MetalFatigu

Shot Peening California

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.

 

Shot Peening New York

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Comparison of peened and unpeened fatigue limits for smooth and notched specimens as a function of ultimate tensile strength of steel.

ManufacturingProcesses – Effect on Fatigue Life

Manufacturing processes are known to have a significanteffect on meal fatigue properties of parts. These effectscan be either detrimental or beneficial, as representedbelow:

DETRIMENTAL BENEFICIAL
Hardening Carburizing
Grinding Honing
Machining Polishing
Plating Burnishing
Welding Rolling
EDM and ECM Shot Peening

On the detrimental side grinding, machining and weldingall can leave the surface of the part in tension, aseedbed for metal fatigue cracks. Hardening, platingand EDM can leave a hard brittle surface. ECM can damageor weaken surface grain boundaries.

On the beneficial side all the listed processes improvemetal fatigue life by virtue of the compressive stressesthey induce. Shot peening is the most versatile of thelist because it provides the highest magnitude of compressivestress in the greatest variety of materials and partconfigurations.

The graph below presents “s/n” (stress vs. number ofcycles to metal failure) curves for different typesof grinding. The base line curve is that for “gentlegrind” specimens and shows metal fatigue strength of60,000 psi. The following “severe grind” graph representsthat condition produced from faster cutting speeds and/orthe taking larger cuts. In this case large amounts ofsurface tensile stress, the seedbed of tensile metalfatigue cracks, are generated. As shown, metal fatiguestrength decreases to 45,000 psi. The last graph presentsthe metal fatigue strength of “severe grind plus shotpeened” specimens. As shown, these specimens increasedwell beyond even the baseline “gentle grind”, providingmetal fatigue strength of over 80,000 psi. The compressivestresses generated by shot peening overcame the tensilestresses from severe grinding.

Shot Peening California

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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.