Chap. 4+5 - Materials

• The structure of a material can be split into two broad categories: Crystalline and Amorphous.
• A crystalline structure has a regular, organised structure whereas an amorphous structure has a random and disordered arrangement.
• Polycrystalline structures are ones which have regular crystalline fragments (grains), but these are arranged in a disordered way.
• Dislocations are structures in a pure metal that help them be malleable and ductile.
• Alloying can make metals less ductile by producing a less regular structure due to different sized atoms, pinning dislocations into position and reducing the metal’s ductility.
• The bonds in ceramics are directional whereas the bonds in metals are non-directional.
• Polymers are long repeating chains of monomers.
• Crosslinks are structures in a polymer that reduce the rotation and flexibility of monomer chains.
• Rayleigh’s Oil Drop experiment was designed to estimate the size of an atom.
• Elastic deformation is one where the object will return to its original shape, once the deforming forces are removed.
• A plastic deformation is one where the object remains permanently deformed, even once the deforming forces are removed.
• There must be a pair of forces, acting on the object in opposite directions, causing it to stretch for an object to experience tension.
• There must be a pair of forces, acting on the object in opposite directions, causing it to compress for an object to experience compression.
• The stress at and beyond which the object will undergo plastic deformation is known as the yield stress of an object.
• The fracture stress of an object is the stress that causes the material to break.
• A stiff object is one that only produces a small extension per unit force.
• A tough object is one that can absorb a lot of energy before fracture.
• The limit of proportionality is the point beyond which the object will no longer obey Hooke’s law.
• The unit of Young Modulus is Pascals since stress is measured in Pascals and strain doesn’t have a unit.
• The extension of an object is directly proportional to the force applied to it, up to the limit of proportionality, given that external conditions remain constant.
• The Young Modulus of a material is the ratio of stress to strain and is a measure of a material’s stiffness.
• If an object is ductile, it means that it can be easily stretched into wires.
• The area under a force-extension graph is equal to the energy stored.
• The unit for a spring constant is Nm ⁻ ¹.
• Strain is the ratio of extension to original length.
• Stress is measured in Pascals.
• Hooke’s Law governs the extension of an elastic object.
• The equation for the energy stored when a spring is stretched is E = ½ kx².
• Elastic potential energy is the type of energy stored when an object is stretched or compressed.
• If an object is brittle, it will undergo very little plastic deformation before a fracture occurs.
• Stress is the force per unit area applied to an object.