Mechanical properties – in physics, and design
Materials selection charts are a novel graphical way of presenting material property data. Most mechanical characteristics extend over several orders of magnitude, so logarithmic scales are used. Individual properties could be plotted as bar charts, but a 2D plot of a pair of properties is much better, as we will see.
Here is an example – Young modulus against density – showing where the data for the different classes of materials fall.
Notice that the bubbles for a type of metal or polymer (steels, or aluminium alloys, or nylon) are mostly small. The density and Young modulus of steel only depends on the way the iron atoms are packed, and the bonding between them – alloying or heat treatment have almost no effect. We call these 'microstructure-insensitive' properties. The spread of the data on a selection chart therefore enhances our appreciation of the underlying physics of each property. As the name suggests, materials selection charts are also a valuable tool in engineering design. Designers have a challenging task in choosing materials for products, as they usually have to consider many competing objectives and constraints at once – light and stiff, strong and cheap, tough and recyclable (or maybe all of these at once!). Materials selection in design is therefore a matter of assessing trade-offs between several competing requirements. Traditionally designers have used extensive handbooks and their own experience to guide the choice of material in design. Selection charts provide insight into these trade-offs by pairing properties which must commonly both be considered, avoiding the need to work with tedious tables of numerical data. As an example – what materials might be used for a light, stiff bike frame? Consider what falls towards the top left corner of the first chart – woods, composites, some metals, and ceramics. Ceramics don't sound very likely, but we have only considered two properties.
- Stiffness measures how much something stretches elastically when a load is applied. Young modulus measures stiffness and is a material constant, i.e. it is the same whatever the size of the test-piece.
- Young modulus and density both depend on the atomic packing within the material, and Young modulus depends on the type of bonding between the atoms (electron bond, covalent, ionic etc.)
- Note how the materials all lie roughly on a diagonal – Young modulus is strongly correlated to density.
- The metal and polymer bubbles are small – this is because material composition and processing do not have a significant effect on density or Young modulus.
- Woods have very different stiffnesses depending on whether they are loaded 'with' or 'across' the grain. This is because of the aligned stiff cellulose micro-fibres. Both paper and MDF are made from wood pulp and so have similar densities, but have little directional variation in Young modulus.
- Foams have the lowest densities because they have pores full of air.
- Note that the scales are logarithmic, because of the large ranges of values.
Applications of the chart
- Stiff lightweight materials are hard to find, for things like sports products and bicycles – composites appear to offer a good compromise, but they are usually quite expensive, and wood is still used for cheaper products (e.g. oars).
- Many applications require stiff materials, e.g. roof beams.
- Many applications require low density materials, e.g. packaging foams.
- Polymers don't seem like a good choice for stiff, lightweight products – but they can be reinforced by incorporating stiffening ribs into the design (for instance, look inside a plug).
- Ceramics are quite light and very stiff – but their poor tensile strength and toughness means they are likely to fracture.