The
graph below compares fatigue strength with ultimate tensile strength
for both smooth and notched
specimens. Without shot peening optimal fatigue properties for machined
steel components are obtained at approximately 30 HRc (700 MPa). At
higher strength/hardness levels, materials lose fatigue strength due
to increased notch sensitivity and brittleness. With the addition of
compressive stresses from shot peening, however, fatigue strength increases
proportionately to increasing strength/hardness. For example, at a
52 HRc (1240 MPa), the fatigue strength of the shot peened specimen
is 144 ksi (988 MPa), more than twice the fatigue strength of the unpeened,
smooth specimen.
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Click
image to enlarge |
Comparison
of peened and unpeened fatigue limits for smooth
and notched specimens as a function of ultimate tensile
strength of steel. |
Manufacturing
Processes - Effect on Fatigue Life
Manufacturing
processes are known to have a significant effect on fatigue properties
of parts. These effects can be either detrimental or beneficial, as represented
by the chart below:
| DETRIMENTAL |
BENEFICIAL |
| Hardening |
Carburizing |
| Grinding |
Honing |
| Machining |
Polishing |
| Plating |
Burnishing |
| Welding |
Rolling |
| EDM
and ECM |
Shot
Peening |
On the
detrimental side grinding, machining and welding all can leave the surface
of the part in tension, a seedbed for fatigue cracks. Hardening, plating
and EDM can leave a hard brittle surface. ECM can damage or weaken surface
grain boundaries.
On the
beneficial side all the listed processes improve metal fatigue life by
virtue of the compressive stresses they induce. Shot peening is the most
versatile of the list since it provides the highest magnitude of compressive
stress in the greatest variety of materials and part configurations.
The graph
below presents "s/n" (stress vs. number of cycles to failure) curves
for different types of grinding. The base line curve is that for "gentle
grind" specimens and shows fatigue strength of 60,000psi. A "severe grind" graph
is shown just below and represents that condition produced from faster
cutting speeds and/or the taking larger cuts. In this case large amounts
of surface tensile stress, the seedbed of tensile fatigue cracks, are
generated. As shown, fatigue strength decreases to 45,000 psi. The last
graph presents the fatigue strength of "severe grind plus shot peened" specimens.
As shown these specimens increased well beyond even the baseline "gentle
grind", providing fatigue strength of over 80,000 psi. The compressive
stresses generated by shot peening overcame the tensile stresses from
severe grinding.
There
are several ways of considering these benefits. First, shot peening allows
an increased amount of stress to achieve the same component fatigue life.
Second, shot peening extends the life of any part if the existing stress
level is maintained. Thirdly, shot peening permits a greater range of
acceptable manufacturing operations by providing a consistent surface
compressive stress.
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Click
the graph to view larger image
Shot
peening improves endurance limit of ground components.
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