The significance of fractography for investigations of fatigue crack growth under variable-amplitude loading

Fatigue crack growth tests were carried out on 2024-T3 and 7075-T6 centrall y cracked specimens. Variable-amplitude (VA) load spectra were used with pe riodic overload (OL) cycles added to constant-amplitude (CA) cycles. The fa tigue fracture surfaces were examined in the SEM to obtain more detailed in formation on crack growth contributions of different load cycles. The stria tion patterns could be related to the load histories. SEM observations were related with (i) delayed retardation, (ii) the effect of 10 or a single OL on retardation, (iii) crack growth during the OL cycles, and (iv) crack gr owth arrest after a high peak load. Fractographs exhibited local scatter of crack growth rates and sometimes a rather tortuous 3D geometry of the crac k front. Indications of structurally sensitive crack growth under VA loadin g were obtained. Fractography appears to be indispensable for the evaluatio n of fatigue crack growth prediction models in view of similarities and dis similarities between crack growth under VA and CA loading.