"Yielding and Transition Across Dense Granular Flows" 

Donovan Stumpf, MSE PhD Candidate 

Advisor: Professor Paul Mort

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ABSTRACT

Granular materials yield, but what that means depends on how you look at the problem. In bulk-solids mechanics, yielding is usually defined as failure of a consolidated contact network, and is described using shear-cell measurements, Mohr–Coulomb analysis, friction angles, and flow functions. From that perspective, the transition from static to quasistatic (QS) behavior is controlled by consolidation, stress history, and interparticle friction, rather than a single material property. Once the material is already flowing, the picture changes. The shift from QS to dense inertial (DI) flow is gradual, and while approaches like $\mu(I)$ capture the increasing rate dependence, they do not clearly define where one regime ends and the next begins and are unable to capture hysteresis in the transition. As an alternative understanding, it is useful to borrow ideas from yield-stress fluid rheology, where yielding is treated as a gradual increase in unrecoverable deformation instead of a single point. From that viewpoint, the QS to DI transition can be interpreted as an evolving balance between force-chains recoverable deformation and inertial unrecoverable rearrangements, rather than a sharply defined boundary between regimes.