Surface cracked steel plates reinforced with single-side Fiber-Reinforced Polymer (FRP) subjected to cyclic tension are experimental studied. The main purpose is to analyze the effect of FRP reinforcement on the crack growth. The failure modes and their effects are analyzed as well. Given the single-side reinforcement, reinforcing the cracked surface significantly prolonged the fatigue life, while reinforcing the reversed side resulted in the opposite consequence. Most specimens did not encounter debonding failures, indicating such failures are avoidable by improving the reinforcement quality. The results also indicate the bond layer number is an insensitive factor–an optimum number is existed.

As a result of the influence of surface characteristics, it is difficult to identify cracks on complex surfaces using the conventional processing of a single-frame image that misses important infrared thermal image sequences. In this paper, the red-green-blue normalisation method and the interframe difference method are used to make full use of the sequences and to characterise infrared thermal images of cracks on complex surfaces. An experiment to prove the effectiveness of the detection method was carried out on weld steel and carbon fibre-reinforced polymer (CFRP) steel plate. The selected infrared thermal image was divided into an objective area and a background area according to the relationship between three parameters for red-green-blue and objective temperature. The targeted and interfering information of the objective area were separated and processed based on the interframe difference method. The index of crack detection was extracted from the interframe difference matrix, which was constructed from two selected infrared thermal images. This method takes into account the local and global spatial information of the image and the maximum crack length measurement error was approximately 6%.

Fiber-reinforced polymer (FRP) has been increasingly applied to steel structures for structural strengthening or crack repair, given its high strength-to-weight ratio and high stiffness-to-weight ratio. Cracks in steel structures are the dominant hidden threats to structural safety. However, it is difficult to monitor structural cracks under FRP coverage and there is little related research. In this paper, a crack monitoring method for an FRP-strengthened steel structure deploying a microstrip antenna sensor is presented. A theoretical model of the dual-substrate antenna sensor with FRP is established and the sensitivity of crack monitoring is studied. The effects of the weak conductivity of carbon fiber reinforced polymers (CFRPs) on the performance of crack monitoring are analyzed via contrast experiments. The effects of FRP thickness on the performance of the antenna sensor are studied. The influence of structural strain on crack detection coupling is studied through strain–crack coupling experiments. The results indicate that the antenna sensor can detect cracks in steel structures covered by FRP (including CFRP). FRP thickness affects the antenna sensor’s performance significantly, while the effects of strain can be ignored. The results provide a new approach for crack monitoring of FRP-strengthened steel structures with extensive application prospects.