A new, direct approach toward modeling thermo-coupled fatigue failure behavior of metals and alloys (original) (raw)

Thermodynamic Approach to Fatigue Failure Analysis in Metals and Composite Materials

2011

Fatigue is a dissipative process and must obey the laws of thermodynamics. In general, it can be hypothesized that the degradation of machinery components is a consequence of irreversible thermodynamic processes that disorder a component, and that degradation is a time dependent phenomenon with increasing disorder. This suggests that entropy-a fundamental parameter in thermodynamics that characterizes disorder-offers a natural measure of component degradation. The majority of the existing methods for prediction of fatigue are limited to the study of a single fatigue mode, i.e., bending or torsion or tension-compression. Further, the variability in the duty cycle in a practical application may render many of these existing methods incapable of reliable performance. During this research, we put forward the idea that fatigue is a degradation process and that entropy is the most suitable index for assessing degradation. That is, tallying irreversible entropy is more reliable and accurate than many of the other methods presented in the existing papers. We show that in processes involving fatigue, for a given material (metal and composite laminate), there exists a unique threshold of the cumulative thermodynamic entropy beyond which fatigue fracture takes place. This threshold is shown to be independent of the type of the fatigue process and the loading history. This exciting result is the basis of the development of a Fatigue Monitoring Unit (FMU) described in this research. We also propose a general procedure for assessment of damage evolution based on the concept of entropy production. The procedure is applicable to both constant-and variable amplitude loading. Empirical relations between entropy generation and damage evolution for two types of metals (Alumunium 6061-T6 and Stainless steel 304) and a woven Glass/Epoxy composite laminate are proposed and their potential for evaluation of fatigue damage are investigated.

Effect of Constitutive Model on Thermomechanical Fatigue Life Prediction

Procedia Engineering, 2015

In this paper, thermomechanical fatigue life (TMF) prediction is performed on an automotive exhaust manifold using in-house postprocessor code (Hotlife) with four different constitutive models: elastoplastic model with kinematic hardening, two-layer viscoplastic model, and two unified elasto-viscoplastic models based on the equations proposed by Sehitoglu and Chaboche, respectively. Commercial FEA software Abaqus has been used for the analysis. The first two models are already available in Abaqus; the Chaboche and Sehitoglu models were implemented using a user material subroutine (UMAT, available in Abaqus). The stress and strain histories calculated by these four models are output to Hotlife for post-processing. The results from the prediction are then compared to the experimental TMF life obtained from a component thermal fatigue bench test. Result show the Hotlife calculations based on the two implemented elasto-viscoplastic models yield better correlation with test results.

Dissipation and Thermoelastic Coupling Associated with Fatigue of Materials

Lecture Notes in Applied and Computational Mechanics, 2012

The fatigue behaviour is examined in terms of calorimetric effects. Aluminum alloy and steel have been chosen as reference materials. Heat sources accompanying the fatigue mechanisms are derived from thermal images provided by an infrared camera. A processing method allows identifying separately thermoelastic and dissipative sources. Thermoelastic effects are compared to theoretical predictions given by the basic, linear, isotropic thermoelastic model. Dissipation amplitudes are analyzed as a function of the loading frequency and stress amplitude applied to the fatigue specimen. Finally, the heterogeneous character of the fatigue development is studied both in terms of thermoelastic and dissipation sources.

Fatigue design of structures under thermomechanical loadings

Fatigue & Fracture of Engineering Materials & Structures, 2002

A B S T R A C T This paper presents a global approach to the design of structures that experience thermomechanical fatigue loading, which has been applied successfully in the case of cast-iron exhaust manifolds. After a presentation of the design context in the automotive industry, the important hypotheses and choices of this approach, based on a thermal 3D computation, an elastoviscoplastic constitutive law and the dissipated energy per cycle as a damage indicator associated with a failure criterion, are first pointed out. Two particular aspects are described in more detail: the viscoplastic constitutive models, which permit a finite element analysis of complex structures and the fatigue criterion based on the dissipated energy per cycle. The FEM results associated with this damage indicator permit the construction of a design curve independent of temperature; an agreement is observed between the predicted durability and the results of isothermal as well as non isothermal tests on specimens and thermomechanical fatigue tests on real components on an engine bench. These results show that thermomechanical fatigue design of complex structures can be performed in an industrial context.

Thermodynamic entropy generation model for metal fatigue failure

2018

Fatigue damage is comprehensively determined by thermodynamic entropy generation as the damage precursor. For fatigue mechanism, time rate of thermodynamic entropy generation is defined as ratio of plastic strain energy density rate per specimen’s temperature. Recent researches claim that entropy generation is constant at the time of crack initiation, directly related to the type of material, independent of loading and surrounding conditions. In this study, an analytical solution is proposed to evaluate the temperature of specimen during the fatigue test while the Morrow equation employed as plastic strain energy density. Result leads to derive new analytical-empirical model for calculating entropy generation. It is shown that temperature obtained from analytical solution is in good agreement with experimental data. In the next section, uncertainty and sensitivity analysis are accomplished based on proposed model. Monte-Carlo simulation and sigma-normalized derivative methods are em...

Two scale damage model and related numerical issues for thermo-mechanical High Cycle Fatigue

European Journal of Mechanics - A/Solids, 2007

On the idea that fatigue damage is localized at the microscopic scale, a scale smaller than the mesoscopic one of the Representative Volume Element (RVE), a three-dimensional two scale damage model has been proposed for High Cycle Fatigue applications. It is extented here to anisothermal cases and then to thermo-mechanical fatigue. The modeling consists in the micromechanics analysis of a weak micro-inclusion subjected to plasticity and damage embedded in an elastic meso-element (the RVE of continuum mechanics). The consideration of plasticity coupled with damage equations at microscale, altogether with Eshelby-Kröner localization law, allows to compute the value of microscopic damage up to failure for any kind of loading, 1D or 3D, cyclic or random, isothermal or anisothermal, mechanical, thermal or thermo-mechanical. A robust numerical scheme is proposed in order to make the computations fast. A post-processor for damage and fatigue (DAMAGE 2005) has been developped. It applies to complex thermo-mechanical loadings. Examples of the representation by the two scale damage model of physical phenomena related to High Cycle Fatigue are given such as the mean stress effect, the non-linear accumulation of damage. Examples of thermal and thermo-mechanical fatigue as well as complex applications on real size testing structure subjected to thermomechanical fatigue are detailed. * now at BRGM, Orléans, France

A continuum mechanics model for mechanical fatigue analysis

Computational Materials Science, 2005

In this paper, a thermo-mechanical constitutive model for the predictions of fatigue in structures using the finite element method is formulated. The model is based on the damage mechanics of the continuous medium and allows the treatment in a unified way of coupled phenomena such as fatigue with damage, plasticity, viscosity and temperature effects. Basically it is gotten sensitive models to cyclic loads starting from classical non-linear constitutive formulations incorporating the special variable influenced by the characteristics of the cyclic load. A formulation based on the theories of damage and plasticity is developed. The necessary modifications of these theories are outlined in order to include the fatigue phenomena. A brief description of the finite element implementation is given. Finally, results of the performance of the proposed model are shown through the simple fatigue test and the fatigue analysis of an aluminium engine alternator support.

Strain-life approach in thermo-mechanical fatigue evaluation of complex structures

Fatigue & Fracture of Engineering Materials and Structures, 2007

This paper is a contribution to strain-life approach evaluation of thermo-mechanically loaded structures. It takes into consideration the uncoupling of stress and damage evaluation and has the option of importing non-linear or linear stress results from finite element analysis (FEA). The multiaxiality is considered with the signed von Mises method. In the developed Damage Calculation Program (DCP) local temperature-stress-strain behaviour is modelled with an operator of the Prandtl type and damage is estimated by use of the strain-life approach and Skelton's energy criterion. Material data were obtained from standard isothermal strain-controlled low cycle fatigue (LCF) tests, with linear parameter interpolation or piecewise cubic Hermite interpolation being used to estimate values at unmeasured temperature points. The model is shown with examples of constant temperature loading and random force-temperature history. Additional research was done regarding the temperature dependency of the K p used in the Neuber approximate formula for stress-strain estimation from linear FEA results. The proposed model enables computationally fast thermo-mechanical fatigue (TMF) damage estimations for random load and temperature histories.