Damage Reconstruction in Complex Composite Structures using Lamb Waves

Kurzfassung

The current maintenance and inspection strategy for aircraft structures is based on strictly scheduled inspection intervals considering the age and usage of the aircraft. This implies that the structures must be designed in a damage tolerant way, allowing a safe operation with undiscovered damages until the next major inspection. In addition, new aircraft are designed with even longer inspection intervals to reduce downtime and maintenance costs. For fiber-reinforced composite structures this approach is contradictory to the idea of reducing the weight of the aircraft. Especially blunt low-velocity impacts may cause large damages like delaminations that are likely to be missed during visual in-service inspection. It is therefore desirable to have an integrated structural health monitoring (SHM) system that will monitor the structure for damages and allow early damage detection between inspection intervals. This does not only pave the way to a more demand-driven maintenance, but may contribute to further exploiting the light-weight potential of composite materials. In contrast to other monitoring methods the use of Lamb waves allows determining the location of a damage. This has been shown many times for simple structures like plates or pipelines. However, these methods are often not applicable to complex structures that feature curvatures, anisotropic material properties and changing material properties throughout the structure. Therefore a damage reconstruction method is proposed, that is similar to methods known from conventional ultrasonic inspection, but takes into account the complex material properties and their influence on the wave propagation. The base for this approach is a time-of-flight calculation that can be applied to structures with many local changes in material properties and allows each material to be anisotropic. This is combined with a pulse compression to increase the temporal resolution of the signal and therefore the spatial resolution of the reconstruction. The proposed method can be applied to integrated monitoring systems with a limited number of fixed transducers, high-resolution scans of the wave field obtained with air-coupled ultrasound systems or laser-vibrometers and can also be adapted for acoustic emission monitoring.