Thermodynamic Entropy Generation Research Articles

Entropy‐based method considering nonlinear hardening effect to predict the crack propagation life of superalloy GH4169 at elevated temperature

AbstractThis paper aims to determine the relationship between thermodynamic entropy generation and fatigue crack propagation life of superalloy GH4169 at 300–650°C. The entire specimen was considered as the thermodynamic system. The plastic energy dissipation in the crack tip was obtained by finite element simulation utilizing the Chaboche nonlinear hardening model. Then the cyclic entropy generation rate (CEGR) and the accumulated entropy generation are calculated by combining simulation and experimental methods. Results show that the CEGR is a power function of the stress intensity factor range, and it is almost a constant at fatigue failure. The fatigue fracture entropy (FFE) increases as fatigue cycles at failure increase at constant temperature, but it first decreases and then increases when temperature increases from 300 to 650°C. A fatigue life prediction model based on the thermodynamic damage parameter is established and verified by comparison with experimental results and available data in the literature.

Thermodynamic Entropy-Based Fatigue Life Assessment Method for Nickel-Based Superalloy GH4169 at Elevated Temperature Considering Cyclic Viscoplasticity.

This paper develops a thermodynamic entropy-based life prediction model to estimate the low-cycle fatigue (LCF) life of the nickel-based superalloy GH4169 at elevated temperature (650 °C). The gauge section of the specimen was chosen as the thermodynamic system for modeling entropy generation within the framework of the Chaboche viscoplasticity constitutive theory. Furthermore, an explicitly numerical integration algorithm was compiled to calculate the cyclic stress-strain responses and thermodynamic entropy generation for establishing the framework for fatigue life assessment. A thermodynamic entropy-based life prediction model is proposed with a damage parameter based on entropy generation considering the influence of loading ratio. Fatigue lives for GH4169 at 650 °C under various loading conditions were estimated utilizing the proposed model, and the results showed good consistency with the experimental results. Finally, compared to the existing classical models, such as Manson-Coffin, Ostergren, Walker strain, and SWT, the thermodynamic entropy-based life prediction model provided significantly better life prediction results.

Evaluation of Volumetric Strain on Polyamide 6 by Thermodynamic Entropy Generation

Evaluation of Volumetric Strain on Polyamide 6 by Thermodynamic Entropy Generation

Crystal plasticity constitutive model and thermodynamics informed creep-fatigue life prediction model for Ni-based single crystal superalloy

Ni-based single crystal superalloys are the main constituent materials for aeroengine turbine blades. They are subjected to extensive in-service plastic deformation and creep-fatigue interaction, which can cause damage and failure and hence limit the turbine blade durability. In this study, a novel crystal plasticity-based constitutive model is proposed to predict the cyclic inelastic deformation of Ni-based single crystal superalloy under creep-fatigue loads, and the key aspects examined include cyclic strain hardening, ratcheting and stress relaxation behavior. The novelty of the model lies in the introduction of a dislocation density parameter in the kinematic hardening rule to describe the evolutionary characteristics of hysteresis loops. The constitutive model is implemented via the crystal plasticity finite element method and the predictions are in good agreement with experimental results. Furthermore, thermodynamic entropy generation is innovatively adopted as an indicator parameter for analysis of Ni-based single crystal creep-fatigue failure, and the corresponding creep and fatigue damage models are developed to evaluate the degree of damage. The half-life concept associated with the steady-state hysteresis loop is employed in the failure model to predict the creep-fatigue life without being limited by the computational efficiency of the crystal plasticity finite element method. The proposed model can well capture the characteristics of Ni-based single crystal creep-fatigue life, and the prediction falls within a scatter band of factor 2.0 compared to experimental results. The proposed creep-fatigue life prediction model is underpinned by deformation and failure mechanisms, which would provide a basis for accurate analysis and robust assessment of Ni-based single crystal superalloy performance and life.

An improved method for fatigue life prediction of metal materials based on thermodynamic entropy

To determine the relationship between thermodynamic entropy generation rate and cumulative entropy and fatigue performance of metal components during fatigue. The surface temperatures of the specimens under different loading amplitudes and frequencies were recorded by infrared thermography. The effect of internal friction is not considered. Then the entropy generation rate and cumulative entropy are calculated, and the relationship between fatigue life and entropy is constructed. The calculation results show that under the same loading frequency, the entropy generation rate is proportional to the loading amplitude, the fatigue cycle is relatively low, and the cumulative entropy is inversely proportional to the loading amplitude. The cumulative entropy is only related to the loading amplitude, but not the loading frequency. A fatigue life prediction model based on the thermodynamic entropy of metals is established, which can be used for the real-time assessment of fatigue damage of metal components.

Rapid prediction of fatigue life based on thermodynamic entropy generation

The present paper is devoted to rapid prediction of fatigue performance of materials for medium- and high-cycle fatigue based on the thermography approach. During cyclic loading, the temperature of specimens or components increases due to both anelastic and plastic deformation, while only the plastic strain energy contributes to fatigue damage. Therefore, in this work, a model is proposed to estimate the damaging (plastic strain energy) and non-damaging (anelastic energy) parts of the dissipated heat during the cyclic loading. It is also shown that by taking advantage of the concept of fracture fatigue entropy (FFE), one can rapidly generate a stress-life diagram by conducting a stepwise test and only one fatigue experiment until failure. Further, it is shown mathematically that the dissipating energy is directly proportional to the steady-state temperature. Therefore, in situations where determining the dissipation energy is difficult, the fatigue performance can be readily predicted only by measuring the steady-state temperatures during stepwise testing. The predictions of the proposed models are verified by comparison to a series of experimental results for different materials.

On the effect of internal friction on torsional and axial cyclic loading

Failure of materials subjected to torsional and axial cyclic loading is studied by applying the principles of thermodynamics of dissipative processes. Extensive experimental tests are conducted using stainless steel (SS) 304 and SS316 materials to gain insight into both torsional and tension–compression fatigue life. It is shown that the higher magnitude of dissipated energy in torsional loading, as reported in many studies, is due to the anelastic deformation in the material and its associated internal friction. It is further shown that proper assessment of the damaging dissipation energy requires one to exclude the non-damaging internal friction from the total amount of energy dissipation. Accumulation of thermodynamic entropy generation as an index parameter using the modified dissipated energy is estimated to predict the fatigue life. The experimental results are found to be in good agreement with the proposed fatigue life prediction.

Low-cycle fatigue assessment of metallic materials based on thermodynamic entropy generation – Methodology and model development

In this study, thermodynamic entropy generation is employed as a degradation model for analysis of metal fatigue failure. An analytical solution is carried out to evaluate the temperature of the specimen during the test. Based on the analytical solution, two new models are developed for entropy generation. The first one is applicable to the case that plastic work density is measurable. The second one is useful for temperature monitoring techniques. A set of experiments are conducted on specimens Al 2024-T4 as a case study. The results show that cumulative entropy generation increases with respect to the number of cycles.

Thermodynamic entropy to detect fatigue crack initiation using digital image correlation, and effect of overload spectrums

Current approaches to fatigue life prediction are mostly based on classical fracture mechanics and it is difficult to predict lifetime under different loading conditions, particularly under overload spectrums. A thermodynamic entropy-based approach is less sensitive to variability in loading conditions and provides a dependable measure of damage. Volumetric thermodynamic entropy is utilized in this study to measure damage. Digital Image Correlation-DIC is used to measure plastic deformation in close vicinity of the notch in AA7075-T6 specimens. The plastic deformations are converted to a corresponding thermodynamic entropy, using a modified Clausius-Duhem inequality. A comparison of thermodynamic entropy generation using global measurement consisting of load frame data and Linear Variable Differential Transformer-LVDT extensometer, and local measurement using DIC data is done. The entropic-endurance level for fatigue crack initiation regardless of loading spectrums is shown to scatter within a short interval and is represented by a Weibull distribution.

Metal fatigue assessment based on temperature evolution and thermodynamic entropy generation

In the present study, a general model was developed for temperature evolution prediction during metal's uniaxial fatigue loading. The prediction model leads to a reasonably accurate determination of specimen temperature distribution. It also identifies the effective parameters of fatigue temperature variation. It was observed that plastic strain energy is an important contributor to the temperature evolution trend. The proposed model is also employed to evaluate empirical relations, proposed for fatigue life prediction. At the last stage, based on the proposed model for temperature and Morrow model as plastic work density, the proposed model was utilized for the entropy generation prediction.

Thermodynamic entropy generation in the course of the fatigue crack initiation

AbstractEvolution 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.

On the anelasticity and fatigue fracture entropy in high-cycle metal fatigue

The concept of thermodynamic entropy generation in a degradation process is utilized to study the high-cycle fatigue of medium carbon steel 1018. Uniaxial tension–compression fatigue tests are carried out with tubular dogbone specimens at different stress levels and loading frequencies. It is shown that a phase lag between the stress and the strain caused by the internal friction includes a considerable amount of non-damaging anelastic energy in a hysteresis loop when the amplitude of cyclic load is substantially smaller than the yield strength of the material. A methodology is proposed to determine the anelastic energy associated with metal fatigue at a stress level lower than the yield strength of a material. Finite element simulations are carried out with a 3-D model of the specimen to determine the validity of the proposed methodology. The evolutions of the plastic strain energy and temperature are discussed and utilized to calculate the entropy accumulation. It is shown that the accumulation of entropy generation in the HCF of the material—beginning with a pristine specimen and ending at fatigue fracture—is nearly constant within the experimental and loading conditions considered. The concept of tallying entropy is useful for the prediction of the fatigue life evolution of a material undergoing cyclic loading.

Estimation of reliability of structures subject to fatigue loading using plastic strain energy and thermodynamic entropy generation

Plastic strain energy dissipation and thermodynamic entropy generation are explored as means of estimating the likelihood of fatigue crack initiation in reliability assessment of structures. A set of experimental data for aluminum alloy 7075-T651 used in airframe structures are presented to determine the correlation between plastic strain energy dissipation and the thermodynamic entropy generation versus fatigue crack initiation. Cumulative plastic strain energy and thermodynamic entropy generation measured from hysteresis energy and temperature rise proved to be valid physical indices for estimation of the probability of crack initiation. Crack initiation is considered as evidence of fatigue damage and loss of structural integrity. A Bayesian estimation and validation approach is used to determine systematic errors in the developed models as well as other model uncertainties. It was shown that the results from both methods are good predictors of crack initiation and comparable, but the entropy generation method was associated with fewer uncertainties than the plastic strain energy method. Comparisons of the energy-based and entropy-based models are presented and advantages of using one over the other are discussed.

Entropic characterization of metal fatigue with stress concentration

The accumulation of thermodynamic entropy generation in a fatigue degradation process, beginning with a pristine specimen and ending at fracture, is referred to as the fatigue fracture entropy (FFE). In this paper, the concept of FFE is applied to study the effect of stress concentration on metal fatigue. Experiments involve uniaxial tension–compression fatigue tests carried out with solid cylindrical un-notched and V-notched specimens made of both medium-carbon steel 1045 and aluminum 6061. Finite element simulations are also performed with both types of specimens to study their thermal response under cyclic load and to predict the corresponding FFE. Experimental and theoretical results show that the FFE of these materials are nearly constant for each type of the specimens. Under the conditions tested the presence of stress concentration is shown to reduce the amount of hysteresis energy generation in the specimen gage section compared to the un-notched specimen under the identical loading conditions. Also shown is that the FFE decreases significantly due to the presence of stress concentration. An empirical correlation is proposed that can predict the fatigue life of a V-notched specimen based on the hysteresis energy per cycle and the FFE of an un-notched specimen. Predicted and experimental fatigue lives of V-notched specimens are found to be in good agreement.

On the Role of Entropy Generation in Processes Involving Fatigue

In this paper we describe the potential of employing the concept of thermodynamic entropy generation to assess degradation in processes involving metal fatigue. It is shown that empirical fatigue models such as Miner’s rule, Coffin-Manson equation, and Paris law can be deduced from thermodynamic consideration.