numerical simulation of cyclic plasticity in mechanical components under low cycle fatigue loading accelerated material models

Abstract Numerical simulations of components subjected to low-cycle fatigue loading require an accurate modeling of the material cyclic plasticity behavior until complete stabilization. In some circumstances, especially in case of small plastic strains, it may happen that the material model needs a huge number of cycles to reach complete stabilization, which results into an unfeasible simulation time. An acceleration technique, based on a fictitious increase of the parameter that controls the speed of stabilization in the combined (kinematic and isotropic) model, may be used. To check the efficiency and the correctness of the acceleration technique, the case of a welded cruciform joint under low cycle fatigue, taken from the literature, is here considered. The joint can be analyzed with a two-dimensional finite element model, which permits a relatively fast simulation to be completed until stabilization even with a combined kinematic-isotropic plasticity model (reference case). A comparison of this reference case with accelerated models is performed. Results in term of equivalent total strain range show that the acceleration procedure does not alter the welded joint cyclic behavior at stabilization, whereas it drastically reduces the computational time

experimental characterization of a cuag alloy for thermo mechanical applications non linear plasticity models and low cycle fatigue curves

Abstract The cyclic response and low-cycle fatigue strength of a CuAg0.1 alloy for thermo-mechanical applications are investigated by isothermal strain-controlled fatigue tests at three temperature levels (room temperature, 250°C, 300°C). Both cyclic and stabilized stress-strain responses are used for identifying the material parameters of non-linear kinematic (Armstrong-Frederick, Chaboche) and isotropic models. The identified material parameters are used in numerically simulated cycles, which are successfully compared to experiments. Linear regression analysis of experimental fatigue data allows the "mean" low-cycle fatigue curves to be estimated. Approximate statistical methods are finally adopted to evaluate the design low-cycle fatigue curves at prescribed failure probability and confidence levels

Techniques to accelerate thermo-mechanical simulations in large-scale FE models with nonlinear plasticity and cyclic input

A procedure is proposed to reduce the computation time of thermo-mechanical simulations with large nonlinear finite element (FE) models that involve cyclic plasticity. The procedure is helpful when it is practically unfeasible to simulate the huge amount of cycles needed to bring the material model to its fully stabilised state (an unfavourable situation that often occurs when small plastic strains are present), as required before assessing the structural durability. A "reference" test case, with combined kinematic and isotropic nonlinear model calibrated on actual material properties, is compared to accelerated models as well as pure kinematic models. Guidelines on how to set up the accelerated model are finally discussed

Metal plasticity and fatigue at high temperature

The situation in which a component or structure is maintained at high temperature under the action of cyclic thermal and/or mechanical loadings represents, perhaps, one of the most demanding engineering applications—if not, in fact, the most demanding one. Examples can be found in many industrial fields, such as automotive (cylinder head, engine, disk brakes), steel-making (hot rolling), machining (milling, turning), aerospace (turbine blades), and fire protection systems (fire doors). The presence of high temperatures usually induces some amount of material plasticity or creep deformation in the most stressed regions of the structure. Plasticity, if combined with the action of cyclic loading variation, may lead to low-cycle fatigue (LCF) failure. In order to estimate the component fatigue life in such demanding operative condition, it is often necessary to characterize the high-temperature material behavior under cyclic loading, in terms, for example, of cyclic stress–strain response, strain hardening or softening, creep behavior, experimental fatigue strength under isothermal and/or non-isothermal conditions. Moreover, it is also necessary to develop a reliable structural durability approach that is able to include experimental results in numerical and/or predictive models (e.g., plasticity models, fatigue strength curves). The choice of the most appropriate material model to be used in simulations, or even calibrating the model to experimental data, often represents the most critical step in the whole design approach. Experimental techniques and modeling have to be properly managed to guarantee the reliability of the estimated fatigue life

Experimental characterisation of a CuAg alloy for thermo-mechanical applications. Part 2: Design strain-life curves estimated via statistical analysis

Strain-life fatigue data on copper alloys, especially type CuAg, are seldom available in the literature. This work fills this gap by estimating the strain-life curves of a CuAg alloy used for thermo-mechanical applications, from isothermal low-cycle fatigue tests at 3 temperatures (room temperature, 250°C, 300°C). Regression analysis is used to estimate the median fatigue curves at 50% survival probability. The comparison of median curves with the Universal Slopes Equation model, calibrated on monotonic tensile properties, shows a fairly good agreement. Design strain-life curves with a lower failure probability and given confidence are estimated by several approximate statistical methods (“Equivalent Prediction Interval,” univariate tolerance interval, Owen's tolerance interval for regression). When higher survival probabilities are considered, the results show a marked decrease in the allowable design strain at a prescribed fatigue life. The suggested procedure thus improves the durability analysis of components loaded thermo-mechanically

An industry-oriented approach for the numerical analysis of steelmaking components under thermal loads

This works presents some examples of modelling technique for the numerical analysis of steelmaking components under thermomechanical loads. The proposed technique fits an “industry oriented” approach, in which several complex phenomena such as plasticity at high temperature, thermal transient, phase transition are addressed by a simplified, yet effective modelling. Numerical techniques are developed with a special focus on reducing the model complexity and the computational time, as well as to use simplified material models that require a fewer number of material parameter to be identified from experimental tests. The examples discussed are: an anode undergoing partial melting under operative conditions, a hot mill work modelled by onedimensional harmonic finite elements, a copper mold for continuous steel casting used to choose a suitable cyclic plasticity model

Caratterizzazione sperimentale di una lega CuAg per applicazioni termo-meccaniche: modelli non-lineari di plasticità e curve di fatica oligociclica

Questo lavoro intende presentare i risultati di una caratterizzazione sperimentale di una lega CuAg commerciale per applicazioni termo-meccaniche. Nelle prove sperimentali, la risposta ciclica e la resistenza a fatica oligociclica sono state investigate mediante prove di fatica isoterme a deformazione controllata eseguite a tre livelli di temperatura (ambiente, 250 °C, 300 °C). La risposta ciclica e stabilizzata è stata utilizzata per identificare i parametri di modelli non-lineari di plasticità di tipo cinematico (Armstrong-Frederick, Chaboche) ed isotropo. La risposta ciclica ottenuta da simulazioni numeriche che utilizzano i parametri calibrati sui dati sperimentali è poi confrontata con le misure sperimentali. I dati sperimentali sono infine utilizzati per ottenere, tramite regressione lineare, le curve “medie” di fatica oligociclica e quindi rielaborati statisticamente per stimare le curve di progetto associate a prefissati livelli di probabilità e confidenza

Thermal distortion in copper moulds for continuous casting of steel: numerical study on creep and plasticity effect

reserved4siIn this work, the thermal distortion of a copper mould for continuous casting of steel is investigated through numerical models based on the finite element method. Special attention is devoted to the accuracy of the adopted material properties: several elasto-plastic models, with or without creep effects, are considered and compared into the analysis. The early formation of a bulge close to the meniscus is correctly simulated and results are in good agreement with experimental data from the literature.mixedMoro, L.; Srnec Novak, J.; Benasciutti, D.; De Bona, F.Moro, L.; Srnec Novak, J.; Benasciutti, D.; De Bona, F

Copper mold for continuous casting of steel: Modelling strategies to assess thermal distortion and durability

In this work the durability assessment and the permanent deformation of a copper mold for continuous casting of steel have been investigated using mathematical models based on the Finite Element method. The cyclic plasticity behavior of the material is represented by a combined kinematic-isotropic model experimentally validated. Results from thermo-mechanical analysis are in good agreement with measurements. In particular, creep effects included into the model permit the evolution of bulging near the meniscus area to be correctly predicted. A life estimation is performed considering strain-life and stress-rupture time curves according to a cumulative damage law

How material properties affect the thermal distortion of a mold for continuous casting of steel

Copper molds are adopted in continuous casting to provide an initial solidification of steel. The presence of molten steel induces relevant temperature gradients across mold walls. This, in turn, generates high stress levels, exceeding the yielding limit of the material. Recent works confirm that thermal distortion occurs due to both creep and cyclic plasticity. In this work a numerical investigation is developed, simulating the mold behavior under repeated heating and cooling sequences. The aim of this work is to compare the performances in terms of permanent distortion of different copper alloys usually adopted for such application. It can be observed that both material properties and operating temperature have a significant influence in the permanent distortion evolution