Wave Propagation and Dispersion of Lamb Waves in Adhesively-bonded Structures

With an increasing use of adhesively bonded joints in the industry, there is a correspondingly growing need for the inspection of adhered joints. It is necessary to detect defects in the adhesively bonded materials such as voids inclusions, chemical miscure of the adhesive, or delaminations of the bonded layers. In order to use an ultrasonic technique to detect these defects, a thorough understanding of the wave propagation and wave interaction with the adhesive layer must exist. The paper presents the testing and modeling of guided Lamb waves for disbond detection in adhesively bonded layered media using piezoelectric wafer active sensors (PWAS). The interaction of the Lamb waves with the bonded layer was studied. An aluminum lap joint specimen was created and artificial disbonds were introduced in the structure. The guided Lamb waves were generated and introduced in the specimen using PWAS. The dispersion and attenuation of the traveling waves was studied for two cases, across the bond-line and along the bond-line. The focus was on the A0 and S0 modes of the guided Lamb waves. The two methods used in the experiment are pitch-catch (across the bond-line), and the pulse echo (along the bond-line). INTRODUCTION Ultrasonic testing of adhesively bonded joints using guided waves for both aerospace and automotive applications is gaining more and more attention. In the nondestructive evaluation of adhesively bonded joints of particular interest are the Lamb waves. Lamb wave methods have considerable potential for the inspection of adherent 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 pressure 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. Inspection of the joint then would be based on the differences between the signals received on one side of the assembly compared to those transmitted on the other side. An automated structural health monitoring system is not possible using conventional bulk wave transducers. For such a system, the sensors must be as much as possible part of the structure on which they are attached. New technology using piezoelectric wafer active sensors (PWAS) is being developed based on the piezoelectric ceramic wafers (lead-zirconatetitanium – PZT). The direct piezoelectric effect is when the applied stress on the sensor is converted into electric charge. The inverse effect, conversely, will produce strain when a voltage is applied on the sensor. In this way the PWAS can be used as both, transmitter and receiver. The advantage of such sensors is that they are small, unobtrusive, and can be embedded in the structure. In the past several year researchers started to look more and more into the advantages that piezoelectric materials and guided waves can bring to the field of nondestructive evaluation (NDE). Giurgiutiu and Zagrai characterized the PWAS for structural health monitoring and developed quality assurance techniques. Liu et al. investigated the input-output characteristic of piezoelectric structural health monitoring systems for composite plates. Lee and Staszewski studied Lamb waves for damage detection in metallic structures. The reliability of piezoelectric monitoring of adhesive joints was studied by Kwon et al. A single lap adhesively bonded tubular joint was tested during a torsional fatigue test. The results showed that the piezoelectric properties of the joint are related to the crack propagation.