The behaviour of CFRP strengthened steel joints

In recent years, carbon fibre reinforced polymers (CFRP) have been growing in popularity for external strengthening and repairing of deteriorated steel structures. On one hand, compared to traditional strengthening methods such as welding and bolting, CFRP has shown significant advantages such as light weight, high strength, and corrosion resistance. Furthermore, the application process for CFRP is very simple, quick and requires minimal labour. On the other hand, due to the high strength properties of steel and CFRP, failure is most likely to occur at the bond interface. Therefore, it is vital to study the bond behaviour and the failure mechanism of CFRP strengthened steel structures as well as the factors that play crucial roles in the bond quality. These factors including the material properties, the surface treatment prior bonding or the bond length must be studied before applying in a real structure. The current research program studied the bond behaviour of double strap joints which were formed by using CFRP to connect two steel plates. Experiment was conducted to investigate the effect of various material properties, bond lengths and surface roughness levels on the bond quality and the failure modes. It was found that there existed an effective bond length for CFRP-to-steel structures; once it was exceeded no improvement in the load carrying capacity could be obtained. This effective bond length varied with material properties, especially the steel grade. It was also found that variations of surface roughness level between 0.73μm and 7.75μm had no significant effect on the ultimate loading capacity of the specimens. However, the CFRP and the adhesive types played very important roles in the bond strength and the failure modes. Experiments were also conducted to study the fatigue behaviour of double strap joints. These joints were prepared by using strand CFRP with and without primer resin. After the curing process the joints were subject to fatigue tensile loading until failure. The fatigue lives and failure modes were recorded. Results showed that the fatigue lives of the double strap joints prepared with strand CFRP were comparable to those of specimens strengthened with CFRP plates and other high modulus CFRP sheets. However, a reduction in fatigue life was observed for the samples with primer resin. A cohesion zone finite element model was established to simulate the bond behaviour and to investigate some factors that could affect the load carrying capacity of the joints such as the steel grade. The results obtained from the model were compared with the experimental data in order to verify the cohesion zone model. The results showed an agreement between the experiments and numerical analysis. The cohesion zone was proved to be suitable for predicting the ultimate load of the joints as well as the stress transfer from CFRP-to-steel. It was found that the steel grade had a significant effect on the bond capacity and the effective bond length