Changes in dislocation structures adjacent to fatigue crack tips induced by intermittent overstressing

Abstract

A study was made on the effects of intermittent overstressing on fatigue crack propagation using Fe specimens in as-normalized and as-stretched to 10% strain conditions. A few cycles of fully reversed overstress applied intermittently during very large numbers of cycles of understress below the threshold stress intensity caused significant acceleration of crack propagation. Ultrahigh voltage electron microscopic observation of the dislocation structures adjacent to fatigue crack tips revealed the occurrence of remarkable recovery during understressing and the formation of a radial band structure within the recovered region on overstressing. These structure changes induced by varying the stress are thought to be responsible for the acceleration of crack propagation in the material. The previously reported characteristics of acceleration of crack propagation under intermittent overstressing: (i) the increase of acceleration ratio R _ac as the number of cycles of understress n ₁ increases, and the saturation of R _ac beyond n ₁ = 10⁵ cycles, (ii) the decrease of acceleration ratio as the number of cycles of overstress n ₂ increases, (iii) the existence of a threshold in the understress for acceleration of crack growth, and (iv) the dependence of acceleration ratio on the level of strength of materials, are discussed in connection with the structure changes.