Shape memory alloys (SMA) have a unique ability to alter their mechanical and physical characteristics in response to temperature change, making them immensely popular in military, safety, medical and robotic fields as actuators. In today’s article, Matmatch will investigate how SMAs are used in plumbing projects to facilitate safety as water or atmospheric temperatures fluctuate.
Shape Memory Alloys – a brief history
Nickel titanium alloys, composed in equal parts, remain the most widely used SMA today, known collectively as “NiTinol”. The discovery of the unique property of NiTinol alloys to remember their original shape happened by accident. In a laboratory management meeting in Maryland in 1961, a strip of nickel titanium alloy that had been bent out of shape repeatedly was lit with a pipe light, and to the surprise of everyone present, it morphed back into its original shape once heat was applied. Other SMAs have also been discovered, such as iron-manganese-silicon, copper-zinc-aluminum and copper-aluminum-nickel. All the same, NiTinol remains one of the most suitable SMAs for commercial use today.
Atomic structural changes make SMAs possible
It was soon determined that nickel titanium alloys experience phase changes in their solid state; particle positioning is restructured at a crystalline level. Exposure to a memory transfer temperature is the key to shifting between two phases, identified as martensite (below a memory transfer temperature) and austenite (above the memory transfer temperature). Fluctuation between these two phases can occur many millions of times. Essentially, even after its shape is deformed, the SMA reverts to its “parent shape” once it enters the austenite phase. An SMA that exhibits memory effect upon heating and cooling has a “two-way shape memory”. Its texture is rubbery and malleable in the martensite phase and becomes hard in its austenite phase.
Anti-scalding and SMAs
Since heat is paramount to the reversion of a shape memory alloy to its original shape, this is the characteristic most often used to manipulate the alloy’s performance. A good example of this is an SMA in an anti-scalding valve, used in household plumbing and for fire sprinklers. When installed into a faucet or a showerhead, the anti-scalding valve responds to the memory transfer temperature by changing shape and thus shutting off the flow of water or simply introducing more cold water.
Once the temperature reaches a level underneath the memory transfer temperature, the SMA resets itself. This is particularly useful in residential buildings with older plumbing systems, the water temperature of which is suddenly and sometimes dangerously affected by another flow of water in a different part of the building.
Shape memory alloy springs can be applied to thermostatic mixing valves at the beginning of the water flow to control the ratio of cold and hot water accurately and safely before it reaches the showerhead.
SMA valves are the reason that fire sprinklers can respond so quickly to a rising temperature in a building caused by a fire. Exposure to heat causes the SMA component to expand. Once the transitional temperature has been reached, the element expands quickly to the point at which a bolt breaks and activates the sprinkler automatically.
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