Benjamin Reedlunn

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

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.

Shape memory alloy cables

Conventional structural cables (or wire ropes) are composed of steel wires helically wound into strands, which, in turn, are wound around a core. Cables made from shape memory alloy (SMA) wires are a new structural element with promising properties for a broad range of new applications. Among the many potential advantages of this form are increased bending flexibility for spooling/packaging, better fatigue performance, energy absorption and damping, reduced thermal lag, redundancy, and signicant design flexibility. Currently there are no known studies of SMA cables in the literature, so exploratory thermo-mechanical experiments were performed on two commercially available cable designs as part of an ongoing research program to systematically characterize their thermomechanical behavior and demonstrate their potential utility as adaptive or resilient tension elements.

Uniaxial, Pure Bending and Buckling Characterization of Superelastic NiTi Rods and Tubes: An Experimental Study

Although the tensile behavior of shape memory alloys (SMAs) is relatively well understood, the effects of other modes of loading on SMA behavior are still unknown. To examine the influence of loading and specimen geometry on structural behavior and phase transformation morphology, this work presents an experimental characterization of commercially-available superelastic NiTi rods and tubes under tensile, compressive, pure bending and column buckling loading. At room temperature and under isothermal loading conditions, the mechanical response for each loading configuration and structural geometry is measured and stereo digital image correlation is used to capture local displacement and strain phenomena (such as strain localization and propagating phase boundaries).It is found that the length and cross-sectional geometry of NiTi structures affects the nature of stress-induced phase transformation; in particular, transformation induced strain localization. During uniaxial tension, the rod specimens exhibit strain localization and propagating phase transformation fronts along well-defined load plateaus, and phase boundaries manifest as diffuse propagating necks. The tube specimens also exhibit localization/propagation phenomena, but fronts consist of distinct angled finger-like features due to the relatively thin-walled cross-section. During uniaxial compression, both forms exhibit (now well-known) tension-compression asymmetry in their mechanical responses, no propagating fronts are observed, and distinct load plateaus are absent. During pure bending, the moment-curvature responses of both forms exhibit plateaus during localizing forward and reverse transformations. In the case of the rod, high curvature localizes and propagates along the specimen; whereas, finger-like high strain regions develop along the tensile side of the tube. During column buckling, the structures are loaded into the post-buckling regime yet recover their straight forms upon unloading. Localization is observed, but only for high length/diameter aspect ratio tubes and has little affect on the mechanical response. Therefore, due to the varying phase transformation behaviors observed, understanding geometric and material effects on phase transformation, along with the resulting effects on the material response, is necessary to predict SMA beam behavior.Copyright

Pseudoelastic shape memory alloy cables

Conventional structural cables (wire ropes) are composed of steel wires helically wound into strands that are then wound around a core. Cables made from shape memory alloy (SMA) wires are a new structural element with promising properties for a broad range of applications. Among the many potential advantages of this form are increased bending flexibility for spooling/packaging, better fatigue performance, energy absorption and damping, reduced thermal lag, redundancy, and significant design flexibility. Currently there are few studies of SMA cables in the literature. This paper describes exploratory thermomechanical experiments that were performed on two commercially available cable designs.

Shape Memory Alloy Cables: Exploratory Experiments

Conventional structural cables (or wire ropes) are composed of steel wires helically wound into strands, which, in turn, are wound around a core. Cables made from NiTi shape memory alloy (SMA) wires are a new structural element with promising adaptive and enhanced structural properties for a broad range of applications. Potential advantages of this form include increased bending flexibility for spooling/packaging, better fatigue performance, energy absorption and damping, reduced thermal lag, redundancy, and significant design flexibility. Exploratory thermomechanical experiments were performed on a conventional cable construction: the right regular lay 7 × 7, consisting of 7 strands with 7 wires per strand. Uniaxial tension experiments characterize the cables’ sensitivity to strain rate, temperature, and lubrication. Experiments were also performed on individual strands and wires from the cable to study the hierarchical nature of the cable construction. Special attention was paid to the propagation of phase transformation fronts, similar to that seen previously in uniaxial tension of NiTi wire.© 2009 ASME

Tension-Torsion Experiments on Superelastic Shape Memory Alloy Tubes

Shape memory alloys have been studied extensively in pure tension, but careful studies of multi-axial behavior are far less common in the published literature. Here, we present room temperature tension-torsion experiments on superelastic NiTi tubes, using stereo digital image correlation (DIC) for the first time to measure the strain on the tube surface. DIC can accurately measure large strains, which permitted us to capture the mechanical response at the beginning and end of the phase transformation. These responses were then used to generate the first measurements of the saturation stress and strain surfaces in the published literature. The full field nature of DIC was also important in this work. The strain fields revealed propagating transformation fronts in pure tension, no propagating fronts in pure torsion, and a progression of behaviors in between, similar to the optical microscopy observations of Sun and Li [1]. By quantitatively measuring the strain fields with DIC, however, we found new features. In tension-dominated tests, the transformation front appeared as a near discontinuity in not only the axial strain field, but also the shear strain field. Also, the shear strain fields in pure torsion were not uniform. Although there were no propagating fronts, torsion caused vertical columns of shear strain to gradually appear and disappear during phase transformation.Copyright

Bending of Superelastic Shape Memory Alloy Tubes

Many shape memory alloy (SMA) applications exploit superelasticity in a bending mode, yet the large displacements and rotations associated with bending of slender structures make controlled experiments difficult. A custom pure bending fixture was built to perform experiments on superelastic NiTi tubes. To understand the bending results, the tubes were also characterized in uniaxial tension and compression, where a custom fixture was utilized to avoid buckling. In addition to measuring the global mechanical response, stereo digital image correlation (DIC) was used in all the experiments to capture the local surface displacement and strain fields. Consistent with the tension/compression data, our bending experiments showed a significant shift of the neutral axis towards the compression side. Also, the tube had strain localization on the tension side, but no such localization on the compression side. Detailed analysis of the strain distribution across the tube diameter revealed that the usual assumption of beam theory, that plane sections remain plane, did not hold along the tension side. Averaged over a few diameters of gage length, plane sections remain plane is a reasonable assumption and can be used to predict the global moment–curvature response. However, this assumption should be used with caution since it can under/over predict local strains by as much as 2× due to the localized deformation morphology.Copyright