Low Cycle Thermomechanical Fatigue Research Articles

Combined Gleeble physical welding simulation and low-cycle thermo-mechanical fatigue for heat-affected zone material for 9Cr steel: Experimental testing and through-process model

Combined Gleeble physical welding simulation and low-cycle thermo-mechanical fatigue for heat-affected zone material for 9Cr steel: Experimental testing and through-process model

Multiaxial low-cycle thermo-mechanical fatigue of a low-alloy martensitic steel: Cyclic mechanical behaviour, damage mechanisms and life prediction

Axial–torsional Low-Cycle Fatigue tests (LCF) and Thermo-Mechanical Fatigue tests (TMF) were performed on a low-alloy martensitic steel for temperatures between 300°C and 600°C, as both proportional and non-proportional. The results show that non-proportional loading leads to an increase in fatigue damage and to a decrease in the observed lifetime. The TMF damage model is proposed here in order to incorporate the effects of multiaxial loading and varying temperatures. This damage model is based on a critical plane approach, and it incorporates the fatigue, oxidation and creep forms of damage. Finally, the proposed model is validated for a large experimental database.

Experimental and numerical modeling approach for thermomechanical low cycle fatigue analysis of cyclically non-stabilized steels

The widely used fatigue life prediction models, such as the Coffin–Manson model or S–N curve related models are based on the assumption that the response of a material experiencing low cycle fatigue loading is stabilized during some period. However, for many materials such a stabilized state is hardly observed, and the activated mechanisms for cyclic hardening or softening depend on test conditions. In general, the selected test conditions (stress or strain control) should depend on the intended use of the obtained material data. If testing conditions do not correspond to the operation mode of the considered mechanical facilities, the above mentioned life prediction models will produce inaccurate results. Hence, selecting and identifying proper fatigue parameters, which would represent the state of a material during the whole fatigue life, is extremely important in reliability evaluation of structures.In the case of non-stabilizing steels, the common challenges are:•Selecting and performing a suitable set of experimental tests to recognize various aspects of the material behavior under low-cycle thermomechanical fatigue;•Adjusting a proper constitutive modelling, reflecting the real physical phenomena taking place in the material microstructure;•Effective numerical implementation and optimal parameter identification.

Probabilistic Fatigue Software Platform (ProbFat) for Optimizing Life-Cycle Decisions

Fatigue is inherently uncertain. Thus, the use of technology for prediction of fatigue damage to ensure the long-term reliability of equipment must embrace such uncertainty. The proposed probabilistic fatigue software platform (ProbFat) accomplishes just that. The stochastic engine behind ProbFat is a collection of Bayesian Decision Networks (BDN) that assess risk to recommend the optimal life-cycle strategy for maximizing overall return (i.e. return = benefit – cost). Benefit is a measure of profitability (e.g. annualized revenue) and cost is a measure of expense (e.g. design, inspection, repair, replacement, failure, et cetera). To optimize the return, the BDN must incorporate all aspects of LCM including: (i) design and construction, (ii) monitoring, inspection, and maintenance scheduling, (iii) identification of damage, (iv) assessment of observed damage, and (v) decision strategies to either run, repair, rerate, replace, or retire the component(s) of interest at the best possible times. BDNs are structured as graphical cause-effect relationships, represented by probabilistic nodes with directional links. Populating conditional probabilities associated with nodes is accomplished through assessment of laboratory and site-specific data, contributions from subject matter experts and simulations from in-house deterministic and stochastic fatigue models. In this paper, two industry examples are used to demonstrate the effectiveness of ProbFat: (i) low cycle thermo-mechanical fatigue of a coke drum, and (ii) very-high cycle vibration fatigue of a piping system. For both applications the fatigue methods available in API 579-1/ASME FFS-1 are utilized for deterministic/probabilistic prediction of fatigue damage accumulation and remaining life. Methods available in API RBI are used for inspection planning and maintenance scheduling.

A variational formulation for thermomechanically coupled low cycle fatigue at finite strains

In this paper, a constitutive model suitable for the analysis of Low Cycle Thermo-Mechanical Fatigue in metals is elaborated. The model is based on finite strain elastoplasticity coupled to continuum damage theory. It is embedded into a thermodynamical framework allowing to consistently capture the interplay between mechanics and thermal effects. It is shown that the fully coupled constitutive model can be re-written into a variationally consistent manner such that all (state) variables follow jointly and naturally from minimizing an incrementally defined functional. By discretizing this time-continuous functional in time by means of implicit integration schemes a numerically efficient implementation is proposed. In order to predict the temperature increase caused by plastic deformations realistically, the pre-loading history of the considered specimen is accounted for by non-zero initial internal variables. A comparison of the results predicted by the novel constitutive model to those corresponding to experiments (Ultimet alloy) shows that the predictive capabilities of the final model are excellent.

Low cycle thermomechanical fatigue of VVER-440 reactor pressure vessel steels: Investigation the fatigue kinetics and development of a life assessment model

The fatigue life of the structural materials 15Ch2MFA (CrMoV-alloyed ferritic steel) and 08Ch18N10T (CrNi-alloyed, Ti-stabilized austenitic steel) of the VVER-440 reactor pressure vessels was investigated under fully reversed total strain controlled low cycle fatigue tests. The measurements were carried out in isothermal conditions at 260 °C and with thermal-mechanical conditions in temperature range of 150 to 270 °C using the GLEEBLE 3800 servo-hydraulic thermal-mechanical simulator.Owing the nominal fatigue lifetime for different testing conditions interrupted fatigue tests were carried out to investigate the kinetics of the fatigue evolution. Microstructural evaluation of the samples was performed using transmission electron microscopy as well as X-ray diffraction, and measurement of the dislocations was completed. The course of dislocation density in relation to cumulative usage factor was similar for both materials. However, the nature and distribution of dislocations were different in the individual steels and this resulted in different mechanical behaviours.Using scanning electron microscopy the crack shapes and fracture surfaces were observed and analysed. Crack propagation was assessed in relation to the actual crack size and the loading level.A new low cycle fatigue criterion is presented based on the stored energy, which accumulates in the material during fatigue loading. The new damage parameters are based on the assumption that only the stored part of the introduced energy causes the changes in the microstructure. The proposed model is physically consistent and its prediction accuracy is higher than by the classical strain amplitude and strain energy based approaches.The low cycle fatigue behaviour investigated with the developed engineering model can provide a reference for the remaining life assessment and possible operation life extension of nuclear power plant components.

Low cycle thermomechanical fatigue of reactor steels: Microstructural and fractographic investigations

The fatigue life of the structural materials 15Ch2MFA (CrMoV-alloyed ferritic steel) and 08Ch18N10T (CrNi-alloyed austenitic steel) of a VVER-440 reactor pressure vessel were investigated under fully reversed total strain controlled low cycle fatigue tests. The measurements were carried out in isothermal conditions at 260°C and with thermal-mechanical conditions in the range 150–270°C using a GLEEBLE-3800 servo-hydraulic thermal-mechanical simulator. The low cycle fatigue results were evaluated with the Coffin–Manson law, and the parameters of the Ramberg–Osgood stress–strain relation were investigated. Fracture mechanics behavior was observed using scanning electron microscopic analysis of the crack shapes and fracture surfaces. Crack propagation was assessed in relation to the actual crack size and the loading level. Interrupted fatigue tests were also carried out to investigate the kinetics of the fatigue evolution of the materials. Microstructural evaluation of the samples was performed using light, scanning and transmission electron microscopy as well as X-ray diffraction, and measurement of dislocations was completed using TEM and XRD. The course of dislocation density in relation to cumulative usage factor was similar for both steels. However, the nature and distribution of dislocations were different in the individual steels and this resulted in different mechanical behaviors. The nature of the fracture surfaces of both steels appeared similar despite differences in dislocation arrangement. The distances between striation lines initially increased with increasing crack length and then became saturated. The low cycle fatigue behavior investigated can provide a reference for the remaining life assessment and lifetime extension analysis of nuclear power plant components.

Thermo-mechanical fatigue design of automotive heat exchangers

Automotive heat exchangers are manufactured using brazed thin aluminium tubes and are submitted to pressure pulses, thermal shocks and corrosion. Thermal shocks induce low cycle thermo-mechanical fatigue that leads to failure after several thousand cycles. We propose a thermo-mechanical fatigue design approach for automotive heat exchangers based on the experimental identification of the thermal history during the thermal shock by infrared thermography, the identification of the cyclic elasto-plastic behaviour of the brazed aluminium tubes and the development of a multiaxial lifetime criteria. A finite element simulation of the cyclic thermo-mechanical response in the critical area of the heat exchanger and the use of a multiaxial energy based lifetime criteria lead to the prediction of the heat exchanger’s life.

Numerical Simulation of Fatigue Fracture of the Turbine Disc

This paper presents the results of the crack propagation analysis of an aircraft engine turbine disc. In the first part of the work the finite element method was used for calculation of the stress state and the stress intensity factor (SIF, KI, K-factor) in the turbine disc with an embedded quarter-elliptical corner crack, subjected to low-cycle thermo-mechanical fatigue. To refine the K-factor calculation, specially degenerated finite elements were used. These elements provide stress singularity suitable for the linear-elastic material of the disc. The performed calculations yielded the stress intensity factor KI for different crack sizes. Subsequently, ΔK parameter was determined as a difference of the KI values calculated for the turbine’s speeds equal to 6373 and 14200 RPM. Based on the Paris-Erdogan equation and the obtained ΔK values, the fatigue crack growth plot for the turbine disc subjected to complex thermo-mechanical loads was determined.

Thermal cracking in disc brakes

Disc brakes are exposed to large thermal stresses during routine braking and extraordinary thermal stresses during hard braking. High- g decelerations typical of passenger vehicles are known to generate temperatures as high as 900°C in a fraction of a second. These large temperature excursions have two possible outcomes: thermal shock that generates surface cracks; and/or large amounts of plastic deformation in the brake rotor. In the absence of thermal shock, a relatively small number of high- g braking cycles are found to generate macroscopic cracks running through the rotor thickness and along the radius of the disc brake. The analysis herein shows that rotor failure is a consequence of low cycle thermo-mechanical fatigue. An analysis of the vehicle dynamics was used to find a heat flux equation related to braking forces. The heat flux equation was then used in a finite element analysis to determine the temperature profile in the brake. Once the brake temperature was obtained, a simplified shrink fit analysis was used to estimate the stresses that arise during hard braking. This approach shows that plastic deformation occurs due to the large thermal strains associated with high- g braking. The calculated strain amplitude was then used in a Coffin–Manson law to predict the number of high- g braking cycles to failure. Good agreement was obtained between reported braking cycles to failure and the proposed theoretical approach

Directional γ-γ′ coarsening in a monocrystalline nickel-based superalloy during low-cycle thermomechanical fatigue

The coarsening behavior occurring in nickel-base superalloys hardened by initially cuboidal {gamma}{prime}-particles and having a negative misfit parameter is a well known phenomenon observed in the laboratory on specimens deformed under high-temperature creep conditions and also in turbine blades after engine operation. Less information is available on the coarsening behavior of these alloys under well-defined laboratory tests under simultaneously varying temperature and loading conditions such as in thermomechanical fatigue (TMF) tests. In this paper, the influence of different total strain-temperature cycle shapes on the coarsening behavior is described for the monocrystalline alloy CMSX-6.

Aspects of high-temperature low-cycle thermomechanical fatigue of a single crystal nickel-base superalloy

The thermo mechanical fatigue behaviour of a ferritic–austenitic duplex stainless steel has been investigated in a temperature range of 350–600 °C. The Curie temperature (TCURIE) was determined by an abrupt increase of the power of the inductive heating device and a small control deviation of the temperature. Thus, according to Tavares et al. [1] the evolution of the spinodal decomposition in the DSS was characterised by the shift of the Curie temperature for each test. The results are compared with ageing and annealing experiments in combination with tensile tests at room temperature. In addition, microstructure observations were performed in a field-emission scanning electron microscope.