A mobile crack detection system for plate-like structure based on non-linear ultrasonic fibre Bragg grating

Many researchers have highlighted the fact that non-linear ultrasonic technology is more suitable than linear ultrasonic technology for detecting micro-cracks. In most existing non-linear ultrasonic detection systems, piezoelectric patches are pasted directly onto specimens and used to detect ultrasonics. In this paper, a fibre Bragg grating (FBG) was adopted for sensing ultrasonics because FBGs possess many advantages over piezoelectric patches. Non-linear ultrasonic technology was combined with FBG sensing technology to detect cracks in thin aluminium plates. This study proposed a mobile system in which the excitation and monitoring points were easily moved on the surface of a specimen because piezoelectric patches and FBGs were glued to acrylic plates and placed in contact with the specimen surface using industrial ultrasonic couplants. The influence of acrylic plate thicknesses on the characteristics of mobile piezoelectric patches (MPPs) and FBGs was discussed. This mobile system was more flexible than traditional systems, in which piezoelectric patches and FBGs are fixed to the structures, and it did not destroy the surface of the specimen. Experimental results showed that the mobile system can detect non-linear sideband signals caused by cracks at every excitation and sensing point. The factors influencing sideband amplitude, including excitation amplitude and frequency, and the relative positions of two excitations and a FBG were studied.