Lamb wave approach to identify hidden cracks in hard-to-inspect areas of metallic structures

This thesis reports the results of a study of the scattering of the Lamb wave field due to the presence of a small crack in isotropic structures. The study addressed the challenge of detecting and quantifying a small defect at a hard-to-inspect location for structural health monitoring (SHM). The need for improved aircraft performance and efficiency has made manufacturing design and technology far more complicated, and consideration of nondestructive inspection is usually not factored into these designs. Thus, conventional methods are insufficient, as they do not to account for these advances in design.
   A fuel weep hole is a classical configuration in metallic aircraft structure, which is vulnerable to fatigue cracks in hard-to-inspect locations. Conventional diagnosis methods, such as the Eddy Current Technique, are not optimal for damage detection especially in such cases where built-in sensors are restricted to very limited areas. Previous studies involved using high-frequency bulk-wave wedge transducer techniques for weep hole inspection, but the difficulty of reliably detecting and quantifying the small hidden crack still remains in SHM. A reliable and novel analytical model is needed as a reference for monitoring and assessing the early development of a small crack before reaching it reaches a critical size.
   This study investigated the scattered Lamb waves on the defect, which carries information such as the severity, location, and size of the crack for SHM purposes. The isotropic specimen used in this research is aluminium plates with different geometry, which depends on the specific investigation. There is significant interest in the use of Lamb waves for hidden crack detection and quantification due to their advantageous properties of rapid wide area inspection with minimal attenuation.
   It is also anticipated that the point source equivalence, which consists of a particular combination of body-force doublets, applies for Lamb wave scattering. This research has demonstrated that the point source model can represent the Lamb wave mode scattering by a small crack. This research first investigated edge cracks on the straight edge of the plate, then on the hole boundary, and, lastly, on the top boundary of a rectangular slot. This approach will guide us to the next problem by using the previous study’s findings to determine the later study’s configuration and to compare its results. The Finite Element method and experimental method are used for this research to explore not only the common scattered Lamb waves amplitude but also the mode, pattern, and wave directivity as a conjunction for small crack detection and characterisation.
   The study of the scattering of fundamental Lamb waves by a small edge crack showed a quadratic-like relationship between crack length and scattered wave amplitude and the scattered wave pattern remains independent from the crack length. Within the small crack length-to-wavelength limit, these relationships highly correlate to point source model. It is noteworthy that the dominant scattered Lamb wave modes by an edge crack are the edge-guided and SH0 waves.
   The leaky circumferential edge wave is an interesting scattering phenomenon that can direct and coalesce with SH0 waves to the geometric shadow zone. The rectangular slot study has experimentally and computationally demonstrated that leaky symmetric edge-guided waves could propagate to hard-to-inspect location impinged with a hidden crack. The scattered waves are then redirected back to the location of excitation.
   The findings in this thesis give a fundamental understanding of the scattering of Lamb waves by crack on boundaries and holes as well as cracks in hard-to-inspect locations. The novelty of this research, which are the scattered wave amplitude, the wave pattern, and its directivity are important measurements to locate and quantitatively evaluate small cracks, based on the point source model. Under specific conditions, edge-guided waves can propagate around a curved edge to the blindsight area and redirected back, as shown in the hole and rectangular slot studies. These findings will assist in the development of Lamb wave propagation for SHM as a damage diagnosis tool.