疲劳极限
- 当疲劳循环次数（N）变得非常大时，平均疲劳强度的极限值（Graham，第 23 页）。由于疲劳数据固有的分散性，即使应力低于疲劳极限，个别疲劳失效仍可能发生。 （见 Maennig[1]，特别是第 638–642 页——“The proof of the existence of a fatigue limit and the determination of Ng”（疲劳极限存在性的证明及 Ng 的确定），其中 Ng 为与疲劳极限对应的疲劳循环次数）。
- 中位 S-N 曲线斜率变为零时的应力——即低于该应力时不会发生疲劳失效（寿命无限）。该应力取决于多种因素。
- 钢 — 107 次循环（第 207 页）。
- 钛 — 106–107 次循环（第 218 页）。然而，其他证据表明可能高达 109–1010 次循环。
- 铝 — 5x108 次循环（第 213 页），尽管铝可能并不存在真正的疲劳极限。
并非所有材料都具有这种意义上的疲劳极限（即可能具有无限寿命）。黑色金属材料和钛通常被认为具有疲劳极限，但这可能取决于环境。铝一般没有疲劳极限。因此，无论应力水平如何，只要振动时间足够长，铝最终都会发生疲劳。
为确认某种材料确实具有疲劳极限所需的最小疲劳循环次数取决于材料本身。Juvinall（第 207 页）建议如下：
Maennig（见上）给出了一种统计实验方法，用于在特定材料确实存在疲劳极限时，确定与疲劳极限对应的疲劳循环次数。
Fatigue limit
- The limiting value of the mean fatigue strength as the number of fatigue cycles (N) becomes very large (Graham, p. 23). Due the to the inherent scatter in fatigue data, individual fatigue failures may still occur at stresses below the fatigue limit. (See Maennig[1] , particularly pages 638 - 642 - "The proof of the existence of a fatigue limit and the determination of Ng", where Ng is the number of fatigue cycles corresponding to the fatigue limit).
- The stress at which the median S-N curve attains zero slope -- i.e., the stress below which fatigue failure will not occur (infinite life). This stress depends on a number of factors.
- Steel - 107 cycles (p. 207).
- Titanium - 106 - 107 cycles (p. 218). However, other evidence suggests that this may be as high as 109 - 1010 cycles.
- Aluminum - 5x108 cycles (p. 213), even though aluminum may not have an actual fatigue limit.
Not all materials possess a fatigue limit in this sense (i.e., the possibility of infinite life). Ferrous materials and titanium are generally presumed to have a fatigue limit, although this may depend on the environment. Aluminum generally does not have a fatigue limit. As a result, aluminum will eventually fatigue if it is vibrated long enough, regardless of the stress level.
The minimum number of fatigue cycles that is needed to assure that a material does, indeed, have a fatigue limit depends on the material. Juvinall (p. 207) suggests the following:
Maennig (above) gives a statistical experimental method for determining the number of fatigue cycles corresponding to the fatigue limit, if indeed a fatigue limit exists for the particular material.