• Starink, M. J.
• Connolley, T.
• Reed, Philippa A. S.

Abstract

Fatigue tests at 600°C in air were performed on U-notch specimens of wrought IN718, using l-l-l-l and 1-20-l-l trapezoidal cycles. SEM examination and acetate replication was used to study crack initiation and growth. In support of the fatigue tests, thermal exposure experiments were performed on unstressed specimens to study surface and sub-surface primary carbide oxidation. Compared to a 1 second dwell, a dwell time of 20 seconds at maximum load had a beneficial effect on fatigue lifetime. Polishing the U-notch to remove broaching marks also had a beneficial effect on fatigue life. Multi-site crack initiation along the root of the U-notch was observed in fatigue specimens. Many of the cracks initiated at bulge-like features, associated with the oxidation of sub-surface primary carbides. Crack initiation was first observed as early as 12% of the total fatigue life, with further crack initiation occurring as tests progressed. Short cracks in the U-notches grew at a constant rate, except when interactions between parallel cracks resulted in crack arrest or coalescence. Significant crack coalescence occurred towards the end of the fatigue life, producing a dominant defect which propagated rapidly to failure. Surface eruptions and localised surface deformation were observed in fatigue and unstressed thermal exposure specimens, demonstrating a considerable volume expansion when primary (Nb,Ti)C carbides oxidised. It is proposed that the misfit strains due to primary carbide oxidation were superimposed on the plastic strain field in the U-notch due to external loading. This would create local strains high enough to cause rupture of the matrix, hence initiating a fatigue crack.

Topics

• impedance spectroscopy
• surface
• polymer
• scanning electron microscopy
• experiment
• crack
• carbide
• fatigue
• polishing