of material complicated the dynamic FEA coding. A statically
contact model between the friction pairs, which was simplified/
specified with elastic or plastic deformation of the disc material
across the cross-sectional profile of the contact surfaces (Fig. 13a),
was established for some stage of indentation in the period of
commencing a test and under the maximum load of approximately
60 N (transient high load as shown in Fig. 3c) after the
excitation of the spring-connecting load. The FEA model ignored
the plastic and wear volume of the GB GCr15 ball since it had
hardness beyond twice of that of the 45# steel disk specimen.
Results of FEA (Fig. 13b) indicated that high stress to cause severely
plastic deformation took place. When impaction combined with
sliding, it would lead to complex stressing system within the
contact–sliding interface. The region of highest stress distribution
beneath the surface would be susceptible to firstly result in
cracking in the layer, followed by detaching from substrate in
causing severe wear loss in comparing with that around its two
side edges. This subsequently resulted in wear trace with crosssectional
profiles as shown in Fig. 7c for the three loading
conditions.
Statically contact model for the loading condition of 40 N
(steady load including dead load of 5 N and spring-connecting
load of 35 N as shown in Fig. 3c) and at the moment of impacting
ball to be retrieving from the impacted substrate was also built on
the basis of Fig. 13c. Basically, the elastic and plastic deformation
led to the contacting substrate having larger the contact radius r1
around the edge zones than contact radius r2 for the central region
(Fig. 13c). In the FEA, the radii of r1 and r2 were preset according
to the geometries of the mild wear and severe wear regions,
respectively, in Fig. 7c. Subsequently, the larger radii at either edge
from the central region formed the major supports to the ball. As
the contact surfaces were subjected to the steady load of 40 N
(Fig. 3c), the contact bound mainly occurred in the region with
radius r1. FEA results (Fig. 13d) predicted the occurrence of two
regions of stress concentration, namely, (i) region / around the
edge boundaries of the whole wear track and (ii) region II around
the boundaries of the central region of the wear track. The stress
concentration in region I tended to squeeze material sideway for
building some worn material up in either vicinity rims of the wear
track (Fig. 7c). Although some material in region I tended to be
squeezed inwardly to meet the building-up of worn material from
the high stress concentration region II, such building-up material
was plastically shorn to flow and to build-up material perpendicular
to the sliding direction around the boundaries of the central
region (Fig. 8b).
5. Conclusion
The tribological properties of sliding pairs under DHE loading
were assessed by a self-design ball-on-disc tester incorporating
with a force sensor and an OLVF system. Profilometer, SEM, and
FEA were then used to analyze surface conditions and to search for
possible causing mechanisms. The fluctuation behavior and peak
magnitudes of the test COF values under different springconnecting
loads suggested that: (i) adequate selection of loading
on a sliding pair with specific roughness was possible to
reduce the COF values, and (ii) frequently transient high loading
on the disc surface was likely to harden the metallic material on its
wear track. Increase in spring-connecting loads generally led to
the increase in wear rate, side flow of material, squeezed debris,
and deep and wide grooves on the wear tracks. It also reduced
asperities to certain amount which tended to be constrained to the
central region in the bottom of cross-sectional profile of the wea