Materials Flashcards
Glover (14 cards)
Young’s modulus
A property of a material describing how easily it can stretch and deform (doesn’t matter the size and shape of the object, as it’s a material property)
What does the extension of a wire depend on?
The wire’s cross sectional area, length and material
What do brittle stress-strain graphs look like and why?
Straight line through the origin (where it stops, the material has broken) - they don’t stretch much, but when the stress gets too high, the object breaks suddenly
May curve slightly at the end
What do ductile material stress-strain graphs look like and why?
Ductile materials have a big elastic region (linear line through the origin) but after the elastic limit the material can no longer return to its original form and the graph curves towards the x-axis with a lowish gradient - the line is “drawn out”. There is a second peak (UTS) where the object cannot be stressed anymore or it will stretch and break (fracture point).
plastic region
Levels of stress on a stress-strain graph where the material has plastically deformed (i.e. after the elastic limit)
plastic material (e.g. plastic spoon) stress-strain graph
Plastic materials aren’t very strong but can withstand a lot of strain - initially very small gradient that decreases
plastic deformation loading then unloading stress-strain graph with explanation
As the load is decreased, the bonds in the material are re-aligned permanent extension -> doesn’t go through origin
The gradient remains the same as when loading because the intermolecular forces are identical to before.
yield point
point just before a small increase in stress leads to a large increase in strain (and so extension)
Why does the gradient change after the yield point?
Bonds between particles holding material together start to break -> increasing extension at a higher rate
What is the graph like between the UTS and point of fracture on a stress-strain graph and why?
The tension in the material decreases as it stretches -> strain increases as stress decreases
importance of UTS
It’s the maximum stress a material can withstand before it weakens and breaks
Explain a rubber loading and unloading curve before its elastic limit.
loading curve is concave then convex, but the unloading curve is steeper at higher extensions and less steep at lower extensions (unloading curve is below loading curve)
This is because the rubber chains stretch out and don’t re-tangle when unloading; weak molecular bonds are broken during loading, which requires more force
How to find elastic strain energy from a graph?
area under force-extension graph i.e. 1/2 F Δl as it’s a triangle (= 1.2 ke * e)
= energy stored in object
How is energy converted between stores in a vertically hanging spring with a load?
Max Ep -> completely unstretched
Max Ek -> when spring is being stretched and it’s extending the fastest when in the equilibrium position
Max Ee -> full extension (no Ek)
