When engineers describe things as strong or tough, they are talking about something very specific. These are terms that have been well defined to the point we have equations so we know exactly how strong or tough something is. What we are describing using these words are a material’s properties. There are a lot of material properties we can describe as we have the physical properties, thermal properties, and electrical properties, just to name some. At the ISM we research mainly mechanical properties, surface properties, and manufacturing properties.
When we talk about mechanical properties we want to know how the whole component will respond when put in a certain situation. The simplest mechanical property is density, in other words how much does a material weigh for a given size. In the section ‘Stress, Strain, Strong & Tough’ we go through a range of mechanical properties and what they mean.
The two other types of properties the ISM is interested in are surface properties and manufacturing properties. Surface properties are all about the outside of a component and how it deals with the environment we put it in. We explain what some of these surface properties are in ‘Hard, Wear & Rust‘. Manufacturing properties are the way we describe the effect of how we make a component impacts on how it performs. These are described in ‘Form & Finish‘.
So before we start talking about properties we need to know a few technical terms:
Elastic: When you think about something that is elastic, if you stretch it and then let go, it returns to its original shape. This is the same in materials like metals and ceramics although over a smaller range than things like elastic and rubber.
Plastic: This is when a material no longer acts elastically. In other words, when you stretch it it no longer returns to its original shape. There are many things that go on at the atomic scale that make a material unable to return to its original shape and we call these mechanisms. The important thing about plasticity is that it is time dependent, in that it doesn’t happen instantly and takes time to happen.
Failure: When most people think of failure they think of something catastrophically breaking, but this is only the most extreme type of failure. Failure is a word engineers use to describe the point at which a component or material becomes useless for the purpose it is designed for. This means failure is often situation specific and hence defined by the engineer. Failure can be obvious things like breaking, cracking, and stretching, but also it can be simply becoming too hard/soft, not being able to handle the forces like it used to, rusting, etc.
Ductile & Brittle: When we apply a force on a component/material sometimes engineers describe it as behaving as either ductile or brittle. To use a simple example, think about Mozzarella cheese. When it is cold and you tear it apart it ‘rips’ rather than stretches therefore it is acting brittle. If the cheese is hot, when you try to tear it apart it stretches inelastically or plastically which is why you get cheese strings hanging off your pizza. This is ductile behaviour. Now the difference between whether a material is ductile or brittle comes down to the mechanisms that are happening.