Subject: Machine Design -I

Topic: Machine Design consideration

Difficulty: Medium

A static load is a force, which is gradually applied to a mechanical component or system which does not change its magnitude or direction with respect to time. Engineering materials are classified into two groups ductile and brittle materials. A ductile material e.g Al, structural steels is one which has a relatively high tensile strain before fracture takes place. On the other hand, a brittle material e.g. cast iron has a relatively low tensile strain before fracture.

Three Modes of Failure:

(a) Failure by elastic deflection.

(b) Failure by general yielding.

(c) Failure by fracture

Failure by elastic deflection: In applications like transmission shaft supporting gears, the maximum force acting on the shaft, without affecting its performance, is limited by the permissible elastic deflection. Lateral or torsional rigidity is considered as the criterion of design in such cases.

Failure by general yielding: A mechanical component made of ductile material loses its engineering usefulness due to a large amount of plastic deformation after the yield point stress is reached. Considerable portion of the component is subjected to plastic deformation, called general yielding

Failure by fracture: Components made of brittle material cease to function satisfactorily because of the sudden fracture without any plastic deformation.