Abstract: The structural morphology in the mechanical interlock of self-piercing riveted joints determines the static strength and fatigue performance of structures, and it is an important evaluation indicator for the joining quality. Based on the characteristics of sound pressure distribution on the rivet focal plane a theoretical formula of the sound intensity distribution in the vicinity of the structure edge is established and a method to measure the rivet expansion diameter is proposed. Both theoretical derivation and experimental results show that the change rate of sound intensity tends to increase and then decrease when the sonic beam of the probe moves to the rivet toe edge, and the grey-scale curves derived from the C scan images near the rivet toe edge demonstrates the same rule. And the“peak”of the change rate of the grey-scale can be used to locate the boundary of the rivet expansion diameter. The measurement results obtained by using this“peak”as the rivet toe edge point have a small error, and the relative error is 0.14%-1.87% compared with the actual profiling results, which verifies the feasibility of the method. And riveting defects: crack and off-center riveting can be judged by using an ultrasonic C-scan image extracted from the expansion plane of the rivet.

Key words: self-piercing riveted joints, interlock structure, ultrasonic detection, gray-scale distribution