Materials

• Tensile force


  Produces extension, stretches an object
• Compressive force


  Reduces the length of the material, compresses it
• Hooke's law


  Applied force is directly proportional to the extension
• Experiment to demonstrate Hooke's law

  1. Suspend masses from a spring

  2. Measure extension

  3. Plot force vs extension graph

  4. Gradient of graph = spring constant k
• Area under force-extension graph


  Represents work done or elastic potential energy
• Stress


  Force applied divided by cross-sectional area
• Strain


  Extension divided by original length
• Young's modulus


  Stress divided by strain
• Experiment to determine Young's modulus

  1. Clamp wire horizontally

  2. Attach mass to end of wire

  3. Measure diameter, original length, extension

  4. Plot force vs extension graph

  5. Gradient = Young's modulus * A/L
• Ductile material


  Can be easily drawn into wires
• Elastic deformation


  Material returns to original shape after force is removed
• Plastic deformation


  Material does not return to original shape after force is removed
• Stress-strain graph for ductile material


  • Proportional up to elastic limit
  • Plastic deformation beyond elastic limit
  • Ultimate tensile strength before breaking
• Stress-strain graph for brittle material

  • Straight line through origin up to breaking point
  • Obeys Hooke's law up to breaking point
  • Elastic deformation only
• Stress-strain graph for polymeric material

  • Non-linear curve
  • Elastic behaviour - returns to original shape
  • Does not obey Hooke's law
• Stress-strain graph for plastic (e.g. polythene)

  • Initially proportional, then plastic deformation
  • Does not return to original shape