Strength - Cost

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)
• Many applications require strong materials, e.g. screw drivers, safety belts - these lie at the top of the chart
• Unfortunately there are few cheap high strength materials (top left)

Physical Insights

• Ceramics and glasses have directional covalent bonds. They are weak in tension because they are sensitive to small cracks or flaws.
• Metals do not have directional bonds and have similar tensile and compressive properties
• Cellulose microfibres make wood strong in tension along the grain
• Alloys are much stronger than pure metals
• Metals can be strengthened by heat treating to change the microstructure

Example Uses

• Teflon as a non stick surface for frying pans
• Ceramic for fire bricks and for coatings for jet engine blades
• Tungsten for light bulb filaments

Simple Questions

• Why is wood used for beams rather than stone, when stone has a higher strength?
• Why is pottery weak?
• Why is steel the most commonly used ‘strong’ material?
• Select materials for a screwdriver.
• Select materials for a glass-cutting tool.

Further Questions

• Why can polymers operate only at low temperatures when their polymer chains contain covalent bonds like ceramics?
• What is the operating temperature of a light bulb filament?

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