Strength - Density

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 1 (just sink), most woods slightly less than 1 (just float)
• High strength at low weight is so often important that a property called specific strength is defined as strength/density

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?
• Select materials for a rucksack.
• Select materials for a car wheel.

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

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