Martensitic Magnetomechanical Measurements
Shape and Applied Stress Effects
Shape effects
Because of demagnetization effects, increased axial/transverse length
aspect ratios require larger applied fields to saturate the specimen. Because variant
rearrangement is a saturation process, it is expected that larger fields will be required
to produce variant rearrangement as well, leading to decreased strain vs. field slopes
analagous to the magnetization curves. This is shown schematically below:
Applied stress effects
When a field is applied to saturate an FSM material under load, a competition arises between
rotation and rearrangement processes. Consider a specimen with axial volume fraction k
, transverse volume fraction (1-k), and applied load s .
Note that the anisotropy is quadratic with respect to q , the amount
of rotation present in the axial variant, while it is linear with respect to changes in volume
fraction. Because of this, we expect some degree of rotation to always occur prior to variant rearrangement.
The degree of initial rotation that will occur will depend on both the volume fraction
and the applied load. The degree of rotation can be calculated by equating the mechanical
energy and anisotropy energy for a fixed value of the volume fraction k.
When this rotation occurs, it forms a plateau region on the strain vs.
field curve, since there is little shape change associated with the rotation (only ordinary
magnetostriction effects). For small applied loads, we would expect less initial rotation,
and curves with smaller plateau regions at the start; for larger loads, longer plateau regions.
These results are illustrated below:
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