It is also observed that laminates with unidirectional fibres have the best performance under elevated temperatures, while laminates with randomly distributed fibres are the most vulnerable type. The performance of laminates with woven fibres are almost between those two other types. However, all types of the laminates lost almost all their flexural strength and impact energy absorption capacity at 300 °C. [ABSTRACT FROM AUTHOR]

Abstract In this study, a novel retrofitting system is developed to strengthen existing metallic I-girders using prestressed unbonded carbon fiber reinforced polymer (CFRP) plates. The system relies on a pair of mechanical clamps. Each clamp holds two CFRP plates (each having cross-sectional dimensions of 50 × 1.4 mm) and anchors their prestressing forces to a metallic I-girder via friction. A finite element (FE) model was established to optimize the design of the required mechanical components, and a set of pull-off tests was performed to evaluate the capacity of the optimized system. The proposed flat prestressed unbonded retrofit (FPUR) system was then applied on a 6.4-m-long steel I-beam, and the excellent performance of the system, in terms of stress reduction in the beam bottom flange, was confirmed based on the results of a set of static four-point bending tests. Moreover, a fatigue four-point bending test was conducted on the steel I-beam, strengthened using the proposed FPUR system with a prestressing level of 53% (i.e., approximately 1120 MPa prestress in the CFRPs). Experimental results of the high-cycle fatigue test confirmed the excellent fatigue performance of the proposed FPUR system, as no prestress loss was observed after 20 million fatigue cycles. An analytical model is proposed to accurately predict the stress state in an I-girder strengthened with the proposed FPUR system, while design recommendations are provided concerning the practical use of the system. Relying on the laboratory experimental test results, the proposed FPUR system was used to strengthen the metallic cross-girders of a 121-year-old bridge in Australia. [ABSTRACT FROM AUTHOR]

The bond characteristics of double strap joints between carbon fibre reinforced polymer (CFRP) laminates and steel with carbon nanotubes (CNTs) modified epoxy, under moderately elevated temperature (23–70 °C) were studied. The effect of CNTs on the failure modes, bond interface, and bond strength were presented. Results show that increasing test temperature causes transition of failure from epoxy-CFRP interface to steel–epoxy interface and to cohesive layer in the joints with neat epoxy. The cohesive failure could be avoided in the joints with CNT-epoxy. The observations from scanning electron microscopy revealed that CNTs bridge the cracks in epoxy matrix indicating a reinforcing effect. Moreover, CNT-epoxy can provide a significant increase (about two fold) of bond strengths at moderately elevated temperatures when compared with neat epoxy. It demonstrated that the incorporation of CNTs into the bond adhesive could allow better exploiting the potential of CFRP system in strengthening of steel structures at moderately elevated temperature. [ABSTRACT FROM AUTHOR]

The effects of carbon nanotube (CNT) modified epoxy adhesive on CFRP-to-steel interfaces were investigated using double strap joints. The bond behaviours studied were failure modes, bond interface at microlevel, bond strength, effective bond length, CFRP strain distribution and bond-slip relationships. For the first time, a novel type of failure in the CFRP-steel joint was discovered, attributable to weak bonding between woven mesh and CFRP fibres. This failure mode prevented exploitation of the full potential of the carbon fibres and the CNT modified epoxy adhesive. Joints bonded with CNT-epoxy adhesive had an effective bond length of about 60 mm, whereas that of joints bonded with pure epoxy was about 70 mm. The CNT-epoxy adhesive can transfer more load from the host structure to the bonded CFRP laminates, consequently modifying bond behaviour. It is therefore expected that CNT-epoxy nanocomposites will assist in the strengthening and rehabilitation of steel infrastructures using CFRP laminates. [ABSTRACT FROM AUTHOR]