The study investigating the electrical properties of dense lead-free piezoelectric ceramics in the (1-x)Sr2NaNb5O15-xCa2NaNb5O15 (SCNN) system with x ranging from 0.05 to 0.35 is reported here. Materials were produced with Spark Plasma Sintering (SPS) method. X-Ray diffraction patterns all showed a single tetragonal tungsten bronze phase, peak broadening being seen on increasing the Ca content. The lattice constant calculation indicated distortion and shrinkage of the crystal structure with Ca substitution. Ferroelectric and piezoelectric properties of SCNN greatly depended on the Ca content. The composition with x =0.15 exhibited the greatest polarization with Pr =3.0 μC cm-2and piezoelectric constant of d33 =96 pC/ N, whereas the compositions with x =0.30 and 0.35 almost lost their ferroelectricity and piezoelectricity. Not only investigation of materials properties but also application possibility as a sensor is surveyed. The ultrasonic waves generated by SCNN material as a sensor are much clearer that that by commercially available PZT materials. This study apparently indicates that lead-free SCNN piezoelectric ceramics have potential for electromechanical application.
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.
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.
Conference Committee Involvement (3)
Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems
12 March 2012 | San Diego, California, United States
Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems
7 March 2011 | San Diego, California, United States
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