The thermodynamics of degradation
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Material degradation occurs as a result of irreversible dissipative processes and forces. Various forms of degradation mechanisms exist such as friction, chemical reactions, plasticity, dislocation movements and corrosion all irreversibly leading to failure of a particular system or component. The first and second laws of thermodynamics describe states of a system from the perspective of energy content and exchanges. The first law prescribes energy conservation while the second law introduces the concept of irreversibility in systems as thermodynamic energies decrease, also known as entropy. It has been shown severally that entropy generation accompanies all degradation mechanisms simply by the irreversible nature of the dissipative processes involved. Hence, predicting and quantifying the effect of these processes based on accurate estimate of entropy produced led to the formulation of the Degradation-Entropy Generation (DEG) Theorem by Michael Bryant, Michael Khonsari and Frederick Ling (2008). The DEG theorem also establishes that if a critical value of degradation measure exists, at which failure occurs, there must also exist critical values of accumulated irreversible entropies, and the relationship between them has also been formulated in an independent study in Russia. A close look at 2 classical theories: Holm’s wear equation, w = kNx/H (subsequently modified to the more commonly used Archard’s equation) and Coulomb friction law, F = μN, shows a direct proportionality between wear and energy dissipated by friction, w ∝ Fx. Application of the DEG theorem to a similar sliding friction between two surfaces and the resulting wear characterized by the accompanying entropy generated (or energy dissipated) is shown to define an equivalent wear coefficient k as the Holm-Archard equation. Currently, this study focuses on further development and validation of the DEG theorem primarily in the area of its application to friction wear, grease degradation, battery ageing and fatigue analysis. A consistent thermodynamic approach for evaluating entropy generation accumulation is proposed. An investigation into the dissipative processes relevant to the degradation mechanisms is carried out for correlation to entropy generation. In addition to mathematical formulations, this work includes theorem verification using empirical fatigue data from previously published studies as well as seminal work - new battery and grease experiments to measure DEG parameters.