Permanently Installed Monitoring Systems
There are many measurements that are not suitable for periodic NDE because the measurement is sensitive to defects, but is equally or more sensitive to other factors that vary in an uncontrolled way across the structure, so leading to missed defects and an unacceptable false call rate; permanent attachment at a fixed position means that, provided the other factors are constant with time, changes in signal give a reliable indication of damage growth. This potentially gives excellent sensitivity to defect initiation and growth, without excessive false call issues, and opens the door to the solution of currently intractable inspection problems.
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.
Reliable damage detection from monitoring data
If permanently installed systems are to be deployed widely, it is vital that the data is high quality and processed so that false calls are avoided, and it is presented to operators in a form that enables skilled operator attention to be focused on the areas where action is needed. This requires research on the causes of drift in readings and means of compensating for them. Automatic change detection algorithms will be required that optimise the probability of detection (POD)/false call trade-off without requiring intervention by skilled operators; this is a significant area of research and will have applications across a range of monitoring methodologies.
EPSRC and RCNDE industrial members via RCNDE3 core research programme