Relaxation oscillations and negative strain rate sensitivity in the Portevin - Le Chatelier effect

A characteristic feature of the Portevin - Le Chatelier effect or the jerky flow is the stick-slip nature of stress-strain curves which is believed to result from the negative strain rate dependence of the flow stress. The latter is assumed to result from the competition of a few relevant time scales controlling the dynamics of jerky flow. We address the issue of time scales and its connection to the negative strain rate sensitivity of the flow stress within the framework of a model for the jerky flow which is known to reproduce several experimentally observed features including the negative strain rate sensitivity of the flow stress. We attempt to understand the above issues by analyzing the geometry of the slow manifold underlying the relaxational oscillations in the model. We show that the nature of the relaxational oscillations is a result of the atypical bent geometry of the slow manifold. The analysis of the slow manifold structure helps us to understand the time scales operating in different regions of the slow manifold. Using this information we are able to establish connection with the strain rate sensitivity of the flow stress. The analysis also helps us to provide a proper dynamical interpretation for the negative branch of the strain rate sensitivity.Comment: 7 figures, To appear in Phys. Rev.

Dynamic Necking of Rods at High Strain Rates

The dynamic necking instability of a rod of a non-linear viscoplastic material as formed in shaped charges is investigated. A two-dimensional linear Lagrangian perturbation method leads to a single fourth order partial differential equation with time-dependent coefficients. The growth of disturbancies depends of the interplay between the stabilizing inertial and viscous effects, and the destabilizing geometrical softening of the rod. Inertia slows down the growth of long wavelengths, while viscosity damps preferentially the short wavelengths. A time-increasing critical wavelength of maximum perturbation growth is selected at each moment. The latter is characteristic of the length scale of a multiple necking phenornenom.On étudie la striction dynamique d'un jet (comportement viscoplastique non linéaire), obtenu par exemple à l'aide de charges creuses. Une méthode de perturbation linéaire Lagrangienne bi-dimensionnelle permet d'obtenir une équation aux dérivées partielles du quatrième ordre à coefficients dépendant du temps. La croissance des perturbations dépend d'une part des effets stabilisants de la viscosité et de l'inertie, et d'autre part des effets déstabilisants de l'adoucissement géométrique. L'inertie ralentit la croissance des grandes longueurs d'onde, alors que la viscosité influe plutôt sur les courtes longueurs d'onde. A chaque instant, il existe une longueur d'onde critique de croissance maximum des perturbations, caractérisant l'échelle de longueur d'un phénomène de striction multiple

Effects of liner grain size on shaped – charge jet performance: A combined experimental/numerical/analytical approach

Experimental data on the fragmentation of copper shaped-charge jets are presented. A combined numerical/analytical analysis is designed to describe shaped-charge jet break-up. The method overcomes drawbacks from exclusively numerical or analytical analyses, such as mesh sensitivity or oversimplified description. It yields predictions for break-up time, total number and cumulative length of fragments in fairly good agreement with the experimental data. The dependence of fragmentation characteristics in the liner grain size is well predicted, except for the leveling out of performances at very large grain size. A discussion is provided

Effects of the impact of a low temperature nitrogen jet on metallic surfaces

International audienceThe technology of nitrogen jets impacting surfaces at low temperature has recently been introduced for surface cleaning/stripping. Under the impact of the jet, the material surface undergoes a thermomechanical shock inducing complex transformation mechanisms. Depending on the material and test parameters such as standoff distance, dwell time, upstream pressure, the latter include cleavage, cracking, spalling, blistering, grain fragmentation, phase transformation and ductile deformation. Quite often, these modes are superimposed in the same test, or even in the same material area. In this study, an overview of these mechanisms is proposed for metallic materials. Measurements of thermomechanical variables in the impacted area are presented and the influence of the test parameters on surface transformation is investigated. Grain fragmentation and ultrafast transport of nitrogen in a deep layer below the surface are explored