Structural Health Monitoring (SHM) systems are widely regarded as capable of significantly reducing inspection costs of safety-critical structures in industries such as aerospace, nuclear, and oil and gas, among others. Successful SHM systems can be considered those which combine good sensitivity to defects, preferably with the capability of localization and identification, with a low sensor density. Techniques based on sparse arrays of sensors which generate and receive guided waves are among the most promising candidates. Guided waves propagate over large distances and certain modes have the ability to transmit through a variety of structural features leading to a relatively small number of distributed sensors being able to cover the structure. In complex structures, which contain high densities of structural elements, the timetraces obtained are often too complex to be directly interpreted due to the large number of overlapping reflections. In this case, the Baseline Subtraction technique becomes attractive. In this method a current signal from the structure is subtracted from a signal which has been acquired during the initial stages of operation of the structure. This eliminates the need for interpretation of the complex raw time signal and any defects will be clearly seen provided the amplitude of the residual signal obtained after subtraction of the baseline signal is sufficiently low when the structure is undamaged. However, it is well known that environmental effects such as stress, ambient temperature variations and liquid loading affect the velocity of guided waves; this modifies the time-traces and leads to high levels of residual signal if a single baseline, taken under different conditions, is used. Of these effects, temperature variations are the most commonly encountered and are critical since they affect not only the wave propagation but also the response of transducers. The present work aims to demonstrate the potential of guided wave health monitoring of large area complex structures. It starts with a general literature review on inspection and monitoring of large area structures, in which the advantages and disadvantages of this technique compared to other well-established SHM techniques are presented. The design and behaviour of two different temperature-stable transducers generating high A0 or S0 mode purity in the sub-200kHz frequency region are described. The effciency of different signal processing techniques aimed at reducing or eliminating the influence of temperature on wave propagation is evaluated and a temperature compensation signal processing strategy is proposed. Finally, a large metallic structure is used to demonstrate a sparse-array SHM system based on this signal processing strategy, and imaging algorithms are used to combine the information from a large number of sensor combinations, ultimately leading to the localization of defects artificially introduced in the structure.

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The concept of 'inheritance' in stone decay studies is well established. Histori~ stone may inherit the effects of background environmental factors (including salt weathering, temperature and moisture cycling, chemical alteration) and more extreme, 'exceptional', events (including lime rerrdering I removal, fire, Little Ice Age frost events, inappropriate intervention), which determine its decay pathway. r'\ Building on this conceptual framework, this thesis draws on three main themes to assess the impact of complex stress histories on the decay of historic sandstone: 1) Fieldwork (Northern Ireland and western Scotland) is used to map the decay on of selected fayades, using connectivity analysis and the Unit Area Spread (UAS) Staging System to give a meaningful rating of fayade condition; 2) Historical research is essential for building a clear picture of the stresses historic sandstone structures have experienced since their construction; 3) Laboratory simulation, informed by fieldwork and historical research, explores the effects of lime rendering, fire, frost events, and salt weathering cycles on representative sandstones (Peakmoor and Dunhouse) blocks in isolation and in combination. Special emphasis is given to the study of 'process combinations', where each sample is given a carefully designed stress history before moving on to the next stage in simulation. Thus, different groups of stone blocks with different stress histories can be created. Each stress history group experiences salt weathering cycles and the response of each group to salt weathering is monitored by weight loss. . Results highlight the non-linear nature of the stone decay system, with different stress· histories producing divergent response of sandstone over time. Over the perIod of the experimental run (75 cycles, 150 days), the effects of lime rendering tended to suppress debris release (although this does not preclude rapid future decay), while other stress histories (fire and frost) increased breakdown in response to salt weathefing. The impact of fire on subsequent performance is especially complex, with soot cover potentially hindering soluble salt ingress, and fracturing from the stresses created in the fire leading to spalling events during salt weathering cycles. In future, conservation practitioners should approach the management of historic sandstone with a knowledge of the pathology of structures (potential inherited and concealed stresses) in mind.

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