LOCALIZATION OF PLASTIC-FLOW - SPATIAL VS TEMPORAL INSTABILITIES

The initiation of unstable plastic deformation associated with dynamic strain ageing in controlled load rate tests has been studied. Experimental measurernents on an Al-Mg-Si alloy show the existence of two regimes of instability: temporal and spatial. The initiation of the instability is found to occur by the growth of spatially uniform (temporal) strain rate oscillations. At a larger strain, spatial strain localization is found to develop. The existence of an intermediate regime of purely temporal instability is confirmed by a (linear) stability analysis, taking into account stress triaxiality due to non-uniform deformation. Interaction between adjacent matefial elements via dislocation cross-slip is also considered. Both effects are shown to cause a delay in the onset of spatial instabilities relative to purely temporal instabilities.