Modes of Failure
When a mechanical component is designed then there are mode of failure also be considered in designing of the component.
There are following three modes of failure :-
1- Failure by elastic deflection
2- Failure by general yielding
3- Failure by fracture
1- Failure by elastic deflection:- The failure which takes place within the elastic limit of stress - strain curve is known as failure by elastic deflection.
In applications like transmission shaft supporting gears , the maximum force acting on the shaft, without affecting it's performance, is limited by the permissible elastic deflection.
Lateral or torsional Rigidity is considered as the criterion of design in such cases.
2- Failure by general yielding:- A considerable portion of the component is subjected to plastic deformation , is called general yielding and the failure in any material due to general yielding is known as failure by general yielding.
A mechanical component made of ductile material losses it's engineering usefulness due to a large amount of plastic deformation after the yield point stress is reached.
3- Failure by fracture:- The failure in brittle materials due to sudden and total fracture is known as failure by fracture.
This type of fracture is happened in brittle materials because these materials have the ability of the sudden fracture without any plastic deformation.
In applications like transmission shaft supporting gears , the maximum force acting on the shaft, without affecting it's performance, is limited by the permissible elastic deflection.
Lateral or torsional Rigidity is considered as the criterion of design in such cases.
2- Failure by general yielding:- A considerable portion of the component is subjected to plastic deformation , is called general yielding and the failure in any material due to general yielding is known as failure by general yielding.
A mechanical component made of ductile material losses it's engineering usefulness due to a large amount of plastic deformation after the yield point stress is reached.
3- Failure by fracture:- The failure in brittle materials due to sudden and total fracture is known as failure by fracture.
This type of fracture is happened in brittle materials because these materials have the ability of the sudden fracture without any plastic deformation.
-: Factor of safety :-
The ratio of maximum stress to the working stress is known as factor of safety ( FOS ).
Selection of FOS:-
Before selecting a proper factor of safety, a design engineer should consider the following points -
1- The reliability of the properties of the material and change of these properties during service.
2- The reliability of test results and accuracy of application of these results to actual machine parts.
3- The reliability of applied load.
4- The certainly as to exact mode of failure.
5- The extent of simplyfing assumptions.
6- The extent of localised stress.
7- The extent of initial stresses set up during manufacturing .
8- The extent of loss of life if failure occurs.
9- The extent of loss of property if failure occurs.
Consideration of higher FOS :-
A higher factor of safety is chosen under the following conditions-
1- When the material of the machine has a non - homogeneous structure.
2- When the machine components is subjected to impact force in service.
3- When the machine components is working in a corrosive atmosphere.
4- When the machine part is subjected to high temperatures during the operation.
5- When higher reliability is demanded in applications like components of aircraft.
6- When the quality of manufacture of the machine part is poor.
-: Factors affecting FOS :-
There are following factors which affects the factor of safety ( FOS ) -
1- Effect of failure
2- Type of load
3- Degree of accuracy in force analysis
4- Material of components
5- Reliability of component
6- Cost of component
7- Testing of machine element
8- Service conditions
9- Quality of manufacture
No comments:
Post a Comment