More about stress and strain Flashcards
What is the characteristic of a metal wire loading and uloading curve? What is permanent extension?
the curve are the same provided the elastic limit is not exceeded
returns to original shape when unloaded
the unloading line is parallel to the loading line beyond its elastic limt - this is permanent extension
What on the axis of a unloading loading graph?
load (N) on the y
extension on x (xxxtension)
What is the characteristics of a loading-unloading graph of an elastic band?
the change of length during unloading is greater than during loading for a given change in tension
the rubber band returns to the same unstretched length, but the unloading curve is below the loading curve except at zero and maximm extension
the rubber band regains it initial length, but it has a low limit of propotionality
What are the characteristics of the loading-unloading graph of a polythene strip?
the extension during unloading is also greater than loading
the strip does not return to the same initial length when it is completely unloaded
the polythene strip has a low limit of propotionality and suffers plastic deformation
What does the area under the line of a force-extension graph show?
equal to work done to stretch the wire
What is the work done to deform an object?
referred to as the strain energy
What happens to molecules in a polythene strip that is being stretched?
before the strip is stretched, the molecules are tangles together
weak bonds form between the molecule
when it is under tension, it easily stretches as the weak bonds break
in this stretched state, new bonds form, and when the tension is removed the polythene strip stays stretched
What happens to the molecules in rubber when stretched?
also is a polymers, but its molecules are curled and tangled together when it is in an unstretched strain
when placed under tension, its molecules are straightened out, but these curl up again when the tension is removed, the rubber regains its initial length
Provided the limit of propotionallity is not exceeded, what is the equation for the elastic energy stored in a stretched wire or spirng?
= 1/2 TΔL
because the elastic limit is not reached, the work done is stored as elastic energy in the wire
because the graph of tension against extension is the same for loading and unloading, all the energy stored in the wire can be recovered when the wire is unloaded
What is the work done to stretch the rubber band represented as?
the area under the loading curve
What is the work done by the rubber band when unloaded?
area under the unloaded curve
What does the area between the loading curve and the unloading curve of an elastic band represent?
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 does this difference in the curve occur?
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 curve in polythene?
represents work done to deform the material permanently
as well as the internal energy retain by the polythene when it unstretches
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