Deformation (change of shape) of a solid is caused by a force that can either be compressive or tensile when applied in one direction (plane). Compressive forces try to compress the object (make it smaller or more compact) while tensile forces try to tear it apart. We can study these effects by looking at what happens when you compress or expand a spring. Hooke’s Law describes the relationship between the force applied to a spring and its extension.
Hooke’s Law - the relationship between extension of a spring and the force applied to it.Deviation from Hooke’s Law
We know that if you have a small spring and you pull it apart too much it stops ’working’. It bends out of shape and loses its springiness. When this happens Hooke’s Law no longer applies, the spring’s behaviour deviates from Hooke’s Law. Depending on what type of material we are dealing the manner in which it deviates from Hooke’s Law is different. We give classify materials by this deviation. The following graphs show the relationship between force and extension for different materials and they all deviate from Hooke’s Law. Remember that a straight line show proportionality so as soon as the graph is no longer a straight line, Hooke’s Law no longer applies.
Brittle material :
This graph shows the relationship between force and extension for a brittle, but strong material.
Note that there is very little extension for a large force but then the material suddenly fractures. Brittleness is the property of a material that makes it break easily without bending. Have you ever dropped something made of glass and seen it shatter? Glass does this because of its brittleness.
Plastic material :
Here the graph shows the relationship between force and extension for a plastic material. The material extends under a small force but it does not fracture.
Here the graph shows the relationship between force and extension for a plastic material. The material extends under a small force but it does not fracture.
Ductile material :
In this graph the relationship between force and extension is for a material that is ductile. The material shows plastic behavior over a range of forces before the material finally fractures.
Ductility is the ability of a material to be stretched into a new shape without breaking.
Ductility is one of the characteristic properties of metals.
A good example of this is aluminium, many things are made of aluminium. Aluminium is used for making everything from cool drink cans to aeroplane parts and even engine blocks for cars. Think about squashing and bending a cool drink can. Brittleness is the opposite of ductility.
When a material reaches a point where Hooke’s Law is no longer valid, we say it has reached its limit of proportionality. After this point, the material will not return to its original shape after the force has been removed. We say it has reached its elastic limit.
