Factor of Safety and Margin of Safety

The factor of safety is something that design engineer always want to have.

Because it not only ensures a safe design but also gives the estimate cost of the components.
The American Civil Engineer,Henry Petroski book's title "Success through Failures",itself tells the essence of design engineers.

We get the point where the component fails under a load.And then the load applied will be in the limitations of that specified load.
Simple!
To specify the relation between that maximum load and the load we like the component to bear,the concept of factor of safety comes in light.
That maximum load is actually the strength specified by failure criteria and latter is called the working stress.So as we will always work under this strength .We can say,


Factor of Safety =     Strength      
                              Working Stress

Then what is Margin of Safety?
Are they both same?
NO!!!!
Margin of safety is a concept used in fields where material used must be less.Like aerospace industries for light weight structures.
Here we challenge the structure by decreasing the ratio of strength and working stress by 1.
So,load will be more with respect to the design dimensions or vice versa.
Simply we can say that
Margin of safety =Factor of safety - 1