Ultrasonic Measurement of Gap Size in an Epoxy Layer between a Steel Sheet and Fiber Reinforced Plastics

Detection and sizing of empty gaps caused by missing adhesive are of great importance for quality assurance of multi-layer structures with a thin epoxy adhesive layer. This paper describes an approach and instrumentation for sizing empty gaps existed in adhesive layer between a thin steel sheet and fiber reinforced plastics by the combination of higher-frequency pulse-echo measurement and lower-frequency resonance spectroscopy of ultrasound. In this approach, the ultrasonic reflection from the interface between the steel sheet and the epoxy adhesive is measured with a high-frequency pulse-echo setup in order to identify contact disbanding and missing adhesive. Then, the steel sheet is excited to resonance by low-frequency ultrasound, and the gap size underneath the measuring location is estimated from the resonance responses. For practical application in industry an automated testing system has been developed where the proposed approach is implemented. The performance of the proposed approach has been verified by actual measurement of gap sizes from the cross-sections of cut specimens using an optical microscope.