Transition from static to kinetic friction: Insights from a 2D model

We describe a 2D spring-block model for the transition from static to kinetic friction at an elastic slider/rigid substrate interface obeying a minimalistic friction law (Amontons-Coulomb). By using realistic boundary conditions, a number of previously unexplained experimental results on precursory micro-slip fronts are successfully reproduced. From the analysis of the interfacial stresses, we derive a prediction for the evolution of the precursor length as a function of the applied loads, as well as an approximate relationship between microscopic and macroscopic friction coefficients. We show that the stress build-up due to both elastic loading and micro-slip-related relaxations depend only weakly on the underlying shear crack propagation dynamics. Conversely, crack speed depends strongly on both the instantaneous stresses and the friction coefficients, through a non-trivial scaling parameter.Comment: 5 pages, 4 figures, accepted for publication in Physical Review Letter

Avalanche dynamics, surface roughening and self-organized criticality - experiments on a 3 dimensional pile of rice

We present a two-dimensional system which exhibits features of self-organized criticality. The avalanches which occur on the surface of a pile of rice are found to exhibit finite size scaling in their probability distribution. The critical exponents are $\tau$ = 1.21(2) for the avalanche size distribution and $D$ = 1.99(2) for the cut-off size. Furthermore the geometry of the avalanches is studied leading to a fractal dimension of the active sites of $d_B$ = 1.58(2). Using a set of scaling relations, we can calculate the roughness exponent $\alpha = D - d_B$ = 0.41(3) and the dynamic exponent $z = D(2 - \tau)$ = 1.56(8). This result is compared with that obtained from a power spectrum analysis of the surface roughness, which yields $\alpha$ = 0.42(3) and $z$ = 1.5(1) in excellent agreement with those obtained from the scaling relations.Comment: 7 pages, 8 figures, accepted for publication in PR

Mechanisms of overburden deformation associated with the emplacement of the Tulipan sill, mid-Norwegian margin

The emplacement of igneous intrusions into sedimentary basins mechanically deforms the host rocks and causes hydrocarbon maturation. Existing models of host-rock deformation are investigated using high-quality 3D seismic and industry well data in the western Møre Basin offshore mid-Norway. The models include synemplacement (e.g., elastic bending-related active uplift and volume reduction of metamorphic aureoles) and postemplacement (e.g., differential compaction) mechanisms. We use the seismic interpretations of five horizons in the Cretaceous-Paleogene sequence (Springar, Tang, and Tare Formations) to analyze the host rock deformation induced by the emplacement of the underlying saucer-shaped Tulipan sill. The results show that the sill, emplaced between 55.8 and 54.9 Ma, is responsible for the overlying dome structure observed in the seismic data. Isochron maps of the deformed sediments, as well as deformation of the younger postemplacement sediments, document a good match between the spatial distribution of the dome and the periphery of the sill. The thickness t of the Tulipan is less than 100 m, whereas the amplitude f of the overlying dome ranges between 30 and 70 m. Spectral decomposition maps highlight the distribution of fractures in the upper part of the dome. These fractures are observed in between hydrothermal vent complexes in the outer parts of the dome structure. The 3D seismic horizon interpretation and volume rendering visualization of the Tulipan sill reveal fingers and an overall saucer-shaped geometry. We conclude that a combination of different mechanisms of overburden deformation, including (1) elastic bending, (2) shear failure, and (3) differential compaction, is responsible for the synemplacement formation and the postemplacement modification of the observed dome structure in the Tulipan area

Crack propagation driven by crystal growth

Crystals that grow in confinement may exert a force on their surroundings and thereby drive crack propagation in rocks and other materials. We describe a model of crystal growth in an idealized crack geometry in which the crystal growth and crack propagation are coupled through the stress in the surrounding bulk solid. Subcritical crack propagation takes place during a transient period, which may be very long, during which the crack velocity is limited by the kinetics of crack propagation. When the crack is sufficiently large, the crack velocity becomes limited by the kinetics of crystal growth. The duration of the subcritical regime is determined by two non-dimensional parameters, which relate the kinetics of crack propagation and crystal growth to the supersaturation of the fluid and the elastic properties of the surrounding material

Effect of Humans on Belief Propagation in Large Heterogeneous Teams

Members of large, heterogeneous teams often need to interact with different kinds of teammates to accomplish their tasks, teammates with dramatically different capabilities to their own. While the role of humans in teams has progressively decreased with the deployment of increasingly intelligent systems, they still have a major role to play. In this chapter, we focus on the role of humans in large, heterogeneous teams that are faced with situations, where there is a large volume of incoming, conflicting data about some important fact. We use an abstract model of both humans and agents to investigate the dynamics and emergent behaviors of large teams trying to decide whether some fact is true. In particular, we focus on the role of humans in handling noisy information and their role in convergence of beliefs in large heterogeneous teams. Our simulation results show that systems involving humans exhibit an enabler-impeder effect, where if humans are present in low percentages, they aid in propagating information; however when the percentage of humans increase beyond a certain threshold, they seem to impede the information propagation