Carbon and glass hierarchical fibers : influence of carbon nanotubes on tensile, flexural and impact properties of short fiber reinforced composites

Dense carbon nanotubes (CNTs) were grown uniformly on the surface of carbon fibers and glass fibers to create hierarchical fibers by use of floating catalyst chemical vapor deposition. Morphologies of the CNTs were investigated using scanning electronic microscope (SEM) and transmission electron microscope (TEM). Larger diameter dimension and distinct growing mechanism of nanotubes on glass fiber were revealed. Short carbon and glass fiber reinforced polypropylene composites were fabricated using the hierarchical fibers and compared with composites made using neat fibers. Tensile, flexural and impact properties of the composites were measured, which showed evident enhancement in all mechanical properties compared to neat short fiber composites. SEM micrographs of composite fracture surface demonstrated improved adhesion between CNT-coated fiber and the matrix. The enhanced mechanical properties of short fiber composites was attributed to the synergistic effects of CNTs in improving fiber–matrix interfacial properties as well as the CNTs acting as supplemental reinforcement in short fiber-composites

Theoretical prediction of CNT-CF/PP composite tensile properties using various numerical modeling methods

Development of effective models to predict tensile properties of ‘carbon nanotube coated carbon fibre reinforced polypropylene (CNT-CF/PP)’ composites is briefly discussed. The composite taken as the reference is based on the highest growth mechanism of CNTs over carbon fibres. Halpin-Tsai and Combined Voigt-Reuss model has been implemented. Young's modulus for CNT-CF/PP composites has been found 4.5368 GPa and the tensile strength has been estimated 45.367 MPa considering the optimum operating condition of chemical vapor deposition (CVD) technique. Stiffness of the composite is represented through the stress-strain plots; stiffness is proportional to the steepness of the slope. There are slight deviations of results that have been found theoretically over the experimental issues

Polymer Fibre Artificial Muscle

Large-scale torsional actuation occurs in twisted fibres and yarns as a result of volume change induced electrochemically, thermally, photonically, and other means. When formed into spring-like coils, the torsional actuation within the fibre or yarn generates powerful tensile actuation per muscle weight. For further development of these coil actuators and for the practical application of torsional actuators, it is important to standardise methods for characterising both the torsional stroke (rotation) and torque generated. This thesis introduces such a method for use in the free rotation of a one-endtethered fibre, when operating against an externally applied torque (isotonic) and during actuation against a return spring fibre (variable torque). The torsion mechanics approach has been verified and allows the prediction of torsional stroke under any external loading condition based on the fundamental characteristics of the actuator: free stroke and stiffness. The second thesis aim was to develop a better understanding of the link between fibre / yarn volume change and the induced torsional actuation. The developed theoretical analysis was based on experimental investigation of the effects of fibre diameter and inserted twist on the torsional stroke and torque measured when heating and cooling nylon 6 fibres over a certain temperature range. The results show that the torsional stroke depends only on the amount of twist inserted into the fibre and is independent of fibre diameter

Experimental and theoretical evaluation of the tensile properties of carbon nanotube-coated carbon fibre hybrid composites

Hierarchically structured hybrid composites are ideal engineered materials to carry loads and stresses due to their unconventional in-plane specific mechanical properties such as tensile modulus, strength, and stiffness. Growing carbon nanotubes (CNT) on the surface of high performance carbon fibres (CF) provides a means to tailor the mechanical properties of the fibre-matrix interface of a composite. The growth of CNT onto the surface of CF was conducted via floating catalyst chemical vapor deposition (CVD) technique. The mechanical properties of the resultant fibres, CNT density and alignment morphology were shown to depend on the CNT growth temperature, growth time, carrier gas flow rate, catalyst amount, and atmospheric conditions within the CVD chamber. The evidence of intensive CNTcoating on CF was shown at a CVD temperature of 700 °C and 30 minutes reaction time by using Scanning electron microscope (SEM). Single fibre/Epoxy composite coupons were fabricated by using both neat and CNT-coated CF to conduct single fibre fragmentation test (SFFT). For neat-CF/Epoxy composite coupons, IFSS was found to be 12.52 MPa. A CNT-coated CF demonstrated approximately 45% increase in calculated IFSS when treated at 700 °C and 30 minutes reaction environment over that of the untreated fibre from which it was processed. Carbon nanotube coated short carbon fibre reinforced polypropylene (CNT-CF/PP) composites were fabricated. The resulting hybrid composite samples were characterized using the tensile testing method. For neat-CF/PP composite, Young’s modulus and tensile strength were found to be 1.72 GPa and 20.5 MPa respectively. In contrast with the neat CF/PP composite, CNT-CF/PP composite has shown enhanced Young’s modulus by approximately 104% and tensile strength increased to approximately 64%. The fibre-matrix adhesion was analyzed by using SEM on cryogenically fractured surface of both types of composites. The proper justification of fibre-matrix interfacial adhesion featuring the composite tensile properties was explained through interfacial shear strength (IFSS). Composites with high IFSS was found to show a high Young’s modulus and tensile strength. Theoretical prediction of hybrid CNT-CF/PP composite tensile properties was accomplished by using a hierarchical model which comprises Halpin–Tsai equations, Combined Voigt-Reuss model, simple rule-of-mixtures (RoM) and Krenchel approach. When the internal geometry of composite was a key factor RoM was utilized to study the fibre orientation distribution in the composite. A comprehensive fractographic investigation was carried out with scanning electron microscope (SEM) to analyze the fibre orientation distribution on the CNT-CF/PP composite fracture surfaces. Then, a thorough analysis was done on the SEM images using Bersoft and Geozebra image analyzing software packages to evaluate the fibre orientation distribution factor (η 0 ). In the context of this approach, when the fibre orientation effect is ignored a noteworthy deviation in tensile modulus with 51% was notified rather than experimental result of 1.72 GPa. When η0 is considered a more acceptable validation with the experimental results of tensile modulus was obtained which shows amoderate deviation with 30% to the predicted value of 4.57 GPa. Finally, the discrepancies between the predicted and experimental values were explained in terms of stress-strain behavior

Torsional artificial muscles

Large stroke torsional actuators are the newest class of artificial muscle technology that produces rotary motion or generate torque in response to various stimuli. A number of materials comprising twisted fibres or filaments have been shown to display high degrees of reversible untwist and retwist under various experimental conditions such as heating, electrochemical charging, chemical absorption or photonic excitation. Torsional actuators are of potential application in areas that include microfluidic mixing, microsensors, photonic displays, and energy-harvesting devices. Furthermore, the torsional actuation in fibres can be translated into a linear, or tensile, actuation when the fibres are formed into coils. These coil tensile artificial muscles are of potential use in soft and wearable robotics, medical devices and prosthetics. This review will provide a comprehensive overview of torsional actuators constructed from different functional materials, their actuating mechanism, potential applications, and their current limitations. The review will conclude with recent developments and future trends of torsional actuators as well as critical issues that need to be addressed and resolved

Wet-Spun Biofiber for Torsional Artificial Muscles

The demands for new types of artificial muscles continue to grow and novel approaches are being enabled by the advent of new materials and novel fabrication strategies. Self-powered actuators have attracted significant attention due to their ability to be driven by elements in the ambient environment such as moisture. In this study, we demonstrate the use of twisted and coiled wet-spun hygroscopic chitosan fibers to achieve a novel torsional artificial muscle. The coiled fibers exhibited significant torsional actuation where the free end of the coiled fiber rotated up to 1155 degrees per mm of coil length when hydrated. This value is 96%, 362%, and 2210% higher than twisted graphene fiber, carbon nanotube torsional actuators, and coiled nylon muscles, respectively. A model based on a single helix was used to evaluate the torsional actuation behavior of these coiled chitosan fibers

Artificial Muscles from Hybrid Carbon Nanotube-Polypyrrole-Coated Twisted and Coiled Yarns

Electrochemically or electrothermally driven twisted/coiled carbon nanotube (CNT) yarn actuators are interesting artificial muscles for wearables as they can sustain high stress. However, due to high fabrication costs, these yarns have limited their application in smart textiles. An alternative approach is to use off-the-shelf yarns and coat them with conductive polymers that deliver high actuation properties. Here, novel hybrid textile yarns are demonstrated that combine CNT and an electroactive polypyrrole coating to provide both high strength and good actuation properties. CNT-coated polyester yarns are twisted and coiled and subjected to electrochemical coating of polypyrrole to obtain the hierarchical soft actuators. When twisted without coiling, the polypyrrole-coated yarns produce fully reversible 25° mm-1 rotation, 8.3× higher than the non-reversible rotation from twisted CNT-coated yarns in a three-electrode electrochemical system operated between +0.4 and –1.0 V (vs Ag/AgCl). The coiled yarns generate fully reversible 10° mm-1 rotation and 0.22% contraction strain, 2.75× higher than coiled CNT-coated yarns, when operated within the same potential window. The twisted and coiled yarns exhibit high tensile strength with excellent abrasion resistance in wet and dry shearing conditions that can match the requirements for using them as soft actuators in wearables and textile exoskeletons

Twist-coil coupling fibres for high stroke tensile artificial muscles

A new concept for tensile artificial muscles is introduced in which the torsional actuation of a twisted polymer fibre drives a twist to writhe conversion in a serially attached elastomeric fibre. Thermally induced torsional rotation of the twisted fibre caused formation of coils in the elastomeric fibre which resulted in overall muscle length contraction. Theoretical predictions of the muscle strain were developed by means of a modified single-helix theory. Experimental tests were conducted to measure the isotonic contraction strains for elastomeric fibres of different diameters and lengths. A good agreement between the measured and calculated results was found. Practical applicability of this muscle is evaluated by using different mechanical loading conditions. Actuation contraction strains as high as 10% were observed with excellent reversibility. Unlike original coiled fibre tensile actuators, these twist-coil artificial muscles did not require any pre-conditioning cycles

Thermomechanical effects in the torsional actuation of twisted nylon 6 fiber

Thermally induced torsional and tensile actuators based on twisted polymeric fibers have opened new opportunities for the application of artificial muscles. These newly developed actuators show significant torsional deformations when subjected to temperature changes, and this torsional actuation is the defining mechanism for tensile actuation of twisted and coiled fibers. To date it has been found that these actuators require multiple heat/cool cycles (referred to as training cycles) prior to obtaining a fully reversible actuation response. Herein, the effect of annealing conditions applied to twisted nylon 6 monofilament is investigated and it is shown that annealing at 200°C eliminates the need for the training cycles. Furthermore, the effect of an applied external torque on the torsional actuation is also investigated and torsional creep is shown to be affected by the temperature and load

Controlled and scalable torsional actuation of twisted nylon 6 fiber

Large-scale torsional actuation occurs in twisted fibers and yarns as a result of volume change induced electrochemically, thermally, photonically, and other means. A quantitative relationship between torsional actuation (stroke and torque) and volume change is here introduced. The analysis is based on experimental investigation of the effects of fiber diameter and inserted twist on the torsional stroke and torque measured when heating and cooling nylon 6 fibers over the temperature range of 26-62 °C. The results show that the torsional stroke depends only on the amount of twist inserted into the fiber and is independent of fiber diameter. The torque generated is larger in fibers with more inserted twist and with larger diameters. These results are successfully modeled using a single-helix approximation of the twisted fiber structure