The graph below compares metal fatigue strength
with ultimate tensile strength for both smooth and
notched specimens. Without shot peening, optimal
metal 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, metal fatigue
strength increases proportionately to increasing
strength/hardness. For example, at a 52 HRc (1240
MPa), the metal fatigue strength of the shot peened
specimen is 144 ksi (988 MPa), more than twice the
metal 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 meal fatigue properties of parts. These effects
can be either detrimental or beneficial, as represented
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 metal 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 because 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 metal failure) curves for different types
of grinding. The base line curve is that for "gentle
grind" specimens and shows metal fatigue strength of
60,000 psi. The following "severe grind" graph 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 metal
fatigue cracks, are generated. As shown, metal fatigue
strength decreases to 45,000 psi. The last graph presents
the metal fatigue strength of "severe grind plus shot
peened" specimens. As shown, these specimens increased
well beyond even the baseline "gentle grind", providing
metal fatigue strength of over 80,000 psi. The compressive
stresses generated by shot peening overcame the tensile
stresses from severe grinding.
Click
image to enlarge |
Shot
peening improves endurance limit of ground
components.
|
There are several ways of considering these benefits.
- First, shot peening allows an increased amount of
stress to achieve the same component metal 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 for combating
metal fatigue.
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