V. Torra

Pseudoelastic fatigue of NiTi wires: frequency and size effects on damping capacity

The damping properties associated with hysteretic behavior of the pseudoelastic stress–strain (σ–e) curves of NiTi shape memory alloy (SMA) wires were studied. Damping was characterized for wires of 2.46 and 0.5 mm diameter using samples of 120 mm in length. The effect of the frequency and size of the wire on the σ–e curves were studied in the 3 × 10 − 5–3 Hz range, with 8% maximal strain. Damping associated parameters, such as hysteresis width, dissipated energy and specific damping capacity (SDC), defined as the ratio between the hysteretic energy and the maximum strain work over a complete pseudoelastic cycle, show maximum values at a specific frequency for each size diameter. These findings were explained in terms of the temperature effects associated to the heat of transformation. Results show that NiTi wire of 0.5 mm diameter has the highest SDC when cycling around 0.1 Hz while wire of 2.46 mm diameter has the highest SDC at 0.01 Hz. At 1 Hz, the SDC for 0.5 mm diameter wire is around twice that of 2.46 mm diameter wire.

Damping in civil engineering using SMA Part 2 – particular properties of NiTi for damping of stayed cables in bridges

Abstract Shape memory alloys (SMAs) show particular properties associated to their martensitic transformation between metastable phases. Their use requires a deep knowledge of the SMA behaviour according to the requirements of the application. In this paper, the following are studied: the required fatigue life of the NiTi SMA that easily overcomes 1 million of oscillations for a complete storm; the suitable thermomechanical treatment to minimise or stabilise the SMA creep; and the temperature effects on the damper and the dynamic phenomena effects modifying the hysteretic energy via the self-heating associated to the release and the absorption of the latent heat were studied. The study includes the effects on SMA of the external summer–winter temperatures. La transformation martensitique entre phases métastables est à l’origine des propriétés particulières des Alliages à Mémoire de Forme (AMF). L’utilisation des AMF nécessite une connaissance approfondie de leur comportement associé aux exigences de l’application. Dans cet article, on étudie: (a) la durée de vie requise de l’AMF NiTi, qui peut facilement avoir à surmonter 1 million d’oscillations lors d’une tempête entière. (b) Le traitement thermomécanique approprié afin de minimiser ou stabiliser le fluage de l’AMF. (c) Les effets de température sur l’amortisseur ainsi que des phénomènes dynamiques modifiant l’hystérésis de l’énergie par l’auto-échauffement associé au dégagement et à l’absorption de la chaleur latente. On étudie également l’effet des températures extérieures de l’été et de l’hiver sur l’AMF.

On the partial recovery of residual strain accumulated during an interrupted cyclic loading of NiTi shape memory alloys

It is well known that the behaviour of shape memory alloys (SMAs) evolves when subjected to cyclic loading. For instance, during superelastic cycling of an SMA, an increasing residual strain is observed, mainly during the first cycles. This paper first reports that (i)?this residual strain, believed to be irreversible, can be partially recovered and (ii)?a stress increase is observed after a pause during cyclic loading. Also, it suggests an explanation of these experimental observations based on the endo and exothermic phase transformations of the alloy, the heat exchange by convection around the sample and the Clausius?Clapeyron relation characterizing the SMA.

Effects of strain aging at 373 K in wires of NiTi shape memory alloy

Abstract Strain and temperature aging for a NiTi shape memory alloy is performed at 373 K, using wires of 2·46 mm and eventually 0·5 mm in diameter. Aging under large strains modifies the hysteresis shape and induces an increase in the maximal stress in the transformation from 600 to 800–1000 MPa (wires of 2·46 mm). On applying a strain up to 7–8%, the effect according to the initial state is close to 50 MPa per month of strain–temperature aging. The results enable efficient NiTi damper application from 233 to 323 K. A partial study of wires of 0·5 mm shows quantitatively different effects.

SMA Passive Elements for Damping in the Stayed Cables of Bridges

In this paper, the properties of shape memory alloy (SMA) wires relevant for their application as passive elements to mitigate the vibrations of stayed cables in bridges are discussed. These properties are investigated by carrying out experimental tests on samples of diameter 2.46 mm. Future research will address the scalability issues.