Infinite life of a material, Fatigue Strength

Question

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?

Answer 1

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.

Answer 2

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.