stress strain and force extension graphs

• point E is the elastic limit - at this point, the material starts to behave plastically, and the material would no longer return to its original shape once the stress was removed

  
• point Y is the yield point - here the material starts to stretch without any extra load - this is the stress at which a large amount of plastic deformation takes place with a constant or reduced load

  
• the area under the first part of the graph gives the energy stored in the material per unit volume

  
• before point p, the graph is a straight line, showing the material is obeying Hooke's law

  
  

  point P is the limit of proportionality - after this, the graph is no longer a straight line, as the material stops obeying Hooke's law, however it would still return to its original shape if the stress was removed.

  
• this is a graph for a brittle material. it is a straight line so it obeys Hooke's law, but when the stress reaches a certain point, the material suddenly breaks - it doesn't deform plastically.

  
• ceramics are an example of a brittle material, as they break without deforming plastically
• force extension graphs are specific for the tested object and depend on its dimensions. however, stress strain graphs describe the general behaviour of a material, as stress and strain are independent of the dimensions
• this graph is for a wire that has been stretched past its limit of proportionality, so the graph has started to curve. when the load is removed, the extension decreases. the unloading line is parallel to the loading line because the stiffness is still the same. but because the wire was stretched beyond its elastic limit and deformed plastically, it has been permanently stretched so the unloading line doesn't go through the origin

  
• the area between the two lines on this force extension graph is the work done to permanently deform the wire