Ductile fracture topography: Geometrical contributions and effects of hydrogen

Abstract

A volume element which is deformed becomes elongated and narrowed with increasing strain, for reasons of constant plastic volume; inclusion spacings experience the same changes. Equivalent plasticity expressions permit a description of this effect as a function of fracture strain. Such an effect must be taken into account in comparison of initial inclusion spacings with dimple sizes observed in tensile or toughness specimen fractures. It also plays a role in the assessment of dimple size changes in hydrogen-assisted ductile fracture. The most convenient way to display the effect in the hydrogen case is on a plot of ductility lossvs dimple size ratio. This comparison demonstrates that the geometrical effect generally should lead to large increases in size ratio as ductility decreases; thus the common observations of dimple sizereduction with hydrogen evidently represent large increases in dimple nucleation, while dimple sizeincreases may represent microvoid growth effects.