5. Materials Flashcards
What is density?
‘compactness’ of a substance
What is the approximate density of water at atmospheric pressure and room temperature?
1000 kgm^3
What is the relationship between change in length and force applied?
change in length is directly proportional to force applied
What is Hooke’s law?
force is directly proportional to the change in length up to the limit of proportionality
What is the equation for Hooke’s law?
force = stiffness constant x change in length
What is stiffness constant?
force needed to extend an object by 1 metre
What does stiffness constant depend on?
- material
- length
- shape
On a force against extension graph, what is the point where the straight line ends known as?
limit of proportionality
How is the force shared when springs are arranged in parallel?
force is shared equally
How is the force shared when springs are arranged in series?
they don’t share the force, they each carry the full weight
What happens when an object undergoes an elastic stretch/deformation?
it returns to its original shape once the load has been removed
What happens when an object undergoes an plastic stretch/deformation?
it doesn’t return to its original shape once the load is removed
When does plastic deformation occur?
when you stretch an object past it’s elastic limit
What does the area under a force against extension graph represent?
- work done to stretch the object by that change in length
- elastic potential energy now stored
How are energy and extension related?
energy is directly proportional to extension squared
What happens to the work put in to stretching a string in an elastic stretch when the force is removed?
it is released
What happens so some of the energy in a plastic stretch?
- it is used to rearrange the bonds
- it is lost as heat
What is stress?
force divided by cross-sectional area
What is strain?
the change in length divided by the original length
What is the gradient of the straight section of a force against extension graph?
stiffness constant
What is the area under a force against extension graph?
elastic potential energy
What is the gradient of the straight section of a stress against strain graph?
Young’s modulus
What is the area under a stress against strain graph?
elastic potential energy per unit volume
What is ultimate tensile stress?
the maximum stress a sample can withstand
What is breaking stress?
the stress at which a material breaks
What is Young’s modulus?
a measure of the stiffness of the material
What is yield stress?
the force per unit cross-sectional area after which the material exhibits a large increase in strain for no increase in (or less) stress
How does a brittle material break?
will break without warning and leave flat surfaces
How does a ductile material break?
will deform and stretch slowly before breaking and leaving a deformed surface
State two examples of brittle materials
glass, china
State two examples of ductile materials
rubber, metals
What is ‘necking’?
- where the object gets thinner
- concentrates force over a smaller cross-sectional area meaning force can be reduced but still cause an increase in strain
What is brittle fracture?
when stress is applied, any tiny cracks on the material’s surface focus the stress and get bigger until the materials breaks completely leaving a flat break
