Cyclic Thermomechanical Loading Research Articles

Thermomechanical fatigue life prediction of 316L compact heat exchanger

Compact welded heat exchangers are designed to be used in severe operating conditions (temperature, pressure, aggressive fluid, etc.). Thus fatigue failure can be observed after large cyclic strain due to thermomechanical cyclic loads. In this paper, a 316L stainless steel structure solicited in the (extremely) low cycle fatigue regime is analyzed through a multi-scale approach. A finite element analysis method has been developed and correlated to experiments. The thereto-elastic response of the heat exchanger to thermal cycling of various amplitudes has been firstly investigated. The stress concentration locations are identified and the local thermo-elastic stored energy on these points is calculated. By the use of a combined isotropic/ldnematic hardening previously determined by alternated bending tests, these data are then used in a micromechanic approach. It consists in the consideration of the material elastic-plastic behavior under uniaxial mechanical solicitation. Based on energy equivalence, a local nonlinear description is so adopted to an expected equivalent strain amplitude. This estimation is used to predict the lifetime of the heat exchangers through an adapted rule relying this deformation to cycle number to failure through the Coffin Manson law. The methodology, simple to handle, is thought to improve the design of heat exchangers depending on their operating conditions. (C) 2016 Elsevier Ltd. All rights reserved.

Assessment of the Cyclic Behavior of Structural Components Using Novel Approaches

To extend the life of a structure, or a component, which is subjected to cyclic thermomechanical loading history, one has to provide safety margins against excessive inelastic deformations that may lead either to low-cycle fatigue or to ratcheting. Direct methods constitute a convenient tool toward this direction. Two direct methods that have been named residual stress decomposition method (RSDM) and residual stress decomposition method for shakedown (RSDM-S) have recently appeared in the literature. The first method may predict any cyclic elastoplastic state for a given cyclic loading history. The second method RSDM-S that is based upon RSDM is suggested for the shakedown analysis of structures. Both methods may be directly implemented in any finite-element (FE) code. An elastic perfectly plastic material with a von Mises yield surface has been assumed. In this work, through their application to structures that are used as benchmarks in the literature, both methods, applied together, prove their efficiency and capacity to determine shakedown boundaries and reveal unsafe conditions.

Thermo-mechanical coupling in constitutive modeling of dissipative materials

In the present paper a thermodynamics based constitutive modeling of coupled dissipative phenomena is discussed. In the proposed analysis special attention is paid to the problem of thermo-mechanical coupling in rate equations of thermodynamic conjugated forces. As an example of thermo-plastic coupling, the non-isothermal fatigue behavior of tempered martensitic hot work tool steel is considered. The numerical simulations are performed, to illustrate the qualitative and quantitative influence of temperature rate on the response of constitutive model when cyclic thermo-mechanical loading is applied.

Tribological behavior of high thermal conductivity steels for hot stamping tools

To improve the performances of new hot stamping processes for automotive industry, as well as to overcome their numerous drawbacks-such as oxidation, dies wear, severe thermal and mechanical cycles-new dies materials are being developed to obtain shorter heating and cooling cycles as well as improved strength and service-life.The paper investigates the tribological behavior of two new high thermal conductivity steel grades, by applying thermo-mechanical cyclic loads in the temperature range 600–800°C. The results show that a material microstructure characterized by agglomerates of Mo allows better performances than the case of finer elements of Mo homogeneously distributed in the metal matrix, with reduced friction and abrasion wear more relevant at the lowest testing temperatures.

Thermo-mechanical relaxation of compressive residual stresses induced by shot peening

Experimental and theoretical studies have shown that the compressive residual stresses induced by shot peening, have been identified, generally, as the principal factor for fatigue strength improvement. However, it is worth noticing, that the initial induced compressive shot peening residual stresses may be redistributed and relaxed under mechanical, thermal or thermo-mechanical cyclic loading. It is important to consider the relaxation phenomenon in the prediction of the fatigue behaviour of shot-peened parts. This paper presents a numerical approach to evaluate the mechanical, thermal and thermo-mechanical relaxation of the compressive residual stresses induced by shot peening.

РАСЧЕТ КОНЦЕНТРАЦИИ ВАКАНСИЙ ПРИ ТЕРМОМЕХАНИЧЕСКОМ НАГРУЖЕНИИ

The evolution of vacancy system and pore emergence under the influence of cyclic thermo-mechanical loading determines the durability of the nanocrystalline two-phase alloys such as superalloys, which are composite materials. The emergence of excess non-equilibrium vacancies, the activation of diffusion processes and the pore growth occur under the influence of thermo-mechanical loading. However, kinetics of the emergence and evolution of the excess vacancy concentration at various kinds of thermo-mechanical loading arising during the operation process have not been investigated yet. In this paper, the vacancy concentration in the one-dimensional model for thermo-mechanical loading including cyclic tensile stresses, thermal stresses and heating to high temperature taking into account microstructure is calculated. The considerable supersaturation of non-equilibrium vacancies arises in nanostructured alloys under the influence of thermo-mechanical loading. The rising of diffusion may occur at a critical value of excess vacancies as part of the diffusion-deformation stability mechanism, when small local fluctuation of excess vacancy concentration begins to grow. The growth occurs due to a Gibbs energy decrease in the increased vacancy concentration caused by the influence of thermo-mechanical stresses

Cracks path growth in turbine blades with TBC under thermo – mechanical cyclic loadings

Blades of combustion turbines are extremely loaded turbojet elements, which transmit operative energy onto a rotor. Experiences of many years indicate, that cracks initiation and propagation in the blades during the operation time can cause destruction not only of the engine, but sometimes an airplane. In high temperature one of the most often occuring interactions in the turbine engine are time variable force fields, caused by non-stationary flowing of an exhaust gas and aerodynamical interaction of the engine elements. The extremal thermo-mechanical loadings initiate gradual degradation process of the blades as a result of fatigue and material creep. More often Thermal Barrier Coatings (TBCs) are applied on the turbine blade surface to provide protection not only against the high temperature but also against aggressive environment. The paper presents the advantages of applying of the TBC layers for increase of the cracks resistance to gradual degradation of the turbine blades. The level of save values of thermo-mechanical loading was estimated. Analysis of critical values of loading leading to crack initiation, further growth and the final blade fragmentation was performed. The most efforted places of the turbine blades were selected and crack paths due to thermo-mechanical cyclic loading were determined.

Basic Mechanisms Leading to Fatigue Failure of Structural Materials

General features of the damage evolution in cyclic loading of structural materials are summarized. The attention is paid to the comparison of the damage mechanisms in materials that are used for service both at room and at elevated temperatures, namely austenitic stainless steel Sanicro 25. Principal mechanisms leading to fatigue fracture at room and at elevated temperature are documented. While cyclic slip localization is a decisive process in the initiation of fatigue cracks at room temperature, the localized oxidation plays an important role in isothermal high temperature cyclic loading. Specific mechanisms of the early fatigue damage in thermomechanical cyclic loading are studied. The in-phase thermomechanical loading leads to the intergranular crack initiation due to preferred grain boundary oxidation and intergranular crack growth. The out-of-phase thermomechanical loading results in oxide cracking and localized oxidation of the metal in the area of cracked oxides and to transgranular crack growth. The damage mechanisms can explain differences in fatigue life under various loading and temperature conditions.

Cyclic degradation of titanium–tantalum high-temperature shape memory alloys — the role of dislocation activity and chemical decomposition

Titanium–tantalum shape memory alloys (SMAs) are promising candidates for actuator applications at elevated temperatures. They may even succeed in substituting ternary nickel–titanium high temperature SMAs, which are either extremely expensive or difficult to form. However, titanium–tantalum alloys show rapid functional and structural degradation under cyclic thermo-mechanical loading. The current work reveals that degradation is not only governed by the evolution of the ω-phase. Dislocation processes and chemical decomposition of the matrix at grain boundaries also play a major role.

Experimental characterization of laser cladding of CPM 9V on H13 tool steel for die repair applications

H13 tool steel with excellent hot working properties is commonly used for manufacturing dies. However, the damage of die surface due to cyclic thermo-mechanical loading is detrimental to the service life. In order to enhance the die life, it has been observed that cladding based repair is superior to welding or thermal spraying repair techniques. In this paper, experimental study of laser cladding of H13 has been carried out. CPM 9V steel powder has been deposited on H13 tool steel plate for repairing the die surface damage using a CW CO2 laser in conjunction with powder injection system. The effect of laser parameters on clad geometry and clad quality has been investigated. The microstructure of laser cladded samples has been characterized using optical microscope (OM) and scanning electron microscope (SEM). The phases and the residual stresses present in the clad have been determined via X-ray diffraction. The micro-hardness profiles obtained in the clad–substrate system and the hardness change due to cyclic thermal loading have also been characterized. Optical micrographs of the clad microstructure shows existence of vanadium carbide particles embedded in martensite and retained austenite. The hard vanadium carbide particles increase the clad hardness to an average of four times greater than the substrate hardness. It has been observed that compressive residual stresses are generated in clad which is desirable for repair applications as it will impede the crack propagation resulting in enhanced die life.

An evaluation of the segmented expanding cone-mandrel test to assess hydride re-orientation and ductility reduction for Zircaloy-2 cladding tubes

This paper presents the segmented expanding cone-mandrel (SECM) test for thin-walled cladding tubes and quantifies its sensitivity to inherent uncertainty factors such as friction. The SECM test is then applied to study hydride re-orientation and embrittlement of pre-hydrided Zircaloy-2 cladding by cyclic thermo-mechanical loadings to induce radial hydrides followed by a ductility test. The paper shows that the SECM can be used to induce hydride re-orientation but the repeatability of the re-orientation needs to be improved. The loss of ductility from radial hydrides can be well quantified with respect to the radial hydride content. The ductility reduction from radial hydrides is however a three-dimensional problem and accurate quantification requires a three-dimensional model.

Finite Element Modeling of Shot Peening Residual Stress Relaxation in Turbine Disk Assemblies

Surface enhancement techniques such as shot peening are extensively used to increase the fatigue life of components in gas turbine engines. Due to the combined thermomechanical nature of the loading encountered within an engine, aeroengine designers have avoided incorporating the beneficial effects in their analysis. This can lead to overdesign and early retirement of critical engine components. A finite element modeling procedure is introduced that incorporates the shot peening residual stresses on a fir-tree turbine disk assembly. Unlike traditional equivalent loading approaches, the method models the actual impact of shots on the assembly and is the first time this approach is used to introduce peening residual stresses in turbine disks. In addition, the stability of these residual stresses in response to cyclic thermomechanical loadings at the contact interface is also studied. The results reveal that thermomechanical overload can nearly fully relax the shot peening residual stresses within the first cycle due to the combined effects of decreased material yield strength and plastic deformation. This work will enable aeroengine designers to assess critical surface treated components for structural integrity, optimal design, and residual life.

Synthesis of Copper/Silicon-Carbide Composites in a Thermodynamic Disequilibrium

Composites with interpenetrating metal-ceramic microstructures (IPC, interpenetrating composites) can be tailored for specific applications, such as high thermal conductivity combined with low thermal expansion, e.g. for heat sinks. Heat sinks are required in power electronic devices or in future fusion reactor technology where extreme conditions and high cyclic thermo-mechanical loads appear. Due to its rigid ceramic backbone IPCs are expected to reveal high thermal stability. Pure silicon carbide exhibits high thermal conductivity, low coefficient of thermal expansion, high corrosion and wear resistance. But it is also known as a very brittle material when mechanical loads are applied. Thus a composite of silicon carbide with ductile and highly conductive copper seems to be a promising new material for a number of applications.This paper reports the synthesis of Cu-SiC composites using a unique high temperature squeeze casting process (HTSC). Microstructural design of SiC-preforms with open porosity and its synthesis progress is reported. Influence of preform properties, temperature, pressure and atmosphere during HTSC were investigated. A qualitative and quantitative description of the microstructure of the composites and their composition allows the creation of structure-property correlations that take effect retroactively to the casting process.

Calculation of fatigue life of cylindrical parts at multilayer surfacing and service cyclic thermomechanical loading

Calculation of fatigue life of cylindrical parts at multilayer surfacing and service cyclic thermomechanical loading

Effect of Temperature Rate Term while Predicting Thermal Ratcheting of a Thin Cylinder due to Cyclic Temperature Variation

The occurrence of progressive plastic deformation due to ratcheting under thermal cycling governs the integrity and life of industrial components such as the main vessel of sodium cooled fast reactors (SFR). Present work firstly discussed a constitutive model with temperature rate terms (TRT) to highlight the effect of thermal load rate (TLR) during the ratcheting behavior of the two-bar mechanical system (SS 316 LN). The influence of TLR is studied for various stress controlled cyclic thermo-mechanical loadings considering temperature dependent material parameters. Secondly, the approach is used to reveal the effect of TRT while predicting thermal ratcheting of a thin cylinder caused by change in peak temperature during temperature front movement. Such loading situations are more relevant to the SFR when the pool temperature varies with free-level variations. Visco-plastic formulation with and without TRT is used for analysis, which is validated by comparing the published experimental and numerical results.

Marginal adaptation of CAD/CAM zirconia-based crown during fabrication steps

BackgroundExcessive marginal discrepancy of crowns favors the rate of cement dissolution and microleakage that may cause pulpal inflammation. Besides, it may increase plaque retention leading to the onset of periodontal disease. Therefore, this research was carried out to study the effect of fabrication stages and artificial aging on the marginal adaptation of CAD/CAM Zirconia-based crowns which have become increasingly popular among the patients due to their natural esthetics and excellent strength. PurposeTo evaluate the marginal adaptation of CAD/CAM Zirconia-based crowns on their respective prepared teeth during different fabrication stages namely; framework, after veneering, after crown cementation and after thermo-mechanical loading. Materials and methodsA total of twenty zirconia frameworks were fabricated using (Cercon smart ceramics, DeguDent, Germany) manufacturing system, conventionally veneered with (Cercon Ceram kiss, DENSPLY, Germany), cemented with conventional glass ionomer to their corresponding prepared human teeth. The cemented crowns were finally aged through a process of thermo-mechanical cyclic loading. The assessment of vertical marginal gap was performed on the prepared teeth using an optical microscope with image-j software analysis system at 40× magnification for frameworks, for restorations before and after cementation then, after thermo-mechanical loading. Differences between the vertical gap during fabrication stages and the effect of artificial aging were statistically analyzed using a one-way analysis of variance ANOVA and Independent-samples T test. ResultsIn this study, the marginal gap increased after every tested stage. The vertical marginal gap recorded its smallest mean value (41.08 ± 3.23 μm) during core stage. A significant increase in the measurement of vertical marginal gap was observed after firing the veneering layer reaching (46.87 ± 3.94 μm). After cementation, the marginal gap was (46.87 ± 4.65 μm). Finally; thermo-mechanical loading corresponding to one year of clinical use was found to cause a significant increase the vertical marginal gap to record its highest value (56.73 ± 7.21 μm). ConclusionWithin the limitations of this study, it was found that every examined stage had a negative effect on the vertical marginal adaptation of zirconia crowns.

A Unified Shakedown Assessment Method for Butt Welded Joints With Various Weld Groove Shapes

A unified shakedown assessment approach for butt welded pipes or cylindrical pressure vessels with common circumferential butt welded joints for U-groove, V-groove, X-groove, double U-groove (DU-groove), UV-groove, and single-side welding was established under cyclic thermomechanical loadings based on the simplified finite element method. Individual effects of material properties of weld metal (WM), heat affected zone (HAZ), and parent material (PM), the ratio of inner radius, and wall thickness were further considered. Results showed that the proposed method has a high accuracy for shakedown evaluation of weld pipes with various weld groove shapes. Moreover, U-groove is fit for weld pipes to improve the load-carrying capacity of thick-walled cylinders under cyclic thermomechanical loading. Furthermore, the ratio of the yield stress between WM and PM should be greater than the transition point for weld pipes with various weld groove shapes to avoid ratchet first occurs in the WM region.

Elasto-Plastic Analysis of a Functionally Graded Rotating Disk Under Cyclic Thermo-Mechanical Loadings Considering Continuum Damage Mechanics

The goal of this work is to study the influence of continuum damage mechanics on a functionally graded rotating disk subjected to cyclic temperature gradient loading through nonlinear kinematic hardening rule employed to model the back stress. The formulations have been developed on the basis of von Mises' yield criterion. The material properties are assumed to be independent of temperature and vary according to a power law volume fraction relation but Poisson's ratio is assumed to be constant. Return mapping algorithm (RMA), an incremental method, has been used in the numerical procedure. Material behaviors such as elastic shakedown, plastic shakedown and ratcheting were specified in the existence of continuum damage mechanics to obtain the Bree's interaction diagram for different temperatures and angular velocities.

Site Specific Microstructural Evolution of Thermo-mechanically Fatigued Copper Films

Copper metallizations used in modern power semiconductors have to withstand cyclic thermo-mechanical loading over the full operational life-time. Hence, detailed knowledge about the microstructural changes is required to better understand ongoing fatigue mechanisms and specifically to enable improvement of the fabrication process. Therefore, a microstructural characterization method was applied which combines a site-specific analysis using atomic force microscopy (AFM) and electron backscatter diffraction (EBSD) throughout thermal cycling. Studying the same surface area with AFM and EBSD enables a determination of global microstructural parameters, such as surface roughness, grain size, and grain boundary characteristics with cycling, and hence exhibits a great potential for universal microstructural analysis regardless of the loading process. We report about combining topographical and crystallographic information by tracking the identical surface area. Our observations indicate diffusional mass transport at grain boundaries, quantified by height profile evolution, grain growth, and twin boundary migration.

Modeling of Deformation and Functional Properties of Shape Memory Alloys Based on a Microstructural Approach

This microstructural model of the functional-mechanical behavior of shape memory alloys (SMA) includes a description of the reversible phase deformation, microplastic deformation due to the accommodation of martensite and the evolution of the deformation defects. The laws of these phenomena are formulated in terms of the generalized thermodynamic forces. The microplastic flow rule accounts for isotropic and kinematic hardening, which are related to the accumulation of the deformation defects. The model gives a good description both of reversible and irreversible deformation under one-side or cyclic thermomechanical loading of SMA and opens a way for the fatigue life prediction. The model can be applied for solving of the mechanical problems for SMA parts such as dampers or base isolators used in seismic protection devices.