The widespread use of adhesive bonding is indicative of its significant advantages as compared to other fastening methods (welding, soldering, brazing, bolts, rivets, etc.). From an acoustical point of view, adhesives are represented by a steplike change which occurs in the characteristic acoustic impedance. Despite great attention to quality in the development of manufacturing processes, joints are still the weakest link in the mechanical integrity of components. This is partially due to stress concentrations and thermal residual stresses inherent in the designing and manufacturing process. Also, interfaces are very susceptible to flaw formation during manufacture. Often the failure of a jointed, laminated structure is initiated at the interface regions and joints. These regions may contain manufacturing defects, or they may degrade due to environmental and load-induced effects. As a result of damage at the interface of the adherent and adhesive, disbonds may develop. The mechanical behavior of an adhesive layer containing a distribution of disbonds is of a complex nature. Many investigators have considered the progressive degradation of the adhesive/adherent interface as a transition from `welded' boundary conditions between the adhesive layer and the adherent (continuity of both normal and tangential displacements across the interface), to `slip' boundary conditions (continuity only of the normal displacements since the slip interface will not transmit shear stresses).
Effective nondestructive testing techniques are necessary for quality control and in-service inspection of bonding conditions. Commonly encountered bonding problems can be classified into three groups: delamination, cohesive weakness and adhesive weakness. The former two types can be detected by conventional ultrasonic techniques. The last type is the most difficult due to physically perfect contact between the adhesive and the adherend. The challenge is to evaluate imperfect interfaces in tight mechanical contact without chemical or metallurgical bond. The difficulty is to discriminate and quantitatively describe such interfaces by non-destructive measurements. The extensive use of adhesive bonded structures has given stimulus to development of methods of nondestructive testing of such products [1]-[5]. Ultrasonic measurements seem most promising for NDE of bonds since they are extremely sensitive to the state of contact at the interface and can be utilized to directly measure interfacial properties.
Ultrasonic inspection of adhesive joints is usually based on the analysis of the amplitude or frequency dependence of the coefficient of reflection at normal or oblique incidence of the ultrasonic waves [6]-[7]. Ultrasonic inspection of adhesive joints was analyzed also by the Lamb wave technique in which one measures the attenuation and phase or group velocity of ultrasonic modes propagating along the joint [4, 8], i.e., in the adherend-adhesive-adherend sandwich as a whole. Lamb waves are sensitive to adhesive-type defects as well, but the bond is rather difficult to evaluate since the wave is dominated by the adherend plates, while the adhesive layer causes weak, subtle effects.
For very thin adhesive layers the alternative is the application of guided waves or leaky guided waves in the bonded area, combining the advantages of the above discussed conventional techniques [9]. These types of waves produce shear stresses at the interface and propagate along the interface; they are therefore mainly sensitive to variations of adhesive quality.
Wave propagation in layered media has been of major interest in fields like seismology, geophysics, microelectronics, NDE and others. In seismological models, the layers are usually assumed to be perfectly bonded at their common interface. For engineering applications it is necessary to model the interface more precisely. There are several advantages in employing guided waves for interface testing. The intensification of the measured effects along the path of the wave propagation, the possibility of testing very thin adhesive layers, only one dimensional scanning is necessary and they are less sensitive to the variations in the properties of the adherends relative to the method of ordinary bulk ultrasonic wave. The leaky guided waves are substantially attenuated, therefore they yield localized information. Full inspection of the adhesive joint will naturally necessitate the combination of the leaky wave technique with a scanning mechanism.
This work is based on the modified boundary conditions approach. The purpose of this paper is to analyze the possibility of measuring the quality of bonded joints and an extensive investigation of the effects of bonding quality on the propagation of guided acoustic modes. The results presented here, are of a series of experiments, in which a number of aluminum samples with bonds of varying thickness and quality were probed using surface traveling ultrasonic waves. These waves are detected by a Michelson interferometer.
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