Distinguish elastic and plastic deformation
Use recovery after unloading to distinguish elastic deformation from permanent plastic change and identify the elastic limit.
AS Level; Cambridge International Physics 9702, 2025–2027 syllabus, Topic 6. The wire investigation is an original study guide and data-analysis practice, not a supervised laboratory activity or proof of practical competence. Full timed assessment and human subject-teacher review remain outstanding.
Before you begin
- Use force, area, length, extension and graph gradient with consistent SI units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What evidence indicates plastic deformation?
What you will learn
- Define elastic and plastic deformation
- Identify elastic limit from unloading behaviour
- Distinguish elastic limit from limit of proportionality
Recovery determines whether deformation is elastic
Elastic deformation is reversible: after the load is removed, the material returns to its original shape and length, within measurement resolution. Plastic deformation leaves a permanent change after unloading.
The elastic limit is the greatest deformation or load from which the material still fully recovers when the load is removed. Beyond it, permanent deformation occurs. This boundary is about recovery, unlike the limit of proportionality, which marks departure from a straight-line force-extension relation.
Material behaviour can change gradually
A material can stop obeying Hooke's law before it reaches its elastic limit. A curved force-extension graph may still return to the original length when unloaded, so non-linearity alone does not prove plastic deformation.
Compare the original and final unloaded lengths to identify permanent extension. In practice, apply loads within a safe range and follow supervised lab procedures; this lesson focuses on interpreting the stated model and evidence.
Pause and explain
Can a material deform elastically outside its proportional region?
Worked example
A sample extends 5.0 mm under load. After unloading, its length is unchanged from the original measurement. Classify the deformation and explain the evidence.
Show the worked solution
- Compare the unloaded final length with the original length.
- There is no measured permanent extension.
- The response is elastic for this loading, within measurement resolution.
Answer Elastic deformation: the sample returns to its original length within the measurement resolution.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A wire returns to its original length after a load is removed. Is the observed deformation elastic or plastic? State the evidence.
Give me a hint
Focus on the unloaded length rather than the graph shape alone.
Compare my reasoning
- Compare the final unloaded length with the original length.
- They are equal within measurement resolution.
- So the deformation is reversible in this test.
Elastic deformation, because no permanent change remains after unloading.
Look for these in your work
- Recovery checked
- Evidence stated
- Term used correctly
A metal strip remains 2.0 mm longer after the load is removed. Classify its deformation.
Give me a hint
A residual extension is a permanent change.
Compare my reasoning
- The unloaded length differs from the original length.
- The 2.0 mm residual extension remains without the load.
- Therefore the deformation includes plastic change.
Plastic deformation: 2.0 mm of permanent extension remains.
Look for these in your work
- Residual measured
- Permanent change identified
- Classification correct
A sample's force-extension graph begins to curve, but it returns to its original length after unloading. What can you conclude about proportionality and elasticity?
Give me a hint
Use the graph shape for Hooke's-law range and recovery for elasticity.
Compare my reasoning
- The curved graph means force is no longer proportional to extension in that region.
- Returning to original length shows the response remains elastic for the applied loading.
- The limit of proportionality has been passed, but the elastic limit has not been shown to be exceeded.
It is outside the proportional region but still elastically deformed for the applied load; the two limits are distinct.
Look for these in your work
- Proportionality inferred from graph
- Elasticity inferred from recovery
- Limits distinguished
Common mistakes
- Calling any curved force-extension response plastic
- Confusing elastic limit with the limit of proportionality
- Claiming perfect recovery beyond the elastic limit
What can you explain now?
What evidence indicates plastic deformation?
Compare with the explanation
A permanent change remains after unloading
Plastic deformation is not fully reversible.
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Make it stick
What observation distinguishes elastic from plastic deformation after a load is removed?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.