Yongjun He

Two-dimensional analysis to improve the output stress in ferromagnetic shape memory alloys

Most existing experiments investigating the martensite-variants reorientation (switching) of ferromagnetic shape memory alloys (FSMA) are in a simple 1D condition: An axial compressive stress and a transverse magnetic field. To obtain field-induced variant switching, however, the compressive stress (output stress) is limited by a small blocking stress (<10 MPa, mainly governed by the materials’ magnetic anisotropic energy). In this paper, to overcome the stress limit, we suggest using the materials in two-dimensional (2D) configurations: Two (axial and transverse) compressive stresses and a magnetic field. Based on a 2D magneto-mechanical energy analysis, it is found that only the difference between the two stresses is limited; each of the two stresses can be larger than the blocking stress. The energy analysis is also incorporated into the field-stress phase diagrams (including hysteretic effect) to study the variant switching in different loading paths: rotating/non-rotating field-induced strain and fie...

Reversible-strain criteria of ferromagnetic shape memory alloys under cyclic 3D magneto-mechanical loadings

Recent researches revealed that ferromagnetic shape memory alloys (FSMA) in 2D/3D configurations (with multi-axial stresses) had much more advantages (e.g., higher working stress and more application flexibility) than that in 1D configuration (with uniaxial stress). In literature, however, there is no simple criterion to judge whether a cyclic 3D magneto-mechanical loading can induce a large reversible strain (via martensite reorientation in FSMA). In this paper, a 3D magneto-mechanical energy analysis is proposed and incorporated into a phase diagram in terms of deviatoric stresses (including mechanical and magneto-stresses) to study the path-dependent (hysteretic) martensite reorientation in FSMA under 3D cyclic loadings. Based on the phase diagram (a plane graph), general criteria for obtaining reversible strain under cyclic magneto-mechanical loadings are derived, which provide basic guidelines for FSMA’s applications under multi-axial loadings. Particularly for FSMA actuators driven by cyclic magneti...

Energy Dissipation of Martensite Reorientation in NiMnGa Single Crystals under Biaxial Compression

Experiments of biaxial compression were conducted to study the energy dissipation (stress-hysteresis) of martensite reorientation in NiMnGa single crystals. From the experiments, the observed stress-hysteresis consists of two parts ― the material intrinsic friction due to martensite reorientation, and the external friction between the loading clampers and the sample surfaces. It is found that the former one is independent of the 2D stress state while the latter one strongly depends on the 2D stress levels. Both kinds of friction are important and need to be considered in real applications.

Simulation of bacterial flagellar phase transition by non-convex and non-local continuum modeling

Abstract Bacterial flagellar filament can undergo a stress-induced polymorphic phase transition in both vitro and vivo environments. The filament has 12 different helical forms (phases) characterized by different pitch lengths and helix radii. When subjected to the frictional force of flowing fluid, the filament changes between a left-handed normal phase and a right-handed semi-coiled phase via phase nucleation and growth. This paper develops non-local finite element method (FEM) to simulate the phase transition under a displacement-controlled loading condition (controlled helix-twist). The FEM formulation is based on the Ginzburg-Landau theory using a one-dimensional non-convex and non-local continuum model. To describe the processes of the phase nucleation and growth, viscosity-type kinetics is also used. The non-local FEM simulation captures the main features of the phase transition: two-phase coexistence with an interface of finite thickness, phase nucleation and phase growth with interface propagation. The non-local FEM model provides a tool to study the effects of the interfacial energy/thickness and loading conditions on the phase transition.

3D Energy Analysis of Magnetic-Field Induced Martensite Reorientation in Magnetic Shape Memory Alloys

This paper explains the magnetic-field induced martensite reorientation in Ferromagnetic Shape Memory Alloys (FSMA) through a simple energy analysis from which the role of the martensite’s magnetic anisotropy is emphasized. In particularly, with a three-dimensional (3D) energy analysis, we study the switching between the three tetragonal martensite variants driven by a rotating magnetic field (with a constant magnitude) and a non-rotating magnetic field (with a fixed direction but varying magnitudes). Finally, a simple planar phase diagram is proposed to describe the martensite reorientation in general 3D loadings.