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A B S T R A C T Closure is investigated with an optical interference technique which measures the transverse displacement associated with a propagating fatigue crack. The results of this study require that "closure" be devided into two phenomena, each of which affects the propagating mechanism differently. The plastic region is partitioned into the plastic wake zone behind the crack front and the active plastic zone ahead of the crack. The work to be presented deals with the effects of the plastic wake zone on fatigue crack propagation. It is found that the contact length between the crack faces can be several times the specimen thickness dimensions and that contact occurs only in the plane stress region of the specimen. It is concluded that the contact forces between the crack faces act like some external tensile load, modifying the Klmin of the loading cycles with low R-ratios. But, contact between the fracture faces (and the plastic wake zone) cannot account for any overload effects. The occurence of contact in the plane stress region of the specimen explains the observed thickness dependence of fatigue crack growth rates. Nomenclature0376--9429/80/020133-21500.20[0 Int. Journ. o[ Fracture, 16 (1980) 133-153

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