Materials

• Strength


  • A strong material needs a large force to break it
  • Example: Steel
• Stiffness


  • A stiff material does not deform very much when a force is applied
  • Example: Steel
• Brittle


  These materials are prone to cracking
• Tough


  These materials are resistant to cracking
• Malleable


  Malleable materials can be hammered into shape
• Examples of malleable materials


  • Copper
• Ductile:A ductile material can be drawn into a wire
• Examples of Ductile materials


  • Copper
• Density


  • Mass per unit volume
  • Unit is kg m−3
• A block of iron has a volume of 0.15 m3 and a mass of 1180 kg. Calculate the density of iron.


  7867kgm-3
• Measuring stiffness


  Apply a range of tensile force F
• Hooke's Law


  • The strain of the material is proportional to the applied stress within the elastic limit
  • Mathematically: σ = -k Δx
  • The spring obeys Hooke's Law (up to a limit)
  • Its extension is proportional to the applied force: F = k Δx
  • k is the stiffness of the spring
• If we stretch a wire then, as long as we don't go past the elastic limit, it will behave the same way. It will obey Hooke's law.
• The stiffness of a wire will depend on its dimensions - its length and its cross sectional area.
• The stiffness of a material should be independent of its dimensions.
• Elastic potential energy or strain energy


  Energy stored when a material is stretched elastically: U = 1/2 kΔx^2
• Elastic potential energy calculations

  • A spring has a stiffness of 800 Nm-1. How much will it extend when it supports a tensile load of 40N? How much energy will it store?
  • The spring in the shooter of a pinball machine needs a force of 30N to compress it 6 cm. Calculate its stiffness. Calculate the energy it will give to the ball when you let go.
• Young's Modulus


  • Measurement of the mechanical properties of linear elastic solids
• A wire has a cross sectional area of 2.1 × 10−7 m2. It supports a tensile load 25 N. What stress will this load produce?The length of the wire is 78 cm. The stress produces an extension of 2.4 mm.

  
  

  Calculate the strain that this stress has produced.

  Calculate the Young's Modulus of the Material that the wire is made from..

  1.19x10^8 Pa.

  3.077x10^-3 strain.

  3.87x10^10
• • The Young's Modulus of Aluminium is 69 × 10^9 Pa. A component in a mechanical device supports a maximum load of 500N. The length of this component is 30 cm and its cross sectional area is 14 × 10−6 m2. Calculate the stress that this load produces, the strain that this load produces, and the maximum extension of the component.
  • 11408.17Pa
  • 0.09358 strain
  • 0.3996
• Fatigue


  The process in which small cracks in a component initiate and grow under the influence of a repeated (normally mechanical) loading, possibly to a final failure
• Creep


  A form of slow mechanical deformation that occurs when a material is exposed to high-stress levels for a long period of time. Creep is time-dependent. High temperature and stresses can cause creep in metals.
• Hysteresis


  Energy is transferred as heat as the rubber molecules slide past each other. The deformation is not permanent.
• Ductile


  Can be drawn into wire
• Stress


  Internal resistance offered by the body to the eternal load applied to it pper unit cross-secrional area
• Strain


  Extension per original length