Please wait while page loads... This should be < 1 minute on a 56k modem
Strength - Toughness
General Information
Physical Insights
Example Uses
Simple Questions
Further Questions
This chart is useful for selecting materials that are not only strong (carry a high static load before deforming) but also tough (can withstand impact loads).
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.
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
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?
A. Concrete has almost no strength in tension, but in pre-stressed concrete the steel is in tension and the concrete in compression. Steel is very strong in tension, so that even under high tensile loads the material does not fail because the concrete is still in compression.
Select materials for a child’s cup or spoon.
It needs to be soft not strong so something like polythene is ideal, a metal spoon could hurt their gums.
Select materials for a 13A plug casing for a vacuum cleaner.
A vacuum is dragged all round the house and the plug often has to follow down stairs, etc. As a result a reasonable tough plastic is ideal (plastics are good insulators). Something like ABS would be better than urea formaldehyde (a brittle thermoset).
Select materials for a bullet/knife proof vest.
These two applications are actually different, bullets are easier to stop than knives. A kevlar fibre weave is good for stopping bullets and is comfortable to wear, but knives can penetrate the weave. Ceramic vests are not so comfortable but can stop knives.
Select materials for a bus shelter window.
These need to be tough to cope with vandals. Usually polycarbonate (Perspex) is used which is very tough (about a hundred times tougher than glass).
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)?
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.
