Statistical and multifractal analysis of the Portevin-Le Chatelier effect

The stress time series associated with the Portevin-Le Chatelier (PLC) effect in Al-2.5%Mg polycrystals deformed in constant applied strain rate have been analyzed by statistical and multifractal methods. The objective was to check the conjecture that jerky flow should exhibit distinct dynamical regimes. Statistical distributions of stress drops and drop durations exhibit a well-defined sequence as the applied strain rate increases. Peaked distributions are followed by monotonically decreasing ones at higher strain rate. For a large grain size, the high strain rate distributions are consistently fitted by power laws suggesting a self-organized critical (SOC) dynamics. At smaller grain sizes, a multifractal fit of the distributions is more appropriate. Multifractality increases with strain rate. A sharp upturn is observed at the boundary between type B and type A PLC bands. The correlation between independant statistical and multifractal analyses suggests a fundamental crossover of the underlying dynamics. It is also an indication that the multifractal analysis has the ability to provide sensible quantitative characterizations of jerky flow