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After 10^6 cycles a component is considered to have infinite life. What makes 10^6 cycles a deciding factor for infinite life? Why not 10^8?

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    $\begingroup$ Divide 10^6 cycles by the number of cycles used per year or per day or per use. How many times can it be used? $\endgroup$
    – Solar Mike
    Mar 29, 2023 at 7:08
  • $\begingroup$ Right: if a cycle takes $1 ns$, then the material will be gone while you watch it ... $\endgroup$
    – MS-SPO
    Mar 29, 2023 at 8:14
  • $\begingroup$ No one knows the answer. It is just an empirical observation, and it works fine. $\endgroup$ Mar 29, 2023 at 10:13

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Studies on fatigue life estimations was first done on steel axis in trains and continued on for other steel constructions. The majority of the fatigue publications have been based on fatigue estimations for steel and it has become a "standard" that $10^6$ cycles is "infinite life", because the Stress - Number-of-cycles (S-N or Whöler) curve flattens out after $10^6$ cycles for most steel alloys with minimal inpact on life after more cycles.

However, new research in the field has started looking into "Very High Cyclic Fatigue" (VHCF), which is beyond the "infinite life" region. See the article by S. Sharma et. al. [1] for more info. In short, there is no such thing as "infinite life" for many alloys and more research is required, the $10^6$ rule has been a rule of thumb.

  1. A. Sharma, M. C. Oh, B. Ahn, “Recent advances in very high cycle fatigue behavior of metals and alloys – a review”, Metals, vol. 10, no. 9, pp. 1200, Sep. 2020, doi: 10.3390/met10091200.
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  • $\begingroup$ DNV-RP-C203 specifies the fatigue limit for curves at 10 mil cycles, and also has the kink at 10 M cycles. It is simply not true that there is little damage after 1 M cycles. I believe the 1M cycles is just a practical choice made for tests. Waiting for more than a million cycles is just not practical. If the test show more that a million cycles at a certain load level, it makes sense to assume it is infinite and not wait for 10 or 100 M cycles to see if it breaks. $\endgroup$
    – Orbit
    Mar 30, 2023 at 13:56
  • $\begingroup$ Yes, in the article I linked they also mention $10^7$ as a limit before VHCF, but I have seen $10^6$ as well in some literature. I guess you are right, that it is about practical choice. $\endgroup$ Apr 4, 2023 at 8:29
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No, the only metal that has an indefinite life at some stress level is steel. All other metals have a limit to fatigue life. A rule of thumb for steel is unlimited life with maximum stress cycles less than half of the tensile strength.

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