When deforming force acting on a body produces change in the volume of the body , the stress is called -
a)Tensile stress
b)Hydraulic stress
c)Shear stress
d)Longitudinal stress
Answers
further increased from Q onwards up to R, then for a small increase in the stress shows a larger increase in strain. Now if the wire is unloaded at this point R, the graph will traverse a new path RO', instead of RQPO thus material can be said to have a permanent set. This shows that even if stress is zero, strain will have some value. So a permanent strain represented by OO� as shown in figure is produced after the point Q. In portion PQR of graph Hooke�s law fails as the extension in the wire is partly elastic and partly plastic in behaviour. The curve OPQR corresponds to plastic behavior of material. A plastic behavior is irreversible. Beyond the point R, the wire shows virtual increase in strain with no increase in stress. Beyond point R, the graph indicates that the length of the wire increases even if the wire is unloaded. The thinning of the wire starts and the necks and waists (i.e., constrictions) are developed at few weaker portions in the wire and finally the wire breaks at a point denoted by S called breaking point. The stress corresponding to point S is called breaking stress or ultimate stress or tensile stress of the wire. If large plastic deformation takes place between elastic limit and breaking point, the metal is said to be ductile but if, in the case the material experience fracture just after elastic limit is crossed, then the material is said to be brittle.
Now we are in a position to redefine materials types
Ductile materials
Materials which have large plastic range beyond elastic limit whose breaking point is widely separated from the point of elastic limit on the stress-strain graph are called ductile materials, e.g., mild steel, copper etc.
Brittle materials
The materials which break as soon as the stress is increased beyond the elastic limit are called brittle. For such materials, the breaking point is near the elastic limit, e.g., cast iron or glass.
Elastomers
Elastomers are those materials for which the stress and strain variation is not a straight-line within the elastic limit and strain produced is much larger than the stress applied. The breaking point lies just near the elastic limit, e.g., rubber.
Elastic after-effects
We know that when the deforming force is removed the bodies regain their original state. It is found that some bodies return to their original state immediately, but others may takes some time to recover their original states. This delay in regaining the original state after the removal of the deforming force is called elastic after-effect. In measuring instruments such as galvanometer and the electrometers the suspensions are made from quartz and phosphorus bronze are used instead of glass fiber, as elastic after-effect is negligible in quartz and phosphorus bronze wires while for glass fiber it is comparatively large.
Elastic fatigue
Elastic fatigue is the property of an elastic body by virtue of which its behavior becomes less elastic under the action of repeated alternating deforming forces. Let us take an example of wire under vibration. The wire will continue to vibrate till its vibrations dies out.
If the wire is made to vibrate once again, then its vibrations dies out quick y due to repeated alternating stress. The elastic body is relived of the fatigue when allowed to rest for some period similar is the case with us.
Tension and compression
We have already discussed the concepts of tensile and compressive stresses. Now consider a wire of uniform cross-sectional area that is suspended vertically from a rigid support through one end and attach known weights at the other end and note down the extensions as shown in the figure below
Measurement of extensions produced in the wire
The stress and strain for various observations are then calculated and a graph between stress and strain for the stretched wire under observation is plotted. The stress equals the force acting per unit area
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If elastic forces developed are perpendicular to the area of cross section of the body then the stress developed is known as normal stress. The normal stress are of two types, tensile and compressive stress, accordingly as there is a increase or decrease in length or volume of body on application of force.