Machine Design
The cracks in helical springs used in railway carriages usually start on the inner side of the coil because of the fact that

It has a lower curvature than the outer side
It is subjected to a higher cyclic loading than the outer side
It is subjected to a higher stress than the outer side
It is more stretched than the outer side during the manufacturing process

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Machine Design
The plasticity is the property of a material which enables it to

Regain its original shape after deformation when the external forces are removed
Draw into wires by the application of a tensile force
Retain deformation produced under load permanently
Resist fracture due to high impact loads

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