This paper proposes a highly stretchable strain sensor using viscous conductive materials as resistive element and introduces a simple and economic fabrication process by encapsulating the conductive materials between two layers of silicone rubbers Ecoflex 00-30. The fabrication process of the strain sensor is presented, and the properties of the viscous conductive materials are studied. Characterization shows that the sensor with conductive gels, toothpastes, carbon paint, and carbon grease can sustain a maximum tensile strain of 200% and retain good repeatability, with a strain gauge factor of 2.0, 1.75, 3.0, and 7.5, respectively. Furthermore, strain sensors with graphite and carbon nanotubes mixed with conductive gels are fabricated to explore how to improve the gauge factor. With a focus on the most promising material, conductive carbon grease, cyclic stretching tests are conducted and show good repeatability at 100% strain for 100 cycles. Lastly, it is demonstrated that the stretchable strain sensor made of carbon grease is capable of measuring finger bending. With its easy and low-cost fabrication process, large strain detection range and good gauge factor, the conductive materials-based strain sensors are promising for future biomedical, wearable electronics and rehabilitation applications.

The piezoresistive effect in conductive nanofiller-modified polymer, cementitious, and ceramic composites has immense potential to enable multifunctional properties such as intrinsic self-sensing. To date, much work has been done to study the piezoresistive effect under quasi-static loading. Some work has also been done to study the piezoresistive effect under cyclic loading such as when a piezoresistive patch is adhered directly to an oscillating substrate. However, little-to-no work has been done with regard to general dynamic loading conditions such as strain waves originating from a remote source. This is an important gap in the state of the art for two reasons: One, coupling the self-sensing nature of nanocomposites with general elastodynamics is a possible pathway to enabling the study of full-field dynamics (i.e. using the piezoresistive effect to study internal dynamics as opposed to just surface measurements available via tools such as accelerometers and laser vibrometry). And two, coupling piezoresistive self-sensing with damage detection via vibratory methods could lead to transformative gains in the areas of structural health monitoring (SHM) and nondestructive evaluation (NDE). Therefore, we herein work towards addressing this gap in the state of the art by developing basic knowledge on the relation between elastic strain waves and piezoresistive response. Specifically, an electromagnetic-piezoelectric shaker is used to inject highly-controlled strain waves into a long and slender carbon nanofiber (CNF)-modified epoxy rod. Resistance changes along the length of the rod are then measured as strain waves travel along the length of the rod. It is shown that the measured resistance response closely matches the applied mechanical loading. Results from this preliminary study suggest the establishment of an exciting new field — piezoresistive elastodynamics.

Fibre-reinforced polymer (FRP) composite materials are limited in high temperature applications by the matrix glass transition temperature, T g . At and above this temperature, significant mechanical performance is lost, and degradation processes accelerated. This research explores the use of internal passages, or vascules, within the laminate to carry a coolant fluid, absorbing heat energy and cooling the material. A custom thermal chamber and four-point flexural test fixture were developed to perform in-situ thermo-mechanical testing. Vascular and non-vascular carbon/epoxy specimens were manufactured, containing arrays of four 1.1 mm diameter vascules. Specimens were exposed to temperatures from ambient to 170 °C (T g = 200 °C). Flexural modulus varied little with temperature across all tests. Non-vascular specimens at 170 °C showed a reduction in ultimate strength of 21 % compared to under ambient conditions. The presence of vascules caused a small improvement in flexural modulus and strength, due to displacement of a small number of 0° fibre tows further from the neutral axis as a result of the manufacturing process. At 15 L·min −1 coolant flow, vascular specimens showed full retention of strength compared to non-vascular specimens at ambient, demonstrating the potential mechanical performance benefits.

Carbon nanotubes (CNTs) show an active behavior when they are positioned within an electric field, immersed into an electrolyte and charged. Several explanations are given, ranging from nanoscopic to macroscopic effects. This paper presents experimental proven explanations of the paper actuation and results using continuous CNTs of a CNT-array. For the first test series specimens are cut off a paper manufactured of single-walled, μ-long CNTs working in series. The second test series uses specimens which are prepared of free standing multi-walled CNTs. Their CNT lengths reach macroscopic dimensions of almost 3 mm and they can be considered as connected in parallel. Both series are electromechanically tested. The paper tests reveal their strong condition-dependent microstructure. Generally, the observed effects can be explained by diffusion of ions into the flexible CNT microstructure. In contrast, the CNT-array based specimens show almost no condition dependency which can be explained by the strong carbon bonds. Due to specimen orientation and test set-up, macroscopic effects can be excluded. The found actuation can be attributed to an elongation of the carbon structure as result of ion-interaction. However, it must be assumed that there are further superimposing effects which might not be distinguished from each other down to the last detail.

There has been increasing focus in the area of in-situ structural health monitoring since the advent of embedded nano-composites. This experimental research investigates the structural health monitoring abilities of polymer bonded energetics embedded with a uniformly dispersed but randomly oriented carbon nanotube (CNT) sensing network within the polymer binder. A common formulation of the recent solid propellants consists of ammonium perchlorate (oxidizer) and aluminum powder (combustive fuel)-often shaped using a polymer binder, rather than the older techniques of power pressing. Since this study focuses on the structural health of the material and not its thermal properties, monoclinic sugar crystals were used as a substitute for ammonium perchlorate as it has very similar mechanical properties and is much safer in terms of material handling. Thus, a combination of sugar crystals and aluminum powder bound by a Polydimethylsiloxane (PDMS) binder is fabricated in varying concentrations to simulate actual solid rocket propellants. The main focus of this study lies in characterizing the mechanical and electrical properties of the CNT embedded energetic material through subjecting it under mechanical loads; followed by a detailed observation and study of the real time electro-mechanical response under tension and compression. The addition of carbon nanotubes to the polymer binder, thus translates to a real time sensing technique for detection of multi-scale damage in polymer bonded energetics. The results of this study aim to establish a proof of concept for CNT embedded structural health monitoring systems.

An exact solution that investigates the pre-buckling characteristics of nonlocal carbon nanotube (CNT)-based mass sensor subjected to thermal load under clamped-clamped boundary condition is determined. The uniform temperature rise is utilized to study thermal effects on the sensitivity of the mechanical resonator in pre-buckling configuration. Using Eringen’s nonlocal theory, along with the Hamilton’s principle, the governing equations considering small scale and geometric nonlinearity are derived. The influences of important parameters including nonlocal parameter, temperature change, length, and diameter of the CNT on the pre-buckling behavior and frequency shift of the CNT-based mass detector are also studied. Results show that these parameters have significant impact on the dynamic characteristics of the CNT-mass sensor.

Conductive nanofiller-modified composites have received a lot of attention from the structural health monitoring (SHM) research community in recent years because these materials are piezoresistive (i.e. they have deformation and damage-dependent electrical conductivity) and are therefore self-sensing. To date, the vast majority of work in this area has utilized direct current (DC) interrogation to identify and/or localize damage. While this approach has been met with much success, it is also well known that nanofiller-modified composites possess frequency-dependent electrical behavior. This behavior can be roughly modeled as a parallel resistor-capacitor circuit. However, much less work has been done to explore the potential this frequency-dependent behavior for damage detection. To this end, the work herein presented covers some preliminary results which leverage high-frequency electrical interrogation for damage detection. More specifically, carbon nanofiber (CNF)/epoxy specimens are produced and connected to an external inductor in both series and parallel configurations. Because the CNF/epoxy electrically behaves like a resistor-capacitor circuit, the inclusion of an inductor enables electrical resonance to be achieved. Changes in resonant frequency are then used for rudimentary damage detection. These preliminary results indicate that the potential of SHM via the piezoresistive effect in nanofiller-modified composites can be considerably expanded by leveraging alternating current (AC) interrogation and resonant frequency principles.

The actuation mechanisms of cnt-based materials are still controversially discussed. It is not common sense whether it is a macroscopic volume effect caused by ion intercalation or electrostatic repulsion of equally charged cnts or a nanoscopic effect of filled electron anti-bonding orbitals of the carbon atom or interactions with ions docking on the carbon surface. In the presented paper arrays of highly aligned multi-walled carbon nanotubes (mwcnts) are used which are stabilized by a polypyrrole-coating. The samples are tested along the cnt-orientation and in perpendicular mode to analyze the influence of the structure-ion interaction. The mwcnt-arrays exhibit only a total length of approximately 2.8 mm but by coating with polypyrrole larger geometries can be tested. The actuation is analyzed using an in-plane test and an actuated tensile testing. Free strain can be detected using the first set-up, the second method is carried out to evaluate the mechanical stability of the samples. As might be expected, the material shows a strong anisotropic active behavior with the actuation along the tube axis being only half of the value detected at the perpendicular oriented samples. The findings point out that an intercalation of ions into the charged CNT-architecture seems here to be the dominating mechanism.

Recently, carbon fiber-reinforced thermoplastics (CFRTPs) have become popular choices in desktop-based additive manufacturing, but there is limited information on their effective usage. In Fused Deposition Modeling (FDM), a structure is created by layers of extruded beads. The degree of bonding between beads, bead orientation, degree of interlayer bonding, type of infill and the type of material, determines overall mechanical performance. The presence of chopped fibers in thermoplastics increases melt viscosity, changes coefficients of thermal expansion, may have layer adhesion issues, and causes increased wear on nozzles, which makes FDM fabrication of thermoplastic composites somewhat different from neat thermoplastics. In the current work, best practices and the effect of annealing and infill patterns on the mechanical performance of FDM-fabricated composite parts were investigated. Materials included commercially available PLA, CF-PLA, ABS, CF-ABS, PETG, and CF-PETG. Two sets of ASTM D638 tensile and ASTM D790 flexural test specimens with 3 different infill patterns and each material were fabricated, one set annealed, and all tested. Anisotropic behavior was observed as a function of infill pattern. As expected, strength and stiffness were higher when the beads were oriented in the direction of the load, even for neat resins. All fiber-filled tensile results showed an increase in stiffness, but surprisingly, not in strength (likely due to very short fiber lengths). Tests of annealed specimens resulted in clear improvements in tensile strength, tensile stiffness and flexural strength for PLA, CF-PLA, and PETG, CF-PETG but a reduction in flexural stiffness. Also, annealing resulted in mixed improvements for ABS and CF-ABS and is only useful in certain infill patterns. This work also establishes ‘Best Practices’ of FDM-type fabrication of thermoplastic composite structures and documents the minimum critical fiber lengths and fiber fractions of several CF-filled FDM filaments.

Inspired by the fibrillar network in plant cell walls and the helical fibers found in soft bodied hydrostats (e.g. worms, squid, elephant trunks, and octopus arms), fluidic flexible matrix composites (F2MCs) are composite tubes that consist of multiple layers of oriented, high performance fibers, such as carbon, precisely placed in a flexible matrix resin to form high-mechanical advantage actuators and variable stiffness materials. Unique to the F2MC tube is its ability to generate high pressures and volume change with a small external load as a result of the stiff reinforcement fiber orientation in the wall of the tube and the soft supporting elastomer. When a load is applied to the tubes, the volume of the composite pump is reduced and fluid is forced out of the tube by the reinforcing fibers. The objective of this research is to design, fabricate and characterize F2MCs for use in wave energy conversion where ocean waves provide the axial load to drive fluid through the pumps. F2MCs pumps are tested in a water basin and mechanically cycled between 0 Hz and 2 Hz at up to 17 percent strain. Instantaneous input power is found by measuring the displacement and applied force to the actuators, while output power values are derived from pressure and flow rate measurements at the tube outlet. From these measurements the actuator efficiency is subsequently determined.

This research introduces an integrated vibration energy harvester and electrochemical energy storage device that can effectively convert ambient vibrations directly into stored electrochemical energy. The electrochemical energy storage device is an electrical double layer capacitor (EDLC) with an ionic redox transistor as its membrane separator. This ‘ smart ’ membrane separator directly rectifies the electrical energy generated by the transduction from the nonlinear energy harvester, creating an ionic polarization across the membrane separator for storage. This electrochemical gradient can be subsequently used for powering sensor electronics as required in various applications, including structural condition monitoring. The alternating voltage developed by the energy harvester (+/−5V around 100 Hz) is connected to an aqueous supercapacitor fabricated from nanofibrous carbon paper electrodes and a polypyrrole-based (PPy(DBS)) smart membrane separator. A potential below −400mV from the energy harvester applied to the supercapacitor turns the smart membrane separator ‘ON’ and results in a unidirectional ionic current of Li+ ions. As the potential developed by the harvester cycles above ∼50 mV, the membrane separator switches ‘OFF’ and prevents the discharge of the rectified current. This leads to a continuous polarization of ions towards electrical fields relevant for powering electronics. This article is the first description and demonstration of an energy harvesting and storage system that can directly convert the electrical energy from a vibration energy harvester into electrochemical energy without the use of passive circuit components for power rectification.

Spiral Torsion Springs (STS) are generally manufactured employing medium/high-carbon steel alloys shaped as thin rods with rectangular cross section. Meanwhile, plastic materials (e.g. ABS or PLA), currently used in freeform manufacturing processes, may not be suited for several applications, owing to the low material yield strength and the rather poor fatigue life. Despite the above-mentioned limitations, the main advantages of a 3D printing process, as compared to more traditional manufacturing techniques, are the design flexibility and the possibility to directly integrate elastic components within a joint mechanism produced as a single (monolithic) part. In particular, provided that the external forces acting on the spring coils are maintained within a certain threshold and that the spring geometry is suitably optimized, a reliable 3D-printed STS alternative to traditional steel springs is actually feasible. Given these premises, the main purpose of the present paper is to propose a model-based optimization algorithm that allows to optimally size STS for user-specified torque-deflection characteristics. Optimal STS geometries are then realized in ABS via Fused Deposition Manufacturing, and subsequently tested with a purposely-designed experimental set-up. Furthermore, the behavior of each STS sample (in terms of stiffness and equivalent Von Mises stress) is evaluated by means of non-linear finite elements analysis, in order to check the correspondence with the expected behavior. Finally, numerical and experimental results are provided, which demonstrate the prediction capabilities of the proposed modeling/optimization techniques, and confirm that well-behaved STS can be conceived and produced. Envisaged applications concern the development of smart structures for robot design, such as multi-articulated compliant robotic chains that can be used as low-cost manipulators (i.e. arm) or as mini-manipulators (i.e. fingers). The proposed approach effectively simplifies the production and the assembly of the mechanism, also allowing for an easier integration of embedded sensory-actuation systems.

Over the past two decades, numerous research studies have been conducted to explore behavior of self-sensing cementitious composites with different functional fillers. Most of these studies investigated the use of fillers such as carbon nanofiber (CNF), carbon black, and carbon nanotubes (CNTs) in cement composites to develop a multifunctional material. Since its discovery in 2004, graphene has also raised significant attention as 2D nanoscale reinforcement for composite materials. The planar structure of graphene sheets provides more contact area with the host material. However, high cost and dispersion difficulties are among the drawbacks of graphene. More recently, graphene nanoplatelets (GNPs), which have very thin but wide aspect ratio, are drawing the graphene market due to their advantages such as ease of processing and excellent material properties at a very low cost. The application of two-dimensional graphene nanoplatelets in cementitious composites has yet to gain widespread attention. This paper investigates the self-sensing capabilities of GNP-reinforced hydraulic Portland cement composites. In particular, the effects of GNP content on the electrical properties and piezoresistive characteristics of mortar specimens are explored. In addition, a simple fabrication method that does not require special treating procedures such as ultrasonication and chemical (covalent) treatments for the dispersion of GNPs is pursued. The GNPs used in this study have an average thickness of 8 nanometers and a diameter of 25 microns. Standard prismatic mortar specimens containing different GNP concentrations are prepared using three different mixing procedures. The resistivity of the specimens is measured using a four-point probe method. The piezoresistive response of GNP-reinforced cement composites is evaluated under cyclic compressive loads.

Carbon nanofiller-modified composites have incredible potential for self-sensing and structural health monitoring (SHM) because they are piezoresistive. That is, the electrical conductivity of a nanocomposite is inherently coupled with mechanical perturbations such as damage and strain. Because of the correspondence of strain and damage with conductivity changes, non-invasive conductivity imaging techniques such as electrical impedance tomography (EIT) can enable unprecedented insight into the mechanical state of a nanofiller-modified composite. Furthermore, because of the potential of nanocomposites for self-sensing and SHM, considerable effort has been dedicated to studying the effect of strain on nanocomposite conductivity. That is, these efforts seek to determine the change in conductivity of a nanocomposite for a prescribed strain. However, from a SHM perspective, knowing the inverse relation would be much more useful. In other words, for an observed conductivity change, what is the underlying strain state? In light of the potential of EIT to provide insight into the conductivity distribution of a strained nanocomposite, we herein develop a method of estimating the infinitesimal strain tensor of a piezoresistive nanocomposite for observed conductivity changes. This is done by formulating an inverse problem that seeks to minimize the difference between an observed conductivity and a piezoresistivity model that predicts nanocomposite conductivity as a function of the strain in the least-squares sense. Next, this approach is specialized to the finite element method such that the nodal displacements giving rise to an observed conductivity change can be ascertained. Lastly, this method is tested analytically with noisy data. It is found that the proposed method can accurately reproduce nodal displacements and therefore strains from conductivity data.

The unique micro/nano-structure of an intrinsically conducting polymer can be tuned to get higher gauge factors (GF) and reliability, which make them better materials for piezo-resistive applications than conducting carbon based composites and metallic composites. This work reports a highly sensitive conducting polyaniline (PANI)-based composite film that showed a GF ∼66. This high GF was achieved by forming a particular microstructure of conducting PANI particles in a free standing film of PANI-DBSA/EVA. The paper also attempts to explain the mechanism for the observed high sensitivity using the electronic percolation theory, shape and size of the conducting particles and the changes in the microstructure, due to strain. The high sensitivity, high stability during cyclic loading and low electrical hysteresis together with high mechanical strength make PANI-DBSA/EVA conducting composite film a promising material for piezo-resistive strain sensing applications.

Inflatable deployable structures are practical and promising candidates for serving various aerospace missions, for instance, as solar sails, antennas, space suits, and especially Lunar and Mars habitats. These structures feature flexible composites folded at high packing efficiency, which can drastically reduce launch costs. However, they can also be damaged due to the harsh extraterrestrial operating conditions, which can propagate to cause catastrophic mission failure and endanger crew safety. Therefore, it is imperative to integrate a robust structural health monitoring (SHM) system, so that damage and faults can be detected for ensuring their safe and reliable operations. While a variety of SHM technologies have been developed for monitoring conventional, rigid, structural systems, they are faced with challenges when used for these unconventional flexible and inflatable systems. Therefore, a flexible carbon nanotube-fabric nanocomposite sensor is proposed in this study for monitoring the integrity of inflatable space structures. In particular, CNT-based thin films were fabricated by spraying and then integrated with flexible fabric to form the lightweight sensor. By coupling fabric sensors with an electrical impedance tomography (EIT) algorithm, the fabric’s distribution of spatial resistivity can be mapped using only electrical measurements obtained along the material’s boundaries. The severity and location of localized pressure and impact damage can be captured by observing changes in the EIT-calculated resistivity maps. They can be embedded in inflatable habitat structures to detect and locate abnormally high pressure regions and impact damage.

The heat treatment in carbon powder is a safe and effective strategy to enhance the photocatalytic activity of titanium dioxide (TiO 2 ) coatings. Firstly, Ti coatings were prepared by mechanical coating operation with Ti powder on alumina balls. Secondly, the target TiO 2 coatings were prepared using the prepared Ti coatings by the multi-heat treatment (pretreatment in carbon powder, oxidation in air and reduction in carbon powder). During the pretreatment, thin films of Ti 2 CO and micro-cracks form in the surface of the Ti coatings. The formed thin films and micro-cracks are helpful for forming a nano-fiber morphology of rutile TiO 2 , during the subsequent oxidation. During the reduction, oxygen vacancies generated in the lattice of rutile TiO 2 are in favor of narrowing the band gap. The target TiO 2 coatings with a nano-fiber morphology and the narrowed band gap, effectively enhances the photocatalytic activity by more than 3 times, compared with the pristine TiO 2 coatings.

This paper focuses on the actuation mechanisms of CNT-papers and CNT-arrays. CNT-papers represent architectures of CNTs which are connected by van der Waals forces or structural entanglement. In contrast CNT-arrays are vertically aligned CNTs. Single CNTs are favored for investigation of the active behavior of the hexagonal carbon structure formed by covalent C-bonds. CNT-arrays feature contiuous tubes of 3 mm length which allows the test the tubes themselves. Thus they are clamped at each end they represent samples for testing covalent bonds. Both sample types are tested within an actuated tensile test set-up under different conditions to identify the specific influence. Furthermore different electrolytes are used to investigate the influence of the ion-radius on the CNT-paper. CNT-papers are tested in water-based electrolytes CNT-arrays are tested in an ionic liquid. It was found that the performance of CNT-papers strongly depends on the conditions which indicates ion-diffusion as actuation mechanism. However, CNT-arrays are almost unaffected by the conditions, considering their active response and sample composition quantum mechanical reasons seem to be the most appropriate explanation for the array actuation.

This paper presents an analysis of optimization for multifunctional nanocomposites. A carbon nanotubeepoxy composite is optimized for maximum resistance change and minimum strain energy. Analysis uses a finite element method and includes the coupled physics of mechanics, electrostatics, and piezoresistivity. The problem is solved first for minimum strain energy, then two resistance maximization problems are solved. For all optimization, sensitivities are obtained analytically. After solving the individual problems a weighted sum approach is used in the multi-objective optimization of both minimizing the strain energy and maximizing the resistance change. Comments are made as to the effect of the topology optimization method as a design tool, on the shape of the optimized cross sections, and on the possible extensions on using the coupled physics topology optimization algorithm.

Manned and unmanned aircraft are commonly manufactured from lightweight composite materials, replacing heavier metallic aircraft materials. Here, we describe a collaborative industrial-academic effort to develop novel microvascular composites that offer thermal and structural multifunctionality via embedded micro-capillary array-based heat exchangers for thermal management embedded within aerospace-grade carbon prepreg material. The current research establishes a route to create prepreg materials suitable for heat exchanger network manufacture by automated fiber placement processes. Potential network geometries are investigated with analytical fluid models to predict flow characteristics and heat distribution for pre-defined heat exchanger configurations within a carbon composite panel. Based on these predictions, two geometries are fabricated for thermal management experiments. The mechanical effect of the embedded microvascular networks on the panel structural properties is also investigated analytically.