Materials

Materials

1. Stress and Strain

   • Stress causes strain
     • Stress is the force applied divided by cross sectional area
     • Strain is the change in length/original length
       • no unit
   • Graphs
     • always the same for a material, doesn't change with dimensions
   • Forces that....
     • Stretch = tensile
       • positive
     • squash = compressive
       • negative

2. Hooke's law

   • Extension directly proportional to Force.
     • F=kx
       • K is the force constant of the spring or wire
       • Applies to extension AND compression
   • Stops when the load is great enough
     • Limit of Proportionality
     • If stretched too much, the elastic limit is reached
       • Elastic deformation
         • Atoms move relative to one another, but do not return to their original position when load removed
           • Permanently stretched
       • Plastic deformation
         • Atoms move relative to equilibrium positions, without changing position in material.
           • return to equilibrium distance apart once load removed
   • Measuring extension
     • Measuring extension
   • Combining K
     • series = k/2
     • parallel = 2k

3. Elastic potential energy

   • If stretched beyond elastic limit work is done separating atoms
     • not stored as elastic potential energy, so not released when force removed

4. The Young Modulus

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