The nondestructive evaluation (NDE) of adhesively bonded structures is a complex process. Earlier work has confirmed that ultrasonic waves are influenced by the properties of the material in which they travel. Acousto-ultrasonic methods have been widely used by previous researchers to generate ultrasonic waves in plates and bonded structures for flaw detection, visualization, and measurements of the local properties of the jointed materials. This paper will present the methods and principles used for generation and propagation of ultrasonic guided waves (Lamb waves) using piezoelectric wafer active sensors (PWAS).
Adhesively bonded joints between metallic and composite plates are gaining increasing acceptance in safety critical applications such as automotive and aerospace structures. Lamb wave methods have considerable potential for the inspection of adhesive joints and assemblies for two reasons: they do not require direct access to the bond region, and they are much more amenable to rapid scanning than are compression wave techniques. Lamb waves can be excited in one plate of a bonded assembly, propagated across the joint region, and received in the second plate of the assembly. The paper will present a study of the use of guided Lamb waves for disbond detection in adhesively bonded layered media using piezoelectric wafer active sensors (PWAS). The focus is on developing and compare methods for damage and disbond detection in adhesively bonded structures. For far-field detection, propagating Lamb waves will be used in pitch-catch and pulse-echo modes. The pitch-catch method will send Lamb waves across the adhesive joint, while the pulse-echo method will send Lamb waves along the joint line. For near-field detection, high-frequency standing Lamb waves will be used to evaluate the presence of disbond from the changes in the mechanical impedance of the bonded joint. The standing wave approach is achieved with the self-sensing electromechanical impedance method. Both, the propagating and the standing Lamb waves are generated with the same PWAS installation. The combination of far-field and near-field damage detection in the same sensor installation is a unique feature of our work.