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Aerogels with low density and high porosity exhibit outstanding performances as thermal or acoustic insulators. However, most aerogels are mechanically brittle and need further chemical or physical surface treatments to obtain elasticity and superhydrophobicity. Structurally new types of polysilsesquioxane (PSQ) aerogels containing thiourethane as their bridging groups are now reported. These novel PSQ aerogels were prepared from thiourethane bridged silsesquioxane through a simple sol–gel process and vacuum-drying method. The PSQ aerogels exhibit inherent superhydrophobicity, excellent mechanical toughness and good thermal insulation properties. Moreover, the morphology and mechanical performance of these aerogels can be tailored by changing the rigidity of the bridging group.

Porous aerogels are an inspiring absorbent for oil–water separation and water purification. In particular, hydrophobic silica aerogels from abundant and cheap sources offer both economical and environmental benefits. Conventional silica aerogels from alkoxy siloxane precursors involve time-consuming and laboursome multistep processes. Herein, we demonstrate the formation of flexible superhydrophobic polysiloxane aerogels through a facile hydrosilylation from functionalized polydimethylsiloxane in supercritical carbon dioxide. This robust aerogel exhibits a high oil absorption capacity, superior recyclability and extraordinary mechanical properties even under harsh heating and cooling cycles. These characters favor this polysiloxane aerogel as being more competitive than common alkoxy silica ones for oily water treatment, oil spill clean-up and oil recovery.

In this thesis, several numerical approaches for the development of structural health monitoring (SHM) methodologies for engineering structures are described. In particular, the first boundary element models of three-dimensional piezoelectric smart structures are introduced. Comparing to the finite element method (FEM), the boundary element method (BEM) demonstrates higher numerical stability and requires less computational resources. Also, the dual boundary integral formulation provides a natural and efficient approach for replicating the targets of SHM techniques – material discontinuities. A boundary element formulation for the ultrasonic guided wave based damage detection strategy is firstly presented. The semi-analytical finite element model of piezoelectric patches is coupled with the boundary element model of substrates via the variables of the BEM. The first systematic approach for determining the number of Laplace terms to be used for an elastodynamic boundary element analysis is also introduced. The above-mentioned formulation is then transformed to the Fourier domain for simulating the electro-mechanical impedance (EMI) based damage detection strategy. The key to attaining accurate EMI signatures is the inclusion of appropriate damping effects. In addition to the detection of the damages in substrates, a partially debonded coupling condition between substrates and piezoelectric patches is derived for modelling the diagnosis of faulty transducers. The computational efficiency of the BEM is further enhanced by the implementation of high-order spectral elements. The difficulties associated with the applications of these elements in the BEM are among the key emphases. The accelerated BEM is used to reformat the models of the two damage detection strategies. The performances of the two strategies are more deeply investigated and understood. At the end of this thesis, a technique for the characterisation of cracks in plate structures is established. By utilising a two-stage approach, the long-existed difficulty of the simultaneous localisation and sizing of arbitrary cracks can be overcome. The technique is developed mathematically using analytic models and the FEM, and is extensively assessed by numerically simulated extreme scenarios. Throughout this thesis, physical experiments are heavily relied on for validation studies. A summary of the skills and the experiences, which the author has gained on experimental testing, is reported in this thesis for further reference. Open Access