The transition from slow frictional to intermediate granular flows is of interest in such process applications as binder agglomeration, powder mixing and precise metering and pneumatic conveying. Slow frictional (I.e., quasi-static) flows are typically characterized using a Mohr-Coulomb analysis of shear-cell data. The quasi-static analysis (tensile intercept and curvature approaching the critical state) is very sensitive to curve fitting of this data. The present paper builds on previous work using a scaling approach of a family of yield loci to supply additional curve-fitting constraints [Mort et al., 2002]. While methods for the characterization of intermediate granular flows are not as well developed, the use of stress transducers to measure stress fluctuations has been successfully demonstrated [Talu et al., 2001 and Tardos et al., 2003]. The current paper considers characterization of both flow regimes with a focus on the transition from one regime to the other. On one hand, the slope of yield loci generated by Mohr-Coulomb analyses can be extrapolated to obtain an estimate of cohesive and tensile intercepts. In theory, the tensile should be directly related to the inter-particle attractive forces that are acting on the particles in the quasi-static state. On the other hand, the curvature of the yield locus as it approaches the critical state (I.e., the point of fully-consolidated flow in the quasi-static regime) is relevant to the difference between the shear stresses at incipient and steady state flow. While steady-state flow is typically described as a straight line on the Mohr- Coulomb plot, the incipient yield locus is often curved. Increased curvature of the incipient locus generally results in reduced internal friction at the critical state (I.e., under full consolidation). The magnitude of internal friction at the critical state [and/or the relative difference between the incipient and steady-state shear stress] appears to correlate to the magnitude of stress fluctuations measured in the transition from static to intermediate bulk flows. Preliminary evidence is presented in this paper to support the above correlation.
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