CH4 Materials

• Deformation


  Materials change shape, often being stretched or compressed, when multiple forces are applied to them
• Elastic deformation


  Temporary change to the shape of an object within the limit of proportionality, material returns to original shape when forces are removed
• Plastic deformation


  More permanent change to the shape of an object, material does not return to original shape when forces are removed
• Tension


  Two forces acting in opposite directions on an object to make it stretch
• Compression


  Two forces acting in opposite directions on an object to make it squash
• Yield stress

  Stress which produces plastic deformation, any stresses less than this will produce elastic deformation
• Fracture stress


  Stress which causes the material to break
• Force-extension graph


  • Linear part represents elastic deformation, non-linear part shows plastic deformation, graph stops at fracture stress
• Stiff


  Small extension per unit force (high Young modulus)
• Elastic


  Returns to unstretched form when stresses are removed
• Plastic


  Permanent deformation
• Ductile


  Can be drawn into wires
• Hard


  Resists indentation on impact
• Brittle


  Undergoes little/no plastic deformation before fracture
• Tough


  Absorbs a lot of energy (deforms plastically) before fracture
• Strong


  Can withstand high stresses
• Hooke's Law


  Extension produced is directly proportional to the force applied
• Elastic potential energy


  Energy stored when something is stretched or compressed, equal to 1/2 kx^2
• Stress


  Force per unit area on an object
• Strain


  Ratio of extension to original length
• Young's Modulus


  Ratio of stress to strain, indicates stiffness of an object
• Rayleigh's Oil Drop Experiment


  Used to estimate the size of an atom by measuring the height of an oil film on water
• Crystalline structure


  • Regular, organised arrangement of particles
• Amorphous structure


  • Random, disordered arrangement of particles
• Polycrystalline structure

  • Regular crystalline fragments (grains) arranged in a random/disordered way
• Malleable

  Can be easily shaped
• Dislocations


  • Gaps in the crystalline structure that are mobile and allow metals to deform more easily
• Alloying


  Adding other types of atoms to a metal to make it less ductile by pinning dislocations in place
• Brittle


  Easy propagation of cracks through the material
• Tough


  Can deform plastically around a crack to reduce stress
• Bonding in ceramics


  • Directional bonds make it harder for them to deform plastically
• Bonding in metals


  • Non-directional bonds allow ions to slip over each other and rearrange relatively easily
• Polymers


  Long chains of repeating monomers with a wide range of properties
• Crosslinks


  Bonds between polymer chains that reduce rotation and flexibility, making the material stiffer