Thermodynamic entropy generation in the course of the fatigue crack initiation

Abstract

Evolution of the thermodynamic entropy generation during fatigue crack initiation life of notched specimens is studied. A set of experimental results of AA7075‐T651 is examined to determine applicability of the thermodynamic entropy generation as an index of fatigue crack initiation. Entropy accumulation is calculated from hysteresis energy and temperature rise. An increasing trend of entropy accumulation with the number of cycle to failure is observed on macroscale measurements. Results also determine that the entropy generations from the samples under the same operating conditions are similar as the crack grows. Scanning electron microscope analysis is performed on the fractured surfaces to observe the fatigue striations, and persistent slip bands are observed employing an optical microscope. A discussion is presented regarding the length scales on which crack initiation occurs and entropy calculation is made.

Measures of Entropy to Characterize Fatigue Damage in Metallic Materials

“…Naderi et al [19] numerically calculated the dissipative entropy by using only the mechanical work term, assuming that plastic deformation is the dominant term and the heat conduction effect is negligible, as presented in Figure 1. This assumption was also empirically verified by Imanian et al [15] and Ontiveros et al [20], In addition, the concept of entropic endurance was further confirmed by Imanian et al [15], who measured the interacting thermodynamic forces in a coupled failure mechanism, corrosion-fatigue. Summation of entropies from both mechanical work (fatigue) and chemical reactions (corrosion) contributed to the total entropic endurance at the point of fatigue failure.…”

“…As described in Equation (2), thermodynamic entropy generation is computed by the bilinear equation of force and flux for each energy dissipation mode. In the fatigue damage process, mechanical work is the dominating term, as experimentally proved from previous studies [14,20,26]. Plastic strain energy is computed numerically using discrete stress-strain data.…”

“…They reviewed in more detail the DEG theorem, including the concept of entropic endurance introduced by Imanian and Modarres [26]. Experimental results have supported this theorem for fatigue failures [15,19,20] by demonstrating that fatigue fracture occurs at a relatively fixed entropic endurance level regardless of the underlying loading profiles. Figure 1 presents an example confirming this entropic theorem.Entropy 2019, 21, x 3 of 21 second law of thermodynamics suggests a universal methodology that applies to all the scales discussed above.…”

“…In the case of fatigue damage, irreversible energy dissipations include plastic mechanical work, heat, and acoustic emission [18]. A popular entropic approach in fatigue is to use plastic strain energy and surface temperature [19,20]. In this approach, the existence of a fixed entropic endurance, irrespective of the underlying conditions that lead to fatigue damage and failure, is experimentally verified.…”

A Model of Mechanothermodynamic Entropy in Tribology

“…The above results were obtained for steels, polymers, titanium and its alloys and aluminum alloys. It seems appropriate to apply the rules in Figures 2 and 3 and Equations (39), (40), (52) to specific polymers, composites and natural materials like seashells for example, which are characterized by high strength and wear resistance [55][56][57][58][59]. Such an application could be realized based on the model of many contact interactions between the elements of the multibody tribo-fatigue system described in [5,60].…”