Contribution of the cyclic loading portion below the opening load to fatigue crack growth

A simple test procedure involving stress ratio changes at the fatigue threshold is proposed to reveal the role of the lower portion of the loading cycle below Kop in the fatigue crack growth behaviour. It is observed for both Al 2024-T3 and Al 7475-T761 alloys that the initially non-propagating fatigue crack at the fatigue threshold resumes growth upon diminishing Kmin while keeping Kth,max constant. These experimental findings can be interpreted by means of a modified crack closure concept in which the contribution of the lower portion of the loading cycle below Kop (including also a part of compressive loading if R < 0) to the variation in the stress state experienced by the fatigue crack tip is taken into account.