Kazuhiro Otsuka

Damping properties of Ti50Ni50-xCux alloys utilizing martensitic transformation

Abstract The internal friction (IF) and elastic modulus of rigidity were investigated for Ti50Ni50−xCux shape memory alloys for the composition range of 8≦x≦20 by utilizing a low frequency inverted torsion pendulum method. The observed extraordinarily large internal friction and large softening in rigidity are discussed, paying attention to low twinning shears of B19 (orthorhombic) martensite. The attained large internal friction Q−1=0.2 for Ti50Ni30Cu20 at peak temperature, and the shoulder temperature of IF indicate that the alloy system is a good damping material which can be used at temperatures higher than room temperature.

Additive nature of recovery strains in heavily cold-worked shape memory alloys

Abstract The effects of cold-working and subsequent low temperature heat-treatment on transformation temperatures and recovery strains were studied for heavily cold-drawn Ti–Ni. Consequently, the reverse transformation temperatures were found to increase further by the heat-treatment. The reason for the effect and how to use it in practical applications are fully explained.

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.

Premartensitic attenuation in Ti–30Ni–20Cu shape memory alloy

Abstract Rectangular parallelepiped resonance (RPR) method has been mostly used to measure elastic constants of crystals. Here we show that this method can also yield useful information about the attenuation of samples. We measured the change in the height of resonance peak with temperature for a Ti–30Ni–20Cu shape memory alloy prior to its transformation temperature. From this measurement we found that attenuation increases with approaching transformation temperature. This is consistent with the result obtained from pulse-echo method and strongly suggests there exists premartensitic attenuation well above M s temperature. We further show that the premartensitic attenuation increase just corresponds to a softening in zone boundary TA2 phonon, as well as an increase in diffuse scattering intensity at Brillouin zone boundary. All these results are consistent with the existence of hetero-phase fluctuation prior to martensitic transformation.

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.

Origin of As temperature increase by predeformation in Ti-Ni alloys

Ti-Ni-Nb shape memory alloys show a wide transformation temperature hysteresis, which is caused by the As increase due to predeformation. We confirmed the same effect in Ti-Ni alloys as well. To find out the origin for the As increase by predeformation, the systematical investigation was made by tensile tests, electrical resistance measurements, optical microscopy and transmission electron microscopy (TEM), for Ti50Ni50 and Ti49.5Ni50.5 alloys. The As increase was found to be closely related with the introduction of the permanent strain in polycrystals. We propose a new mechanism for the effect, by focusing attention on the elastic strain energy stored during the thermoelastic martensitic transformation, which can interpret the observed effect well.