Tips and Tricks for Characterizing Shape Memory Wire Part 5: Full-Field Strain Measurement by Digital Image Correlation

Abstract

This is the fifth paper in a series on the experimental characterization of shape memory alloy (SMA) wires. In this installment we focus on the use of digital image correlation (DIC) to measure the strain field on the surface of the wire. After a brief overview of the principles and mathematics behind DIC, two different thermo‐mechanical tension tests using DIC are presented to demonstrate the technique. The first experiment consists of Joule heating a shape memory (SM) wire to induce the shape memory effect, using 2‐D DIC to measure the strain field. The second experiment measures the response of a superelastic (SE) wire to mechanical cycling at room temperature, using 3‐D DIC to measure the strain field and an infrared camera to measure the temperature field. In addition to describing the experimental results, attention is paid to specimen preparation and the two experimental setups. Many of the challenges and precautions associated with using DIC are discussed, along with practical recommendations for specimen speckle patterns, digital photography, and data post processing.

Local Mechanical Response of Superelastic NiTi Shape-Memory Alloy Under Uniaxial Loading

“…5d, several declining transformation boundaries, which are also referred to as Lüders deformation bands (LDBs), pass through the width of the sample. In light of the in situ EBSD studies of Mao et al [21,22], it is pointed out that LDB does not purely keep to the principal of the maximum shear stress, but is possibly a result of the interaction between the mechanics and the martensite crystallography. In the present experiments, the declining angle of the LDB ranges from 57°to 62°, which agrees with the detailed studies indicating that shear angle of the LDB varies from 48°to 61° [23][24][25].…”

“…5a, 6a, 7a, and 8a. Choosing three material [21], and Bewerse et al [22], the sharp transition between high-strain and low-strain regions is attributed to transformation fronts associated with the local martensite transformation, and thus regions undergoing different magnitudes of local strain (large vs. small) are corresponding to regions with different phases (martensite vs. austenite). As is demonstrated in Fig.…”

Spatio-temporal characteristics of plastic instability in AA5182-O during biaxial deformation

“…The second consisted of black spray paint droplets applied such that each droplet was no more than $10% of the chosen square pixel subset area in the DIC post-processing step. This random contrast pattern facilitated post-processing of displacements, strains, and strain rates with the DIC Vic-3D software from Correlated Solutions, Inc. Two-5 megapixel Grasshopper cameras were positioned above the clamped sheet specimen prior to application of the fluid pressure and subsequently calibrated [27]. Up to 700-4.8 megabyte.tiff images of the specimen surface recorded its deformation from the start of the test to a maximum hydraulic pressure of 8.8 MPa, which was 0.5 MPa below the pre-determined burst pressure for AA5182-O (9.3 MPa).…”

Experimental Validation of a Shape Memory Alloy Constitutive Model by Heterogeneous Thermal Loading Using Infrared Thermography and Digital Image Correlation

“…The change in temperature of an SMA actuator device is instigated by a source of heat, which is often applied such that the temperature fields are not spatially uniform. Additionally, while SMA wires and some ASME standard material specimen shapes have been tested extensively [1], [2], the behavior of non-standard SMA material specimen shapes should also be considered, for two reasons. First, because there are many configurations of SMA actuator devices which exhibit complicated geometries [3].…”

“…DIC is a widely used method [1], [4][5][6] for non-uniform strain measurement. Using DIC, the strain at any point on the material specimen may be measured.…”