疲劳极限（Fatigue limit）、持久极限（endurance limit）及疲劳强度（Fatigue strength），都是和材料的周期应力及疲劳有关的材料性质。

一材料试片在不同大小的周期应力下，使材料破坏需要的周期数也随之不同。应力大小和周期数的关系可以用S-N图表示。一般而言，周期应力越小，需材料破坏需要的周期数越多。但铁合金和钛合金有一特性，当周期应力大小低于一特定数值，材料可以承受无限次的周期应力，不会造成疲劳，此数值对应S-N图右侧的水平线。

其他的结构金属（如铝和铜）没有类似的限制值，即使是很小的周期应力，只要周期持续增加，最后材料就会疲劳破坏。这类的材料一般会用一特定数字（通常为10）为其疲劳寿命周期数N_f。

Fatigue limit, endurance limit, and fatigue strength are all expressions used to describe a property of materials: the amplitude (or range) of cyclic stress that can be applied to the material without causing fatigue failure. Ferrous alloys and titanium alloys have a distinct limit, an amplitude below which there appears to be no number of cycles that will cause failure. Other structural metals such as aluminium and copper, do not have a distinct limit and will eventually fail even from small stress amplitudes. In these cases, a number of cycles (usually 10) is chosen to represent the fatigue life of the material.