Shape Memory Alloys by the Numbers

About this book

A few metal alloys can be bent out of shape when cool and then, when warmed, return by themselves to the shape they were given in manufacture, pulling with surprising force as they do. Others, at room temperature, can be bent or stretched by several per cent and spring back completely, many times further than any spring steel. Both behaviours come from the same source, a reversible change in the metal’s crystal structure called a martensitic transformation, and the most useful alloy that shows them is nickel-titanium, often called Nitinol.

Shape memory wire makes a silent, compact actuator that pulls when an electric current warms it. Superelastic wire and tube make springs, clips and guides that survive strains that would permanently bend steel. Both are easy to misuse, because their behaviour depends strongly on temperature and stress in ways that ordinary metals do not prepare a designer for. This book works through the numbers that decide whether a shape memory design works: transformation temperatures and how stress shifts them, stroke and force, bias springs, heating current, cooling time, fatigue and the behaviour of superelastic parts across a temperature range.

It suits mechanical and mechatronic designers and automation engineers. It assumes school physics and algebra, the idea of stress and strain, and Ohm’s law. No previous knowledge of metallurgy is needed.

The running case

Every chapter follows one fictional product. This is an illustration. A maker of electrical enclosures designs VL-25, a latch release for a pressure-relief vent flap on an outdoor control cabinet. A spring-loaded latch pin holds the flap shut. To open the vent remotely, a controller passes current through a 150 mm length of 0.25 mm (0.010 in) nickel-titanium actuator wire, which contracts and pulls the latch pin back 4.0 mm. A steel bias spring stretches the wire again when it cools. The cabinet’s interior can reach 60 °C in summer sun. A superelastic nickel-titanium wire clip, introduced in Chapter 9, holds the flap against its seal.

The enclosure maker, the latch and every mistake are invented and do not describe KEVOS® work or a real product. Wire data are taken from manufacturers’ published sheets and identified in the references; the case’s transformation temperatures and test results are illustrations.

Source limits and professional responsibility

Shape memory alloys’ properties depend strongly on composition and processing, and published values are typical, not guaranteed. The transformation temperatures, stress–temperature slope and fatigue behaviour used here are illustrations within the ranges published sources describe, and every calculation was checked by running it. A real design must use the supplier’s data for the actual wire and confirm it by test.

VL-25 is a convenience release, not a certified safety device. Pressure relief, emergency venting and any function on which safety depends are governed by the applicable standards and must be designed and approved by competent people. Medical devices, which are a major use of nickel-titanium, are outside this book’s scope. The thermodynamics text in the references was used only as a map of ideas; nothing from it is reproduced.