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• W3148994060 abstract "This thesisreports the results of a study of the scattering of the Lamb wave field due tothe presence of a small crack in isotropic structures. The study addressed thechallenge of detecting and quantifying a small defect at a hard-to-inspectlocation for structural health monitoring (SHM). The need for improved aircraftperformance and efficiency has made manufacturing design and technology farmore complicated, and consideration of nondestructive inspection is usually notfactored into these designs. Thus, conventional methods are insufficient, asthey do not to account for these advances in design.    A fuel weep hole is a classical configuration in metallicaircraft structure, which is vulnerable to fatigue cracks in hard-to-inspectlocations. Conventional diagnosis methods, such as the Eddy Current Technique,are not optimal for damage detection especially in such cases where built-insensors are restricted to very limited areas. Previous studies involved usinghigh-frequency bulk-wave wedge transducer techniques for weep hole inspection,but the difficulty of reliably detecting and quantifying the small hidden crackstill remains in SHM. A reliable and novel analytical model is needed as areference for monitoring and assessing the early development of a small crackbefore reaching it reaches a critical size.    This study investigated the scattered Lamb waves on thedefect, which carries information such as the severity, location, and size ofthe crack for SHM purposes. The isotropic specimen used in this research isaluminium plates with different geometry, which depends on the specificinvestigation. There is significant interest in the use of Lamb waves forhidden crack detection and quantification due to their advantageous propertiesof 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 forLamb wave scattering. This research has demonstrated that the point sourcemodel can represent the Lamb wave mode scattering by a small crack. Thisresearch first investigated edge cracks on the straight edge of the plate, thenon 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’sfindings to determine the later study’s configuration and to compare itsresults. The Finite Element method and experimental method are used for thisresearch to explore not only the common scattered Lamb waves amplitude but alsothe mode, pattern, and wave directivity as a conjunction for small crackdetection and characterisation.    The study of the scattering of fundamental Lamb waves by asmall edge crack showed a quadratic-like relationship between crack length andscattered wave amplitude and the scattered wave pattern remains independentfrom the crack length. Within the small crack length-to-wavelength limit, theserelationships highly correlate to point source model. It is noteworthy that thedominant scattered Lamb wave modes by an edge crack are the edge-guided and SH0waves.    The leaky circumferential edge wave is an interestingscattering phenomenon that can direct and coalesce with SH0 waves to thegeometric shadow zone. The rectangular slot study has experimentally andcomputationally demonstrated that leaky symmetric edge-guided waves couldpropagate to hard-to-inspect location impinged with a hidden crack. Thescattered waves are then redirected back to the location of excitation.    The findings in this thesis give a fundamental understandingof the scattering of Lamb waves by crack on boundaries and holes as well ascracks in hard-to-inspect locations. The novelty of this research, which arethe scattered wave amplitude, the wave pattern, and its directivity areimportant measurements to locate and quantitatively evaluate small cracks,based on the point source model. Under specific conditions, edge-guided wavescan propagate around a curved edge to the blindsight area and redirected back,as shown in the hole and rectangular slot studies. These findings will assistin the development of Lamb wave propagation for SHM as a damage diagnosis tool." @default.
• W3148994060 created "2021-04-13" @default.
• W3148994060 creator A5026327402 @default.
• W3148994060 date "2017-03-27" @default.
• W3148994060 modified "2023-09-23" @default.
• W3148994060 title "Lamb wave approach to identify hidden cracks in hard-to-inspect areas of metallic structures" @default.
• W3148994060 doi "https://doi.org/10.4225/03/58d8851b5dd70" @default.
• W3148994060 hasPublicationYear "2017" @default.
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