Evaluation of surface fatigue cracks using Rayleigh waves

An acoustic scattering model for a planar distribution of cracks in an infinite space is derived based on the generalized reciprocity and Kino\u27s scattering formalism. Subsequently, an acoustic scattering model of Rayleigh waves for a linear distribution of surface cracks in a half space in the form of the local stress intensity factor of the surface cracks is developed by using the elastostatic energy balance of crack formation. The weight function estimation method is introduced into the scattering model to approximately evaluate the local stress intensity factor of the crack in the stress fields of Rayleigh waves to improve the evaluation results and to extend the maximum crack depth region which can be evaluated. The initiation and growth behavior of the surface cracks which are related with the reflection coefficient are studied by evaluating the reflection coefficient varying with respect to; crack aspect ratios, frequencies and the number of cracks for several aerospace materials such as; Al 7075 T6, Al-Li 2090 T6, Ti-24Al-11Nb and Ti 6Al-4V. The results show that by using the newly developed model not only the evaluation of the reflection coefficient of Rayleigh waves for a linear distribution of surface cracks become possible but also the evaluation results are improved. Contacting surface acoustic wave wedge transducers are optimally designed and fabricated using principles of physical acoustics, and the impedance matching networks are introduced to increase the efficiency of the transducer. The new transducers used in combination with the variable impedance matching networks increase the efficiency of SAW devices produced by a factor of five. An automated detection system including the control software is developed. Isolated surface fatigue microcracks in the size range from 120 to 135 $\mu$m are detected on hourglass shaped laboratory test specimens of Al-Li 2090 using the detection system. The use of a simple algorithm for detecting the presence of specular reflections from the initiating microcracks superimposed on the nonspecular reflections from the microstructure proved to be effective