Other physical properties
What is strength?
Strength is a property with obvious benefits. If you were building a bridge or a skyscraper they need to be made from something strong.
An articulated lorry is made from a mixture of materials, some of which must be strong. Just imagine the life of the lorry – a lot of stresses and strains!
It's not just these grand feats of engineering that have to be strong. Many other things need strength: shopping bags, CD cases, keyboards, mugs, screwdrivers. In fact - almost everything!
But the idea of strength is a complicated one. Avoiding too much detail, let’s just look at some of the considerations when thinking about strength …
We need to consider what sort of strength we want. Does the material need to withstand a tensile force (stretching), or a compressive force (squeezing)?
Does the material need to resist impact forces or static forces? If you hit a table with a hammer you’d be applying an impact force. If the hammer was just sitting on the table, its mass would provide a static force.
Will the material break or deform? Will deformation be permanent (when the force is removed, it stays deformed), or temporary (when the force is removed, it springs back to its original shape).
Different materials have different types of strength.
Metals with a memory
Shape-Memory Alloys (SMAs) are metals that, after being strained, can go back to their original shape. The technology is used to create, among other things, mobile phone aerials and superelastic frames for glasses. The alloys are usually nickel-titanium.
Ordinary metals get weak when they’re bent out of shape and bent back into shape. It’s called metal fatigue.
Muscle wires
SMAs can also be used to make “Muscle Wires”. These are thin wires of a nickel-titanium alloy called Nitinol which can also be made into springs. At room temperature, they are easily stretched by a small force. When the wire is heated it returns to its original shape.
They’re used in the aerospace and automotive industries. They also have medical applications, for example, dental braces, microsurgical instruments and stents.
But perhaps the most fun is had by robot builders who use them to mimic human muscles!