Methods for adhesion/friction reduction of novel wire-shaped actuators, based on shape memory alloys, for use in adaptive fiber-reinforced plastic composites

For fiber-reinforced plastic composites, fiber-matrix adhesion is a significant aspect of composite properties. While conventional lightweight structures are always aiming for high fiber-matrix adhesion, innovative and unconventional functional constructions require different concepts. The research work treating adaptive fiber-reinforced plastic composites with shape memory alloy wires presented here uses the approach of actuators freely movable within the composite. This is supposed to prevent mechanical tensions in the interfaces of actuator and composite structure, which would otherwise cause damages of the composite. This work examines hybrid yarns based on friction spinning technology, with shape memory alloy wires as their core component as well as glass fibers, and partly polypropylene, as their sheath component. Additionally, the surface properties of the shape memory alloy wires being used are modified by sanding and coating. The results of a characterization by pull-out testing clearly show that a coating of the shape memory alloy wires with an abherent causes considerable decrease in adhesion and friction in the interface and leads to the mobility of the shape memory alloy wires in the later composite. An even greater effect is attained by sheathing the hybrid yarns in an additional layer of polypropylene, compacting the yarn cross-section. Thus, the pull-out force could be reduced to 35–40% of the reference structure

Technological Development of a Yarn Grip System for High-Speed Tensile Testing of High-Performance Fibers

Particularly in terms of carbon fiber (CF) rovings and further high performance fibers, it is a highly demanding task to clamp technical yarns with low elongations at break during high-speed tensile tests due to their sensitivity to shear stress. For fibers to be tested, a low elongation at break results in short testing times and requires high acceleration. In this paper, four different yarn grips that can be applied with various test machines will be introduced and compared to a wedge screw grip. By using most sensitive CF rovings, advantages and disadvantages of these gripping devices will be qualitatively evaluated by means of testing machines with test speeds of up to 20 m/s and strain rates of up to 200 s−1, respectively. Hence, the reproducibility and precision of test results were considerably enhanced by optimizing the geometry and mass of yarn grips. Moreover, theoretical approaches and calculations for the design of yarn grips suitable for test speeds of up to 100 m/s will be presented

Non-Monotonic Sensor Behavior of Carbon Particle-Filled Textile Strain Sensors

Carbon particle-filled elastomers are a widely researched option to be used as piezoresistive strain sensors for soft robotics or human motion monitoring. Therefore, various polymers can be compounded with carbon black (CB), carbon nanotubes (CNT) or graphene. However, in many studies, the electrical resistance strain response of the carbon particle-filled elastomers is non-monotonic in dynamic evaluation scenarios. The non-monotonic material behavior is also called shoulder phenomenon or secondary peak. Until today, the underlying cause is not sufficiently well understood. In this study, several influencing test parameters on the shoulder phenomena are explored, such as strain level, strain rate and strain history. Moreover, material parameters such as CNT content and anisotropy are varied in melt-spun CNT filled thermoplastic polyurethane (TPU) filament yarns, and their non-monotonic sensor response is evaluated. Additionally, a theoretical concept for the underlying mechanism and thereupon-based model is presented. An equivalent circuit model is used, which incorporates the visco-elastic properties and the characteristic of the percolation network formed by the conductive filler material. The simulation results are in good agreement when compared to the experimental results

Characterization of the Viscoelastic Properties of Yarn Materials: Dynamic Mechanical Analysis in Longitudinal Direction

Warp knitting is a highly productive textile manufacturing process and method of choice for many products. With the current generation of machines running up to 4400 min−1, dynamics become a limit for the production. Resonance effects of yarn-guiding elements and oscillations of the yarn lead to load peaks, resulting in breakage or mismatches. This limits material choice to highly elastic materials for high speeds, which compensate for these effects through their intrinsic properties. To allow the processing of high-performance fibers, a better understanding of the viscoelastic yarn behavior is necessary. The present paper shows a method to achieve this in longitudinal yarn direction using a dynamic mechanical analysis approach. Samples of high tenacity polyester and aramid are investigated. The test setup resembles the warp knitting process in terms of similar geometrical conditions, pre-loads, and occurring frequencies. By recording the mechanical load resulting from an applied strain, it is possible to calculate the phase shift and the dissipation factor, which is a key indicator for the damping behavior. It shows that the dissipation factor rises with rising frequency. The results allow for a simulation of the warp knitting process, including a detailed yarn model and representation of stitch-formation process

Monitoring the Joint Area of Composite Membrane Materials

Textile membranes are suitable for a wide range of applications due to their user-adjustable properties, which can be modified based on both the textile reinforcement structure and the coating material. Complex dynamic loads are involved in typical usage scenarios for load-bearing components of textile architecture, e.g., unsupported convertible roofs of halls or stadiums, temporary buildings, large-volume consumable media storage and the main sail of sailing boats. It is generally known that particularly in the area of membrane joints, successive degradation of seam strength may occur. This paper addresses the realization of an in situ measurement system for textile surface formation in textile membranes, which is introduced locally in the area of the joining zone and is compatible with the materials as well as the ultrasonic welding process itself. These development efforts are supported by a numerical investigation in terms of the serviceability and residual load-carrying capacity of the joining zone and the textile membrane surface area

Melt Spinning of Highly Stretchable, Electrically Conductive Filament Yarns

Electrically conductive fibers are required for various applications in modern textile technology, e.g., the manufacturing of smart textiles and fiber composite systems with textile-based sensor and actuator systems. According to the state of the art, fine copper wires, carbon rovings, or metallized filament yarns, which offer very good electrical conductivity but low mechanical elongation capabilities, are primarily used for this purpose. However, for applications requiring highly flexible textile structures, as, for example, in the case of wearable smart textiles and fiber elastomer composites, the development of electrically conductive, elastic yarns is of great importance. Therefore, highly stretchable thermoplastic polyurethane (TPU) was compounded with electrically conductive carbon nanotubes (CNTs) and subsequently melt spun. The melt spinning technology had to be modified for the processing of highly viscous TPU–CNT compounds with fill levels of up to 6 wt.% CNT. The optimal configuration was achieved at a CNT content of 5 wt.%, providing an electrical resistance of 110 Ωcm and an elongation at break of 400%

Development of an Elastic, Electrically Conductive Coating for TPU Filaments

Electrically conductive filaments are used in a wide variety of applications, for example, in smart textiles and soft robotics. Filaments that conduct electricity are required for the transmission of energy and information, but up until now, most electrically conductive fibers, filaments and wires offer low mechanical elongation. Therefore, they are not well suited for the implementation into elastomeric composites and textiles that are worn close to the human body and have to follow a wide range of movements. In order to overcome this issue, the presented study aims at the development of electrically conductive and elastic filaments based on a coating process suited for multifilament yarns made of thermoplastic polyurethane (TPU). The coating solution contains TPU, carbon nanotubes (CNT) and N-Methyl-2-pyrrolidone (NMP) with varied concentrations of solids and electrically conductive particles. After applying the coating to TPU multifilament yarns, the mechanical and electrical properties are analyzed. A special focus is given to the electromechanical behavior of the coated yarns under mechanical strain loading. It is determined that the electrical conductivity is maintained even at elongations of up to 100%