  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.
