Smart Multifunctional Composite Materials for Improvement of Structural and Non-Structural Properties

The principal aim of this thesis is to analyse the effectiveness of multifunctional smart materials as intelligent structures to improve mechanical properties and activate additional non-structural features. In order to investigate these multiple aspects, a comprehensive literature review has been presented focusing on the stale of the art in multifunctional and smart materials. From this analysis, jive different systems based on different designing solutions and manufacturing techniques were developed and experimentally validated Multiscaled composites are a typical example of multifunctional materials and are based on the addition of engineered nanoscaled reinforcements to traditional mesoscopic systems. To test the effectiveness of nanomodijication, an experimental campaign has been carried out, aimed to the characterisation of a nanocomposite obtained embedding Graphene Nanoplatelets (GNPs) in the polymeric structure of Low Density Polyethylene films at difference concentrations. Nanoscaled fillers were subsequently used to manufacture a threephasic multi-scaled composite based on the inclusion of nanometric Si02 particles in a traditional carbon fabric/epoxy system. Following a different approach, hybrid structures with embedded Non-Newtonian fluids have been manufactured and tested and the results showed that nonlinear viscosity can be exploited to dynamically enhance material properties during an impact event. The possibility to intervene both on structural and non-structural properties has been investigated with another hybrid system, based on the embodiment of Shape memory Alloys (SMA) within a traditional unidirectional CFRP. The study of the impact properties pointed out that the superelasticity effect and the hysteretic stress/strain behaviour of the embedded wires reduce the extent of the internal delamination for samples subjected to low velocity impacts. Moreover, by exploiting the SMAs thermoelectrical properties it is possible to use the embedded metallic network as a strain sensor by measuring the electrical resistance variation and as an embedded heat source to be used for rapid thermographic damage location and evaluationEThOS - Electronic Theses Online ServiceGBUnited Kingdo

An Analytical Model for Defect Depth Estimation Using Pulsed Thermography

The use of pulsed thermography as a non-destructive evaluation tool for damage monitoring of composite materials has dramatically increased in the past decade. Typically, optical flashes are used as external heating sources, which may cause poor defect definition especially for thicker materials or multiple delaminations. SMArt thermography is a new alternative to standard pulsed thermography as it overcomes the limitations on the use of external thermal sources. Such a novel technology enables a built-in, fast and in-depth assessment of both surface and internal material defects by embedding shape memory alloy wires in traditional carbon fibre reinforced composite laminates. However, a theoretical model of thermal wave propagation for SMArt thermography, especially in the presence of internal structural defects, is needed to better interpret the observations/data measured during the experiments. The objective of this paper was to develop an analytical model for SMArt thermography to predict the depth of flaws/damage within composite materials based on experimental data. This model can also be used to predict the temperature contrast on the surface of the laminate, accounting for defect depth, size and opening, thermal properties of material and defect filler, thickness of the component, and intensity of the excitation energy. The results showed that the analytical model gives good predictions compared to experimental data. This paper is one of the first pioneering work showing the use thermography as a quantitative non-destructive tool where defect size and depth could be assessed with good accuracy

A novel bistable energy harvesting concept

Bistable energy harvesting has become a major field of research due to some unique features for converting mechanical energy into electrical power. When properly loaded, bistable structures snap-through from one stable configuration to another, causing large strains and consequently power generation. Moreover, bistable structures can harvest energy across a broad-frequency bandwidth due to their nonlinear characteristics. Despite the fact that snap-through may be triggered regardless of the form or frequency of exciting vibration, the external force must reach a specific snap-through activation threshold value to trigger the transition from one stable state to another. This aspect is a limiting factor for realistic vibration energy harvesting application with bistable devices. This paper presents a novel power harvesting concept for bistable composites based on a 'lever effect' aimed at minimising the activation force to cause the snap through by choosing properly the bistable structures' constraints. The concept was demonstrated with the help of numerical simulation and experimental testing. The results showed that the actuation force is one order of magnitude smaller (3%–6%) than the activation force of conventionally constrained bistable devices. In addition, it was shown that the output voltage was higher than the conventional configuration, leading to a significant increase in power generation. This novel concept could lead to a new generation of more efficient bistable energy harvesters for realistic vibration environments

In-situ damage detection in SMA reinforced CFRP

The purpose of this paper is to analyse the possibility to manufacture and verify the self-sensing capability of composite materials plates with an embedded network of NiTi shape memory alloys (SMA) used as transducers for structural integrity. Firstly, the thermo-electrical material properties of SMAs were investigated to assess their capability to sense strain within. The results showed that the electrical resistance variation provided by the shape memory alloys network enables a built in and fast assessment of the stress distribution over the entire structure. Then, by transmitting a low amperage current, results in an electric and thermal flow through the entire SMA network. Using an IR Camera it is possible to capture the emitted thermal waves from the sample and create an image of the thermal field within the material. Consequently, analysing the behaviour of the heating curves on different points of the sample, it is possible to identify potential variation in the apparent temperature of the composite, leading to the identification of damages within the composite structure

Recent advances in active infrared thermography for non-destructive testing of aerospace components

Active infrared thermography is a fast and accurate non-destructive evaluation technique that is of particular relevance to the aerospace industry for the inspection of aircraft and helicopters’ primary and secondary structures, aero-engine parts, spacecraft components and its subsystems. This review provides an exhaustive summary of most recent active thermographic methods used for aerospace applications according to their physical principle and thermal excitation sources. Besides traditional optically stimulated thermography, which uses external optical radiation such as flashes, heaters and laser systems, novel hybrid thermographic techniques are also investigated. These include ultrasonic stimulated thermography, which uses ultrasonic waves and the local damage resonance effect to enhance the reliability and sensitivity to micro-cracks, eddy current stimulated thermography, which uses cost-effective eddy current excitation to generate induction heating, and microwave thermography, which uses electromagnetic radiation at the microwave frequency bands to provide rapid detection of cracks and delamination. All these techniques are here analysed and numerous examples are provided for different damage scenarios and aerospace components in order to identify the strength and limitations of each thermographic technique. Moreover, alternative strategies to current external thermal excitation sources, here named as material-based thermography methods, are examined in this paper. These novel thermographic techniques rely on thermoresistive internal heating and offer a fast, low power, accurate and reliable assessment of damage in aerospace composites