During fatigue crack propagation,resiaual deformations are built up in front of the crack tip and left behind the propagating crack. As a result of these residual deformations the crack will close(at least partly) during unloading. At zero load the presence of these deformations excersises residual compressive stresses in the wake of the crack normal to the fractured surfaces. The load at which the crack closes is therefore tensile rather than zero o/ compressive. This phenomenon is refferedteascrack closure. The main objective of this study is to correlate fatigue crack gr-owth rates with crack closure under different types of loadings: Constant-amplitude with two different cycle ratio R,(R= E. / Er ),
mln. max. Programmed block loading and A single tensile overload. The incidence of crack closure is examined and the concept of equ-ivalent constant-amplitude is applied. The delayed crack growth retardation after a single overload cycle isiiterpreted using crack closure concept. Aspects covered include microscopic and fracto-graphic observations confirming the crack closure to provide the fatigue striations on the surfaces ruptured by cyclic loads. Materials used in this study are two Aluminium alloys,widely used in aeronautical structures: 2124T351 (AU4G1) and 2618AT851(AU2GN). Several interaction effects might be explained by changes in the crack closure levels due to variable amplitude loads.