Using EMATs with Composite Materials
An electromagnetic acoustic transducer (EMAT) requires no couplant and can be noncontact in the generation and reception of ultrasound. Measurements using EMAT probes can therefore be done with a high degree of reproducibility. In an NDE project EMATs have been applied to a number of composites, including poorly conducting graphite/epoxy composites and nonconducting glass/epoxy and ceramic matrix composites. In order to generate sound waves via the Lorentz force mechanism, the surface of the composite must be conducting. An aluminum tape (0.003" aluminum foil with adhesive layer) is applied to the composite surface to achieve this. Ultrasound is generated in the aluminum foil and the adhesive bond between the metal and the composite allows the propagation of sound waves into the bulk of the composite.
EMATs on aluminum foil which in turn is bonded to composite material
Two types of EMAT probes are used: shear horizontal (SH) wave probes originally designed to study rolled plates in a stress and texture project in CNDE and, more recently, EMATs that generate normal incidence shear waves. Using the SH wave probes, the mechanical anisotropy of composite laminates were investigated in a configuration somewhat akin to the "acousto-ultrasonic" technique. Directivity of the received EMAT signals 'Showed excellent correlation with fiber' directions in the laminate. The SH wave probes, although not intended for generating bulk waves, had a sufficient fringing field to be used in a transmission measurement through a full inch of graphite epoxy laminate. EMAT-generated plate modes were also used in the detection of skin-core separation in a honeycomb sandwich structure (a rudder skin).
A large rise in amplitude (signal to noise ratio of 4-5) was detected over the defect, as expected from damping considerations. Some polymer composites contained a metallized layer in the form of foil or mesh (for EMI and lightning protection purposes). These composites did not require the help of the aluminum tape in using EMATs. In a graphite epoxy panel with 0/90 lay-up of graphite fiber, also containing a copper mesh, azimuthal scans using a pair of EMATs showed the combined effects of the fiber tows at 0° and 90° and the direction of the copper wires at ±35°. In addition, it has been found that EMATs can also be applied directly to a graphite epoxy panel that contains a top ply of nickel-plated graphite fibers.