Dry-coupled low-frequency ultrasonic wheel probes: Application to adhesive bond inspection

One of the factors limiting the more widespread use of adhesive bonding by industry is the lack of fast and reliable non-destructive testing methods. This paper describes a novel low frequency (<500kHz) dry coupled wheel probe which can be used to assess the integrity of three layered adhesive joints. A key novel element of this wheel probe is its operation in pulse-echo mode at low frequencies with a wide bandwidth. The system measures the first through thickness resonance of the joint which is then used to infer bondline thickness. The system is demonstrated on a range of adhesively bonded specimens of automotive specification. Also its sensitivity to alignment and coupling are discussed as are other factors limiting its operation.