Oxide induced crack closure in the near threshold regime: The effect of oxide debris release

Within this work the influence of oxide particle transport during fatigue loading on the fatigue propagation rate was investigated in detail by means of load-shedding tests in the near threshold region (crack propagation rate between 0.1 and 10 nm per cycle). To this purpose, standard fatigue specimens (SENB - single edge notch bending) were machined. At selected specimens, a stretchable nitrile rubber patch was glued onto the sample surfaces in the ligament region. This patch was expected to prevent the emergence of iron oxide particles near the sample surface, and indeed subsequent fracture surface analysis showed obvious differences in the shape of the arising oxide debris layer. It is shown that the build-up of oxide debris on the fracture surface is higher in the specimen with the nitrile rubber patch applied. This leads to increased oxide induced crack closure and, in turn, to a higher fatigue crack growth threshold. In this context, also the influence of the specimen thickness on the fatigue crack growth threshold is examined. Finally, also the influence of the load ratio on the oxide debris layer is briefly discussed.