Shape memory alloys (SMAs) are widely used in technological applications due to their unique properties. Recently, many investigations have been done to improve the performance of SMA wires as thermal actuators. Near-equiatomic Nickel-Titanium alloys are the most employed materials for this purpose because of their cycling stability and their large recoverable strain values. Consequently, a comprehensive evaluation of their actuation behavior and their fatigue resistance could be very useful for their optimal use. In doing so, an experimental rig able to perform this purpose was constructed. In particular, the changes in actuation response of the SMA wires were studied by varying the functioning parameters such as stress, strain, current and shape of the current pulse (step and ramp). The thermo-mechanical fatigue tests were carried out on SMA wires to study the changes of functional properties of the wires. During the fatigue test, the actuation behaviors of the wires were compared within different number of cycles (1st, 100th, 1000th and 7500th). Tensile tests in the pseudo-elastic regime were done on SMA wires before and after cycling in order to observe the noticeable changes in stress-strain curve. Changes in resistivity of the wires during the heating process were investigated and compared. The DSC scans were performed on SMA wires before and after cycling in order to observe the transformation temperature changes. Finally fracture surfaces were analyzed by scanning electron microscope.
Le leghe a memoria di forma (SMA), grazie alle loro uniche proprietà, sono ampiamente utilizzate in molte applicazioni tecnologiche. Recentemente, molti studi sono stati effettuati per migliorare le prestazioni di fili SMA utilizzati come attuatori termici. Le leghe quasi-equiatomiche di Nichel-Titanio sono i materiali più utilizzati a questo scopo per via della loro stabilità al ciclaggio e le loro capacità di recuperare grandi valori di deformazione. Di conseguenza, una valutazione completa della loro risposta quando attuati elettricamente e la loro resistenza a fatica potrebbe essere molto utile per il loro uso ottimale. Quindi, un apparato sperimentale in grado di effettuare tale tipologie di prove è stato progettato e costruito. In particolare, i cambiamenti nella risposta di fili SMA sono stati studiati variando i parametri di attuazione come stress, strain recuperato, intensità di corrente e la forma dell'impulso elettrico. Prove di fatica termo-meccanica sono state effettuate su fili SMA per studiare le variazioni delle loro proprietà funzionali. Durante le prova di fatica, i comportamenti dei fili soggetti ad attuazione tramite stimolo elettrico sono stati confrontati dopo un diverso numero di cicli (1, 100, 1000 e 7500 cicli). Prove di trazione e DSC prima e dopo il ciclaggio termomeccanico sono stati fatti su fili SMA per osservare i notevoli cambiamenti delle proprietà termiche e meccaniche indotti dal ciclaggio. Infine, variazioni resistività dei fili durante il processo di riscaldamento sono stati studiati e confrontati.
Thermo-mechanical cycling of thin Ni-Ti shape memory wires
SAGHAFI, FARSHAD
2013/2014
Abstract
Shape memory alloys (SMAs) are widely used in technological applications due to their unique properties. Recently, many investigations have been done to improve the performance of SMA wires as thermal actuators. Near-equiatomic Nickel-Titanium alloys are the most employed materials for this purpose because of their cycling stability and their large recoverable strain values. Consequently, a comprehensive evaluation of their actuation behavior and their fatigue resistance could be very useful for their optimal use. In doing so, an experimental rig able to perform this purpose was constructed. In particular, the changes in actuation response of the SMA wires were studied by varying the functioning parameters such as stress, strain, current and shape of the current pulse (step and ramp). The thermo-mechanical fatigue tests were carried out on SMA wires to study the changes of functional properties of the wires. During the fatigue test, the actuation behaviors of the wires were compared within different number of cycles (1st, 100th, 1000th and 7500th). Tensile tests in the pseudo-elastic regime were done on SMA wires before and after cycling in order to observe the noticeable changes in stress-strain curve. Changes in resistivity of the wires during the heating process were investigated and compared. The DSC scans were performed on SMA wires before and after cycling in order to observe the transformation temperature changes. Finally fracture surfaces were analyzed by scanning electron microscope.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/101809