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Strength - Density
General Information
Physical Insights
Example Uses
Simple Questions
Further Questions
This chart helps in selecting strong materials that are also lightweight.
General Information
Strength measures the resistance of a material to failure, given by the applied stress (or load per unit area).
The chart shows yield strength in tension for all materials, except for ceramics for which compressive strength is shown (their tensile strength being much lower).
This chart is useful for identifying materials for components which require high strength combined with low weight (top left).
Most polymers have densities slightly greater than water (1000kg/m3) and so just sink, most woods have densities slightly less than water and so they float.
High strength at low weight is so often important that a property called specific strength is defined as strength/density.
The 'hardness' of a material is often used to indicate things like wear resistance. Hardness is
measured by making a small surface indentation. It is not a material property, but is closely
related to the strength, so strength gives a good indicator of wear resistance.
Physical Insights
The bubbles are elongated along the strength axis, but not density. This is because alloying and heat treatments have a strong effect on strength but little on density.
Strength is correlated to density so that most materials lie on the bottom left-top right diagonal.
Composites provide a means of achieving high strength at low weight because they exploit very strong fibres in light matrices.
Woods are somewhat like polymer foams having pores full of air and so float in water.
Wood achieves high strength at low density by its efficient cellular microstructure.
Example Uses
Hang gliders tend to be constructed from light and strong materials (metal tubes with nylon or similar coverings).
Zimmer frames often use aluminium tubes for low weight
Simple Questions
Many aluminium alloys are heat treatable – what does this mean and how is strength affected?
This means their mechanical properties change as a function of heat treatment, i.e. they age with time. This is because the strength of the alloys depends on the amount of alloying elements in solution and in precipitates. Small precipitates (particles) are best for stopping dislocations and preventing plastic flow. Too small and the particles are not good obstacles, too large and they are too far apart to stop flow. Since the particles grow bigger with time strength rises and then after peak hardness falls.
Select materials for a rucksack.
Frames tend to be of tubular aluminium for light weight (good shape as well as strength/density can reduce weight), while strong waterproof nylon is often used for the fabric.
Select materials for a car wheel.
Rotating and moving parts benefit tremendously from weight savings because both linear and rotational kinetic energy is saved and faster acceleration can also be achieved. Aluminium alloy wheels were used on Thrust SCC, the fastest land vehicle, and are replacing steel in the sports car market.
Select materials for an anchor.
Further Questions
Why are forged alloy components often stronger than cast ones?
Explain at a molecular level why low density polythene has a lower density than high density polythene
There are 2 separate populations for this class to improve clarity. Move the mouse over different parts of the class name to reveal each one.
Rollover class name to view individual materals, click chart to return to class view. Hover mouse over property for brief definition.
