This document discusses factor of safety in engineering design. It explains that factor of safety is used to account for uncertainties in loading conditions, material properties, manufacturing processes, and other factors by using an allowable stress lower than the material's theoretical strength. It also briefly outlines different failure theories for brittle versus ductile materials, noting that ductile materials experience plastic deformation before failure while brittle materials fail suddenly without yielding.

1. Module 3: Design for Strength
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2. Why Factor of Safety (FOS)
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The external loads cause internal stresses in the elements and the
component size depends on the stresses developed. Determination of stresses in structural
or machine components would be meaningless unless they are compared with the material
strength.
If the induced stress is less than or equal to the limiting material strength then the
designed component may be considered to be safe and an indication about the size of the
component is obtained. For design purpose an allowable stress is used in place of the critical
stress to take into account the uncertainties including the following:
Uncertainty in loading.
In-homogeneity of materials.
Various material behaviors. e.g. corrosion, plastic flow, creep.
Residual stresses due to different manufacturing process.
Fluctuating load (fatigue loading).
Safety and reliability.

3. Theories of Failure
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Failure occurs when Material starts exhibiting In-elastic behavior
Brittle and Ductile materials – different modes of Failures – mode of Failure Depends on
Loading
Ductile Materials – exhibit Yielding – Plastic Deformation before Failure
Brittle Materials – no Yielding – Sudden Failure
Yield/Ultimate Stress – Material property
Factor of safety (FS)