Strength of Materials
Prof. M. S. Sivakumar
Indian Institute of Technology Madras
1.14. Design considerations:
A good design of a structural element or machine component should ensure that the
developed product will function safely and economically during its estimated life time.
The stress developed in the material should always be less than the maximum stress it
can withstand which is known as ultimate strength as discussed in section 1.6.
During normal operating conditions, the stress experienced by the material is referred to as
working stress or allowable stress or design stress.
The ratio of ultimate strength to allowable stress is defined as factor of safety.
Ultimatestress
Factorof safety
Allowablestress
=
1.26
Factor of safety can also be expressed in terms of load as,
Ultimateload
Factor of safety
Allowableload
=
1.27
Equations 1.26 and 1.27 are identical when a linear relationship exists between the load
and the stress.
This is not true for many materials and equation 1.26 is widely used in design analysis.
Factor of safety take care of the uncertainties in predicting the exact loadings, variation in
material properties, environmental effects and the accuracy of methods of analysis.
If the factor of safety is less, then the risk of failure is more and on the other hand, when
the factor of safety is very high the structure becomes unacceptable or uncompetitive.
Hence, depending upon the applications the factor of safety varies. It is common to see
that the factor of safety is taken between 2 and 3.
Stresses developed in the material when subjected to loads can be considered to be
uniform at sections located far away from the point of application of loads.
This observation is called Saint Venant’s principle and was discussed in section 1.3.
But, when the element has holes, grooves, notches, key ways, threads and other
abrupt changes in geometry, the stress on those cross-sections will not be uniform.