Finite elements prediction of thermal stresses in work roll of hot rolling mills

AbstractA simplified numerical approach based on finite elements to compute thermal stresses occurring in work roll of hot rolling mills is here proposed. To decrease both the complexity of the analysis and the computational effort, this approach implements a plane finite element model of the work roll alone, loaded on its surface by the rotating thermal actions due to the cyclic sequence of the conductive heating caused by the contact with hot strip and cooling provided by water jets. Results from thermal analysis are preliminary compared to an analytical solution available in the literature and then applied as thermal input in the subsequent mechanical finite elements simulations, which provide thermal stress in the work roll and the elastic-plastic evolution of elements, close to the work roll surface

Sulla stima della vita a fatica di giunti saldati soggetti a carichi multiassiali ad ampiezza variabile

Nel presente articolo viene proposta una nuova metodologia di progettazione a fatica, basata sull’utilizzo del metodo delle Curve di Wöhler Modificate, per la previsione della vita a fatica di giunzioni saldate, sia in acciaio che in alluminio, soggette a carichi multiassiali ad ampiezza variabile. In particolare, il criterio delle Curve di Wöhler Modificate è stato applicato determinando l’orientazione del piano critico mediante il Metodo della Massima Varianza, ovvero definendo il piano critico come quello contenente la direzione che sperimenta la massima varianza della tensione tangenziale risolta. L’accuratezza della metodologia di progettazione a fatica proposta nella presente memoria è stata valutata mediante due serie di dati sperimentali di letteratura ottenute sollecitando, sia ad ampiezza costante che variabile, giunti saldati tubo-piastra in acciaio e lega di alluminio con carichi di flesso/torsione in fase e sfasati di 90°. Il criterio delle Curve di Wöhler Modificate, applicato in concomitanza con il Metodo della Massima varianza, si è dimostrato capace di fornisce stime accurate della durata a fatica anche in presenza di sollecitazioni multiassiali ad ampiezza variabile, e questo sia quando applicato in termini di tensioni nominali che in termini di tensioni di “hot-spot”

Metal plasticity and fatigue at high temperature

In several industrial fields (such as automotive, steelmaking, aerospace, and fire protection systems) metals need to withstand a combination of cyclic loadings and high temperatures. In this condition, they usually exhibit an amount—more or less pronounced—of plastic deformation, often accompanied by creep or stress-relaxation phenomena. Plastic deformation under the action of cyclic loadings may cause fatigue cracks to appear, eventually leading to failures after a few cycles. In estimating the material strength under such loading conditions, the high-temperature material behavior needs to be considered against cyclic loading and creep, the experimental strength to isothermal/non-isothermal cyclic loadings and, not least of all, the choice and experimental calibration of numerical material models and the selection of the most comprehensive design approach. This book is a series of recent scientific contributions addressing several topics in the field of experimental characterization and physical-based modeling of material behavior and design methods against high-temperature loadings, with emphasis on the correlation between microstructure and strength. Several material types are considered, from stainless steel, aluminum alloys, Ni-based superalloys, spheroidal graphite iron, and copper alloys. The quality of scientific contributions in this book can assist scholars and scientists with their research in the field of metal plasticity, creep, and low-cycle fatigue

Fracture, Fatigue, and Structural Integrity of Metallic Materials and Components Undergoing Random or Variable Amplitude Loadings

When quickly reviewing engineering and industrial fields, one often discovers that a large number of metallic components and structures are subjected, in service, to random or variable amplitude loadings. The examples are many: vehicles subjected to loadings and vibrations caused by road irregularity and engine, structures exposed to wind, off-shore platforms undergoing wave-loadings, and so on. Just like constant amplitude loadings, random and variable amplitude loadings can make fatigue cracks initiate and propagate, even up to catastrophic failures. Engineers faced with the problem of estimating the structural integrity and the fatigue strength of metallic structures, or their propensity to fracture, usually make use of theoretical or experimental approaches, or both. Counting methods (e.g., rainflow) provide information on the fatigue cycles in the load, whereas damage accumulation laws (as the celebrated Palmgren–Miner linear rule) establish how to sum up the damage of each counted cycle. In structural integrity, this is named as the “time-domain” approach. Over recent years, the “frequency-domain” approach has also received increasing and widespread use, especially with random loadings; this approach estimates fatigue life based on load statistical properties represented, in the frequency domain, by a power spectral density. Neither of the previous approaches, however, can do without the support of experimental laboratory testing, which provides a means to collect material strength data under specific loading conditions, or to verify preliminary estimations. The purpose of this Special Issue is to collect articles aimed at providing an up-todate overview of approaches and case studies—theoretical, numerical or experimental—on several topics in the field of fracture, fatigue strength, and the structural integrity of metallic components subjected to random or variable amplitude loadings

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

A numerical approach for the analysis of deformable journal bearings

This paper presents a numerical approach for the analysis of hydrodynamic radial journal bearings. The effect of shaft and housing elastic deformation on pressure distribution within oil film is investigated. An iterative algorithm that couples Reynolds equation with a plane finite elements structural model is solved. Temperature and pressure effects on viscosity are also included with the Vogel-Barus model. The deformed lubrication gap and the overall stress state were calculated. Numerical results are presented with reference to atypical journal bearing configuration at two different inlet oil temperatures. Obtained results show the great influence of elastic deformation of bearing components on oil pressure distribution, compared with results for ideally rigid components obtained by Raimondi and Boyd solution

An isotropic model for cyclic plasticity calibrated on the whole shape of hardening/softening evolution curve

This work presents a new isotropic model to describe the cyclic hardening/softening plasticity behavior of metals. The model requires three parameters to be evaluated experimentally. The physical behavior of each parameter is explained by sensitivity analysis. Compared to the Voce model, the proposed isotropic model has one more parameter, which may provide a better fit to the experimental data. For the new model, the incremental plasticity equation is also derived; this allows the model to be implemented in finite element codes, and in combination with kinematic models (Armstrong and Frederick, Chaboche), if the material cyclic hardening/softening evolution needs to be described numerically. As an example, the proposed model is applied to the case of a cyclically loaded copper alloy. An error analysis confirms a significant improvement with respect to the usual Voce formulation. Finally, a numerical algorithm is developed to implement the proposed isotropic model, currently not available in finite element codes, and to make a comparison with other cyclic plasticity models in the case of uniaxial stress and strain-controlled loading

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

Uncertainty of Estimated Rainflow Damage in Stationary Random Loadings and in Those Stationary per partes

The uncertainty of rainflow fatigue damage is evaluated for stationary loadings and for non-stationary switching loadings with a finite number of stationary states. The approach is based on confidence intervals constructed after direct analysis of stress-time histories. The accuracy of confidence intervals is verified first by numerical simulations, and then by experimental data measured in a mountain bike traveling under various driving and road surface conditions, yielding stationary and non-stationary switching loadings. Stationarity and non-stationarity of loading records is checked by a statistical method (run test). In experiments, a small set of records (validation set) is also collected and used to approximate the expected damage, which serves for verification purposes. Not only do numerical and experimental results confirm the correctness of the proposed confidence interval for damage, but they also emphasize its usefulness in real engineering applications