Biomedical Engineering Reference
In-Depth Information
the form of macroscopic strain, and apparently, no stress increase
is observed and the relation receives a plateau character. When the
stress level achieves
σ
f
, the process of detwinning is complete and
we can consider it, as the end of plateau. The structure is orienting
twinned planes in one preferred manner by mechanical load
direction. Further stress increase causes elastic deformation of
detwinned structure and the maximum elongation is revealed. If
the stress increase were sustained (to amount dependent from
material composition and receiving process — for the bulk tests
8-10% of elongation), it could generate dislocation movement that
plastically deformed the material, and it would not be recovered in
the characteristic temperature range.
Next, the material is unloaded from point C to D where we can
observe small amount of elastic spring-back; it also now retains the
deform state. During the temperature increase from point D, the
process of the release of accumulated strains starts, and at point
E, detwinned martensite begins to transform into austenite, and at
point F, reversible transformation ends with fully austenite structure.
For a better understanding, the sequence of transformation is shown
in Fig. 8.18., with macro and atomic scale of behavior.
The maximum amount of elongation regain strongly depends on
the system; for the polycrystalline structure, it is usually 1-7%.
Figure 8.18
Sequence of transformation in SMA [13, 42].
8.4.4
TWSME — Two-Way Shape Memory Effect
The behavior discussed earlier is the result of the reversible
transformation occurring in the material. To obtain the two-way
