11.4 More about stress and strain Flashcards
How is the tension in a strip of material increased
By increasing the weight it supports
For a metal wire, Its loading and unloading curves are the same provided that…
Its elastic limit is not exceeded.
This means it returns to its original length when unloaded.
What happens to the unloading line beyond the elastic limit
It is parallel to the loading line
If the wire is slightly longer when unloaded, it has a ______ extension
Permanent
For a rubber band, the change of length during unloading is…
…greater than during loading for a given change in tension.
If the rubber band returns to the same unstretched length but the unloading curve is below the loading curve, the band remains elastic but has a …
…low limit of proportionality
Is the extension during unloading greater or less than the extension during loading
Greater
But the strip does not return to the same initial length when it is completely unloaded.
Has a low limit of proportionality AND suffers plastic deformation
What is strain energy
The work done to deform an object
What is the area under the line of a force extension graph
The work done to stretch the wire
What type of material is polyethene
A polymer
What does it mean if a material is a polymer
Its molecules are long chains of atoms
Before a polythene strip is stretched:
The molecules are tangled together. Weak bonds or cross links form between the molecules.
When polythene is under tension:
It easily stretches as the weak cross-links break. In this stretched state, new weak cross-links form and when the tension is removed, the strip stays stretched
What type of material is rubber
A polymer
Unstretched rubber:
Its molecules are curled and tangled together when it is unstretched
Rubber under tension
Its molecules are straightened out but these curl up again when the tension is removed. The rubber regains its initial length
The elastic energy stored in a stretched wire =
1/2 TΔL
What is the work done to stretch a rubber band represented by
The area under the loading curve.
What is the work done by the rubber band represented by
The area under the unloading curve
What does the area between the loading and unloading curves therefore represent for a rubber band
The difference between energy stored in the rubber band when it is stretched and the useful energy recovered from it when it is unstretched
Why is there a difference between energy stored in the rubber band when it is stretched and the useful energy recovered from it when it is unstretched
Because some of the energy stored in the rubber band becomes the internal energy of the molecules when the rubber band unstretches
What does the area between the loading and unloading curves represent for polythene
As it does not regain its initial length, the area between the loading and unloading curves represents work done to deform the material permanently as well as internal energy retained by the polythene when it unstretches
