accelerated cyclic plasticity models for fem analysis of steelmaking components under thermal loads

Abstract Steelmaking components are often subjected to thermo-mechanical loads applied cyclically. In this case the choice of a suitable cyclic plastic model to be used in the numerical simulation is a crucial aspect in design. Combined (kinematic and isotropic) model permits the cyclic material behavior to be captured accurately. On the other hand, such model often requires unfeasible computational time to arrive at complete material stabilization. Simplified or accelerated models have then been proposed to make simulation faster. In this work, the thermo-mechanical analysis of a round mold for continuous casting is addressed as a case study. Due to axi-symmetry, a plane model can be adopted. This permits a finite element (FE) analysis with a combined model to be performed until complete stabilization. A comparison with other models able to speed up the simulation (accelerated models with increased values of saturation speed, Prager and stabilized models) was performed. It was found that only accelerated models give equivalent strain range values that do not significantly differ from the (reference) combined model, independently from the speed of saturation adopted

Cyclic Plasticity and Low Cycle Fatigue of an AISI 316L Stainless Steel: Experimental Evaluation of Material Parameters for Durability Design

AISI 316L stainless steels are widely employed in applications where durability is crucial. For this reason, an accurate prediction of its behaviour is of paramount importance. In this work, the spotlight is on the cyclic response and low-cycle fatigue performance of this material, at room temperature. Particularly, the first aim of this work is to experimentally test this material and use the results as input to calibrate the parameters involved in a kinematic and isotropic nonlinear plasticity model (Chaboche and Voce). This procedure is conducted through a newly developed calibration procedure to minimise the parameter estimates errors. Experimental data are eventually used also to estimate the strain–life curve, namely the Manson–Coffin curve representing the 50% failure probability and, afterwards, the design strain–life curves (at 5% failure probability) obtained by four statistical methods (i.e., deterministic, “Equivalent Prediction Interval”, univariate tolerance interval, Owen’s tolerance interval for regression). Besides the characterisation of the AISI 316L stainless steel, the statistical methodology presented in this work appears to be an efficient tool for engineers dealing with durability problems as it allows one to select fatigue strength curves at various failure probabilities depending on the sought safety level

fem strategies for large scale thermo mechanical simulations with material non linearity

n/

Parameter estimation of cyclic plasticity models and strain-based fatigue curves in numerical analysis of mechanical components under thermal loads

The aim of this thesis is to set up a methodological approach to assess a fatigue life of components under cyclic thermal loads. Therefore, a copper mould used for continuous steel casting is considered as a case study. During the process, the molten steel passes through a water cooled mould. The inner part of the component is subjected to a huge thermal flux. Consequently large temperature gradients occur across the component, especially in the region near to the meniscus, and cause elastic and plastic strains. The finite-element thermo-mechanical analysis is performed with a three-dimensional numerical model. One of the challenging tasks is choosing a suitable material model which is going to be applied in a simulation; since the amount of resulting plastic and elastic strain is strongly controlled by the material model implemented to perform the analysis. Therefore, four different material models (linear kinematic, combined, stabilized and accelerated material model) are investigated and compared in this thesis. It has been found that the combined model requires huge computational time to reach a stabilized stress-strain loop. On the other hand, the use of the stabilized model overestimates the plasticization phenomena already in the first cycle. Accordingly, the alternative accelerated material model, where stabilization is reached earlier, is thus proposed, proofing that it is able to give suitable and safe life estimation for design purposes. Material coefficients for all applied material and fatigue life models are estimated from experimental, isothermal low cycle fatigue data of CuAg alloy at three temperature levels (20 \ub0C, 250 \ub0C, 300 \ub0C). A strain-based fatigue model, appropriate to assess a service life of the component, is necessary to apply once the material model is chosen and the finite-element analysis is performed. A fatigue model compatible and suitable for a daily industrial practice due to its simplicity; however in the same time able to predict precisely a fatigue life. The fatigue life of analysed component is assessed depending on different material models and fatigue models (Universal Slopes equation, Modified Universal Slopes equation, the 10% Rule and 20% Rule), as well as design curves (deterministic approach, tolerance interval, EPI

Acceleration techniques for the numerical simulation of the cyclic plasticity behaviour of mechanical components under thermal loads

Numerical simulations of components subjected to cyclic thermo-mechanical loads require an accurate modelling of their cyclic plasticity behaviour. Combined models permit to capture monotonic hardening as well as cyclic hardening/softening phenomena, that occur in reality. In principle the durability assessment of a component under thermal loads can be performed only if the cyclic behaviour is simulated until complete material stabilization. As materials stabilize approximately at half the number of cycles to failure, it follows that in case of small plastic strains a huge number of cycles must be considered and an unfeasible simulation time would be required. Accelerated models have thus been proposed in literature. The aim of this work is that of comparing the different acceleration techniques in the case a round mould for continuous casting loaded thermo-mechanically. It can be observed that the usual approach of using the stabilized stress-strain curve already from the first cycle could lead to relevant errors. An alternative method is that of increasing the value of the parameter that controls the speed of stabilization in the combined model. This approach permits the number of cycles to reach stabilization to be drastically reduced, without affecting the overall mechanical behaviour. Based on this approach, a simple design rule, that can be adopted, particularly when relatively small plastic strains occur, is finally proposed

Modeling the cyclic plasticity behavior of 42CrMo4 steel with an isotropic model calibrated on the whole shape of the evolution curve

A durability analysis of a mechanical component generally requires accurate numerical simulations. For this purpose, the adopted cyclic plasticity material model should follow as closely as possible the material behavior observed during experimental testing. This work presents calibration of the isotropic material model for a 42CrMo4 steel, based on a series of cyclic fully-reversed tension-compression strain controlled tests performed at different strain amplitudes. Stress-strain cycles were recorded until end of each test with the goal to capture the isotropic stabilization effect of the material. As the isotropic model calibration gave poor results, if the exponential law proposed by Voce is adopted, an alternative Three parameters (TP) isotropic model is thus considered. The comparison with the experimental results show that the TP model fits significantly better the experimental results in almost all the considered cases. A possible justification of such improvement seems to be related to the fact that the equation governing the TP model contain a parameter that controls also the slope of the “S-shape curve” which describes the evolution of the material from initial to stabilized condition

Material modelling in multi-physics FEM simulation

Nowadays virtual prototyping has a great impact in the design process of an industrial component. Numerical techniques based on the Finite Element Method (FEM) are mature to provide computational tools that permit complex phenomena to be accurately simulated, even when dealing with multi-physical problems. This work puts in evidence that an inaccurate assessment of the material properties may compromise the benefit of such complex modelling techniques. For this purpose, firstly the case of thermo-mechanically loaded structures will be presented. Considering fire walls for naval applications, the influence of the rock wool elastic modulus in the safety behavior will be emphasized. In the case of steel making component, the paper proofs that only a correct cyclic plasticity model of the material (copper alloy) permits a durability analysis to be accurately performed. Finally, in the case of an energy-harvesting device, the importance of taking into account the orthotropic properties of the material will be highlighted

Acceleration techniques for the numerical simulation of the cyclic plasticity behaviour of mechanical components under thermal loads

Numerical simulations of components subjected to cyclic thermo-mechanical loads require an accurate modelling of their cyclic plasticity behaviour. Combined models permit to capture monotonic hardening as well as cyclic hardening/softening phenomena, that occur in reality. In principle the durability assessment of a component under thermal loads can be performed only if the cyclic behaviour is simulated until complete material stabilization. As materials stabilize approximately at half the number of cycles to failure, it follows that in case of small plastic strains a huge number of cycles must be considered and an unfeasible simulation time would be required. Accelerated models have thus been proposed in literature. The aim of this work is that of comparing the different acceleration techniques in the case a round mould for continuous casting loaded thermo-mechanically. It can be observed that the usual approach of using the stabilized stress-strain curve already from the first cycle could lead to relevant errors. An alternative method is that of increasing the value of the parameter that controls the speed of stabilization in the combined model. This approach permits the number of cycles to reach stabilization to be drastically reduced, without affecting the overall mechanical behaviour. Based on this approach, a simple design rule, that can be adopted, particularly when relatively small plastic strains occur, is finally proposed