Ryutaro Oishi

Shape memory alloys as strain sensors in composites

Composites of shape memory alloys (SMA) with a matrix have been widely studied to suppress propagating damage of the matrix, where the embedded SMA is used as an actuator. We use the embedded SMA wire in composites both as an actuator and as a strain sensor. The possibilities of using the SMA wire as a strain sensor to obtain an actuation trigger signal for damage suppression were investigated using the variation of the SMA electric resistance. We determined that the electric resistance variation of the SMA wire is correlated with the strain. The temperature dependence of the correlation, which can be reduced by a compensation method, is also discussed.

Thermomechanical characterization and development of SMA-embedded CFRP composites with self-damage control

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.

Fabrication of TiNi/CFRP smart composite using cold drawn TiNi wires

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.

Smart composite material system with sensor, actuator, and processor functions: a model of holding and releasing a ball

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.

Fabrication technique of SMA/CFRP smart composites

In recent years, pre-strained TiNi shape memory alloys (SMA) have been used for fabricating smart structure with carbon fibers reinforced plastics (CFRP). However, since the curing 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. We have developed a new method to control the transformation temperatures of SMA by proper thermo-mechanical treatments and composition adjustment, which is suitable to fabricate SMA/CFRP smart composite with a curing temperature of 130C. Furthermore, we tried to develop a new fabrication technique which is also suitable to fabricate SMA/CFRP smart composite with a curing temperature of 180C. It was found that by using cold drawn ultra-thin TiNi wires, TiNi/CFRP composites with a curing temperature of 180C could be fabricated without special fixture jigs. The damage suppression effect by embedded ultra-thin wires in the smart composite was confirmed.

Development and applications of TiNi-based smart structure

Smart structures that used the Ni-Ti shape memory alloy in order to develop mounting/dismounting equipment for the radiation protection clothes were produced experimentally. This is merely an instance. 2 kinds of test specimen of the high-temperature closing and high-temperature opening type were fabricated. The analytic solutions on reversible shape change and tightening force or pressing force for these models were deduced. It could be verified that the analysis method in this study was appropriate, because experimental value and analytic value agreed well. Therefore, it was clarified that the real equipment that it is elaborate and that there is the reliability could be made, if present analysis method is used.