Metal Fatigue

Metal Fatigue 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.

Metal Fatigue Graph
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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.

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.

Metal Fatigue Graph
Click image to enlarge

Shot peening improves endurance limit of ground
components.

Benefits of Shot Peening

    • 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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