The influence of stress state on fatigue crack growth in nickel‐base superalloys at high temperature is considered, based on studies in corner crack specimens of Inconel 718 at 600 °C. At high frequency and low R, cycle‐dependent trans‐granular crack growth occurs along the whole crack front, and growth rates are similar at the surface and within the interior of specimens, maintaining the original quarter‐circular shape. For conditions of low frequency and high R, increased crack growth rate per cycle is observed with the crack tunnelling ahead at the centre. A time‐dependent intergranular crack propagation mode occurs in the plane strain interior, attributed to an oxidation mechanism, whereas near the surfaces under plane stress, a trans‐granular cyclic plasticity mechanism is observed. It is proposed that in addition to frequency and R, that stress state influences the competition between the mechanisms controlling crack growth and the transition between them: plane strain in the interior favouring an oxidation‐controlled intergranular cracking mechanism as compared with the plane stress surfaces where cyclic plasticity dominates. An FEM study suggests that this influence of stress state is not associated with variation of ΔK along the crack front.

A B S T R A C T Rehabilitation of a welded structure, which involves repair of cracked joints, is achieved when the local treatment for repair gives a fatigue strength in the joint equal or above the fatigue strength of the uncracked original detail. If the treatment is properly applied the rehabilitation of the detail is assured, and the nature of the weld toe improvement methods can produce a joint, after repair, with a fatigue strength and residual life greater than the initial detail. The paper presents the results obtained on a fatigue study on the rehabilitation of non-load carrying fillet welded joints loaded in bending at the main plate and with fatigue cracking at the weld toes of the attachment in the main plate and though the plate thickness. Residual stresses were measured at the surface, with X-ray diffraction. The residual stresses induced by hammer peening at the weld toe were found to be greater along the longitudinal direction of the plate than in the transverse direction. The peak residual stresses near the weld toe were found to be close to yield in compression, justifying the great benefit of hammer peening. Results of a derived gain factor, g, in fatigue life were obtained as a function of the crack depth repaired by hammer peening.

Anatomic and morphologic details in posterosuperior quadrant were newly described and may explain the greater incidence of retraction pockets and marginal perforations that provide origin for the cholesteatoma.

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