3.4 Materials Flashcards
What is extension
When tensile forces act away from the centre of the spring and stretch it out
What is compressive deformation
Forces act towards the centre of the object and shorten it
What is Hooke’s Law
For a material within its elastic limit, the force applied directly proportional to the extension of the material
What is elastic limit
The greatest stress that can be applied without causing plastic deformation
What is the equation for Hooke’s law
Force (N) ∝ extension (m)
Force (N) = spring constant (Nm^-1) x extension (m)
F = kx
What is the force-extension graph for a string
Linear relationship to a point
- obeys Hooke’s law
- undergoes elastic deformation
- gradient = spring constant
After elastic limit
- plastic deformation
- gradient is curved
What is elastic deformation
The object will return to its original shape / length when the force is removed
What is plastic deformation
Permanent
The object will not return to its original shape / length when the force is removed
What is the force extension graph for a metal wire
Before elastic limit
- obeys Hooke’s law (linear)
- loading curve = unloading curve
After elastic limit
- plastic deformation
- unloading curve shows the wire is permanently extended
What is stress and state its units
The force applied to a material per cross sectional area unit
Newton per square metre (Nm^-2)
What is the equation for stress
Stress (Nm^-2) = force (N) / area (m^-2)
σ = F / A
What is strain and state its unit
The extension or compression of a material per unit of its original length
No units
What is the equation for strain
Strain = extension (m) / original length (m)
ε = x / L
What is Young’s modulus
The ratio of stress to strain
It measures the material’s stiffness, independent of its shape and size
What is the equation for Young’s modulus
Young’s modulus = stress / strain
E = σ / ε
What is a stress-strain graph for brittle marterials
What is stress-strain for brittle materials
- distort very little
- break / fracture under sufficient stress
- little elastic energy stored
What is stress-strain for ductile materials
- can be drawn into a wire (extensive plastic region)
- will break eventually
What is stress-strain for polymeric materials
- small degree of strain
- stretch easily to a point then become harder due to molecules arranged in squashed long chains
- subjected to vulcanisation (impurities make chains stronger and harder to stretch)
What is the area under a force extension graph
The elastic potential energy stored in a material
