Geometrically non-linear modeling of the Portevin-Le Chatelier effect

In this work we investigate the plastic instabilities associated with the Portevin-Le Chatelier (PLC) effect in Al alloy 2024. A semiphenomenological approach is taken. A simple geometrically non-linear elastic-viscoplastic constitutive model is proposed for simulation of material response under various applied strain rates. Using the model we determine numerically the relation between the critical strain for the onset of discontinuous yielding and the applied strain rate. The results obtained are in very good quantitative agreement with the available experimental data and cover both the normal and the inverse behavior of the critical strain. The simulations are performed using non-linear finite element method. Additional verification of the proposed constitutive framework was carried out using statistical analysis of the simulated stress-time series. A transition from a non-linear chaotic regime to self-organized critical behaviour of the localized strain bands were predicted in terms of the temporal two-point correlation function of the stress-time series. Finally we investigated the influence of different factors, such as the geometry of the specimen, its orientation with respect to the rolling direction and loading conditions (strain rate), on the type of PLC instabilities and the critical conditions for their onset

Kinematics of formation and cessation of type B deformation bands during the Portevin-Le Chatelier effect in an AlMg alloy

Using digital image correlation technique, various stages of development of localized deformation bands caused by the Portevin-Le Chatelier (PLC) instability of plastic flow are examined at strain rates corresponding to abrupt stress drops akin to relaxation oscillations. Diffuse deformation bands are found prior to stress drops. Although the respective increase in the local strain rate is relatively weak, it is immediately observed over the entire region of the nascent PLC band. Moreover, diffuse bands are also detected at the beginning of reloading after the end of stress drops, thereby indicating that the termination of plastic instability proceeds through a progressive depletion of the strain localization. Weak transient strain localizations that do not develop into a PLC band are also observed during reloading. Keywords: Aluminum alloys, Portevin-Le Chatelier effect, Deformation bands, Digital image correlatio

Crossover in the scale-free statistics of acoustic emission associated with the Portevin-Le Chatelier instability

International audienceThe acoustic emission accompanying plastic deformation obeys scale-free statistics reflecting avalanche-like dis-location motion. This feature holds out for the macroscopically unstable deformation of alloys. However, stress serrations display peaked distributions at low enough strain rates. This occurrence of a characteristic macroscopic scale was supposed to result from the synchronization of dislocation avalanches. In the present work, the synchronization mechanism is studied using statistical analysis of different subsets of acoustic events. A crossover in the power-law exponents is detected for the events occurring during deep stress drops. It is ascribed to a transition from chaining to overlapping dislocation avalanches

On the similarity of plastic flow processes during smooth and jerky flow in dilute alloys

International audienceThe jerky flow of dilute alloys, or the Portevin-Le Chatelier effect, has a burst-like intermittent character at different fluctuation size levels. Multifractal analysis is applied to both the macroscopic stress serrations and the acoustic emission accompanying the plastic deformation. Multifractal scaling is found for both kinds of time series. The scaling range of the stress serrations is limited from below by their characteristic frequency. Unexpectedly, the scaling range for acoustic bursts not only covers this range but spreads to much shorter time scales with the same scaling exponent. This result testifies that the deformation processes revealed by the acoustic emission at a mesoscopic scale have a similar nature during both stress serrations and smooth plastic flow. The implications on the crossovers in the dynamics of jerky flow are discussed

Intrinsic structure of acoustic emission events during jerky flow in an Al alloy

International audienceScaling behavior is found in acoustic-emission events associated with stress drops observed in velocity-driven plastic deformation of an Al alloy, which exhibits jerky plastic flow. The occurrence of scaling proves that these acoustic-emission events, which are commonly regarded as “elementary” ones, have a small-scale self-organized structure comprising a group of peaks correlated in time. This structure reveals details of the temporal variation in elementary plastic events at a microsecond scale, which are hardly accessible by other measurement techniques

Effect of the surface mechanical attrition treatment on multiscale complexity of the Portevin-Le Chatelier effect in an AlMg alloy

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On the similarity of plastic flow processes during smooth and jerky flow: Statistical analysis

International audienceJerky flow in dilute alloys, or the Portevin-Le Chatelier effect, is investigated using statistical analysis of time series characterizing the evolution of the plastic activity at distinct scales of observation, namely, the macroscopic scale of stress serrations and a mesoscopic scale pertaining to the accompanying acoustic emission. Whereas the stress serrations display various types of statistical distributions depending on the driving strain rate, including power-law, peaked and bimodal histograms, it is found that acoustic emission is characterized by power-law statistics of event size in all experimental conditions. The latter reflect intermittency and self-organization of plastic activity at a mesoscopic scale. This shift in the observed dynamics when the observation length scale is decreased is discussed in terms of the synchronization of small-scale events