Yield stress delined

By virtue of its yield stress, a viscoplastic material in an unsheared state will support an immersed particle for an indefinite period of time. In recent years, this property has been successfiilly exploited in the design of slurry pipelines, as briefly discussed in section 4.3. Before undertaking an examination of the drag force on a spherical particle in a viscoplastic medium, the question of static equilibrium will be discussed and a criterion will be developed to delineate the conditions under which a sphere will either settle or be held stationary in a liquid exhibiting a yield stress. 

Yield stress fluids create caverns around impellers. The cavern boundary is delineated by the surface where the imposed shear force does not exceed the fluid yield stress. Pulp suspensions exhibit this behavior. The approach developed by Soloman et al. (1981) was used to characterize pulp suspension behavior in a laboratory mixer. A force balance at Ihe cavern boundary (assumed cylindrical) gives its radial extent, r, as ... 

The prior work on pure Mg, Mg-Al alloys and Zr-containing alloys showed that SCC is a significant issue, and that SCC can occur for a load condition equivalent to 50% of the yield stress for many combinations of alloy + environment. Since the prior research indicates that all Mg alloys are likely to be more or less susceptible, guidelines are needed to ensure safe application of Mg alloys in service. It would seem that an urgent task would be to delineate safe operational limits for common alloys in likely service environments. 

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