Interaction of Rayleigh Waves Induced by Interdigital Transducer with Fatigue Crack
Compared to bulk waves, Rayleigh waves can propagate much longer distances with lower signal attenuation levels, which are nominally pro portional to r / 1 for a given frequency (r=distance). In addition to the small attenuation, Ra yleigh waves are also non-dispersive and sensitive to surface-breaking defects. The surface penetra tion depth of Rayleigh waves is also approximately one wavelength, which can be beneficial for many NDT applications. However, one major drawback of using Rayleigh waves for nondestructive testing has been the difficulty of generating Rayleigh waves, where the most commonly used method involves the combination of a plastic wedge and a conventional longitudinal mode trans ducer. Unfortunately, a number of wedges with different angles are needed to generate Rayl eigh waves in different materials or at different operating frequencies to satisfy the critical Rayleigh launch angle requirement. In this study, instead of using wedge transducers, we have designed, fab ricated, and tested narrow-band Interdigital Transducers (IDTs), where precision elect rode patterns were etched on piezoelectric ceramic substrate materials using micro laser machining t echniques. These IDTs can be directly applied to the surface of test specimens to generate Rayle igh waves in MHz range. A laser interferometry system was used to characterize the I DT sensors, where a well-defined and directional Gaussian beam profile was observed. Inves tigations were also made for Rayleigh waves interacting with fatigue cracks in aircraft qualifi ed metallic alloys. It was observed that some of the Rayleigh wave energy can propagate through a tight crack at both normal and oblique incidence.
Wavefront Distortion of Acoustic Surface Waves from Apodized Interdigital Transducers
The acoustic surface wave launched by an interdigital transducer is experimentally shown to have a distorted wavefront when the transducer has a varying finger overlap (apodization) and is on a high‐coupling piezoelectric substrate, LiNbO3. Results are shown for different transducer geometries, when an acoustic wave passes under the comb and when the apodized comb itself is driven. One geometry having extra metal fingers significantly reduces the wavefront distortion. The origin of the distortion is a retardation of the wave passing under metal fingers.
Genotypic diversity in field isolates of Babesia bovis from cattle with babesiosis after vaccination.
OBJECTIVE
To determine whether particular genotypes of Babesia bovis were common to field isolates obtained from cattle properties in Queensland where the B bovis vaccine had apparently failed.


DESIGN
A comparative study of polymerase chain reaction genotypes in different populations of B bovis.


PROCEDURE
Two polymerase chain reaction assays were applied to analyse DNA extracts of B bovis vaccine (K, T and Dixie strains) and 27 field isolates from 24 properties where disease outbreaks had occurred despite the use of the vaccine. To evaluate the stability of the genotypes identified, 11 of the field isolates were inoculated into experimental cattle that had either been previously vaccinated with T strain or not vaccinated.


RESULTS
No particular genotype of B bovis was responsible for the problems observed in previously vaccinated herds. None of the isolates had genotypes identical to the vaccine strains used. No geographic trends among the genotypes were observed. Isolates that originated from the same property also had different genotypes. Blood passage of the 11 field isolates in either previously vaccinated or nonvaccinated cattle did not alter the original genotype.


CONCLUSION
No particular genotypes identified by the Bv80 and BvVA1 polymerase chain reaction assays could be associated with vaccine failures.