AQA A Level Physics复习笔记4.7.5 Elastic & Plastic Behaviour

• Materials can undergo two types of deformations:
• Elastic deformation
  • When the load is removed, the object will return to its original shape
  • This is shown in the elastic region of the graph

   

• Plastic deformation
  • The material is permanently deformed
  • When the load is removed, the object will not return to its original shape or length
  • This is beyond the elastic limit and is shown in the plastic region of the graph

   

• These regions can be determined from a Force-Extension graph:

Below the elastic limit, the material exhibits elastic behaviour. Above the elastic limit, the material exhibits plastic behaviour

• The elastic region is where the extension is proportional to the force applied to the material (straight line)
• The plastic region is where the extension is no longer proportional to the force applied to the material (graph starts to curve)
  • These regions are divided by the elastic limit

   

• The plastic region starts at the elastic limit and ends at the point of fracture (the material breaks)

Brittle and Ductile Materials

• Brittle materials have very little to no plastic region e.g. glass, concrete
  • The material breaks with little elastic and insignificant plastic deformation

   

• Ductile materials have a larger plastic region e.g. rubber, copper
  • The material stretches into a new shape before breaking

   

Brittle and ductile materials on a stress-strain graph. These are the same on a force-extension graph too

• To identify these materials on a stress-strain or force-extension graph up to their breaking point:
  • A brittle material is represented by a straight line through the origins with no or negligible curved region
  • A ductile material is represented with a straight line through the origin then curving towards the x-axis

   

(i)

• Since the graph is a straight line and the metal almost fractures, the point after X must be its elastic limit
• The graph starts to curve after this and would fracture beyond point Y
• This curve between X and Y denotes plastic behaviour
• Therefore, the metal exhibits plastic behaviour, but not elastic behaviour

(ii)

• Plastic deformation has occurred which results in permanent extension
• As the load is decreased, the bonds in the metal are re-aligned hence the y-intercept is now not through the origin
• The gradient remains the same because the intermolecular forces (the forces between bonds) are identical to before