题目：Characterization of Magnetic Field Induced Shear and Normal Strains in Ni-Mn-Ga Shape Memory Single Crystals
Ferromagnetic Shape Memory Alloys (FSMA) is one of the new adaptive materials, and its most common composition is Nickel-Manganese-Gallium (Ni-Mn-Ga) in single crystal form. It has been reported that under magnetic field FSMA produces axial strain of up to 10% but low output stresses of a few MPa. These are different from the properties of PZT, which has an actuated strain of the order of ~1% and an actuated stress of the order of 100 MPa. In addition, unlike traditional shape memory alloys which has a low bandwidth of a couple Hz, FSMA material can be actuated in bandwidth of the order of kHz. FSMA is also capable of remote actuation by application of magnetic field. Another unique characteristic is its ability to hold shape (to a certain extent) after actuation once the magnetic field is removed. These unique properties offered by FSMA may offer a competitive edge over other adaptive materials in applications. However it should be noted that, for the Ni-Mn-Ga single crystal orientation presented, there is currently only one axial or longitudinal strain data available in the literature. This paper presents a novel experimental apparatus and a test method for measuring quasi-static uni-axial and shear magnetic-field-induced strain (MFIS) of magnetic shape memory (MSM) Ni-Mn-Ga single crystals. The apparatus consists of a casing, a weight-controlled plunger, two displacement probes (one vertical and one lateral), torsion guide etc. Three Ni-Mn-Ga square prism (3mm by 3mm by 18mm and of composition Ni49.5Ga29.2Mn21.3) samples were purchased and tested. Twin boundary bands were clearly visible in the image after applying magnetic field. A range of material properties were measured repetitively 4 to 5 times for characterizing the three samples under a maximum magnetic flux density 0.6 tesla. These include: (a) the magnetic anisotropy constant; (b) the single variant relative magnetisation versus magnetic field curves for easy and hard axes; (c) the highly nonlinear compressive stress-strain curves for all three samples at room temperature; and (c) the strain versus magnetic field curves (one for longitudinal strain and the other for shear strain) under different compressive stresses (of 0.17, 0.43, 0.68, 0.94, 1.2, 1.46, 1.74 and 2.0MPa). The test results show that: (a) 6.5% longitudinal strain and 7% shear strain can be achieved at low stresses 0.17 to 0.68MPa; (b) the maximum longitudinal and shear strains decrease remarkably as the stress changes from 0.94 to 1.74 MPa; and (c) the maximum strains are also zero at stress 2 MPa.