9702 · 6.2
Elastic and plastic behaviour flashcards
Revision flashcards for Cambridge 9702 Elastic and plastic behaviour (syllabus 6.2). Flip, recall, then mark a real past-paper question.
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What is 'deformation' in physics?
Any change in an object's shape or size due to an applied force.
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Distinguish between 'tensile' and 'compressive' deformation.
Tensile deformation involves stretching by outward forces (tension), while compressive deformation is squeezing by inward forces (compression).
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What defines 'elastic deformation'?
A temporary change in shape; the object fully recovers its original form once the deforming forces are removed.
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What is 'plastic deformation'?
A permanent change in shape; the object does not return to its original form after forces are removed.
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State Hooke's Law.
The extension of an elastic object is directly proportional to the applied force, provided the limit of proportionality is not exceeded.
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Formula for Hooke's Law & what 'k' represents?
$$F = k\Delta L$$ where $F$ is force, $\Delta L$ is extension, and $k$ is the spring constant (a measure of stiffness).
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What is the 'limit of proportionality'?
The point on a force-extension graph beyond which Hooke's Law is no longer valid (the graph ceases to be a straight line).
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Define 'elastic limit'.
The maximum force a material can withstand and still return to its original shape after the force is removed. Beyond this, deformation becomes plastic.
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How is 'Elastic Potential Energy (EPE)' calculated?
For deformation within the limit of proportionality: $$EPE = \frac{1}{2}F\Delta L$$ or $$EPE = \frac{1}{2}k(\Delta L)^2$$
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Define 'stress' and give its formula & unit.
Stress ($\sigma$) is the applied force per unit cross-sectional area: $$\sigma = \frac{F}{A}$$. Unit: Pascals (Pa) or Newtons per square metre (Nm⁻²).
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Define 'strain' and give its formula & unit.
Strain ($\epsilon$) is the ratio of extension to original length: $$\epsilon = \frac{\Delta L}{L_0}$$. It is a dimensionless quantity (no unit).
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What is the 'Young Modulus' (E) and how is it calculated?
The Young Modulus is a measure of a material's stiffness. It's the ratio of stress to strain: $$E = \frac{\sigma}{\epsilon}$$ or $$E = \frac{F L_0}{A \Delta L}$$. Unit: Pascals (Pa).
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What is a ductile material? Give an example.
A material that can be drawn into a wire, showing large plastic deformation before fracturing. Example: Copper.
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What is a brittle material? Give an example.
A material that breaks with little or no plastic deformation. Example: Glass, cast iron.
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What does the area under a stress-strain graph represent?
The work done per unit volume (or strain energy per unit volume) to deform the material.
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What is the Ultimate Tensile Stress (UTS)?
The maximum stress a material can withstand while being stretched or pulled before necking and subsequent fracture. It's the peak of the stress-strain curve.
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What is a hysteresis loop on a stress-strain graph?
A loop formed when the unloading curve does not follow the loading curve. The area inside the loop represents the energy lost (dissipated as heat) per unit volume during a deformation cycle. This is common in polymers like rubber.