Entropy production during the fatigue process can serve as a measure of degradation. We postulate that the thermodynamic entropy of metals undergoing repeated cyclic load reaching the point of fracture is a constant, independent of geometry, load and frequency. That is, the necessary and sufficient condition for the final fracture of a metal undergoing fatigue load corresponds to a constant irreversible entropy gain. To examine validity, we present the results of an extensive set of both experimental tests and analytical predictions that involve bending, torsion and tension-compression of aluminium 6061-T6 and stainless steel 304 specimens. The concept of tallying up the entropy generation has application in determining the fatigue life of components undergoing cyclic bending, torsion and tension-compression.

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The friction characteristic of a journal bearing with dimpled bushings manufactured using the machining and the chemical etching techniques is investigated. A series of experimental results is presented to examine the effect of dimples on the Stribeck curve. Load, oil type, dimple size, depth and shape are varied to explore their influence on the friction characteristics. It is shown that with proper dimensions of dimples, the friction performance of journal bearings can be improved, particularly for light oils. The results reveal that in mixed lubrication regime, the so-called secondary lubrication effect in the dimpled area is the main mechanism responsible for improvement of performance. Experimental results also indicate that a bushing with etched dimples over the entire circumference offer a better frictional performance than a bushing with dimples etched on half of its circumference.

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a b s t r a c tThis paper presents an experimental approach to fatigue damage in metals based on thermodynamic theory of irreversible process. Fatigue damage is an irreversible progression of cyclic plastic strain energy that reaches its critical value at the onset of fracture. In this work, irreversible cyclic plastic energy in terms of entropy generation is utilized to experimentally determine the degradation of different specimens subjected to low cyclic bending, tension-compression, and torsional fatigue. Experimental results show that the cyclic energy dissipation in the form of thermodynamic entropy can be effectively utilized to determine the fatigue damage evolution. An experimental relation between entropy generation and damage variable is developed.