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

Ultrasonic consolidation (UC) debulking of thermosetting prepreg for autoclave curing of composite laminates

Debulking of prepreg (pre-impregnated resin system) layers during hand lay-up manufacturing of carbon fibre reinforced polymers (CFRP) is a key-step to reduce air content and maximise the mechanical properties of the final product. Debulking is usually performed using vacuum-bag cycles of 10–15 min applied after the lay-up of every three or five prepreg layers, leading to a considerable time-consuming process. In this work, the use of ultrasonic stimulation during vacuum is studied to improve the efficiency of the debulking process and reduce the number of operations in order to decrease the overall manufacturing time. Three CFRP laminates were laid-up using the proposed ultrasonic consolidation (UC) with three different exposition times (5, 10 and 15 min) and cured in autoclave. The UC debulking process consists in a vacuum cycle with ultrasonic waves sent to the uncured material through an ultrasonic transducer. In order to evaluate the efficiency of this process interlaminar shear strength (ILSS) and in-plane compressive properties were tested. Experimental results show for 15 min compressive properties comparable with the ones obtained from reference samples manufactured using the traditional debulking technique, and high improvements in terms of ILSS (>20%). Therefore, UC debulking process can be used during hand lay-up of prepreg in order to improve the interlaminar properties of the final part and reduce the debulking time by over 85%

Multifunctional, Smart, Non-Newtonian Polymer Matrix with Improved Anti-impact Properties Enabling Structural Health Monitoring in Composite Laminates

Autonomous Structural Health Monitoring (SHM) has been introduced in composite structures extensively over the last decade in an attempt to proactively monitor potential internal defects, however active/passive control of their integrity status still remains a challenge. In this work, a novel, non-Newtonian multifunctional polymer with unique active/passive capabilities is proposed for impact protection and SHM of composite laminate structures. This Polyboro siloxane(PBS)-based polymer with unique shear-dependant energy absorption characteristics, owed to a phase transition occurrence within its polymeric network, was utilised as scaffold for ferromagnetic iron particles which enabled the manufacturing of the multifunctional matrix for Glass Fibres Reinforced Polymer (GFRP). The iron particles were positioned in the polymer matrix, which was reinforced with glass fibres and employed as outer ply of a laminate structure. Their presence enables a dual functionality of the multifunctional layer: firstly, in the presence of a magnetic field, triggers the phase transition of the polymeric network offering protection to the laminate in case of impacts, and secondly, postimpact allows for the assessment of the internal integrity of the component, acting as an embedded heat source for active Infrared (IR) Thermography. The ability of the iron particles to initiate the phase transition was investigated by means of Low Velocity Impact in the presence/absence of a magnetic field and the laminates were then examined by means of induction thermography, for the evaluation of the internal damage. Results revealed that iron particles in the presence of a magnetic field led to an enhanced protection of the composite laminates, significantly reducing the extent of the internal damage. This novel, low-cost multifunctional layer provides a unique solution for the protection of composite materials, addressing their inherent weak resistance in out-of-plane direction and providing affordable SHM, thus opening new perspectives for smart structural materials which are in great demand in engineering sectors.</p

Impact-responsive layer based on encapsulated solid/liquid non-Newtonian polymers

In this work, spherification was investigated as an encapsulation technique for an impact-responsive gel, with the ultimate objective of the final design being employed as protective equipment in the form of smart layers for protecting delicate goods in transit. The smart protective layers investigated utilised the controlled distribution of a polyborosiloxane based non-Newtonian polymer, namely shear stiffening gel (SSG), which can respond to an external stimulus i.e., a rapid mechanical load, by absorbing a large amount of energy, thus resulting in the protection of the aforementioned goods. At first instance, the constituents of the smart protective layers underwent mechanical characterisation, where the underlying mechanism of the SSG and its ability to absorb energy via means of a phase transition occurrence was established and quantified to be approximately five times higher compared to silicone. At a second stage, a thorough investigation of the optimal encapsulation method and geometrical arrangement was completed. The performance of the final design was assessed via static and dynamic tests which demonstrated that the layers containing SSG displayed superior performance compared to conventional ones, being able to autonomously offer protection to the substrates. In particular, the novel smart layers increased first and final compressive failure stresses by approximately 50%, whereas at the same time the maximum forces prior to failure in low velocity impact (LVI) tests were approximately 50% higher, across the investigated impact energy levels. The results of this work establish these novel smart protective layers as an ideal solution in a wide variety of applications where extremely fragile and valuable goods are in transit and impact forces need to be minimised or eliminated, such as camera lenses, electrical components, blood vials, and other medical products, overcoming the drawbacks of traditional packaging materials.</p

Impact-responsive layer based on encapsulated solid/liquid non-Newtonian polymers

In this work, spherification was investigated as an encapsulation technique for an impact-responsive gel, with the ultimate objective of the final design being employed as protective equipment in the form of smart layers for protecting delicate goods in transit. The smart protective layers investigated utilised the controlled distribution of a polyborosiloxane based non-Newtonian polymer, namely shear stiffening gel (SSG), which can respond to an external stimulus i.e., a rapid mechanical load, by absorbing a large amount of energy, thus resulting in the protection of the aforementioned goods. At first instance, the constituents of the smart protective layers underwent mechanical characterisation, where the underlying mechanism of the SSG and its ability to absorb energy via means of a phase transition occurrence was established and quantified to be approximately five times higher compared to silicone. At a second stage, a thorough investigation of the optimal encapsulation method and geometrical arrangement was completed. The performance of the final design was assessed via static and dynamic tests which demonstrated that the layers containing SSG displayed superior performance compared to conventional ones, being able to autonomously offer protection to the substrates. In particular, the novel smart layers increased first and final compressive failure stresses by approximately 50%, whereas at the same time the maximum forces prior to failure in low velocity impact (LVI) tests were approximately 50% higher, across the investigated impact energy levels. The results of this work establish these novel smart protective layers as an ideal solution in a wide variety of applications where extremely fragile and valuable goods are in transit and impact forces need to be minimised or eliminated, such as camera lenses, electrical components, blood vials, and other medical products, overcoming the drawbacks of traditional packaging materials.</p

Mechanical response of shape memory alloy-based hybrid composite subjected to low-velocity impacts

One of the most common problems with composite materials is their low resistance to impacts with foreign objects because of their tendency to dissipate impact energy through internal delamination, weakening a large area of the structure. One of the possible solutions to increase impact resistance is to use of shape memory alloy wires in order to exploit their unique superelastic behaviour and the hysteresis that characterises their stress–strain curves. In this study, composite laminates were hybridised by embedding a network of shape memory alloy within the laminate structure and were subjected to low-velocity impact in order to analyse their response in comparison with a traditional composite. Ultrasonic C-scan analysis was undertaken on the samples after the impact in order to estimate the extension of the internal delamination. Results show that the shape memory alloy wires embedded in the laminate are able to absorb a large amount of energy, reducing the extension of the internal delamination. </jats:p

Investigation of a dynamic active/passive noise cancellation of polyborosiloxane thin membrane gel.

This study proposes a multifunctional, thin membrane gel based on a formulation of PDMS and boron. The proposed gel offers a dynamic passive stimuli-responsive sound absorption at low frequencies, which can be transformed to active noise cancellation with the use of a secondary sound source. The passive behaviour of the proposed material is the result of a dynamic phase transition in the material’s polymeric network, activated by the interaction with the travelling sound pressure wave. The presence and extent of the phase transition in the material was investigated via Fourier transform infrared spectroscopy and oscillatory rheological measurements, where it was found that the amount of boron in the gel has a crucial role on the occurrence of the phase transition and consequently on its acoustic performance. The passive scenario results revealed a high and dynamic absorption of approximately 80% at the absorption coefficient peaks, which dynamically shifted to lower frequencies while sound amplitudes were increased. The active noise cancellation was successfully demonstrated at the lower frequencies range, as the occurrence of the phase transition was actively controlled via the sound pressure wave introduced. The aforementioned phase transition was intensified, with energy consumed in this process, resulting in a dynamic noise cancellation. These results demonstrated that the proposed gel membrane material can be used to develop active/passive deep subwavelength absorbers with unique properties, which can dynamically tune their performance in response to external stimuli, and that can be further controlled/activated with the use of mechanical transducers

Investigation of a dynamic active/passive noise cancellation of polyborosiloxane thin membrane gel.

This study proposes a multifunctional, thin membrane gel based on a formulation of PDMS and boron. The proposed gel offers a dynamic passive stimuli-responsive sound absorption at low frequencies, which can be transformed to active noise cancellation with the use of a secondary sound source. The passive behaviour of the proposed material is the result of a dynamic phase transition in the material’s polymeric network, activated by the interaction with the travelling sound pressure wave. The presence and extent of the phase transition in the material was investigated via Fourier transform infrared spectroscopy and oscillatory rheological measurements, where it was found that the amount of boron in the gel has a crucial role on the occurrence of the phase transition and consequently on its acoustic performance. The passive scenario results revealed a high and dynamic absorption of approximately 80% at the absorption coefficient peaks, which dynamically shifted to lower frequencies while sound amplitudes were increased. The active noise cancellation was successfully demonstrated at the lower frequencies range, as the occurrence of the phase transition was actively controlled via the sound pressure wave introduced. The aforementioned phase transition was intensified, with energy consumed in this process, resulting in a dynamic noise cancellation. These results demonstrated that the proposed gel membrane material can be used to develop active/passive deep subwavelength absorbers with unique properties, which can dynamically tune their performance in response to external stimuli, and that can be further controlled/activated with the use of mechanical transducers

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