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Composites enable integration of the traditional disciplines of materials science, mechanical engineering, computer science, electrical engineering, numerical modelling and optimization to create high performance intelligent structures. Select from the options above for an overview of work undertaken in ACCIS under this theme.
Polymorphic composite structures are defined as those structures that differ in shape and geometry from the tooling from which they are made, and which may exhibit more than one stable shape in their final form. As such, they differ from conventional composite laminates where components are made to a shape that precisely matches that of their tooling. Polymorphic structures are multistable in the sense that global shape changes are possible and as such, have enormous potential in the field of adaptive structures. Potential applications include morphing aircraft with improved flight characteristics and biomedical devices such as exoskeletons.
Developments in sensors and damage detection techniques mean that a lot of information can now be obtained on the state of the structure. For example full field strain measurements are being used to predict the extent of damage by the effect it has on the surface displacements or strain distributions. This is a sound basis for quantifying the importance of the damage, since it directly measures its effect on the structure. This approach is being applied to damage prognosis by combining techniques for detecting damage with models to predict its effect on the integrity of the structure and its remaining life.
One aspect of this work is the combination of polymorphic composite structures with low profile actuation solutions such as piezoelectric patches. This generates a bi-stable plate that can be snapped between two stable states in a controllable and predictable fashion, thus creating a morphing structure. Considerable effort has been put into the accurate finite element modelling of such structures. Experimental measurements and tests have been performed on both the bi-stable plates and on the actuated morphing plate produced. Another aspect is the use of compliant structures (structures with low stiffness) as these can produce large displacements resulting in shape change. However, these need to be tailored for load carrying applications and combined with novel actuation methods to achieve a useful actuating composite structure.
Classical honeycombs are manufactured using a metal or polymeric base. Although lightweight, these materials provide poor damping capabilities, with limited ranges in terms of cell geometry and size available in the market place. We are developing honeycomb structures made from shape memory alloy material to provide revolutionary cellular solids with enhanced damping capacity, active strain storage and deployment capabilities. Demonstrators are developed using hexagonal and auxetic configurations, with passive and active inserts to provide the actuation capabilities.
High-performance passive vibration damping treatments are developed using cellular configurations, shape memory alloys and nanocomposites. Adaptive control techniques are also being applied in conjunction with nonlinear modelling, sensing and actuation of structural elements to the problem of vibration suppression in composite plate structures.
Structural Health Monitoring (SHM) of large composite structures for localised damage detection can be very efficiently performed using a sparse network of guided acoustic wave sensors operating in either a passive (acoustic emission) or an active mode. Previous research on acoustic emission at Bristol investigated guided wave generation by composite damage mechanisms such as delamination growth and matrix cracking. A major research campaign on active guided wave SHM methods for a variety of both composite and metallic structures is ongoing. This ranges from work on the fundamental physics of guided wave propagation and scattering to long-term experiments acquiring guided wave data on real structures.
One strategy for Structural Health Monitoring (SHM) is to use a sparse network of sensors that have long-range damage detection capability, thus allowing complete structural coverage with a small number of sensors. An alternative that alleviates the challenging requirement of long-range sensitivity is to use a denser network of sensors but here the issues of sensor cost, weight and connectivity become increasingly important. In this project we are investigating the feasibility of very simple wireless ultrasonic sensors that can be embedded just below the surface of a composite material at manufacture. A sensor is activated remotely by an external 'wand' at which point the sensor performs a standard ultrasonic measurement. This could be a conventional ultrasonic measurement to detect delamination directly under the sensor or a short-range guided wave measurement to detect damage in its immediate vicinity.
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