Stress–strain graphs and breaking stress Edexcel International A Level Physics revision
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In plain words
Pull a metal wire until it snaps, plot the whole journey, and the graph tells a story in stages. First the wire stretches in proportion. Then it starts to give. Then it suddenly stretches a lot. Finally it breaks. Each stage has a name.
4 things to know
- In order along the graph: the limit of proportionality (the line stops being straight), the elastic limit (beyond it the stretch is permanent), the yield point (the material suddenly stretches a lot for little or no extra force), and at the end, fracture.
- Breaking stress is the stress at which the material breaks.
- The gradient of the first, straight part of a stress–strain graph is the Young modulus: a steeper line means a stiffer material.
- The same ideas apply to squashing: there are force–compression graphs and compressive stress–strain graphs too.
Tips and tricks
- Stiff means a high Young modulus (a steep line). Strong means a high breaking stress. They are not the same thing.
- A stress–strain graph is the same for any sample of a material. A force–extension graph depends on the size of the sample.
It lands in your notebook with its questions as flashcards.
Stress–strain graphs and breaking stress: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is breaking stress?
It is a stress, so it doesn't depend on the thickness of the sample.
Beyond which point is the deformation of a wire permanent?
Up to the elastic limit the wire returns to its original length.
What happens at the yield point?
The material starts to flow.
What does a steeper straight line on a stress–strain graph show?
The gradient is the Young modulus.
Why is a stress–strain graph better than a force–extension graph for comparing materials?
Stress and strain allow for the sample's area and length.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 7 marks. Write your answers on paper, then check them.
Stress–strain graphs and breaking stress
Edexcel International A Level Physics WPH · 7 marks · papermunch.org
Name ______________________________ Date ______________
Describe what happens to a metal wire at the limit of proportionality, at the elastic limit and at the yield point.[3]
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Limit of proportionality: beyond it, stress is no longer proportional to strain. Elastic limit: beyond it, the wire does not return to its original length when unloaded. Yield point: the wire begins to stretch a great deal with little or no increase in force.
A wire of cross-sectional area 2.0 × 10⁻⁷ m² breaks when the force on it reaches 90 N. Find its breaking stress.[2]
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4.5 × 10⁸ Pa.
Material X has a steeper stress–strain line than material Y, but it breaks at a lower stress. Compare the two materials.[2]
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X is stiffer (it has the higher Young modulus), but Y is stronger (it has the higher breaking stress).
Answers: Stress–strain graphs and breaking stress
- 1. Limit of proportionality: beyond it, stress is no longer proportional to strain. Elastic limit: beyond it, the wire does not return to its original length when unloaded. Yield point: the wire begins to stretch a great deal with little or no increase in force.
- 2. 4.5 × 10⁸ Pa.
- 3. X is stiffer (it has the higher Young modulus), but Y is stronger (it has the higher breaking stress).