Multicycle dynamics of fault systems and static and dynamic triggering of earthquakes

Dynamic simulations of rupture propagation and multiple earthquake cycles for varying fault geometries are presented. We investigate the role of both dynamic and static stress changes on earthquake triggering. Dynamic stress triggering of earthquakes is caused by the passage of seismic waves, whereas static stress triggering is due to net slippage on a fault resulting from an earthquake. Static stress changes represented by a Coulomb failure function and its relationship to seismicity rate change is a relatively well-known mechanism, whereas the physical origin of dynamic triggering remains one of the least understood aspects of earthquake nucleation. We investigate these mechanisms by analysing seismicity patterns with varying fault separation, geometry and with and without dynamic triggering present

Bottlenecks in granular flow: When does an obstacle increase the flowrate in an hourglass?

Bottlenecks occur in a wide range of applications from pedestrian and traffic flow to mineral and food processing. We examine granular flow across a bottleneck using particle-based simulations. Contrary to expectations we find that the flowrate across a bottleneck actually increases if an opti- mized obstacle is placed before it. The dependency of flowrate on obstacle diameter is derived using a phenomenological velocity-density relationship that peaks at a critical density. This relationship is in stark contrast to models of traffic flow, as the mean velocity does not depend only on density but attains hysteresis due to interaction of particles with the obstacle.Comment: Submitted to Phys. Rev. Let

Regularization of continuum damage mechanics models for 3-D brittle materials using implicit gradient enhancement

A well-known problem which arises in local damage models is that they suffer from mesh dependence and strain localization. In this study, we employ the non-local implicit gradient damage formulation for mesh sensitivity analysis of brittle materials. The basic concept of the non-local implicit gradient model is that the strain at a given point depends not only on the strain at that point but also on the nearby strain field. The local equivalent strain is replaced with a non-local equivalent strain, which is constructed by solving a system of partial differential equations (PDEs). We solve this system of PDEs using finite element method (FEM) and split-operator method which has been applied to 3-D microplane damage models. We extend this approach to continuum damage models and apply it to brittle material for three different loading experiments. In these examples, we demonstrate that the implementation of the implicit gradient damage model using the split-operator method can successfully be applied to complex, 3-D geometries requiring large-scale unstructured FEM meshes, and eliminate mesh dependence. In not all cases the simulations were able to obtain the desired fracture widths as this would require meshes at even smaller resolutions which is beyond our current computing capacity

Sensitivity of the damage response and fracture path to material heterogeneity present in a sandstone specimen containing a pre-existing 3-D surface flaw under uniaxial loading

The heterogeneity of natural rock affects its fracture behaviour resulting in a variation of rock strength. To investigate the sensitivity of damage response and failure strength across rock samples numerical simulations are performed with random realizations of the Young’s modulus. As a test case, we consider a rock specimen with a pre-existing 3-D surface flaw under uniaxial compression. To test the sensitivity of the damage response due to material heterogeneity for specific geometrical configurations Young’s modulus is varied at the mesoscale using different randomized distributions while keeping the bulk material properties constant at the macroscale. The study shows that in general, the standard deviation is independent from geometrical parameters of initial fracture, but deviation increases when the orientations of the initial flaw and of the external loading are more aligned. In all cases, the exact fracture path displays some variability and in particular for cases with a high degree of heterogeneity some caution must be used when interpreting fracture patterns from a limited number of samples. As a consequence deviation of damage patterns generated from numerical models when compared to experimental results can confidently be attributed to the particular realizations of the heterogeneity present in the real and numerical sample

Scale effects in simple models for the dynamics of faults

Although the evidence for complexity is overwhelming, the dynamics of faulting is still poorly understood. Whilst it has long been known that discreteness in numerical earthquake models produces complexity, the mathematical structure and form of this complexity has never been fully established. Using a simple 1D nonlinear fault model we show how complexity can arise in discrete models through the presence of nonlinear, scale-dependent (or mesh-dependent) terms. We show that scale-dependencies may be a significant factor in the generation of slip complexity and pulse-like rupture over multiple earthquake cycles. We demonstrate that the introduction of length scales in discrete earthquake models implies that both strongly weakening friction and scale-dependent processes may be necessary in generating the pulse-like rupture mode and earthquake complexity over multiple earthquake cycles

Worldwide outdoor round robin study of organic photovoltaic devices and modules

Accurate characterization and reporting of organic photovoltaic (OPV) device performance remains one of the important challenges in the field. The large spread among the efficiencies of devices with the same structure reported by different groups is significantly caused by different procedures and equipment used during testing. The presented article addresses this issue by offering a new method of device testing using “suitcase sample” approach combined with outdoor testing that limits the diversity of the equipment, and a strict measurement protocol. A round robin outdoor characterization of roll-to-roll coated OPV cells and modules conducted among 46 laboratories worldwide is presented, where the samples and the testing equipment were integrated in a compact suitcase that served both as a sample transportation tool and as a holder and test equipment during testing. In addition, an internet based coordination was used via plasticphotovoltaics.org that allowed fast and efficient communication among participants and provided a controlled reporting format for the results that eased the analysis of the data. The reported deviations among the laboratories were limited to 5% when compared to the Si reference device integrated in the suitcase and were up to 8% when calculated using the local irradiance data. Therefore, this method offers a fast, cheap and efficient tool for sample sharing and testing that allows conducting outdoor measurements of OPV devices in a reproducible manner