Strength - Toughness

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)
• Toughness measures the energy required to crack a material; it is important for things which suffer impact
• There are many cases where strength is no good without toughness, e.g. a car engine, a hammer
• Increasing strength usually leads to decreased toughness
• Tempered steel is tougher but less strong than after quenching.

Physical Insights

• Put a pin-prick in a balloon and begin to blow it up - it will burst when the elastic energy cannot be absorbed by the growing crack
• The tensile strengths of brittle materials are very sensitive to the presence of flaws
• Quenching carbon steel makes it very hard but brittle.
• Tough materials absorb a lot of energy as a crack grows through them
• Metals are tough because they deform plastically while they crack, absorbing energy
• Cast iron is often brittle because it contains graphite flakes which behave like little cracks within the metal

Example Uses

• Steel is often used to absorb energy in car impacts because it is tough and strong
• Saw blades and hammer heads are quench and tempered steel to get moderately high strength with good toughness

Simple Questions

• Why does prestressed concrete have high tensile strength?
• Select materials for a child’s cup or spoon.
• Select materials for a 13A plug casing for a vacuum cleaner.
• Select materials for a bullet/knife proof vest.
• Select materials for a bus shelter window.

Further Questions

• Thermal toughening of glass places the surfaces in compression the interior in tension - why does this increase the strength?
• Is rubber tough or brittle (hint: remember the balloon experiment)?

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