CIE A Level Physics复习笔记6.2.1 Elastic & Plastic Behaviour

• Elastic deformation: when the load is removed, the object will return to its original shape
• Plastic deformation: when the load is removed, the object will not return to its original shape or length. This is beyond the elastic limit
• Elastic limit: the point beyond which the object does not return to its original length when the load is removed
• 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

• Elastic deformation occurs in the ‘elastic region’ of the graph. The extension is proportional to the force applied to the material (straight line)
• Plastic deformation occurs in the ‘plastic region’ of the graph. 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

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

Stress-strain curve for a brittle and ductile material

• 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

ANSWER:   C

• Since the graph is a straight line and the metal fractures, the point after X must be its elastic limit
• The graph starts to curve after this and fractures at point Y
• This curve between X and Y denotes plastic behaviour
• Therefore, the correct answer is C