ABSTRACT
This paper presents an experimental comparison of crack initiation, between free defect
and defect material, under High Cycle Fatigue (HCF) loading. Experiments are carried
out on a C35 steel. Results show that crack initiation and crack propagation
mechanisms for defective material are the same as defect free material: cracks are
initiated in the maximum shear plane then they propagate in the plane perpendicular to
the maximum principal stress direction. But the length of the stage I cracks of defect
materials is always very short in comparison with those of defect free materials. Crack
propagation regime life in defect materials is very short, contrary to defect free material
which shows an early crack initiation However, contrary to the common belief, the
fatigue life initiation is not consumed in defective material and particularly in torsion
loading where the cyclic stress tensor becomes complex around the defect.