Deforming solids
- Stiffness – the ability for a material to resist a tensile force
- Tensile strength – the tensile stress at which a material fails
- In many situations, the force on a material will be tending to reduce the volume, to squash the material. This is known as a COMPRESSIVE force and puts the material under COMPRESSION
- Compressive force per unit area =compressive stress (Nm-2 or Pa) = Compressive force / area of cross-section
- When it comes to I/GCSE Physics, COMPRESSIVE STRENGTH = the compressive strength at which the MATERIAL BREAKS
- Extension per unit length = compressive strain = extension / original length
- Some materials have a very low tensile strength, but are strong when they are subjected to compressive strength – such as brick and concrete. The strength of a material under sheer strength is related to some extent its tensile strength.
STRENGTH –a materials ability to withstand stress, whether is it tensile, compressive or shear
DUCTILE– show plastic deformation
BRITTLE – materials that crack or break with little deformation
TOUGH – materials able to withstand impact forces without breaking and require a large force to produce a small plastic deformation
COMBINATION – more than one material, often gain the best properties of both. Carbon fibre and living wood are good examples
HARD – materials which resist plastic deformation, usually by denting, scratching or cutting MALLEABLE – materials which show large plastic deformation before cracking or breaking. The most malleable material is Gold
In I/GCSE Physics, you can measure hardness by measuring the size of a dent produced by pushing a diamond into the surface with a certain force.
A mineral scale by FRIEDRICH MOHS was used to compare hardness, based on the principle that a material which could scratch another material should be higher (or at least the same) on the scale of hardness. This doesn’t provide accurate values so isn’t often used in engineering
Materials in the real world
For climbing ropes, the material must be a compromise between stiffness, breaking stress, cost and density.
- Climbing helmets have also been developed. Traditional, uncomfortable helmets were made from HDPE (high density polyethene) but newer ones are made from CFRP (carbon-fibre reinforced polymer. These materials are HARD, STRONG and not too BRITTLE
In I/GCSE Physics, The helmets are tested thoroughly, investigating temperature, dropping it from a height with a weight inside it. The compressive stress produced MUST NOT be more than the breaking stress of the material or it will fail. ALSO, by law there must be no more than 8kN of peak force transmitted to the head.
The design is also important, the shell will transfer force out evenly over the skull due to the internal webbing straps. The material it is made out of must also have enough strength in itself. The toughness shows its ability to absorb energy during fracture. The higher, the more energy it can absorb.
That's all~ Thanks for watching.
