A characteristic of all cracks propagated by fatigue is that, to a greater or lesser degree, they display the phenomenon known as crack closure. Zones of localized plasticity are left behind in the wake of the propagating crack-tip. These ‘stretched layers’, as Elber [1] described them, act as props in compression supporting the surrounding bulk of elastic material. As the crack is loaded up, complete opening will be delayed by their presence: similarly, during the decreasing load phase, the surfaces come together early.