Applications of electromagnetic acoustic transducers in the NDE of non-conducting composite materials

Abstract

An electromagnetic acoustic transducers (EMAT) can usually generate or detect an ultrasonic wave in an electrically conducting material across a small gap. The transducer is a unique ultrasonic EMAT probe that does not require a couplant or gel. Therefore it can be applied in a noncontact mode with a high degree of reproducibility. Shear waves and Lamb waves can be easily generated with EMATs in single-sided access. However, the application of EMAT has been limited to electrically conducting materials. In this work, EMAT was applied to the inspection of nonconducting composites by adhering a removable aluminum foil tape on the part surface. Ultrasonic waves generated in the aluminum layer by the EMAT readily propagated into the nonconducting composite. The reversed process was used in the reception of ultrasonic waves by EMAT. For composites that contained surface metallizations (e. g., metal foil or mesh) for lightning protection and electromagnetic interference (EMI) purposes, EMAT probes were placed directly on the composite surface. Finally, EMAT-generated, normal-incidence shear waves were used for the inspection of “green” laminates before curing to detect errors in ply orientation and layup sequence.