Papers by Author: Qing Ping Sun

Abstract: Superelastic NiTi polycrystalline tubes, when subjected to quasi-static stretching, transform from an initial austenite phase to a high-strain martensite phase by the formation and growth of a macroscopic self-organized helical domain as deformation progresses. This paper performed an experimental study on the effects of the externally applied stretching and tube geometry (length L, wall-thickness h and tube radius R) on the martensitic helical domains in the tubes under very slow (isothermal) stretching. The evolution of the helical domains with the applied strain in different tube geometries are quantified by in-situ optical measurement. We demonstrate that the shape of the self-organized helical domain and its evolution are governed by the competition between bending strain energy and domain front energy in minimizing the total energy of the tube system. The former favors a long slim helical domain, while the latter favors a short fat helical domain. The experimental results provide a strong support to the recently developed theoretical relationship.

Abstract: High damping capacity is one of the prominent properties of NiTi shape memory alloy (SMA), having applications in many engineering devices to reduce unwanted vibrations. Recent experiments demonstrated that, the hysteresis loop of the stress-strain curve of a NiTi strip/wire under a tensile loading-unloading cycle changed non-monotonically with the loading rate, i.e., a maximum damping capacity was obtained at an intermediate strain rate (ε^._critical). This rate dependence is due to the coupling between the temperature dependence of material’s transformation stresses, latent-heat release/absorption in the forward/reverse phase transition and the associated heat exchange between the specimen and the environment. In this paper, a simple analytical model was developed to quantify these thermo-mechanical coupling effects on the damping capacity of the NiTi strips/wires under the tensile loading-unloading cycle. We found that, besides the material thermal/mechanical properties and specimen geometry, environmental condition also affects the damping capacity; and the critical strain rate ε^._critical for achieving a maximum damping capacity can be changed by varying the environmental condition. The theoretical predictions agree quantitatively with the experiments.

Abstract: Recent experiment [1] revealed many new characteristics of the domain patterns in a superelastic polycrystalline NiTi tube during tensile loading, such as domain wall instability and branching, dynamic topology transition of domain patterns. In this paper, we use the continuum mechanics approach and model the polycrystal as a phase-transformable continuum described by non-local nonlinear elasticity [2]. We simulate the equilibrium macroscopic domain patterns and their evolution in the tubes under tensile loading by the nonlocal Finite Element Method (FEM). It is revealed that the loading path dependence and dynamic topology transition of domain patterns are mainly due to thermodynamic metastability of the tube system. Our simulations capture all the key features of the domain patterns observed in the NiTi polycrystalline tubes.

Abstract: The experimental results on morphology evolution of macroscopic deformation domain during stress-induced phase transition under uniaxial tension in superelastic NiTi polycrystalline microtubes are reported in this paper. It focuses on morphology of the domain front during unloading process. A series of high-resolution digital photos were taken from the tube surface under equal incremental elongations. The 3D geometry of the front was quantified by an optical profiler. New propagation modes of the front such as rotation and crossing were found.

Abstract: The spherical indentation obeys Hertz contact theory when the applied load is within the elastic limit. Once the applied load is over the elastic limit, the indentation curve starts to deviate from the original purely elastic indentation curve. This deviation point, which indicates the start of the nonlinear deformation, is an important characteristic of a spherical indentation curve. The indentation force corresponding to the deviation point is related to a basic material constant, which is the yield stress for an elastic-plastic material or the transformation stress for a shape memory alloy. This relationship can be applied to measure the yield stress or the transformation stress from a simple spherical indentation curve. Detailed discussion on the relationship and the method is presented in this short paper.

Abstract: Recent uniaxial tension tests have shown that stress-induced phase transformation in NiTi SMAs tubes can lead to helical-type localized deformation and propagation phenomena. Based on detailed experimental observation and possible deformation mechanism, a trilinear stress-strain relationship with intrinsic strain softening is employed to represent the material constitutive behavior in this paper, and a 3-D finite deformation simulation is performed to model the tube under tension by using nonlinear FEM. The simulations successfully reproduce the nucleation and evolution of the helical-type martensite band during stress-induced transformation observed in the experiments.

Abstract: Spherical indentation of superelastic shape memory alloys (SMAs) has been theoretically analyzed. Two characteristic points on the superelastic indentation curve have been discovered. The bifurcation force corresponding to the bifurcation point relies on the forward transformation stress and the return force corresponding to the return point relies on the reverse transformation stress. Based on these theoretical relationships, an approach to determine the transformation stresses of superelastic SMAs has been proposed. To improve the accuracy of the measurement, a slope method to locate the two characteristic points from the slope curves is further suggested. Additionally, the spherical indentation hardness was also analyzed.