• PDF / 1,331,081 Bytes
• 12 Pages / 593.972 x 792 pts Page_size
• 88 Downloads / 188 Views

DOWNLOAD

REPORT

---

RY alloys (SMAs) are slowly but steadily being viewed as a viable, solid-state replacement for hydraulic and pneumatic actuation systems. However, before serious inroads can be made, a better understanding of the macroscopic response of these unique materials, especially under constant stress and thermal cycling conditions, is necessary. To date, most of the research has focused on the isothermal, superelastic properties of SMAs, as these properties are useful in biomedical applications. However, the isobaric, strain-temperature response of the material is more pertinent when dealing with actuation-based applications. Although many SMAs have been studied to ascertain transformation SANTO PADULA II, RONALD NOEBE, and GLEN BIGELOW, Materials Research Engineers, are with the N.A.S.A. Glenn Research Center, Cleveland, OH 44135. Contact e-mail: [email protected] SHIPENG QIU formerly Graduate Student with the University of Central Florida, Orlanda, FL 32816, is now Process Engineer with Intel, Chandler, AZ 85226. DARRELL GAYDOSH, Senior Research Engineer, is with the Ohio Aerospace Institute, Cleveland, OH 44142. ANITA GARG, Senior Research Engineer, is with the University of Toledo, Toledo, OH 43606. RAJ VAIDYANATHAN, Associate Professor, is with the University of Central Florida. Manuscript submitted March 25, 2010. METALLURGICAL AND MATERIALS TRANSACTIONS A

temperatures and shape recovery under stress-free conditions, a much smaller subset has been investigated to determine transformation strain or work output characteristics under bias-stress conditions. Among the materials that have been studied for their actuation characteristics, much of the work has focused on the effect of applied stress on the observed performance. Various authors have shown that increases in the applied stress not only affect the transformation strain[1–6] but also affect the transformation temperatures according to the ClausiusClapeyron relationship.[7,8] However, it has also been reported that the history of the applied stress can affect the level of dimensional stability observed.[4,9] Although the magnitude of the applied stress is extremely important in determining the actuator-specific characteristics of a given material, this work explores the role of upper-cycle temperature (UCT), which is the highest temperature reached during the thermal excursion, on the load-biased thermal-cyclic response of an SMA.

II.

PROCEDURES

A. Material The material used in this study was a commercially available, binary NiTi alloy produced by Special Metals

(New Hartford, NY), with a fully annealed ingot As temperature of 368 K ± 5 K (95 °C ± 5 °C), henceforth designated as 55NiTi. In this case, 55 refers to the wt pct Ni in the alloy. In terms of at. pct, the stoichiometry of the alloy is Ni49.9Ti50.1, which is optimized for a high transformation temperature. The material was delivered as 10-mm-diameter rods in the hot-rolled/hot-drawn and hot-straightened condition. Differential scanning calorimetry measurements on the as-received material using a heat