Fig.9 Macro fracture morphology; (a) P=50MPa, DP; (b) P=250MPa, DP; (c)
P=150MPa, SP; σe=400MPa.
The fretting fatigue fracture of 35CrMoA steel basically includes three
characteristic regions, named with the crack source zone (CSZ), crack
propagation zone (CPZ) and instantaneous fracture zone (IFZ)
(respectively I, II, III). When the contact stress is low, the
proportion of CSZ and IFZ is large, and although CSZ is generated in the
two corresponding fretting regions, there is always a CSZ that
predominates during the crack propagation, which would make IFZ biased
towards another CSZ; With the increase of contact stress, the fracture
area occupied by CSZ and IFZ is reduced. In the whole fracture area, CPZ
occupies the main parts, and the cracks generated by the two
corresponding fretting regions both dominate, so that IFZ is located in
the center of the fracture and always has a nearly symmetrical shape.
When the contact stress is large, the amount of deformation generated by
the sample and the stress concentration at the material damage caused by
fretting are larger, so microcracks are easier to form and are more
liable to propagate into main cracks under this action. Therefore, CSZ
of fracture under high contact stress is small and shows good symmetry.
Compared with DP under the same conditions, there are multiple crack
source regions in the fretting fatigue fracture under SP, but there is
also only one main crack source region, and block spall of the material
also occur in the fracture area. This is due to the fact that the main
crack spreads to connecting with the microcracks that are not generated
at the same level, which causes the material to fall off.
Micro fracture analysis
Fig.10 is the fretting fatigue fracture morphology under DP with
equivalent stress of 400MPa and contact stress of 50MPa. The CSZ, CPZ
and IFZ can be clearly distinguished from the figure. Within a certain
number of cyclic cycles after crack initiation, it mainly expands in the
cross section of the specimen. At this situation, although the specimen
is also subjected to the combined effect of contact stress and
equivalent cyclic stress, the microcrack is closer to the surface of the
fretting zone. Therefore, it is mainly controlled by contact stress;
When the crack grows to a certain length, which is far from the surface,
the effect of cyclic stress on the crack growth gradually strengthens,
so that the crack propagation appears along the longitudinal component,
and as the crack further expands, the longitudinal rate of crack
propagation increases gradually. Fretting fatigue fracture surface
continuously produces stairs along the longitudinal direction.