6 - Materials Flashcards
What do you need to do to express or extend a spring?
Work.
What happens if you extend a spring beyond it’s elastic limit?
Energy/work done is not recoverable and is transferred to the material to permanently change the relative position of the atoms.
What is the elastic potential energy/ work done of the spring?
Area under the line - W = Fx.
What is e?
Permanent extension.
What is the equation for elastic potential energy?
E = 1/2Fx or W = 1/2kx^2.
What is the relationship between elastic potential energy and extension?
E directly proportional to x^2.
What does the graph look like for a metal wire/spring?
Straight line then curved after elastic limit.
What does the graph look like for a rubber band?
.Curved up then curved down but back to same point.
.Doesn’t obey Hooke’s Law.
.Hysteresis loop.
.More work done to stretch than to unload due to change in thermal energy - band heats up.
How do you work out the Young modulus of a material?
.Measure csa.
.Place a marker.
.Add weight on end.
.Measure new distance from og marker place.
.Work out force - mg.
.Plot F/x
.Work out young Modulus.
Define ductile.
Material can be drawn into a wire.
What does the total extension of a wire depend on?
.Initial wire length.
.Initial wire diameter.
.Tension in the wire (forces applied).
.Material the wire is composed of.
What is the equation for tensile stress?
Force/csa (Pa).
What is the equation for tensile strain?
Extension/og length (no units)
What points are there on a stress-strain graph?
.Limit of proportionality - linear up to here.
.Elastic Limit - obeys Hooke’s Law up to here.
.Yield points - material rapidly extends.
.Breaking point.
.UTS - Ultimate tensile stress it withstands before breaking.
What type of materials have a large UTS?
Strong ones.
What is the Young Modulus?
Stress/Strain before limit of proportionality.
What does it mean if a material has a high YM?
Stiff.
What is the stress-strain graph of a brittle material?
.Linear and breaking point.
.Elastic behaviour up to BP.
.Glass/cast iron.
What is the stress-strain graph of a polymeric material e.g. rubber?
.Long molecular chains.
.Elastic properties.
.Hysteresis loop.
.Thermal energy.
What is the stress-strain graph of a polymric material e.g. Polythene?
.Plastic behaviour.
.Linear then plateaus.
