Testing ultrasonic SH waves to estimate the quality of adhesive bonds in small hybrid structures

Shear Horizontal Guided Waves (SH-waves), a special type of ultrasonic waves, were used to investigate the applicability for an assembly of aluminium sheet or tube with a polymer structure. For any kind of joining technology, the polymer-metal phase boundary can be considered as a weak point and prone to failure. In order to get information about the structural integrity of the joining, Nondestructive Testing (NDT)-methods are essential. Concerning adhesive joints, some important aspects are not yet accessible by any ready-to-use method: the strength of adhesive bonding, as well as defects like weak boundary layer and ""kissing bonds"". Surface waves excited in one of the bonded parts exactly interact with this desired region. SH-waves proved beneficial waveform but because of insufficient testing equipment, it has not been possible up to now to examine small structures and samples, e.g. lap joints complying with EN 1465.With state-of-the-art technology, Electromagnetic Acoustic Transducers (EMAT) have been developed to excite the highly pure and short-pulsed waves needed. The study evaluates relevant factors beside a variation in adhesion that can affect the ultrasonic signal in polymer-metal hybrid components in general and lap joints for future tests in particular.