Shape Memory Alloys - SMA
SMAs are metallic alloys with the ability to return to a predetermined shape when heated. After an apparent plastic deformation, the SMAs undergo a thermo-elastic change in crystal structure when heated above its transformation temperature range, resulting in a recovery of the deformation. This effect, known as Shape Memory Effect is used in movement or force generation applications, such as actuators development.
SMA material transformation process
The Martensitic transformation takes place in a certain temperature range. This range is one of the main parameters for the SMA, and is called transition temperatures (see above figure). During cooling, the transformation occurs in the range defined by Ms (Martensite Start Temperature) and Mf (Martensite Finish Temperature). The inverse transformation (austenitic transformation) takes place in the range between As (Austenite Start Temperature) and Af (Austenite Finish Temperature) for the heating process.
- The Shape Memory mechanism can be explained in the following way (see figure below): by reducing the temperature, the material is transformed in martensite, without showing any shape change (self-accommodated martensite).
- Applying a deformation, it is accommodated by the twin movement thus avoiding plastic deformation of the material. This deformation can be achieved by stressing the SMA when a force is applied. When the material is heated up, it returns to its initial shape.
Shape Memory Effect mechanism
In SMA, a change between a mother phase (austenite or high temperature phase) and a product phase (martensite or low temperature phase) is produced. This transformation phase causes the crystallographic movement of the alloy structure, producing a macro scale effect known as shape memory.
SMA is also a well-established technology with numerous applications in several markets since the discovery of SMA effect in NiTi alloys in the Naval Ordnance Laboratory. Most remarkable applications can be found in the field of actuators for deployable structures and in aerospace mechanisms. Other extended applications of SMA make use of their superelastic properties (also in combination with their biocompatibility). Herein a selection of applications is offered:
- Temperature control systems: as it changes shape, it can open or close a valve, activate a switch or a variable resistor to control the temperature.
- Demonstration model heat engines have been built which use SMA to produce mechanical energy from hot and cold heat sources.
- Aeronautical market: design of adaptive structures to control flaps or the chevron shape during taking off and landing phases.
- Actuators for automotive industry, such as latch mechanisms for doors and appendixes, air valves, adaptive structures, etc.
- Actuators for autofocus mechanisms in cameras.
- Triggering actuators for aerospace applications, especially in hold-down and release mechanisms in charge of deploying structures and appendixes in satellites.
- Healthcare: active implants, valves, catheters…
As it can be deduced from the applicability of SMA technology, this is a multifunctional material that can be used in three different manners:
- Metallic material. SMA is above all a metallic material; therefore it maintains the principal structural, conductive, and thermal properties of its main metallic compound (Ti, Cu, etc.).
- Spring material. Used in super elastic mode, this material can become a particularly interesting spring, offering a nearly constant force versus deformation, due to its non-constant K (F=K*x) thus making it ideal for gravity force emulation (constant force with displacement) in a resistive force configuration.
- Artificial Muscle material. In the form of fibers, it becomes a contractile material that can be programmed, just like natural muscles fibers, either as a fast response fiber, or as a slow response fiber. Fibers can be re-programmed almost constantly. The principle of contraction is based on the transformation phase of the material, and the principle of activation is thermal, that can be induced electrically through joule effect, or by other heating means. Above a certain temperature, the transformation occurs. This transformation temperature can be selected during the manufacturing process.
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