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Surface Finish vs. Fatigue strength?

  1. Jun 29, 2010 #1
    Does anyone have access to a chart shown surface finish vs. impact of fatigue strength at 10^6 for various matals?

    In our company we limit the surface finishes to 63 Ra max for highly stressed areas. But where did that value come from? The best way to find out is to see a chart showing the specific affects of finish Ra to fatigue strength.

  2. jcsd
  3. Jun 30, 2010 #2

    Ranger Mike

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    as a product manager of the company that invented the industrial method of measuring surface TEXTURE, I can tell you that Ra is a worthless parameter. It is the average of the peak to valley height of a surface profile over a sampling length..average being the key word.. it is little better than your finger nail is determining suitability of a surface. in addition the tolerancing you mentioned does nothing to restrict the manufacturer form wasting time and money over finishing the surface. there should be a minimum Ra value as well.
    Finally, the value assigned is strictly arbitrary unless some function part that has lasted over time had such a surface texture Average Roughness reading.
    Note attached illustration of two surfaces with the identical parameter Ra ( used to be CLa, AA, and most wrongly RMS pre 1960s), will oil adhere to these two surfaces the same? will light reflect the same/ will they perform identical functions regarding electric conductivity?
    Ra is little better than your finger nail..

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    Last edited: Jun 30, 2010
  4. Jun 30, 2010 #3


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    That chart simply won't exist. Materials are different in so many ways, that there really isn't such a thing as a "standard" chart of parameter vs. parameter.

    Having said that, in my experience, surface finish typically had more of an effect on LCF life than HCF, IIRC.
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