Ultrasonic wave intensity reflected from fretting fatigue cracks at bolt joints of aluminum alloy plates

The in-process ultrasonic measurement was carried out during fatigue testing of the bolted aluminum alloy 2024-T3 plates by using a water bag to obtain the surface acoustic wave (SAW) reflected from a fretting fatigue crack. The echo reflected from the fretting fatigue crack was detected at a position 1.5–2.1 mm ahead of the bolt hole under fastened conditions. In the fatigue process, the intensity of the scattering wave gradually increased in the fretted region ahead of the bolt hole with the number of fatigue cycles, and then a steep increase in the scattering wave intensity was observed. Relationships between the crack length and the average and the peak intensities were also obtained, and the average and peak intensities increased with the crack length. A relationship between the average intensity of the scattering wave and the surface roughness was also obtained. Experimental results suggested that appearance of a fretting fatigue crack in the fretted region caused a steep increase in the scattered wave intensity.