# Retrofitting of Steel Structures with CFRP: Literature Review and Research Needs
## 2 Bond Behavior between CFRP and Steel
### Bond Test Configuration and Failure Modes  
To study the bond behavior between the CFRP and steel substrate, two test setups have been mainly adapted, as shown in Figure 1. In the single strap joint configuration (a), two steel plates are attached by CFRP only on one side, while in the double strap joints configuration (b), CFRPs are attached to both sides of steel plates. In both configurations, the loading is applied to the steel plates. It should be noted that in the single strap joint test setup, the adherends are subjected to bending during loading due to load eccentricity. This asymmetrical configuration can lead to rotation of the bond plane, resulting in significant peel stresses at the adhesive layer ends, which may cause premature adhesive failure and consequently lead to an underestimation of the capacity of the CFRP/steel joint [38]. In contrast, the double strap joint is symmetrical about the mid-plane of the specimen. Therefore, given the adherends, the amount of peel stress due to bond rotation is considerably less than a single strap joint configuration [38]. Possible failure modes of the CFRP/steel joints under tensile loading include the following [5]: (a) adhesion failure at CFRP/adhesive interface; (b) adhesive layer failure; (c) adhesion failure at steel/adhesive interface; (d) CFRP delamination; (e) CFRP rupture; (f) steel yielding. Figure 2 presents a schematic view of these failure modes.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**CFRP Type** & **CFRP Modulus** & **CFRP Modulus Relative to Steel** \\ \hline Low Modulus (LM) & \(<\)100 GPa & E\({}_{\text{CFRP}}\)\(<\) 0.5 E\({}_{\text{steel}}\) \\ Normal Modulus (NM) & 100–200 GPa & 0.5 E\({}_{\text{steel}}\)\(\leq\) ECFRP \(<\) E\({}_{\text{steel}}\) \\ High Modulus (HM) & 200–400 GPa & E\({}_{\text{steel}}\)\(\leq\) ECFRP \(<\) 2 E\({}_{\text{steel}}\) \\ Ultra-High Modulus (UHM) & \(\geq\)400 GPa & E\({}_{\text{CFRP}}\)\(\geq\) 2 E\({}_{\text{steel}}\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: CFRP classification in terms of elastic modulus [33].  
Table 2 presents details of the previous studies on the bond behavior of CFRP/steel joints. It should be noted that in some research studies, the applied CFRP was claimed to be UHM. However, based on the classification presented in Table 1, they should be considered HM or NM CFRP. The elastic modulus of the applied CFRPs in these studies varied from 270 GPa to 640 GPa. However, investigations involving UHM CFRP have not been adequately documented. Based on these studies, it can be inferred that using CFRP with a higher elastic modulus can lead to an improvement in the load-carrying capacity of CFRP/steel joints [39]. Moreover, it has been found that the failure mode is altered as the elastic modulus and type of CFRP change [40; 41]. Nevertheless, the influence of fatigue loading on bond behavior was reported to be insignificant, as only a limited bond region adjacent to the joint, known as the fatigue damage zone, was prone to fatigue damage [19]. It was also found that the failure mode changed as the temperature increased [42]. The dominated failure mode at elevated temperatures is cohesion failure within the adhesive layer [43]. In addition, the debonding loads of the CFRP/steel joints are notably decreased at both low and high service temperatures [44].