INTRODUCTION
Fretting refers to a very small relative displacement (in the order of
microns, usually 5-50 microns) between two surfaces in contact with each
other under a certain contact stress.1-3 Fretting
fatigue is an accelerated failure process under the combined action of
fretting wear and cyclic stress.4,5 In many practical
engineering applications, such as the aerospace
industry,6 the machinery industry,7the transportation industry8 etc, there are inevitable
vibrations and small relative movement between different contact parts
of components,4 the interaction causes cracks to
easily occur at the contact points, which causes fatigue fracture of the
material. The fretting fatigue process is essentially a combination of
fretting wear and fatigue caused by the presence of contact stress and
cyclic stress. Fretting plays a nonnegligible role in the initiation and
propagation of cracks, leading to material failure in
advance,9 therefore, fretting has a great impact on
fatigue life. The current researches on fretting fatigue are mainly
focused on the effect of different influencing factors on different
materials, and it is generally believed that the factors that have a
greater effect on fretting fatigue are contact stress, displacement
amplitude, cyclic stress, coefficient of friction, material
microstructure, temperature and humidity,10-12 such as
C. Giummarra6 and Sachin R.
Shinde,13 et al. studied the effects of different
microstructures of different aluminum alloys on fretting fatigue life,
Yang14 and Jin15 et al. reported
that the effect of contact stress on the fretting fatigue process of
different materials. In addition, many studies had also used the finite
element method to simulate the stress and strain changes of the fretting
process, analyzed the fretting fatigue mechanism on the basis of
experiments, and had proposed some fretting fatigue life prediction
models based on plain fatigue life models, wear damage, and crack
initiation and propagation processes. 14,16,17 In the
process of analyzing fretting fatigue, the fretting maps had a great
effect on analyzing the fretting mechanism and fretting fatigue behavior
prediction of materials. Z.R. Zhou18 et al. made a
good summary of the types and development process of fretting map, and
Sachin Shinde19 et al. also made a supplement to the
development of fretting map.
35CrMoA steel is a medium carbon alloy structural steel that has both
good hardenability and good toughness. It’s also featured in high static
strength, high fatigue limit and excellent stress-rupture properties at
high temperature. Therefore, it was widely used for important structural
parts such as large cross-section gears, heavy-duty transmission shafts,
and turbine generator main shafts.20 In recent years,
due to the requirements of high-speed railways, 35CrMoA steel had been
used to manufacture the axles of high-power electric locomotives. In
actual operation, due to the close cooperation between this component
and its tight fittings, composite fretting usually occurred at the
contact parts. Therefore, the performance research of 35CrMoA steel must
be carried out in accordance with the actual conditions. Currently, some
studies had been done on the fretting fatigue of steels. For example, GH
Farrahi21 et al. studied the fretting fatigue behavior
of 316L stainless steel under the combination of bending and tensile
load, and analyzed the effect of grain size in fretting fatigue process;
Teng22 et al. investigated the high-cycle fretting
fatigue behavior of GCr15 and performed 2D and 3D analysis of the
fretting area morphology; Kozo Nakazawa23 et al.
reported the effect of contact stress on austenitic stainless steel, and
concluded that the effect of relatively low contact stress on the
fretting fatigue life can be neglected, and at high contact stress,
plain fatigue experiments performed before fretting fatigue test can
increase fretting fatigue life. For 35CrMoA steel, there were limited
studies referring its fretting fatigue performance.
Liu24 et al. used experimental and finite element
analysis methods to study the multiaxial fretting fatigue of 35CrMoA
steel, and obtained that with the increase of contact stress, the
fretting fatigue life tended to decrease first, then rose and decreased
again at the next stage, and stress concentration occurs at the edge of
the contact area; Tian25 and Lv26 et
al. also investigated the multiaxial fretting fatigue of 35CrMoA steel,
but levels of variables were quite limited in that study, which didn’t
perform an integrated research on the fretting fatigue behavior of
35CrMoA steel. Moreover, the comparison of the strain response behavior
under different paths was limited, the explanation of the fretting zone
wear mechanism was incomplete, and the analysis of crack initiation and
propagation was lacking.
In this paper, 35CrMoA steels were both used as the specimens and the
fretting pads, and the contact form of the cylindrical surface was used
to explore the multiaxial fretting fatigue behavior and mechanism of
35CrMoA steel under the square path (SP) and diamond path (DP). The
research analyzed the strain response under different loading paths
during fretting fatigue, compared the morphology of the fretting area
under different contact stresses and different loading paths. The
abrasive debris in slip region and stick region of the SP was examined
to analyze the composition and describe the process of abrasive debris
generation. Moreover, the fretting fatigue fracture morphology was
observed to analyze fracture features of crack source zone (CSZ), crack
propagation zone (CPZ) and instantaneous fracture zone (IFZ). The
cracks’ initiation and propagation were also concluded from the fracture
morphology.
EXPERIMENTAL PROCEDURES
The samples and fretting pads materials used in this experiment were
35CrMoA steel. The heat treatment processes used for fretting fatigue
specimens and fretting pads were quenching and tempering. The specific
treatment processes were: solution treated at 850℃ for 25min in a box
type resistance furnace, quenched in oil, and followed immediately by
aging at 550℃ for 1 hour, finally cooling in oil. The chemical
composition and main mechanical properties of the 35CrMoA steel are
shown in Tab.1 and Tab.2, respectively.
Table1 Chemical composition of 35CrMoA steel (wt %)