3.4 Materials Flashcards
What is meant by extension and compression of a spring?
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,
F = 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 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 = 1/2 k (change in)L^2
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
