Progress on nonlinear ultrasonic evaluation of damages within the adhesively bonded composites

Abstract

<p indent="0mm">Adhesively bonded composite structures are widely used in aerospace, shipbuilding and automotive industries because of such advantages as lower structural weight and more uniformly distributed stresses. As an important component of the adhesive joints, properties of the adhesive interface determine the integrity of the whole adhesive structure, making the research on damage detection and evaluation of the adhesive interface an extensively concerned topic in the related fields. The ultrasonic nondestructive testing technique, which has a long history of development since its appearance, is an effective technology for testing, evaluating and characterizing material properties and damages. Because of the advantage of realizing the detection without causing additional destruction to the structure and materials, the ultrasonic nondestructive testing has been widely used in engineering practice. However, the existing traditional ultrasonic nondestructive testing technology mainly concerns the initiation and accumulation of material damages, final failure of the structure on service. It is difficult in characterizing the bonding strength, bonding force, early degradation and micro-damage of the bonding surface directly and quantitatively, which is not capable of realizing the structural health monitoring and life prediction of the adhesive joints. It is promising that the nonlinear ultrasonic technology developed in recent years can effectively solve the problems mentioned above. When nonlinear waves are employed, this technique has higher sensitivity in characterizing the change of micro-structure of materials and micro-damages, whose spatial dimension is much smaller than the wavelength of ultrasonic waves. Considering these advantages, nondestructive testing and evaluation employing nonlinear waves have been widely investigated in recent years; a series of new methods have appeared and many important achievements have been obtained. Detection and evaluation of damages within bonded composite structures and adhesively jointed interfaces are more complicated. There are two types of adhesive failure: Adhesion failure in the interface layer between the adhesive and the adhered, and cohesion failure in the adhesive layer. Due to the existence of adhesive interfaces, complex refraction, reflection and scattering will occur when ultrasonic waves propagate in the adhesive composite structure and interfaces, particularly the existence of the adhesive layer will increase the attenuation of the wave energy. These problems make the investigation of wave propagation in the adhesively bonded structure even more complicated. Based on the nonlinear ultrasonic technique, many investigations have been carried out and many fruitful achievements have been obtained in examining and evaluating the early degradation of the bonded interface. Generally, bulk waves, guided waves and mixing waves are employed in theoretical and experimental research. Bulk waves can be excited and received without much difficulty, and guided waves are effective in examining plate and shell structures. In both cases, the magnitude of high-order harmonic waves and related nonlinear parameters are used in the process of evaluation and characterization. As to the mixing wave method, the amplitude of the mixing resonance waves and the nonlinear parameter based on the mixing resonance waves are employed in evaluating and locating the early damage within the composite structure and the adhesively bonded interface. In this paper, we review the progress of applying the nonlinear ultrasonic waves in evaluating early degradation and micro-damage of the adhesively bonded composite structures and interface damage, including the theoretical and experimental investigations of employing transmitted bulk waves, guided waves and mixed waves. Furthermore, future research related to the characterization of composite structures and adhesive joints is suggested.