Abstract This article discusses an approach to develop innovative carbon fiber composites that have the capability to change shape according to a prescribed input. The approach is based on the study of specific stacking sequences of unidirectional fiber plies cured on a curved mold. The effects of the above-mentioned aspects are investigated on the manufactured specimen. The combined thermo-mechanical response is investigated by performing mechanical tests at various prescribed temperatures and the intensity of the shape change is evaluated together with the corresponding stiffness variation. The experimental campaign is mostly devoted to characterize the response of the manufactured sample and demonstrate the great capability of the proposed approach to develop a smart material with enhanced shape and stiffness variation according the prescribed input.

This article illustrates an approach to develop innovative smart materials based on carbon fiber composites. The proposed approach relies on the use of ultra-light strain sensors that are embedded into the composite and are adopted to monitor in real-time the actual material configuration. Such sensors are composed of electrospun PVDF fibers that exploit piezoelectricity to identify strain and thanks to their extreme lightweight can easily be embedded within the composite layers without affecting the structural integrity. On the other hand, the composite is equipped with a system of internal distributed heaters that can locally and globally vary the composite temperature. Since the adopted epoxy has a considerable temperature-dependent behaviour, it is possible to control its stiffness and thus to control the structural frequencies and damping. By coupling the sensing system with the control system, the structural properties are tuned to match prescribed working conditions, thus optimizing the performance of the proposed smart system. The proposed approach is investigated experimentally by manufacturing prototypes of the smart composite and by performing multiple tests to study the material response and evaluate the obtained performance.

Localized reinforcement of composites employed to manufacture parts for the transport industries is making possible the lightweighting of components that have a much sought-after effect in the reduction of CO 2 and NO x emissions. However, its realization, through the removing of mass where it is not required and reinforcement added to areas more prone to stress from working loads, relies on the development of novel manufacturing processes that can create structures whose performance is on a par with their solid counterparts, but at a fraction of the weight and at an affordable production cost. In this work we exploit the use of a very weak and safe magnetic field to control the location and orientation of functionalized discontinuous carbon fibers within a polymeric structural (polyurethane) foam to create performance-optimized composites. Two wet-chemistry methods (i.e. in-situ precipitation-deposition and amine-co-adjuvated electrodeposition of magnetite) to transform commercial carbon fiber into a magnetically active form were explored. The resulting fibers were analyzed and characterized through a set of physico-chemical tests. The functionalized fibers were then embedded at 3 different %vol contents in the polymeric matrix at given locations and with a desired alignment. Their mechanical performance (incl. compression, tension) was assessed and benchmarked against both a similar %volumetric content but non-functionalized-reinforcement (i.e. randomly distributed) composites and to non-reinforced matrices. In the two sets of reinforced composites (random and aligned) there is a positive correlation between stiffness, yield strength and strain with increasing %vol content. Both sets outperformed the non-reinforced matrix, demonstrating good fiber adhesion within the matrix and successful load transfer from matrix to fiber. The magnetically aligned composites generally outperformed the non-functionalized ones in terms of stiffness and strength at yield.

The demand for clean and sustainable energy sources continuously increases. One of the promising ways to provide electrical power is using fuel cells. Polymer electrolyte membrane fuel cell (PEMFC) represents the most common type of fuel cells. However, PEMFCs have not yet been fully commercialized because of the high cost and low performance. A main part of PEMFC, which significantly contributes to the cost and weight is the bipolar plate (BPP). The US Department of Energy (DOE) has recommended some physical properties for BPP for sustainable commercialization of PEMFC. Those set properties have yet to be met. Conductive polymer composites (CPCs) use conductive fillers such as carbon nanotube (CNT), carbon fiber (CF), and graphite (Gr) to impart electrical and thermal conductivities and can potentially provide an optimum combination of weight, cost, mechanical properties and conductivity characteristics for BPPs. In the current work, CPCs of polycarbonate (PC) filled with singular filler of CNT, binary fillers of CNT and CF and ternary fillers of CNT, CF and Gr were fabricated using melt mixing method followed by compression molding. The through-plane and in-plane electrical conductivities of the CPCs were investigated. The results showed that the electrical percolation thresholds for the PC-CNT is ∼1 wt. % CNT in both the through-plane and in-plane directions. Addition of 3 wt. % CNT to PC composites with 10 - 30 wt. % CF improved the conductivity performance. It was noticed increasing CF content from 20 to 30 wt. % did not yield a big change in conductivity, so that at 20 wt. % CF, the through-plane and in-plane electrical conductivities are 0.11 S.cm −1 and 6.4 S.cm −1 respectively. Moreover, using 20 wt. % CF will allow for higher loading of graphite. To further enhance the conductivities towards the DOE recommendations, 30 wt. % Gr was introduced to the PC composite with binary filler (i.e., 3 wt. % CNT and 20 wt. % CF). The results showed that the through-plane and in-plane electrical conductivities were increased to 1.5 S.cm −1 and 13.5 S.cm −1 , respectively. These properties recommend a potential application of polycarbonate based CPCs for BPP manufacturing.

This study is at attempt to explore the nonlinear behavior of bistable composite laminates for vibration energy harvesting. Asymmetric four-ply [0/90/0/90] carbon-fiber plate with two cylindrical stable equilibria supported at its center and free at all boundaries is used for the experimental testing. Macro-fiber composite (MFC) patches are attached to the plate to transform the mechanical vibration energy into electrical energy. The mechanical bistable property of the plate makes it possible to snap from one stable equilibrium state to the other. This snapthrough motion is highly nonlinear and associated with large-amplitude vibrations. The experimental tests aim at exploiting the nonlinearity due to the snapthrough motion to enhance the energy extraction. First, the resonant frequencies and damping of the plate are identified. A primary-resonance excitations of the first mode are carried out using two schemes: amplitude sweep and frequency sweep. In the first case, amplitude sweep, the excitation frequency is kept fixed at the resonant frequency and the amplitude of excitation is increased. The time history and FFT of the response as well as the output voltage are measured and reported. In the second case, frequency sweep, the excitation frequency is varied around the resonant frequency while the excitation amplitude is kept fixed. In both cases, the response shows a small-amplitude single-well vibrations at low excitation amplitudes and chaotic and periodic snapthrough motion as the amplitude and frequency of excitation are varied. The snapthrough motion has been found to greatly enhance the energy extraction capability. This study can serve as a motive for more testing and modeling efforts in order to understand the complex nonlinear behavior of bistable composite laminates and exploit it for vibration energy harvesting.

The technique of model-based identification is proposed to extract a model for damage in composite materials from experimental data. The proposed method is demonstrated on a unidirectional carbon fiber reinforced polymer (CFRP) beam. Impact damage is seeded in the CFRP beam using a spherical punch, causing localized damage. The specimen is evaluated through modal testing before and after the damage is seeded, with the healthy case modeled using the FEM. Finally, a virtual controller is found which eliminates error in response between the healthy model and damaged experimental system. The virtual controller, being in feedback with the healthy model at the FE node where the damage occurs, reflects the effect of the localized damage. It is found that the seeded impact damage reduces stiffness and is a source of damping inside the composite beam. Interpretation of the local damage is made through the curve fitting of the identified dynamics. To confirm the efficacy of the fit, a closed-loop is made with the healthy model which is then compared to the data from the damaged system.

Structural health monitoring of composite materials is limited by the lack of fundamental understanding of early stage damage at the local material level. This includes damage precursor formation on fiber surfaces, within the matrix, and at the fiber-matrix interface/interphase. In this effort, we present a micro-/nano-scale technique for characterizing damage precursor formation on individual carbon fibers exposed to cyclic tensile loads. Nanoindentation and atomic force microscopy (AFM) were used to study the local properties of the individual microfibers before and after global loading events. An AFM image analysis was used to track evolution of topography on the fiber surfaces. The work is a first step toward understanding damage precursor formation in individual microfibers; the work is expected to enable multiscale composites modeling efforts as well as enable the development of future self-sensing materials.

An experimental investigation was conducted to measure the deformation of flapping wings with distributed piezoelectric modal sensors. The sensors are bonded to the surface of a wing and shaped to extract a specific modal co-ordinate. The experimental setup consisted of a carbon fiber beam with shaped sensors designed to extract the 1 st and 2 nd modal co-ordinates. The beam was mounted to two different excitation devices (shaker or servomotor) that allowed the beam to be excited at a desired frequency and motion. The resulting electrical signals from the strained modal sensors are calibrated to modal co-ordinate measurements. In order to validate modal sensor measurements, the Digital Image Correlation (DIC) technique is used to obtain full field deformation measurements of the beam. The measured deformation from DIC is then used in conjunction with operational modal analysis to extract modal co-ordinates. Comparison of the modal co-ordinates obtained via modal sensors against DIC measurements show good agreement. Furthermore, a sensitivity analysis was performed to gauge the robustness of modal sensors against construction errors. The results obtained show that modal sensors are a simple and accurate method of obtaining the deformation of a flapping wing.

Thermally responsive self-healing polyurethanes (1DA1T, 1.5DA1T, and 2DA1H) with shape memory property were developed and the fully reversible Diels-Alder (DA) and retro Diels-Alder (rDA) reactions were employed for the healing mechanism. The transition temperatures of the DA and rDA reactions were confirmed through a differential scanning calorimetry and the molecular level of analysis on the reversibility and the repeatability between the DA and rDA reactions were completed though a variable temperature proton nuclear magnetic resonance at the reaction temperatures. Also, compact tension specimens were made to observe the healing efficiencies. These specimens were healed without the use of external forces to close the crack surfaces after testing for the repeatable healing ability with three cycles. As a result, the average first healing cycle efficiencies of 80%, 84%, and 96% for 1DA1T, 1.5DA1T and 2DA1H, respectively, were achieved and small drops for the second and third healing cycles were observed. Then, using two of the self-healing polyurethanes as resins, continuous carbon fiber fabric reinforced polymer matrix composites (C1.5DA1T and C2DA1H) were fabricated and short beam shear testing was conducted to determine the healing capability on the delamination. Accordingly, the first healing efficiencies of 88% and 85% were measured without any additional treatments on the fibers; however, an external pressure was applied during the composite healing process.

Morphing wings are of great interest in the aerospace community. Control surfaces with continuously variable geometry can increase the efficiency of aircraft aerodynamics. This research focuses on demonstrating a morphing flap on a small span-wise section of the trailing edge in a wing for a small unmanned air vehicle (UAV). The flaps used flexible matrix composite (FMC) actuators embedded in a flexible structure, rather than hinged control surfaces with conventional actuators. This created a local aerodynamic control force whose effect can be measured using the UAV’s on-board flight control system. After multiple design iterations, in which the FMC actuator material and structure were varied, the final design incorporates a carbon fiber frame with an actuator system embedded in a foam matrix. The FMC control surfaces were successfully demonstrated in flight tests on the eSPAARO unmanned aerial vehicle.

Structural health monitoring can enhance reliability, increase safety, and decrease maintenance costs by detecting damage at an early stage. By taking advantage of the electromechanical coupling, piezoelectric materials have the potential to harvest energy from ambient vibration sources to provide low-power electricity for self-powered electronic devices. In comparison with other piezoelectric transducers, zinc oxide (ZnO) nanowires carry the added advantages of structural flexibility, lower cost, compactness, and lighter weight. In this study, the energy harvesting capabilities of nanoscale ZnO piezoelectric nanowires (NW) grown on the surface of glass fiber fabrics are investigated experimentally. A series of cantilevered carbon fiber beams containing a controlled amount of ZnO nanowires is evaluated. The absolute electrical energy dissipation is quantified by measuring the output power over a broad spectrum of known vibratory loads and frequencies. The maximum amount of power extracted is obtained by employing resistive impedance matching. Here, a maximum peak of ∼6.7 mV was generated when the beam containing ZnO nanowires was excited at 2.90g and connected to a 10 MΩ load. At that excitation level, a maximum of 20.0 pW was generated when an optimal resistor of 1 MΩ is connected. A tip mass of ∼0.6 gram added to the sample with ZnO NWs increased the peak-voltage by 2.21 mV and increased the peak-power by 13.3 pW. A series of DC voltage applied to the ZnO sample suggests the equivalence of poling treatment, where the dipole alignment of the ZnO NWs are disrupted. Here, a maximum peak-power of 45 pW is reported, showing promising potential of scaling-up to harvest ambient energy for low-powered electronics.

The chord-wise bending airfoil wing could be achieved by employing a kind of active morphing skin which was embedded with pneumatic muscle fibers. This camber morphing structure involves the discipline of structural mechanics which is analysed in this paper. Carbon fiber composite plate is utilized for the upper surface skin of the chord-wise bending airfoil structure, so the approach described in this paper starts from deformation analysis of the upper surface skin based on the classical laminate theory. Energy method was also used for solving the shape function of the upper surface skin which was under the condition of pure bending. While the active morphing skin was actuated at a series of discrete actuation pressures, the fixed geometrical shapes of the chord-wise bending airfoil structure could be obtained. Meanwhile the finite element method (FEM) was used for analyzing this chord-wise bending airfoil structure and the deformed shapes of the upper surface skins would be obtained. Deformed shapes of the upper surface skins between numerical analysis result and FEM analysis result were compared in this paper. This structural analysis work provides useful design and camber morphing characteristics for chord-wise bending airfoil structure. Developers of future shape-adaptive air vehicles have been provided with structural design tools.

A new approach to self-healing systems is presented that aims to overcome the inherent drawbacks of conventional liquid resin based healing systems within composites. Finite embedded systems offer limited healing potential for small volume delaminations and as such cannot effectively heal large damage volumes often associated with shear damaged sandwich panel structures or debonding between skin and core. An expanding polymer based approach aims to overcome such limitations. The mechanical and physical properties of a prepared polyepoxide foam are investigated and how the inclusion of a carbon fibre reinforcement within the foam affects processability and performance. The healing efficiency of different polymer foams to heal damaged structures is also investigated. A secondary investigation is also presented that aimed to overcome the drawbacks associated with the requirement for stoichiometric mixing of two part healing agents, or for healing agent to come into direct contact with a catalyst embedded within the matrix material. Different approaches were taken to develop a self-healing system that once deployed required no additional mixing or stimuli for healing to occur.

Traditional composite materials invented to be used in structures with the purpose of high load-bearing with excellent in-plane properties. Continuous fiber reinforced composites are one of the mostly used categories of advanced composites. This class of composites has gained a lot of attention due to their light-weight and decent mechanical properties. However, additional material design is required to tune both mechanical and structural properties of these composites. Since the load transfer between reinforcement phase and polymer matrix happens at the interfacial region, a better interphase might result in a composite with higher vibration damping. In this study, a gradient interphase between carbon fiber and polymer matrix has been created by using ZnO nanowires to engineer the damping loss factor of the carbon fiber composites. For the growth of ZnO nanowires on the carbon fabric, low temperature hydrothermal reaction has been used. Then the carbon fabrics with ZnO nanowires were infiltrated with a low viscosity epoxy using vacuum assisted resin transfer molding technique. The stiffness and structural damping of the composite were examined using dynamic mechanical analysis. The results show that the damping properties of hybrid composites using ZnO nanowires are enhanced compare to the bare carbon fabric composites. Since the growth of ZnO nanowires is a tunable process, the length, diameter and aspect ratio of the nanowires and consequently the architecture of the interphase can be tailored for the desired vibration damping in the system. Thus, the hybrid composites with ZnO nanowire interphase can be used to enhance the energy dissipation in a structural system.

Shape memory polymers (SMPs) are one of the most popular smart materials due to light weight and high elastic deformation capability. In this study, highly conductive carbon nanofibers paper (CNFP) was coated on the surface of SMP as a conductive layer for electro actuation of SMP. To overcome the drawback of low modulus and low strength of shape memory polymer (SMP), continuous carbon fiber reinforcement was also incorporated with SMP by autoclave processing. The dynamic mechanical analysis (DMA) result showed over 600% increase of storage modulus of SMP by introducing carbon fiber reinforcement. Also, the shape recovery time of SMP has been reduced over 150%, while the recovery ratio of SMP has been improved to 99% by incorporating with carbon fiber reinforcement. Additionally, the mechanical property degradation of SMP composites has been investigated after different electro actuation cycles. After 50 actuation cycles, the decrease of flexural modulus of SMP composites is negligible (< 2%), and the ultimate flexural strength of SMP composites only decreased 25%. The SMP composite shows high strength and modulus, and good durability.

Carbon fiber reinforced composites have been used in a wide range of applications in aerospace, mechanical, and civil structures. Due to the nature of material, multiple types of structural damage including micro matrix cracks and delaminations can significant degrade the integrity and safety of composites. It is difficult to detect and repair such damage since they are always barely visible to the naked eye. This paper presents the development of self-sensing and self-healing functions in order to detect damage progression and conduct in-situ damage repair in composite structures. Carbon nanotubes, which are highly conductive materials, are uniformly distributed within epoxy to develop the self-sensing capability. Shape memory polymer is used in the hot spot to obtain the self-healing capability. The developed multi-functional material is applied to carbon fiber reinforced composites for the autonomic detection and heal the matrix cracking. Experimental results showed that the developed composite materials are capable of detecting and healing the matrix cracks and delaminations. The developed self-healing material has the potential to be used as a novel structural material in mechanical, civil, aerospace applications. It can be used to detect and in-situ repair matrix damage induced by low velocity impacts and fatigues.

Fiber-reinforced polymer (FRP) composites have become a primary structural material in many new structures, particularly in the aerospace, wind turbine, automobile, and marine industries, due to their higher strength-to-weight ratios, corrosion resistance, and ease of manufacturing. However, these composite materials have complex damages modes that are different from typical monolithic metallic alloys, such as delamination, fiber breakage, matrix cracking, and fiber-matrix debonding. These avenues of damage tend to manifest internally to the composite structure, making them nearly invisible to visual inspection. Several damage detection approaches have been introduced for the purpose of in situ non-destructive evaluation (NDE) of composites; however, many of these approaches require complex analysis methods, data interpolation for achieving spatial sensing, and/or embedding invasive sensors into the composites themselves. To allow for widespread implementation of a next-generation NDE approach for composites, an easily discernible, highly visual, and fast approach that does not adversely affect the structural performance of the composite laminate is needed. This study introduces the use of a spatially distributed electrical conductivity distribution mapping method called electrical impedance tomography (EIT). EIT reconstructs a material’s 2D or 3D electrical conductivity within a series of boundary electrodes. A 100 mA current is injected between two opposing electrodes while the adjacent differential voltages are measured at the remaining electrodes; this process is repeated for all opposing electrode pairs. Using a linear reconstruction algorithm, changes in electrical conductivity are spatially resolved and plotted for easy detection, localization, and evaluation of damage. This approach is validated by applying EIT to a set of carbon fiber-reinforced polymer composite laminates. First, damage has been simulated in composite parts by selectively removing portions of the structure and then verifying that EIT has captured this occurrence. After validation of the EIT method, pristine composite laminates have been subjected to low velocity impact damage. Before and after impact EIT readings have been taken. The differential conductivity reconstruction is presented. This work demonstrates the value of adopting electrical impedance tomography for in situ NDE of FRP composites.

Presented are the results of an experimental investigation into the effects of particle and carbon fiber (CF) doping in epoxy shape memory polymer (SMP). Motivation for the work originates from the need to increase the thermal performance, and thus decrease the time required to transition the polymer given a finite amount of thermal energy, of a SMP link in a bi-stable linkage. Such a multi-functional link is responsible for structural support, mechanism reconfigurability, as well as system damping. Thus any improvement in thermal properties must be weighed against increases in brittleness and weight as well as altered mechanical properties as a result of the chosen method. Two part epoxy SMP by CRG Industries is doped with Fe 3 O 4 (magnetite) nanoparticles (20–30nm spheres) at a weight fraction of 10% as well as 3mm and 10mm carbon fibers at a weight fraction of 5.4%; resulting in all dopants having a volume fraction of approximately 2.5%. The thermal conductivity, specific heat, and diffusivity are experimentally measured by a Hot Disk Thermal Constants Analyser from ambient through transition and the results compared with several thermal composite models. Changes in the thermal properties of the composites and neat polymers with respect to temperature are presented and the effects these changes have on the predictions of thermal models discussed, specifically the effect of changes in thermal properties near the transition temperature and the resulting change in predicted energy required for transition. The effects of adhesion between the particles and the matrix and particle dispersion on conductive paths and material thermal properties are also discussed.

A novel technique to grow carbon nanotubes (CNTs) on the surface of carbon fibers in a controlled fashion using simple lab set up is developed. Growing CNTs on the surface of carbon fibers will eliminate the problem of dispersion of CNTs in polymeric matrices. The employed synthesis technique retains the attractive feature of uniform distribution of the grown CNTs, low temperature of CNTs’ formation, i.e. 550 °C, via cheap and safe synthesis setup and catalysts. A protective thermal shield of thin ceramic layer and subsequently nickel catalytic particles are deposited on the surface of the carbon fiber yarns using magnetron sputtering. A simple tube furnace setup utilizing nitrogen, hydrogen and ethylene (C 2 H 4 ) were used to grow CNTs on the carbon fiber yarns. Scanning electron microscopy revealed a uniform areal growth over the carbon fibers where the catalytic particles had been sputtered. The structure of the grown multiwall carbon nanotubes was characterized with the aid of transmission electron microscopy (TEM). Dynamical mechanical analysis (DMA) was employed to measure the loss and storage moduli of the hybrid composite together with the reference raw carbon fiber composite and the composite for which only ceramic and nickel substrates had been deposited on. The DMA tests were conducted over a frequency range of 1–40 Hz. Although the storage modulus remained almost unchanged over the frequency range for all samples, the loss modulus showed a frequency dependent behavior. The hybrid composite obtained the highest loss modulus among other samples with an average increase of approximately 25% and 55% compared to composites of the raw and ceramic/nickel coated carbon fibers, respectively. This improvement occurred while the average storage modulus of the hybrid composite declined by almost 9% and 15% compared to the composites of reference and ceramic/nickel coated samples, respectively. The ultimate strength and elastic moduli of the samples were measured using standard ASTM tensile test. Results of this study show that while the addition of the ceramic layer protects the fibers from mechanical degradation it abolishes the mechanisms by which the composite dissipates energy. On the other hand, with almost no compromise in weight, the hybrid composites are good potential candidate for damping applications. Furthermore, the addition of CNTs could contribute to improving other mechanical, electrical and thermal properties of the hybrid composite.

A novel type of morphing twisting structure capable of large deformations is investigated. The structure consists of two flanges subject to a uniform distributed bending moment along their length and joined to introduce two stable twisted configurations. These equilibria are positioned symmetrically with regards to the vertical axis of the device and the structure can be twisted between these configurations by a snap-through action. By tailoring the design parameters of the structure, different twist angles and snap-through moments can be achieved. An analytical model and finite element model (FEM) are presented. A prototype made of carbon-fibre reinforced plastic (CFRP) was manufactured and tested. Agreement in terms of angle of twist, axial force and stiffness is found. Moreover, comparison between the two models confirms their validity in terms of snap-through moment, torsional stiffness and strain energy levels. The influence of the lay-up employed and design parameter is also investigated. Potential applications include deployable and morphing structures.