Thermomechanical Behavior of Shape Memory Alloy Cables and Tubes.

Shape memory alloys (SMAs) are well known for their substantial power density in shape memory mode and their large recoverable strains in superelastic mode. NiTi, the most popular SMA, has been studied extensively in pure tension, but studies of more complex structural forms and other deformation modes are rare in the published literature. Therefore, the first purpose of this research was to characterize and understand the superelastic thermomechanical behavior of one such structural form, cables (or wire ropes). The second purpose was to understand the superelastic tension, compression, and bending behavior of cylindrical NiTi tubes. Cables made from SMA wires are relatively new and unexplored structural elements that combine many of the advantages of conventional cables with the unique properties of SMAs, leading to a number of potential applications. An extensive set of uniaxial tension experiments were performed on two SMA cable constructions, a 7x7 right regular lay, and a 1x27 alternating lay, to characterize their superelastic behavior in room temperature air. Details of the evolution of strain and temperature fields were captured by simultaneous stereo digital image correlation (DIC) and infrared imaging, respectively. Different aspects of the SMA cable responses were considered. First, the nearly isothermal, yet quite different, superelastic responses of the two cable designs were examined. Second, selected subcomponents excised from the two cable constructions were studied to determine the individual contributions of the cables hierarchical construction. Third, the elongation rate sensitivity of the cables and their subcomponents were inspected to compare and quantify their thermomechanical coupling. The tube experiments in the second part of this research should serve to calibrate and validate material models used to simulate SMA cables in the future. Tubes were studied instead of wires to avoid experimental difficulties, but even using tubes, custom built fixtures were required to avoid buckling during uniaxial compression and to avoid axial loads during large-rotation bending. Stereo DIC measurements during the tube experiments revealed that the material instability, which leads to propagating transformation fronts in pure tension, also leads to highly heterogeneous strain fields during bending.Ph.D.Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91514/1/breedlun_1.pd

A review of macroscopic ductile failure criteria.

The objective of this work was to describe several of the ductile failure criteria com- monly used to solve practical problems. The following failure models were considered: equivalent plastic strain, equivalent plastic strain in tension, maximum shear, Mohr- Coulomb, Wellman's tearing parameter, Johnson-Cook and BCJ MEM. The document presents the main characteristics of each failure model as well as sample failure predic- tions for simple proportional loading stress histories in three dimensions and in plane stress. Plasticity calculations prior to failure were conducted with a simple, linear hardening, J2 plasticity model. The resulting failure envelopes were plotted in prin- cipal stress space and plastic strain space, where the dependence on stress triaxiality and Lode angle are clearly visible. This information may help analysts select a ductile fracture model for a practical problem and help interpret analysis results

A review of macroscopic ductile failure criteria.

The objective of this work was to describe several of the ductile failure criteria com- monly used to solve practical problems. The following failure models were considered: equivalent plastic strain, equivalent plastic strain in tension, maximum shear, Mohr- Coulomb, Wellman's tearing parameter, Johnson-Cook and BCJ MEM. The document presents the main characteristics of each failure model as well as sample failure predic- tions for simple proportional loading stress histories in three dimensions and in plane stress. Plasticity calculations prior to failure were conducted with a simple, linear hardening, J2 plasticity model. The resulting failure envelopes were plotted in prin- cipal stress space and plastic strain space, where the dependence on stress triaxiality and Lode angle are clearly visible. This information may help analysts select a ductile fracture model for a practical problem and help interpret analysis results

Tension, Compression, and Bending of Superelastic Shape Memory Alloy Tubes

<p>While many uniaxial tension experiments of shape memory alloys (SMAs) have been published in the literature, relatively few experimental studies address their behavior in compression or bending, despite the prevalence of this latter deformation mode in applications. In this study, superelastic NiTi tubes from a single lot of material were characterized in tension, compression, and pure bending, which allowed us to make direct comparisons between the deformation modes for the first time. Custom built fixtures were used to overcome some long-standing experimental difficulties with performing well-controlled loading and accurate measurements during uniaxial compression (avoiding buckling) and large-rotation bending. In all experiments, the isothermal, global, mechanical responses were measured, and stereo digital image correlation (DIC) was used to measure the evolution of the strain fields on the tube’s outer surface.</p> <p>As is characteristic of textured NiTi, our tubes exhibited significant tension-compression asymmetry in their uniaxial responses. Stress-induced transformations in tension exhibited flat force plateaus accompanied by strain localization and propagation. No such localization, however, was observed in compression, and the stress “plateaus” during compression always maintained a positive tangent modulus. While our uniaxial results are similar to the observations of previous researchers, the DIC strain measurements provided details of localized strain behavior with more clarity and allowed more quantitative measurements to be made. Consistent with the tension-compression asymmetry, our bending experiments showed a significant shift of the neutral axis towards the compression side. Furthermore, the tube exhibited strain localization on the tension side, but no localization on the compression side during bending. This is a new observation that has not been explored before. Detailed analysis of the strain distribution across the tube diameter revealed that the traditional assumption of elementary beam theory, that plane sections remain plane, does not hold. Yet when the strain was averaged over a few diameters of axial length, the tensile and compressive responses input into elementary beam theory predicted the global bending response with reasonable accuracy. While it is encouraging that a simple model could predict the moment-curvature response, we recommend that beam theory be used with caution. The averaged strain field can under/over predict local strains by as much as two-fold due to the localized deformation morphology.</p

Compaction of crushed salt for safe containment: Overview of phase 2 of the KOMPASS project

The KOMPASS project strives to improve the scientific basis behind using crushed salt for long-term isolation of high-level nuclear waste within rock salt repositories. Efforts to improve the prediction of crushed salt compaction began during the first phase of the KOMPASS project (KOMPASS-I, 2020). The second project phase (KOMPASS-II) just started in 2021. Its aim is foremost to quantify the effect of isolated experimental influencing factors on the compaction. Such influencing factors are for instance temperature, moisture or the chosen stress path. Used methods are laboratory tests, microstructural investigations and numerical simulations

Compaction of crushed salt for safe containment: Overview of phase 2 of the KOMPASS project

The KOMPASS project strives to improve the scientific basis behind using crushed salt for long-term isolation of high-level nuclear waste within rock salt repositories. Efforts to improve the prediction of crushed salt compaction began during the first phase of the KOMPASS project (KOMPASS-I, 2020). The second project phase (KOMPASS-II) just started in 2021. Its aim is foremost to quantify the effect of isolated experimental influencing factors on the compaction. Such influencing factors are for instance temperature, moisture or the chosen stress path. Used methods are laboratory tests, microstructural investigations and numerical simulations