Page 301 - Marks Calculation for Machine Design

P1: Shashi
January 4, 2005
Brown˙C07
Brown.cls
stresses, corrosion, electrolytic plating, metal spraying, cyclic frequency, frettage corrosion,
and stress concentration. 15:4 FATIGUE AND DYNAMIC DESIGN 283
Residual stresses can improve the endurance limit if they increase the compressive
stresses, especially at the surface through such processes as shot peening and most cold
working. However, residual stresses that increase the tensile stresses, again especially at the
surface, tend to reduce the endurance limit.
Corrosion tends to reduce the endurance limit as it produces imperfections at the surface
of the machine element where the small cracks associated with fatigue failure can develop.
Electrolytic plating such as chromium or cadmium plating can reduce the endurance limit
by as much as 50 percent.
Like corrosion, metal spraying produces imperfections at the surface so it tends to reduce
the endurance limit.
Cyclic frequency is usually not important, unless the temperature is relatively high and
there is the presence of corrosion. The lower the frequency of the repeated reversed loading
and the higher the temperature, the faster the propagation of cracks once they develop, and
therefore, the shorter the fatigue life of the machine element.
Frettage is a type of corrosion where very tightly fitted parts (bolted and riveted joints,
press or fits between gears, pulleys, and shafts, and bearing races in close tolerance seats)
move ever so slightly producing pitting and discoloration similar to normal corrosion. The
result is a reduced fatigue life because small cracks can develop in these microscopic
areas. Depending on the material, frettage corrosion can reduce the fatigue life from 10 to
80 percent, so it is an important issue to consider.
Stress concentration is the only miscellaneous effect that can be accurately quantified. In
Chap. 6, Sec. 6.1.3, a reduced stress concentration factor (K f ) needed to be applied to the
design of brittle materials. As fatigue failure is similar to brittle failure, stress concentrations
need to be considered for both ductile and brittle materials under repeated loadings, whether
they are completely reversed or fluctuating. The reduced stress concentration factor (K f )
was determined from Eq. (6.23), repeated here.

K f = 1 + q(K t − 1) (6.23)

where the geometric stress concentration factor (K t ) is modified or reduced due to any notch
sensitivity (q) of the material. Values for the stress concentration factor (K t ) for various
types of geometric discontinuities are given in any number of references (Marks). Charts
for the notch sensitivity (q) are also given in these references.
The miscellaneous effects factor for stress concentration (k e ) is therefore the reciprocal
of the reduced stress concentration factor (K f ) and given in Eq. (7.16) as

1
k e = (7.16)
K f

where as (K f ) is usually greater than one, the miscellaneous effects factor (k e ) will be less

than 1 and thereby reduce the test specimen endurance limit (S ) accordingly.
e
Note that the miscellaneous effects factor (k e ) for stress concentration applies to the
3
6
endurance limit (S ) at (N = 10 ) and greater. However, below (N = 10 ) cycles it has

e
no effect, meaning (K f = 1) or (k e = 1). Similar to the process for finite life, between
6
3
(N =10 ) and (N =10 ) cycles define a modified stress concentration factor (K ) where

f

K = aN b (7.17)
f