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Abstract
The effect of surface crack on the wear of polyamide 66 sliding against steel
counterface in dry and wet conditions was investigated in pin-on-plate reciprocating testing
machine. The test rig was designed to perform wear tests under constant and fluctuating
loading conditions at constant sliding speed. Effects of crack orientation, under different
static and cyclic loading parameters, on wear behaviour were examined. Relative change in
wear rate due to surface crack existence, as well as crack orientation, was studied to
provide understanding of surface fatigue wear. Results of the present study provide a basis
for assessing the fatigue wear resulting during the primary stage from pre-existing surface
crack defect.
Under dry sliding, there was consistent evidence of the effect of surface crack on
the wear rate under both loading conditions. A single transverse crack on the rubbing
surface resulting in sever wear rate specially during running-in period. Surface crack may
change the topography and the conformity of the polymer which, in turn, affect the
characteristics of the formed transferred layer. The effect of surface crack on the transfer
film formation was not limited to the existence of surface crack during the initial stage of
wear. It was extended to the steady state stage of wear even the crack becomes worn out.
Polymer wear showed a higher sensitivity to surface crack under cyclic loading rather than
static loading. The correlation between running-in wear and pre-existing surface cracks
explained as a process of ”sever” or ”progressive” surface fatigue wear. Polymer in dry
sliding seems to be more sensitive to surface cracks than in water lubricated condition.
Under wet sliding, the polymer showed generally higher wear rates compared to
dry tests. However, wear process was ruled by the mechanical microploughing and
abrasive wear mechanisms which showed minor sensitivity to surface crack. Plasticization,
swelling and softening of the polymer were determining the effect of water lubricant on the
wear behaviour.
A mode of the wear of surface cracked polymer was designed to study the
feasibility of neural network model that simulate the wear of polyamide under different
working conditions, such as applied load, number of surface cracks and sliding conditions,
using neural network application. The NNs showed good accuracy for the simulation and
acceptable predicted values of wear rates were obtained. The NN is superior in both
training time and accuracy and may be the solution for the cases including more input
parameters such as the dimension and shape of the crack.
Keywords: Wear, Polyamide, surface crack, Cyclic Load.