Mixed mode II and III fatigue crack growth in a rail steel
Résumé
Rolling contact fatigue cracks in rails undergo non-proportional mixed-mode I+II+III, in variable proportions
along their front. In order to determine the crack growth thresholds and kinetics in mixed-mode II/III, asymmetric
four point bending tests are run on a rail steel with different angles between the crack front and the
shearing load, so as to vary the mode mixity ratio. For sufficiently high loading ranges, these tests give rise to
coplanar shear-mode crack growth. The effective stress intensity factors (SIFs) are derived by an inverse method
from the measured crack face relative displacements. It appears to be 10–70% lower than the nominal SIFs and
to allow a reasonable correlation of the measured crack growth rates. The local application, ahead of the crack
front, of shear-driven or tension-driven fatigue damage models – after 3D elastic-plastic computations of local
stress and strain ranges – allows a successful prediction of crack fronts paths and growth rates.
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