Matthieu Mazière

Identification of a strain-aging model accounting for Lüders behavior in a C-Mn steel

Strain-aging constitutive models are suitable to simulate the formation and propagation of Lüders bands in complex specimens and components. The identification of the corresponding material parameters is difficult because the strain localization phenomena associated with the Lüders behavior must be taken into account. The spurious mesh dependence of standard finite element simulations of Lüders band propagation is illustrated in the present work and removed using a strain gradient plasticity model. Furthermore the gradient approach introduces a characteristic size corresponding to the finite dimension of the Lüders band front as observed from strain field measurements. The parameters are identified from the experimental measurement of the peak stress and plastic strain carried by the band for a C-Mn steel over a temperature range from −150°C to room temperature and for several strain rates. The validity of the model is tested on 3D simulations of the Lüders band propagation in a strain gradient plasticity medium.

Numerical modelling of the Portevin-Le Chatelier effect

The model proposed by MacCormick (1989) describing dynamic strain aging is used for a nickel based superalloy. The model is presented for small deformations, and an analytical homogeneous solution is calculated for simple tension. Parameters for the nickelbased superalloy at 500 °C are obtained from tensile tests at constant strain rates. A linear perturbation analysis is performed to evaluate the critical strain (i.e. when serrations begin). The convergence toward this critical value of two numerical integration schemes of material law is studied for the simulation of a plate in tension.

Interaction of the Portevin–Le Chatelier phenomenon with ductile fracture of a thin aluminum CT specimen: experiments and simulations

An attempt is made here to capture numerically slant ductile fracture and its early slant strain precursors via combining a dynamic strain aging (DSA) model with ductile damage models. In recent experimental studies it has been shown that in an AA2XXX alloy strain localization in slant bands preceded the onset of damage, originating slant fracture ahead of a notch. Here tensile tests are performed at different strain rates revealing some negative strain rate sensitivity which is an indication of DSA effect for AA2198-T8. A McCormick-type DSA model in conjunction with a Rousselier damage model, a reduced polycrystalline plasticity model and a Coulomb fracture criterion for slip systems have been used. Full 3D finite element simulations using this model and typical parameters for aluminum alloys capture the early strain localization in slanted bands, their intermittent activity and the final slant fracture. Prior simulation results without the DSA model and others using the von Mises plasticity or the GTN model did not capture the early slant strain localization thereby suggesting that DSA may well be the physical origin of the early slant strain localization and final slant fracture phenomena in this alloy.

Portevin–Le Chatelier effect under cyclic loading: experimental and numerical investigations

The Portevin–Le Chatelier (PLC) effect is generally evidenced by the apparition of serrated yielding under monotonic tensile loading conditions. It appears at room temperature in some aluminium alloys, around in some steels and in many other metallic materials. This effect is associated with the propagation of bands of plastic deformation in tensile specimens and can in some cases lead to unexpected failures. The PLC effect has been widely simulated under monotonic conditions using finite elements and an appropriate mechanical model able to reproduce serrations and strain localization. The occurrence of serrations can be predicted using an analytical stability analysis. Recently, this serrated yielding has also been observed in specimens made of Cobalt-based superalloy under cyclic loading, after a large number of cycles. The mechanical model has been identified in this case to accurately reproduce this critical number of cycle where serrations appear. The associated apparition of localized bands of deformation in specimens and their influence on its failure has also been investigated using finite element simulations.

NUMERICAL SIMULATION OF THE PORTEVIN - LE CHATELIER EFFECT IN VARIOUS MATERIAL AND AT DIFFERENT SCALES

The Portevin Le Chatelier (PLC) effect appears in many metallic materials at different temperatures. Some numerical simulations of this effect for different alloys (aluminium, steel, nickel based superalloy) are presented in this article. The mechanical model remain the same for all studied materials but the behavior parameters and identification methods differ. The scale at which this effect is investigated also varies from the microstructure to aeronautic components.