# Infinite life of a material, Fatigue Strength

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?

• 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? Mar 29, 2023 at 7:08
• Right: if a cycle takes $1 ns$, then the material will be gone while you watch it ... Mar 29, 2023 at 8:14
• No one knows the answer. It is just an empirical observation, and it works fine. Mar 29, 2023 at 10:13

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.
• 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. Apr 4, 2023 at 8:29