- Created by: Charlottelg
- Created on: 09-12-18 11:38
A stress causes a strain
A material subjected to a pair of opposite forces might deform.
Forces stretching material = tensile.
Forces squashing material = compressive.
Stress = tension (force applied) / CSA.
Units of stress = Nm^-2 or pascals (Pa).
Strain = extension (change in length) / original length.
Units of strain = none, given as a number or percentage.
Doesn’t matter whether forces producing stress and strain are tensile or compressive - same equations apply.
Think of tensile forces as positive and compressive forces as negative.
A stress big enough to break a material is called
Greater and greater tensile force applied to material - stress on it increases.
Effect of stress is to start to pull atoms apart from one another.
Stress becomes so great that atoms separate completely, material fractures.
Shown on stress-strain graph where it curves towards the bottom. Fracture stress - the stress at which this occurs.
UTS = ultimate tensile strength - maximum stress material can withstand before breaking.
Engineers have to consider UTS and fracture stress of materials when designing a structure.
Elastic strain energy is the energy stored in a st
Material is stretched or compressed - work is done in deforming material.
Graph of force against extension - work done is given by area under graph.
Before elastic limit - all work done in stretching or compressing material is stored as energy in material.
This stored energy is called elastic strain energy.
You can calculate the energy stored in an elastic
Provided a material obeys Hooke’s Law, strain energy stored inside it can be calculated.
Work done on an elastic material in stretching = energy stored in material as elastic strain energy.
Work done = force X displacement.
Force on material isn’t constant. It rises from zero up to force (F). To calculate work done, use average force between zero and F:
Work done = 1/2FX.
Elastic strain energy (E) = E = 1/2FX
As Hooke’s Law is being obeyed, F = Kx, F can be replaced in the equation to give:
E = 1/2kx^2.
Material stretches beyond elastic limit - some work is done separating atoms. This energy will not be stored as elastic strain energy, and so isn’t released when force is removed.