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 K-op 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 K,,, while keeping K-th,K-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 K-op (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.