Dual role of friction in granular flows: attenuation versus enhancement of instabilities

Flow instabilities driven by the dissipative nature of particle–particle interactions have been well documented in granular flows. The bulk of previous studies on such instabilities have considered the impact of inelastic dissipation only and shown that instabilities are enhanced with increased dissipation. The impact of frictional dissipation on the stability of grains in a homogeneous cooling system is studied in this work using molecular dynamics (MD) simulations and kinetic-theory-based predictions. Surprisingly, both MD simulations and theory indicate that high levels of friction actually attenuate instabilities relative to the frictionless case, whereas moderate levels enhance instabilities compared to frictionless systems, as expected. The mechanism responsible for this behaviour is identified as the coupling between rotational and translational motion. These results have implications not only for granular materials, but also more generally to flows with dissipative interactions between constituent particles – cohesive systems with agglomeration, multiphase flows with viscous dissipation, etc.

A Note on the Kelvin Effect in 100Cr6 Steel with Application to Identification of the Elastoplastic Limit

Experimental and analytical results are presented regarding the temperature evolution in 100Cr6 steel under uniaxial loading. Differently heat-treated conditions of the material are studied at different strain rates. In the annealed state, the materials exhibits a pronounced initial yield stress as it passes from the elastic region to the plastic through discontinuous yielding. In contrast, the quenched and tempered material yields continuously. The focus of the paper is on the temperature decrease during elastic deformation that precedes the more pronounced heating due to inelastic dissipation once the elastoplastic limit stress is surpassed. The applicability of the maximum temperature decrease in the elastic regime as a replacement for the commonly used 0.2%-strain measure to define the elastoplastic limit is discussed. For 100Cr6 steel, the 0.2%-strain measure is found, in some cases, to overestimate the initial yield stress by 50 MPa. The drop in temperature corresponding to the shift from elastic to inelastic material behavior is experimentally determined and compared to predictions by the Kelvin formula which in the current study give a maximum 50% error.