Effect of Fluctuating Load..

Ankush
Member • Feb 25, 2014

Effect of Fluctuating Load..

Hello friends,
We all know that because of dynamic load the strength of a body or member decreases like in the case of bending and unbending the wire in order to break it instead of applying axial force on it. So in order to design a body having dynamic load acting on it, we consider endurance limit instead of U.T.S. . But i want to know that why the strength of material decreases because of dynamic load? what happens inside the material that the strength of material decreases only because of dynamic load not in static load?
Waiting..

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Siddhant Mukherjee

Member • Mar 5, 2014

@#-Link-Snipped-# : Fluctuating load leads to fatigue. When we stress a material, and the stress is somewhere around yield strength the dislocations in a material start to move and when these dislocations move to other place plastic deformations occur. In fatigue, since the stresses are fluctuating, the dislocations tend to move in to and fro manner like a vibration resulting in cracks. These cracks act as stress raisers and material fails at a much less stress than the UTS.

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Sekocan

Member • Mar 12, 2014

As I know, fluctuating loads (which may be seen on a simple rotating shaft) also leads crack propogation. Micro cracks in the material may grow and cause sudden failure.

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Ankush Sharma

Member • Jun 2, 2016

Sekocan
As I know, fluctuating loads (which may be seen on a simple rotating shaft) also leads crack propogation. Micro cracks in the material may grow and cause sudden failure.
But even micro cracks also present in materials which are loaded statically and in these materials crack propagate only when the value of yield strength of material is equal to critical stress (brittle materials) and we also loaded material upto yield strength and when the value of yield strength is less than critical stress( ductile materials) and again we loaded up to yield strength . So why don't the material fail before yield strength in this normal loading as in the case of fluctuating load.

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Ankush Sharma

Member • Jun 2, 2016

S.mukherjee
@#-Link-Snipped-# : These cracks act as stress raisers and material fails at a much less stress than the UTS.
But even micro cracks also present in materials which are loaded statically and in these materials crack propagate only when the value of yield strength of material is equal to critical stress (brittle materials) and we also loaded material upto yield strength and when the value of yield strength is less than critical stress( ductile materials) and again we loaded up to yield strength . So why don't the material fail before yield strength in this normal loading as in the case of fluctuating load.

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