3.4 Materials Flashcards
What is meant by extension and
compression of a string?
Tensile forces act away from the centre of the
spring in both directions and will stretch it out
causing the spring to extend.
Forces acting towards the centre of the spring in
both directions is called compression.
What happens to the spring when
tensile/compressive forces are exerted?
The spring undergoes ‘tensile deformation’ (for
tensile forces) or ‘compressive deformation’ (for
compressive forces).
What is Hooke’s law?
In a material showing Hooke’s law, the force
applied is directly proportional to the extension in
length, up to the limit of proportionality.
[k = spring constant, ∆x = change in length]
What is meant by tensile stress?
The force per unit area.
What is tensile strain?
A measure of how the material stretches: the
extension divided by the original length. This
value is a ratio, so it has no units.
What is the difference between elastic
and plastic deformation?
Elastic deformation: when the force is removed
the object will return to its original shape
Plastic deformation: after the force is removed,
the object will not return to its original shape (limit
of proportionality has been exceeded).
How is energy stored during elastic
deformation?
The work done is transferred and stored as
elastic potential energy.
Describe the energy changes that occur
during plastic deformation.
The material is stretched and the energy from the
work done is used to break the bonds between
the molecules. This causes permanent
deformation.
What is breaking stress?
The amount of stress a material can take without
it breaking.
What is meant when a material is
described as brittle?
It does not extend much when a force is applied
(tensile strain stays low). The material tends to
break rather than stretch under a large force.
What is the elastic limit?
The point after the which plastic deformation
occurs. It is sometimes also referred to as the
‘limit of proportionality’.
What does the area underneath a
force-extension graph represent
The energy stored in the material.
Give the equation that calculates elastic
strain energy in terms of spring constant
and extension.
E = ½ k ΔL2
What is Young’s modulus?
Young’s modulus = tensile stress / tensile strain
How do you find the Young’s modulus
from a stress-strain graph?
Using the gradient of the line.
