Damage Detection in Composites by LAI-PZT Transducer

Abstract

Composite structures are known for being susceptible to manufacturing defects and in-service damage. Since damage in composites is not always visually apparent, NonDestructive Techniques (NDT) such as ultrasonic imaging are often employed to detect and quantify damage [1]. Materials such as carbon-epoxy and fiberglass-epoxy plates are frequently used as structural elements in the both the civil and aerospace industry. Structure made of these materials display high acoustic impedances along the direction of the fibers but behave as Low Acoustic Impedance (LAI) materials in the transverse direction (i.e., through the composite’s thickness). It follows that LAI acoustic transducer are generally more appropriate to scan and detect damage/defects through the thickness in various types of anisotropic structures. When transducers are coupled in a direction orthogonal to the resonant mode they display significantly lower characteristic impedance than if coupled in the same direction. In some cases, this feature can eliminate the need for intermediate “matching†films and makes LAI transducer desirable for acoustic imaging applications. In this paper we introduce the use of low-cost, low effective impedance PZT transducer for damage detection in LAI structures. This manuscript also describes results from both numerical modeling and damage detection testing performed using a prototype transducer. In the future simple and economical transducers resonated in their transverse mode rather than the thickness mode could be used to detect damage such as matrix cracking and delamination of the composite structure.