Guided waves for detection of delamination and disbonding in stiffened composite panels

Composite materials are susceptible to hidden defects that may occur during manufacturing and service (e.g., foreign object impact) and may grow to a critical size, jeopardizing the integrity of the structure. Among the various existing techniques, guided wave methods provide a good compromise in terms of sensitivity to a variety of damage types or defects and extent of the area that can be monitored, given the ability of these waves to travel relatively long distances within the structure under investigation. Wave propagation in composite structures presents several complexities for effective damage identification. The material inhomogeneity, the anisotropy and the multi-layered construction lead to the significant dependence of wave modes on laminate layup configurations, direction of propagation, frequency, and interface conditions. This paper is concerned with the detection and characterization of small emerging or existing defects in composite structural components using a recently developed technique employing an array of surface mounted broadband ultrasonic transducers as actuators and sensors as well as theoretical analysis to interpret the recorded signals. The technique is applied to panels with different thicknesses, including stiffened specimens with stringer-panel disbonding. The major objectives of this research is to extend the current capabilities of ultrasonic methods to wider areas of coverage, faster inspection procedures, lower percentage of false positives and less dependence of manual operations. The method is based on the well known fact that guided waves are strongly influenced by inter-ply delaminations and other hidden defects in their propagation path.