MIC Green Book

C H A P T E R T H R E E
M ANUFACTURING PROCESSES
14
EFFECT ON FATIGUE LIFE
Manufacturing processes have significant effects on fatigue properties of metal parts. The effects can be
either detrimental or beneficial. Detrimental processes include welding, grinding, abusive machining, metal
forming, etc. These processes leave the surface in residual tension. The summation of residual tensile
stress and applied loading stress accelerates fatigue failure as shown in Figure 1-6.
Beneficial manufacturing processes include surface hardening as it induces some residual compressive
stress into the surface. Honing, polishing and burnishing are surface enhancing processes that remove
defects and stress raisers from manufacturing operations. Surface rolling induces compressive stress but is
primarily limited to cylindrical geometries. Shot peening has no geometry limitations and produces results
that are usually the most economical.
The effect of residual stress on fatigue life is demonstrated in the following example. A test by an airframe
manufacturer on a wing fitting showed the initiation of a crack at just 60% of predicted life. The flaw was
removed and the same area of the part shot peened. The fitting was then fatigue tested to over 300% life
without further cracking even with reduced cross sectional thickness [Ref 3.1].
WELDING
The residual tensile stress from welding is created because the weld consumable is applied in its molten state.
This is its hottest, most expanded state. It then bonds to the base material, which is much cooler. The weld
cools rapidly and attempts to shrink during the cooling. Because it has already bonded to the cooler, stronger
base material it is unable to shrink. The net result is a weld that is essentially being "stretched" by the base
material. The heat affected zone is usually most affected by the residual stress and hence where failure will
usually occur. Inconsistency in the weld
filler material, chemistry, weld geometry,
porosity, etc., act as stress risers for
residual and applied tensile stress to
initiate fatigue failure.
As shown in Figure 3-1, shot peening is
extremely beneficial in reversing the
residual stress from welding that tends to
cause failure in the heat affected zone
from a tensile to a compressive state.
Figure 3-1 demonstrates a number of
interesting changes in residual stress
from welding, thermal stress relieving
and shot peening [Ref 3.2]. Tensile
stresses generated from welding are
Figure 3-1 Residual Stresses from Welding
additive with applied load stresses. This combined stress will accelerate failure at welded connections.
When the weld is stress relieved at 1150 °F (620 °C) for one hour, the tensile stress is reduced to almost zero.
This reduction of tensile stress will result in improved fatigue properties.
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