Digital Repository, Giornata IGF Forni di Sopra (UD) 2011

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Study of through thickness effects by means of the stress intensity factor
A. Gonzalez-Herrera, J. Garcia-Manrique, D. Camas, P. Lopez-Crespo

Last modified: 2011-02-25


The study of crack tip fields is often conducted assuming a homogeneous behaviour through the
thickness. Depending on the specimen thickness, a state of plane stress or plane strain is normally presumed.
However, recent studies have shown a more complex behaviour along the thickness. On the one hand,
plasticity-induced crack closure effects affect mainly to a small region close to the specimen surface. On the
other hand, the plastic zone evolution along the thickness is not as simple as the classic dog-bone shape
normally described in Fracture Mechanics textbooks. Unlike what is normally expected, the size of the plastic
zone decreases in a very small region close to the surface, as we move from the interior of the specimen to the
specimen surface. These two effects can be detected if the mesh used in the finite element model is sufficiently
fine. Both effects are probably related to an uneven distribution of the load along the thickness. One of the
consequences of these effects on the fatigue crack growth is the curvature of the crack front. Nevertheless, this
curvature can be explained by two mechanisms. The first one is related to the crack closure effect near the
surface, it would imply a smaller effective ΔK close to the surface, and therefore a slower crack growth rate.
The second one (plastic zone size decrease close to surface) is probably due to ΔK being smaller near the
surface than in the interior. The current work attempts to evaluate numerically both effects in order to separate
their individual influence and their magnitude. This is done by evaluating the K distribution along the thickness
at different planes on an Al 2024-T35 compact tension specimen. The plastic wake effect is removed from the
model in order to distinguish between both effects.

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