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With the growing size of aircraft fleets and the complexity of aircraft structures it has been proposed that there are many cost and operational benefits of installing a structural health monitoring system to monitor the aircraft’s structure throughout its in-service life. A method of achieving this is through monitoring the acoustic emission emitted during a damage event. One of the limiting factors to this however is having sufficient confidence in the placement of the sensors to ensure coverage while limiting the mass associated with the system. A series of five studies were conducted which use both experimental and numerical approaches to investigate Lamb wave propagation and its interaction with damage in both metallic and composite materials. These studies have used some of this data and through the use of genetic algorithms sought to optimise the placement of sensors with the objective of achieving a high probability of damage detection. The use of 3D scanning laser vibrometry has been harnessed along with the use of numerical reasoning using the local interaction simulation approach. This has enabled studies to be conducted which consider both the in-plane and out-of-plane components of the Lamb waves which is an important consideration when selected the appropriate sensing methods. In addition, a novel method of training sensor networks for AE location using the delta-t technique is also presented. The results of these studies has led to the development of two separate methodologies; one for the placement of sensors in an acousto-ultrasonic system for the detection of adhesive disbonds and one for the placement of AE sensors to maximise the coverage of the sensor network on a structure with complex geometry. These methodologies have many advantages, particular in their prompt convergence which makes progress towards enabling a concurrent sensor network-structure development.

The work is devoted to modelling of dry sliding friction between contacting surfaces and related problems of contact mechanics. To limit the number of physical phenomena involved, the studies are targeted to systems used in vacuum conditions, hence there is no need to consider environmental parameters, such as humidity or oxidation films. Although friction has been studied over many years, challenges remain for obtaining a comprehensive understanding and a quantitative description of influence of nanometre scale effects on friction at micro/macro levels. Many existing models are critically re-examined using ideas of nanoscience. The studies start from the classic Zhuravlev model well-known as the Greenwood - Williamson model representing rough surfaces as collections of spherical elastic asperities. Contact problems for bodies of various shapes are investigated using the Galin solution along with the Borodich rescaling formulae. The rescaling method is applied to indentation experiments by spherical and nominally sharp punches and then a way to model dry friction following Zhuravlev’s arguments is outlined. New ways for numerical simulations of dry sliding friction are presented assuming the friction force is defined by the total energy dissipated over the sliding distance. Novel hierarchical, multiscale, multilevel structural models for simulation of sliding dry friction are presented. The models reflect the physical mechanisms which are most relevant to dry friction at the specific length scales: the chemical interactions at the atomic scale, the adhesive (van der Waals) interactions at the nanoscale, and the mechanical interlocking of asperities and their coupling at the micro and macroscales. Although the models include some features of known models, the nanotribological interpretation of the features is novel. It is argued that the nano-asperities do not deform plastically due to the so-called Polonsky-Keer effect. The obtained results are in good agreement with the experimental observations found in literature.