The response of NiMnGa ferromagnetic shape memory alloy to static and dynamic magnetic fields was studied. Tests involving excitation of the samples up to 10 Hz for constant stress and constant strain conditions were conducted. Based on these results, performance parameters were measured and discussed including power density, total power output and electromechanical efficiency. The effects of strain rate and material damping were also measured. It was shown that both power density and total power output were strong functions of applied stress. A maximum volumetric power density of 31 MW/m3 was measured. Once the NiMnGa behavior was characterized, an analytical model based on four experimentally measured parameters was formulated to predict the induced strain in response to a dynamic magnetic field. Comparison of the analytical model to experimental data showed good correlation for applied stresses below 0.6 MPa and above 1.33 MPa. Although requiring further refinement, the model's results are encouraging, indicating that it could be developed into a useful analytical tool for predicting NiMnGa actuator behavior.
A quasi-static model for NiMnGa magnetic shape memory alloy (MSMA) is formulated in parallel to the Brinson and Tanaka thermal SMA constitutive models. Since the shape memory effect (SME) and pseudoelasticity exist in both NiTi and NiMnGa, constitutive models for SMAs can serve as a basis for MSMA behavioral modeling. The quasi-static model for NiMnGa was characterized by nine material parameters identified by conducting a series of uniaxial compression tests in a constant field environment. These model parameters include free strain, Young’s modulus, fundamental critical stresses, fundamental threshold fields, and stress-influence coefficients. The Young’s moduli of the material in both its field and stress preferred configurations were determined to be 450 MPa and 820 MPa respectively, while the free strain was measured to be 5.8%. These test data were used to assemble a critical stress profile that is useful for determining model parameters and for understanding the dependence of critical stresses on magnetic fields. Once implemented, the analytical model shows good correlation with test data for all modes of NiMnGa quasi-static behavior, capturing both the magnetic shape memory effect and pseudoelasticity. Furthermore, the model is also capable of predicting partial pseudoelasticity, minor hysteretic loops and stress-strain behaviors. To correct for the effects of magnetic saturation, a series of stress influence functions were developed from the critical stress profile. Although requiring further refinement, the model’s results are encouraging, indicating that the model is a useful analytical tool for predicting NiMnGa actuator behavior.
A quasi-static model for NiMnGa magnetic shape memory alloy (MSMA) is formulated on the basis of NiTi SMA constitutive models such as the Brinson model, because of the similarities that exist in the behavior of both materials. NiMnGa shows a magnetically induced shape memory effect as well as a pseudoelastic behavior. Quasi-static tests at constant applied magnetic field and stress were conducted to identify the model parameters. The material parameters include free strain, Young's modulus, critical threshold fields and stress-influence coefficients. The Young's moduli of the material in its field preferred and stress preferred states were determined to be 450 MPa and 820 MPa respectively. Critical threshold fields as a function of stress were determined from constant stress testing. These test data were used to assemble a critical stress-temperature profile that is useful in predicting the various states of the material for a wide range of magnetic or mechanical loading conditions. Although the constant applied field and constant stress data have yet to be fully correlated, the model parameters identified from the experiments were used to implement an initial version of the quasi-static model. The model shows good correlation with test data and captures both the magnetic shape memory effect and pseudoelasticity. This introductory model provides a sound basis for further refinements of a quasi-static NiMnGa model.
A comprehensive, experimental characterization of single crystal, NiMnGa rods is the current subject of study. Static tests aimed at characterizing the force-deflection behavior show that the actuator has two stiffnesses, a low stiffness associated with the region of twin boundary motion and a high stiffness when the material is in the fully twinned condition. For a 2×3×16 mm specimen, a block force of 4.35 N and a free strain of 5.02% were determined experimentally. Dynamic test of the material exposed to a 0.85 T peak, AC, inductive field show that the strain and force generated by the actuator consist of mean and dynamic components. For peak performance, the recovery force acting on the NiMnGa actuator must be optimized. Experimental results indicate that NiMnGa has a maximum volumetric energy density of 12.15 kJ/m3 and a maximum weight defined energy density of 1.45 J/kg. Using energy density by weight as a metric, NiMnGa has an energy output on the same order of magnitude as commercial piezostacks.
A comprehensive experimental characterization of single crystal, martensite, NiMnGa rods is the current subject of study. Preliminary actuator force-displacement curves for DC magnetic fields up to 0.6 T in the static regime have been developed. A peak static actuation force of 5.5N at 0.4 mm in was observed. The effects AC magnetic fields below magnetic saturation were also examined. The dynamic force-displacement characteristics of the NiMnGa alloy in the presence of mechanical preloads was studied. Dynamic testing at AC fields of 0.3 T yielded strains of up to 1.2% and dynamic loads of up to 7 N. A possible material dependence on strain-rate and was observed from the experimental dynamic stress-strain curves.
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