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

Almost-stationary motions and gauge conditions in General Relativity

An almost-stationary gauge condition is proposed with a view to Numerical Relativity applications. The time lines are defined as the integral curves of the timelike solutions of the harmonic almost-Killing equation. This vector equation is derived by a variational principle, by minimizing the deviations from isometry. The corresponding almost-stationary gauge condition allows one to put the field equations in hyperbolic form, both in the free-evolution ADM and in the Z4 formalisms.Comment: Talk presented at the Spanish Relativity Meeting, September 6-10 2005 Revised versio

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

Material Modeling in Multiphysics Simulation

Virtual prototyping techniques, generally based on numerical methods, are widely used in the process of designing an industrial product [...

Failure Analyses of Two Gas Turbine Wheels

AbstractThe results of the analysis of the catastrophic failures of two high pressure turbine wheels are discussed in this study. Fractographic and metallographic analyses on both the wheel and a set of failed blades of both wheels were performed to determine the possible events that led to failure. Both wheel materials had an austenitic microstructure, while blade materials were different for each case. One blade material is similar to INCONEL 738 nickel-based superalloy, while the other study is a single-crystal with dendritic growth microstructure. Facing two failures with apparently similar characteristics, once fractographic and metallographic analysis were performed, it was proved that failure modes respond to quite different origins in each case. This led to different corrective actions, according to each particular main contributing factor

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

Global well-posedness of the KP-I initial-value problem in the energy space

We prove that the KP-I initial value problem is globally well-posed in the natural energy space of the equation

Aquatic insects and benthic diatoms: A history of biotic relationships in freshwater ecosystems

The most important environmental characteristic in streams is flow. Due to the force of water current, most ecological processes and taxonomic richness in streams mainly occur in the riverbed. Benthic algae (mainly diatoms) and benthic macroinvertebrates (mainly aquatic insects) are among the most important groups in running water biodiversity, but relatively few studies have investigated their complex relationships. Here, we review the multifaceted interactions between these two important groups of lotic organisms. As the consumption of benthic algae, especially diatoms, was one of the earliest and most common trophic habits among aquatic insects, they then had to adapt to the particular habitat occupied by the algae. The environmental needs of diatoms have morphologically and behaviorally shaped their scrapers, leading to impressive evolutionary convergences between even very distant groups. Other less evident interactions are represented by the importance of insects, both in preimaginal and adult stages, in diatom dispersion. In addition, the top-down control of diatoms by their grazers contributes to their spatial organization and functional composition within the periphyton. Indeed, relationships between aquatic insects and diatoms are an important topic of study, scarcely investigated, the onset of which, hundreds of millions of years ago, has profoundly influenced the evolution of stream biological communities