11.4 - more about stress + strain

how do we increase the tension in a strip of material?

by increasing the weight it supports in steps

for a strip of material, what does increasing the weight it supports in steps do?

increase its tension

when increasing the tension in a strip of material, why do we increase the weight it supports in steps as opposed to all at once?

we need to take extension measurements at each step in order to plot a load-extension graph, so weight is not increased all at once

what is a load-extension graph?

here

• used for showing loading and unloading curves

• y = load

• x = extension

• made by increasing the weight a material strip supports in steps to increase material tension, and measuring extension at each step

what are load-extension graphs used for?

loading and unloading curves

how do we make a load-extension graph?

1. increase the weight a material strip supports in steps to increase material tension

2. measure extension at each step (added weight, for loading)

3. also measure extension at each weight removed (unloading)

4. extension and load are plotted for loading and unloading

what does loading mean?

the extension due to the weight the material supports being added in steps

what does unloading mean?

the return to original length L (reverse extension) with the weight the material supports be removed in steps

what is the load-extension graph for a metal wire when elastic limit is not exceeded?

here

• loading and unloading graphs the same

• linear

here

load-extension graph for metal wire that has not surpassed elastic limit

here

load-extension graph for metal wire that has surpassed elastic limit

what is the load-extension graph for a metal wire when elastic limit is exceeded?

here

unloading line is parallel to the loading line

on a load-extension graph for a metal wire, why is the unloading graph parallel to the loading graph when elastic limit has been exceeded?

wire is slightly longer when unloaded as it has a permanent extension from being plastically deformed

what happens to a metal wire when elastic limit is exceeded?

• wire is slightly longer when unloaded as it has a permanent extension from being plastically deformed

• on a graph, the unloading line is parallel to the loading line

when does a metal wire undergo permanent extension?

when elastic limit is surpassed in loading

what does it mean when a material’s loading and unloading graphs are the same?

• that the material will return to its original length when unloaded

• the material is not plastically deformed

which of these material’s loading and unloading graphs are the same - wire, rubber, or polyethene?

wire, providing elastic limit has not been surpassed

on a load-extension graph, what does it mean when the graph finishes at a different position to the one it started with?

that the material has been plastically deformed (by surpassing its elastic limit) and has permanent extension (because its extension is non zero compared to its unstretched length L)

what does a plastically deformed material look like on a load-extension graph?

unloading graph finishes (touches the x-axis) at a different place to where it starts, as it now has permanent extension from being plastically deformed

what is the load-extension graph for rubber?

here

• extension during unloading > extension during loading , meaning it has a low limit of proportionality

• returns to the same initial point, meaning it remains elastic as it regains initial length

• the loading and unloading graph are only equal at zero and maximum extension