The current investigation conducted in concentric cylinder viscometer and cylindrical tube geometries with a bentonite clay suspension was successful in establishing criteria for the often observed dependence of gel strength or yield stress on what has been inappropriately termed shear rate. Rather than being dependent on the shear rate associated with fluid hydraulics, these thixotropic fluid structural yield stresses are demonstrated to be dependent upon such solid mechanics considerations as shear stress rate and shear strain rate. The concentric cylinder or Couette viscometer was shown to be capable of assessing the gel strength dependence on shear stress rate. Strain rate data were limited because of the poor resolution obtainable at higher stress rates from the instrumentation employed with the viscometer. Similar gel strength-stress rate data were generated by subjecting steel tubes containing gel to loadings imposed by compressed air and suddenly applied pump rates. Theoretical considerations lead to a data correlation of yield stress, Y, as a function of shear stress rate,. This correlation was of the form, Y = b 1n () + C, where C and b are constants. It was further shown that constants C and b could be correlated as exponential functions of rest time.

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