Ultrasonic monitoring of highly textured materials
The ultrasonic inspection of highly textured materials, such as austenitic stainless steels is made difficult by the presence of large grains or colonies of grains with different alignments of their principal axes. This leads to acoustic impedance contrast and hence scattering of the incoming ultrasonic wave from grain/colony boundaries. This means that the outgoing wave is strongly attenuated as it passes through the material, and the received signals are dominated by the scattering that appears as noise. The attenuation reduces the amplitude of reflections from defects and the scattering increases the noise level, so that the resulting signal to noise ratio is often too poor for the test to be viable. The attraction of permanent monitoring, at fixed position, is that the scattering is constant and so can in principle be subtracted out, allowing any damage growth to be monitored.
We are focusing on the particular application of crack detection in the coarse grained super alloys that will be used in the next generation of power stations that work at increased operating temperatures to improve their efficiency. These operate at temperatures in excess of 600°C so either transducers will have to be installed permanently that can give consistent signals under these conditions, or periodic inspections at shutdowns will have to be sufficiently reproducible to allow baseline subtraction. Both possibilities will be investigated and in either case, the key issue will be compensating for inevitable changes in transducer response.