SMA - How it works

Many metals exhibit a phase change as they are heated and cooled. We can illustrate this using a crude ‘stick and ball’ model of the metallic lattice.

The phase change is an instantaneous shear transformation between a body centered cubic structure called austenite and a highly twinned martensite structure.

Shape Memory Alloys are a special class of alloys which not only change phase on cooling or heating but have the particular characteristic of a low temperature phase which gives the appearance of increased ductility.

The higher temperature austenitic structure has the characteristic stress strain curve of most metals.

The lower temperature martensitic structure has a stress strain curve more like that of an elastomer in which there is a ‘plateau’ stress.

All the deformation up to about 8% is “elastic” or in other words it can be recovered but not by simply relaxing the stress while in the martensitic condition.

Let’s go back to the ‘stick and ball’ model. We cool the material and it becomes martensitic. It does not change shape by being cooled, but we can now deform it mechanically. If it stays cold it will remain deformed, but if we allow it to warm up, the austenitic structure reappears and the material returns to its original two.

This cycle from austenitic to martensitic to deformed martensitic and back to austenitic is repeatable indefinitely and is what we call ‘free recovery‘. It is important to note that it is a one way process.

At Aerofit a Tinel coupling or separable end fitting is machined at room temperature, in its high strength (austenitic) state, to an inside diameter slightly smaller than the pipes or tubes it is to join. The component is then cooled in liquid nitrogen and the metallurgical characteristics of the alloy change to a lower strength (martensitic) state. With the coupling in this lower strength, cooled state, a tapered mandrel is driven through its center, expanding the inside diameter of the coupling.

The coupling remains in cold storage in its expanded condition until ready for use. It is then removed from this container and slipped over the tube or pipe ends to be joined. As the coupling warms to room temperature, it reverts back to its high-strength state and attempts to recover to its original shape while exerting tremendous radial force. This permanent, live crimp action establishes a leak free, metal to metal seal between the coupling and the tube or pipe throughout the life of the joint.

This product line includes permanent couplings called CryoFit and separable end fittings called CryOlive (flareless) and CryoFlare (flared). CryoFlare is also available in a lightweight version. Liquid Nitrogen.

The SMA Fitting System is designed for use on 2000, 3000, 4000, and 5000 psi hydraulic systems that incorporate titanium, stainless steel, and aluminum tubing materials. The SMA Fitting System is easier and less costly to install than such traditional joining methods as welding, brazing, and swaging. It meets or exceeds the burst, impulse, and fatigue requirements of welded or brazed joints.

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