Characterization of a Nickel-Titanium (55.3 Ni wt%) shape memory alloy wire

posted on 08.07.2019 by Philippe Hannequart, Karim Benzarti, Jean-François Caron, Michael Peigney

Characterization of a Nickel-Titanium (55.3 Ni wt%) shape memory alloy (SMA) wire supplied by Memry (USA).

Diameter : 0.50mm

Data given by the supplier : Nitinol wire, Hard black oxide, thermally straightened, As = 53°C

Differential Scanning Calorimetry (DSC) test (as received) : a virgin sample of NiTi wire (7.9mg) is submitted to an initial heating up to 250°C, followed by 2 cooling-heating cycles (250°C/-90°C/+250°C). Test speed : 10°C per minute. The DSC apparatus is a DSC 250 (TA Instruments) with nitrogen gas flow. The test files for the software Trios are supplied as well as the .csv data.

Differential Scanning Calorimetry (DSC) test (50 cycles) : the NiTi wire maintained in a water bath at a temperature of 70°C is submitted to 50 mechanical cycles of loading up to 90N and unloading down to 0.5N (superelastic cycles). A sample of this wire (8.1mg) is submitted to a DSC with an initial heating up to 250°C, followed by 2 cooling-heating cycles (250°C/-90°C/+250°C). Test speed : 10°C per minute.

Isothermal traction at 25 degrees : a specific testing apparatus (illustration in the file Apparatus.jpg) has been set up. The SMA wire is tested in a box containing water, a water pumping/heating/cooling apparatus enables to control the water temperature in order to precisely control the SMA wire temperature. One side of the box is made of transparent PMMA in order to monitor the wire strains by Digital Image Correlation (DIC). The tested part of the wire finds itself in water, and its upper end is clamped in the cylinder jaws of a displacement controlled ADAMEL DY31 testing machine. The water temperature is monitored by two type K thermocouples at two different locations inside the water volume. The load in the wire is registered using a 100N load cell and the deformations of the wire were measured by DIC on images acquired by a Pike F421B camera (Allied Vision Technologies). A Matlab routine has been set up to acquire simultaneously the deformation images with the load values. A virgin wire of the same spool has been submitted to axial traction in 25°C water with a speed of 10^-4 s^-1. The first traction test presents a strain localization along the wire. We were not interested in this phenomenon and only present here the 2nd, 3rd and 4th test which show a homogeneous strain along the observed zone of the wire. The strain has been measured with the software GOM Correlate, by computing the mean value of the axial strain on a rectangular zone of several centimeters on the wire. The GOM Correlate files are supplied, as well as the .csv values. Between each test, the wire has been extracted from the testing chamber, placed in a furnace at 100°C during several minutes and then placed in a deep freezer at -20°C during several minutes in order to always test a martensitic wire.

Thermal cycling at 25°C : With the same apparatus, a load of 25N has been applied on the wire at 85°C and then maintained constant with the help of a PID controller in the TestWorks software. The wire being maintained at this constant load, the water has been cooled down to 15°C and heated up to 85°C. The strain field could not be measured accurately, and for this reason the strain named Axial Strain 1, Axial Strain 2 and Axial Strain 3 are computed with virtual gauges in GOM Correlate (only between two points). The strain may be heterogeneous along the wire.