The Mechanics of Neck Formation

The profile also determines the strain rate, being given by 

dz/ dt denotes the velocity of the volume element relative to the shoulder as measured in the moving coordinate system. Since the material flow through the shoulder is a constant, we write 

Equation (10.21) states that the extension rate of the volume element is not a constant but strongly time-dependent. It starts from zero, then increases, passes over a maximum near the position of the largest slope in the profile and finally returns to zero. 

This peculiar time dependence of the deformation rate is associated with a simple law for the evolution of stress. Since the external force during colddrawing is constant, the tensile stress acting on the volume element follows from 

Equation (10.23) is imposed by the conditions of the drawing experiment. Indeed, it is exactly this requirement by which the time-dependent deformation t) is selected and thus the profile determined. 

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The availability of an oxygen tracer permits us to consider several types of marker experiments. One of these which we are undertaking can be understood by reference to Fig. 1. Two spheres can be prepared, one having its surface layer labeled with tracer, and the other without tracer. The distribution of the tracer within the neck area after a sinter anneal as Compared with the initial distribution should provide a clue about the mechanism of neck formation. The following should be mentioned ... 

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