Experimental Determination of Entropy and Exergy in Low Cycle Fatigue

Abstract

Recent works in mechanical fatigue consider that a threshold of entropy exists, the fracture fatigue entropy. The determination of this quantity is usually done considering empirical models for the mechanical power estimation. In this paper, we experimentally observe the existence of a threshold of entropy and exergy in low cycle fatigue for a flat Al-2024 specimen avoiding the use of a model, solely measuring the heat generated during a fatigue test. Results are then compared considering various hypotheses (1D heat dissipation with convection and radiation considered as heat sources, and, heat transfer from a fin with convection and radiation as boundary conditions) to an empirical mechanical model known in the literature and deviations between them are discussed.

Thermodynamic entropy as a marker of high‐cycle fatigue damage accumulation: Example for normalized SAE 1045 steel

“…Simultaneously with the rise of the degradation, the entropy continuously increases as well towards the final FFE. Based on former work, the FFE for a certain material is constant, 35,37 regardless of the type of the mechanical fatigue load, such as tension‐compression, bending or torsion, frequency and geometry, and possibly others, which has been proven through the application of different models of FFE calculation 27,35 . Based on Equation 21, the results of FFE calculations for 15 specimens are given in Figure 9.…”

“…Combining Equations 6 and 10 and using ε p = ε − ε e as a simplification due to a small strain assumption in the case of high‐cycle fatigue, the specific entropy generation flow can be expressed as follows: where 35 : is the specific entropy generation derived from plastic deformation. is the specific entropy generation caused by irreversible deformation, such as strain hardening and phase transformation. is the specific entropy generation provided through heat conduction. …”

“…Thus, the FFE, namely the maximum entropy generation at the onset of fracture generated by irreversibility during the fatigue process, is , where t f is the time to failure. In the case of materials with low hardening potential and at sufficiently high fatigue test frequencies, the second and the third terms of Equation 11 can be neglected as it was proposed in 35 the following: …”

Microstructure-sensitive estimation of fatigue life using cyclic thermodynamic entropy as an index for metals

“…It is responsible for 80-90% of the related failures [1], making the study of the fatigue performance and the design of fatigue-resistant metals crucially important [2][3][4][5][6]. Rich volumes of literature offer different theoretical [7][8][9] and experimental [6,10] approaches to predict fatigue life, among which microstructure-sensitive and thermodynamically-based models are becoming widespread [11][12][13]. Importantly, recent crystal-plasticity models show a strong correlation between energy dissipation and crystalline deformation and call for further development of models that can effectively treat microstructure-sensitive materials [14,15].…”

“…Bayati et al [19] investigated the heat dissipation of additively manufactured NiTi specimens under cyclic loading using an infrared (IR) camera to study and predict its fatigue properties such as the fatigue limit. Ribeiro et al [13] used the concept of exergy (the maximum recoverable and useful work of a system interacting with the environment) to quantify fatigue damage irreversibility. As a result, they measured the Fracture Fatigue Entropy (FFE as introduced by Naderi et al [20]) by measuring temperature during the fatigue test of Al 2024 samples.…”