Electrically conductive fiber-reinforced composites have been designed in order to develop self-diagnosis materials with the ability to memorize damage histories. Irreversible resistance changes dependent on the strain histories of the composites were utilized to achieve this ability. Conductive fiber-reinforced plastics for memorizing maximum strain were prepared by adding carbon fibers or particles into the composites. Pre-tensile stresses in composites containing carbon fibers were found to effectively enhance their residual resistance and to significantly improve the limit of smallest detectable strains. The residual resistances of composites containing carbon particles connected by a percolation structure were found to depend strongly on the volume fractions of carbon particles; composites with high volume fractions of carbon displayed remarkable residual resistance without application of a pre-tensile stress. In order to memorize cumulative damage, composites consisting of a brittle titanium nitride ceramic wire laminated with glass fiber reinforced plastics were prepared. These composites were found to exhibit remarkable residual resistances that increased in proportion to the logarithm of the number of tensile cycles. These results suggest that a simple and low cost monitoring technique without real-time measurement system will be available in wide range of applications using these composites.
A smart composite material system which has three smart functions of sensor, actuator and processor has been developed intend to apply to structure of house for controlling ambient temperature and humidity, hands of robot for holding and feeling an object, and so on. A carbon fiber reinforced plastics (CFRP) is used as matrix in the smart composite. The size of the matrix is 120mm x 24mm x 0.45mm. The CFRP plate is combined two Ni-Ti shape memory alloy (SMA) wires with an elastic rubber to construct a composite material. The composite material has a characteristic of reversible response with respect to temperature. A photo-sensor and temperature sensor are embedded in the composite material. The composite material has a processor function to combine with a simple CPU (processor) unit. For demonstrating the capability of the composite material system, a model is built up for controlling certain behaviors such as gripping and releasing a spherical object. The amplitude of gripping force is (3.0 plus/minus 0.3) N in the measurement, which is consistent with our calculation of 2.7 N. Out of a variety of functions to be executed by the CPU, it is shown to exert calculation and decision making in regard to object selection, object holding, and ON-OFF control of action by external commands.
In recent years, pre-strained TiNi shape memory alloys (SMA) have been used for fabricating smart structure with carbon fibers reinforced plastics (CFRP) in order to suppress microscopic mechanical damages. However, since the cure temperature of CFRP is higher than the reverse transformation temperatures of TiNi SMA, special fixture jigs have to be used for keeping the pre-strain during fabrication, which restricted its practical application. In order to overcome this difficulty, we developed a new method to fabricate SMA/CFRP smart composites without using special fixture jigs by controlling the transformation temperatures of SMA during fabrication. This method consists of using heavily cold-worked wires to increase the reverse transformation temperatures, and of using flash electrical heating of the wires after fabrication in order to decrease the reverse transformation temperatures to a lower temperature range again without damaging the epoxy resin around SMA wires. By choosing proper cold-working rate and composition of TiNi alloys, the reverse transformation temperatures were well controlled, and the TiNi/CFRP hybrid smart composite was fabricated without using special fixture jigs. The damage suppressing effect of cold drawn wires embedded in CFRP was confirmed.
The focus of this work is the thermomechanical characterization and effect of damage recovery on the pre-strained SMA wire embedded CFRP composites for developing the smart composites with self-damage control. The SMA utilized in this work is a Ni-45at percent Ti wire with a diameter of 0.4 mm. A steel mold was specially designed to embed the pre-strained TiNi wire into CFRP preperg and prevent their recovery during the cure cycle. TiNi/CFRP composites were fabricated by hot-pressing in the temperature range of 150-180 degrees C by controlling the applied pressure. The overall research is divided into four parts: fabrication of SMA wire embedded CFRP composites, experimental characterization of thermomechanical behavior on SMA wire by electrical heating, recovery effect of self-damage control in composites and sensing effect by detecting the electrical resistance at SMA wire. Compressive recovery force induced by thermomechanical actuation of SMA depends on pre-strained level and volume fraction of TiNi. The hot-pressed TiNi/CFRP specimens were loaded under tensile test in order to induce a transverse crack or partial damage. Specially, transverse crack easily happen at 90 degrees stacking CFRP layers. The damage degree due to generation of transverse cracks is quantified by real-time measurements of electrical resistance of SMA in composites during tensile load. After electrical heating, the generated transverse cracks at composites successfully repaired due to compressive force introduced by pre-strained TiNi wires and resulting in the self-damage recovery effect.
TiNi/CFRP composites were fabricated by hot-pressing in the temperature range of 130-180 degree(s)C, by controlling the applied pressure. The TiNi wires were embedded as an 1mm interval into the center of CFRP layers and CFRP host materials were stacked as 0, 30, 60 and 90 degrees configuration on tensile direction, respectively. The stress-strain curve and tensile strength of composites strongly depends on stacking direction of carbon fibers. The tensile strength of TiNi/CFRP composites with stacking direction of 0 and 90 degrees configuration are about 1.2GPa and 50MPa, respectively. The microstructural properties of TiNi/CFRP composites were observed by SEM. Pore and/or voids were found to congregate near the embedded TiNi wire and they increased in proportion to stacking direction of carbon fibers. Larger pores and interfacial crack were also observed at interface between TiNi wires and epoxy resin. Furthermore, the fracture behavior was studied by an AE technique during tensile test, to analyze the fracture process. The effects of surface treatment of TiNi wire by acid etching to improve the interfacial bonding strength between TiNi wire and epoxy matrix are also investigated. The average interfacial bonding strength of the TiNi wire embedded in CFRP matrix was evaluated by pull out test. It was confirmed that surface treatment of TiNi wire by acid etching improved the interfacial bonding strength. Acid etching by HF+HNO3 mixed solution significantly increased the interfacial bonding strength. The damage recovery effect of SMA in specimen was successfully confirmed by heating above 70 degree(s)C.
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