Definition Of Factor Of Safety

Definition Of Factor Of Safety

Factor of safety

It is defined as the ratio of maximum stress to permissible or working stress.

In case of ductile material

FOS = yield stress/working stress

In case of brittle material

FOS = ultimate stress/working stress

FOS is number which is always greater than 1.

Stress-strain diagram for ductile material

In case of ductile material the moment a material reaches at point E after that it can break at anywhere. If ductile material reaches at maximum stress after that it can break at any other point. This maximum stress for ductile material. It is called yield stress.

As design engineers are aim should be stop the material by reaching maximum stress value that is we want the material to operate in region where the maximum stress does not reach point E so that we can use factor of safety.

For example,

FOS = 3,

Yield stress (σyt) = 300 N/mm2
σyt = σmax
FOS = σyt/σw
σw = σyt/FOS = 300/3 = 100 N/mm2
Working stress = 100 N/mm2
Because of FOS, The working stress on the material is reduced. Working stress is that stress, with helped of we are designing the material or design of product.
FOS prevent the failure of components.

In case of brittle material, brittle material fail at stress value which is called as ultimate stress. Ultimate stress is maximum stress. So now if we have using brittle material to design some component then that brittle material they fail suddenly. Where brittle material subjected to load from out side we can not see any difference in that material but internally it creaks are developed and finally that creaks reached the surface and material breaks. So that is the behaviour of brittle material.
Brittle material breaks at ultimate stress.
For example,

FOS = 4,

Ultimate stress (σut) = 400 N/mm2
σut = σmax
FOS = σut/σw
σw = σut/FOS = 400/4 = 100 N/mm2
Working stress = 100 N/mm2
Because of FOS the Ultimate stress is reduced.
Hence material will not fail.

Factors on which FOS depends :

1. Material selection : 

Factor of safety value is more then 1. But we have to select the material depending the type of material selection.

2. Type of loading: 

Statics, variable, or shocks.

3. Cost of economy :

If we increase the material cost then over all cost of machine will increase so we have to take FOS and size of relation.

4. Important of machine parts in complete machine :

If we are design any single part of machine. So we have to determine what is the significant of part like even screw, nut and bolts, they are parts of the machine. So have to understand what type of loading can acting in that screw, nuts and bolts. So we analyse the importance of machine parts.

5. Safety Of Human Life.

6. Life of components 

If we increase life of components then we have increase FOS value.