This paper outlines my learning towards basic approach for structural design for fatigue life when physical test data is not available.

1. Structural Design and Fatigue Strength
Ravikesh Khdatare
MS Manufacturing Management
BE Automotive Engineering
Most of the on road and off road vehicles/equipment’s are designed for heavy load carrying capacity
and long life. Structural design is most important part of product design function.
Structural design is basically done by focusing on static and dynamic performance of
component/machine. It is always easy to design a structure for static loading, and with following four
fundamental formulas we can design and predict stress on most of components.
With these static calculations we can decide geometrical dimensions of component and select
appropriate material based on material yield strength. With general practice and guidelines we select
material yield strength based on F.O.S (Factor of safety) allowed for particular component and generally
it’s around 1.5 to 2.
At this stage we have made sure that our design is safe and it assures us that component will not fail
even though loaded two times of the rated load.
Here point comes that customer will not use my vehicle/equipment for just stationary loading like table
or chair, right! As mentioned earlier, it is also important to design structural component of automotive
for dynamic performance. Dynamic is French word which means that, “A force that stimulates change or
progress within a system or process”.
So to deal with this cyclic force now we design components for Fatigue life. In general term fatigue is
defined as failure of component under cyclic loading. So what happens to component in cyclic loading?
Because of repeated load there a failure mechanism occurs wherein small crack can propagate to such

2. an extent that failure of the considered detail occurs. Hence it is important to find out strength of
material to withstand such cyclic loading, which is nothing but fatigue strength.
There are different theories and approaches are defined for life expectancy of product. Generally those
are broadly based on following design approaches;
1. Infinite-Life Design: This is oldest method of design wherein importance is given to unlimited
safety and life of product. To achieve infinite life it requires local stresses or strains to be
essentially elastic and safely below the fatigue limit. This approach is not economical cost and
competitiveness in market. (i.e to reduce stress you are ending up to adding more weight to
components)
2. Safe-Life Design: In this approach component are designed for a finite life. Generally this
approach is used in automotive industry. As we are pre-defining life of product it is easy to
decide allowable stress or strain. A design criterion is based on stress-life, strain-life, or crack
growth relations.
3. Fail-Safe Design: Fail-safe design requires that if one part fails, the system does not fail. Fail-
safe design recognizes that fatigue cracks may occur and structures are arranged so that cracks
will not lead to failure of the structure before they are detected and repaired.
To design component on Safe- Life Design approach we must know life expectancy of component in
terms of hours or cycles of usage. With physical testing we can easily plot S-N curve and predict stress at
particular cycle and accordingly decide fatigue strength. But it is always not possible to do testing each
and every case.
What if you are designing new product and do not have S-N curve data available? Following example will
illustrate how to find fatigue strength of design based on ultimate strength.
Example:
Calculate the approximate fatigue strength vehicle frame at 10^3cycles. Assume that ultimate tensile
strength of the steel (SUT)= 80,000 psi and yield strength (Sy ) = 60 ksi
Given: (SUT) = 80 ksi, (Sy ) = 60 ksi & n = 1000
Formulae’s:
⁄
(common materials lies in the range of -0.12 to - 0.05)

3. = Number of loading cycles corresponding to the endurance limit (Generally cycles)
n = Number of loading cycle for which the fatigue strength need to be calculated
Solution:
Step: 1 (Calculate endurance Strength)
= 0.5 = 40 ksi
Step : 2 ( Calculate fatigue strength coefficient )
Step : 3 (Calculate the fatigue strength exponent )
⁄
= - 10
⁄
= - 0.077
Step : 4 (Calculate the fatigue strength)
= 50.12
The fatigue stress design calculation explained here is useful for a design engineer in case if he/she don’t
have an S-N curve data for the material of their design interests. However, the method has the following
limitations:
1. It should only be used for the component of steel
2. It produces reasonable results only for the high cycle fatigue strength analysis
In above example if we have designed frame considering static loading with F.O.S of 1.5 then our
allowable stress = 60 ksi / 1.5 = 40 ksi
But if we design frame for safe life considering fatigue then allowable stress = 50.12 ksi / 1.5 = 33.4 ksi