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Investigation of Rayleigh-like Wave Propagation and Scattering Characteristics with a View towards Detection of Fatigue Crack Growth
The propagation of Rayleigh-like waves in plates had been predicted theoretically in literature for several decades. However, only very few published experimental studies can be found. This wave type can be interpreted as coupled Lamb waves. Rayleigh-like wave propagation is characterized by an energy transfer between both plate surfaces with a characteristic distance called the beatlength. Working with a post-doctoral researcher funded by the SNSF (Swiss National Science Foundation), the fundamental understanding of this type of mechanical wave propagation has been expanded. During this project a hybrid analytical/numerical model has been developed to describe the wave propagation and the reflection at small surface defects. Experimental results obtained using laser measurements clearly show the beating effect and its dependency on the frequency-thickness product, and excellent agreement has been obtained with analytical predictions. The propagation and scattering were modelled separately for the fundamental A0 and S0 Lamb modes that constitute the incident Rayleigh-like wave. The theoretical model reveals strong dependencies of the reflected field on the ratio between excitation distance and beatlength and on the cut-off frequencies of specific higher Lamb modes. The developed model allows for the evaluation of defect location and damaged plate side using a combination of time-of-flight and frequency measurements. The measurement principle has been extended for the defect detection in realistic aerospace structures. The beating effect of the Rayleigh-like waves offers a simple approach to propagate ultrasonic pulses in manufactured structures with regularly spaced stiffeners, difficult to inspect otherwise. The propagation and scattering characteristics for aerospace structures were modelled using 2D and 3D finite difference code developed by Dr. Masserey. The methodology is currently being extended to investigate the propagation characteristics in multi-layered structures, contributing significantly to the understanding of this field.
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