This document discusses different types of deformation in materials when forces are applied. It defines elastic deformation as when an object returns to its original shape after a distorting force is removed, and plastic deformation as when it is permanently stretched. It describes tensile and compressive forces, Hooke's law of elasticity, and the force constant. It explains the limit of proportionality and elastic limit on a force-extension graph.

1. Deformation of Materials 1

2. Deformation
• When a distorting force is applied to an object and then removed, the object
exhibits
elastic behaviour if it regains its original shape;
plastic behaviour if it is permanently stretched.

3. Tensile Forces
• Forces that stretch objects are known as tensile forces.
• Tensile forces are applied in pairs. To create tension in a fixed wire, equal and
opposite (“pulling”) forces must be applied to either end.
F
F

4. Compressive Forces
• Forces that reduce the length of objects are known as compressive forces.
• Compressive forces are applied in pairs. Two equal and opposite (“pushing”)
forces must be applied to either end, assuming the object is not accelerating.
F
F

5. Extension
• The increase in length of a sample subjected to tensile forces is known the
extension.
extension = length − natural length

6. Hooke’s Law
• Elastic deformation usually produces a straight line force (tension) versus
extension graph. In this case the object obeys Hooke’s law:
The extension of an elastic body is directly proportional to the force that
causes it.

7. Force Constant
• Mathematically, Hooke’s law is written
F = kx
where k is known as the force constant (or “spring constant”).
• The force constant tells us the force required per unit extension. It takes units
newtons per metre (Nm-1).
F
k is the gradient of the line
x

8. Limit of Proportionality/Elastic Limit
• The linear part of a force-extension graph ends at the limit of
proportionality.
• The elastic behaviour of an object ends at the elastic limit. This does not
necessarily coincide with the limit of proportionality.
• Behaviour beyond the elastic limit is described as plastic.
force
limit of
proportionality
elastic limit
Hooke’s law region
extension
permanent
extension