Long-term structural health monitoring of plate-like structures using distributed guided wave sensors

Aircraft, containers, and storage tanks contain plate-like structures that are safety critical. The structures often undergo non-destructive inspections. The inspection frequency tends to be over-conservatively high, and it may be possible to reduce the intervals between inspections to realize cost savings. This goal can possibly be realized by automated structural health monitoring (SHM) of structures using sparse active guided wave sensor arrays. Guided waves are sensitive to small defects and can propagate long distances across feature dense plates. Thus, a guided wave SHM system that enables reliable detection of critical defects or monitoring of their growth can potentially be used to reduce the frequency of inspections for real structures. Industrial guided wave SHM systems must be reliable throughout prolonged exposure to temperature, humidity, and loading changes encountered in operation. Research at Imperial College shows that temperature compensation and subtraction between monitored guided wave signals and baselines acquired from healthy plates enables detection of 1.5% reflection change over areas ~1 m^2 in the presence of thermal swings and uniform liquid layers. These results and findings from scattering studies indicate it may be possible to detect reflections from hole type defects and notches affecting structures during their operation. An issue is that demonstrations of SHM system capabilities have only been shown in controlled laboratory tests within short periods following baseline acquisition. There is concern whether sustained exposure to service conditions will subject transducer elements to irreversible changes and introduce variability in baseline subtraction results that would mask signals due to slowly growing damage. This thesis studies the reliability of guided wave SHM for monitoring plate-like structures over longer time periods. The theoretical characteristics of the fundamental Lamb waves and their use to monitor and detect damage are reviewed. Strategies for sensing and signal processing are described alongside experimental validation of their performance. The effectiveness of the SHM system is tested in experiments where damage-free plates are exposed to British weather as well as thermal variations in an environmental chamber. The monitoring capabilities of bonded piezoelectric sensors are quantified and compared to the performance achieved using electromagnetic acoustic transducers. Experimental results and findings from simulations of bonded piezoelectric transduction establish that performances achieved with bonded sensors degrade due to variations in the properties of adhesives used to attach sensors to plates. EMATs are relatively stable and capable of enabling detection of 1.5% reflection change at points away from the edges of plates after sustained exposure to thermal cycling loads.