STRUCTURAL HEALTH MONITORING OF AEROSPACE STRUCTURES BY MEANS OF GUIDED WAVES
Ultrasonic guided waves, specifically Lamb waves propagate only in structures where the wavelength is comparable or greater than the structure thickness. Therefore plate-like structures and shells of any solid material act as a waveguide. Very low attenuation – propagating over the long distances has already been succesfully utilized in pipeline inspection. Guided wave based monitoring of composite structures plays an important role in the area of structural health monitoring (SHM) of aerospace structures. Despite the advantages and possibility of exploitation guided waves show multimodal and dispersive character. The number and velocity of propagating modes depend on the product of frequency and thickness. The fundamental symmetric (S0) and the fundamental antisymmetric (A0) mode propagate through the whole frequency-thickness range, increasing the frequency-thickness product the number of propagating modes increases. These relations are described by dispersion curves. Due to the complexity of Lamb waves deep understanding of wave propagation and utilization the tools for signal processing is conditional for damage evaluation.
Regarding SHM of aerospace structures there are many different approaches and algorithms for damage detection using Lamb waves especially dependent on the structure design and type of damage. The most common evaluation is based on the comparison of baseline data and signals measured after loading, however baseline-free algorithms have been proposed by some researchers. Damage is indicated by damage-sensitive signal feature – in general called Damage Index (DI). Aerospace composite structures may in operation suffer for impact damages with high concentration in door surround areas. Literature on detection, localization and quantification of either barely visible impact damages (BVID) or visible impact damages (VID) shows a variety of approaches. Tomographic imaging algorithms, especially RAPID algorithm (reconstruction algorithm for the probabilistic inspection of damage) has become very popular. Adhesively bonded composite joints represent a significant challenge in SHM, since they are not certified yet and the bond quality needs to be reliably assessed.