Stretching a spring Strain energy Stretching materials
Describing deformation
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Stretching a Spring
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Hookes Law states that The extension is proportional to the
force The spring will go back to its original length when the force
is removed So long as we do not exceed the elastic limit
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Graphs
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Interpreting Graph Strain Energy
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Stored ability to do work due to stretching or compression or
displacement
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Combination of Springs
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Deformation of Rubber Band Hysteresis
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Deformation of Rubber Band Hysteresis
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Elastic versus Plastic Elastic Behaviour Material has the
ability to go back to its original shape Elastic Limit A point
where beyond it the material is permanently deformed
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Elastic versus Plastic Plastic Behaviour Material has been
permanently deformed but not broken
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Describing Deformation Stress Measure of force required to
cause a particular deformation Force per unit area Pressure Units:
Nm 2 Pascal
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Describing Deformation Strain Resulting deformation Extension
divided by original length Dimensionless quantity
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Testing Materials
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Describing Deformation Young Modulus, Y Ratio of tensile stress
to tensile strain Units: Nm - 2 Pascal
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Some interesting values of Young Modulus DNA ~ 10 8 Pa
spaghetti (dry) ~ 10 9 Pa cotton thread ~ 10 10 Pa plant cell walls
~ 10 11 Pa carbon fullerene nanotubes ~ 10 12 Pa
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Materials
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Graph
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Interpreting Graph Slope = Young Modulus Ultimate Tensile
Stress Maximum stress a material can withstand before breaking
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Measuring Young Modulus
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Describing Deformation Curve A shows a brittle material. Strong
The fracture of a brittle material is sudden and catastrophic
Example: cast iron
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Describing Deformation Curve B is quite brittle and slightly
ductile Brittle but deforms before breaking Example: steel
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Describing Deformation Curve C is a ductile material Deforms
permanently Drawn into thin wires Examples are copper and gold
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Describing Deformation Curve D is a plastic material. Deforms
permanently Deformation is not proportional to stress applied
Example is polyethylene