Bond Healing and Endurance Limit

See how the endurance limit influences fatigue life.

After an extensive literature review and limited amount of data, it is speculated that loads below the knee strength of woven lamina result in infinite life [25]. This corresponds to an endurance limit in which no initial failures give rise to stress concentrations to stem further failures from. Unidirectional composite laminates also exhibit this behavior when loads are well below those of matrix failure [26], which gives supporting evidence for an endurance limit of both unidirectional and woven laminae.

The endurance limit occurs at very low stress levels, and it is proposed that the endurance limit is due to bond healing at these low stress levels. In fact, the stress levels are so low that we can ignore γh in Eq. 53 and focus solely as an energy barrier Uh that contributes to bond healing. Then, under medium to large stress levels, bond breaking will dominate the fatigue process, and under low stress levels bond breaking and healing will interact and could potentially form an endurance limit for the material. We can now write Eq. 53 as