Parameter sensitivity in finite element analysis with constitutive models of the rate type

Non‐linear soil–structure interactions are usually analysed using an incremental finite element approach. There a constitutive subroutine provides for each element the stress increment for a given strain increment. In geotechnical calculations, uncertainties in material parameters and initial conditions are abundant. Sensitivity analysis can be a first step to account for such uncertainties. Sensitivities of the system response with respect to material parameters and initial conditions can be calculated by differentiating the whole numerical scheme. It turns out that the essential information from the constitutive subroutine are the derivatives of the stress increment with respect to the strain increment, as well as the derivatives with respect to material parameters and all state variables involved in the problem. We propose a method to compute these quantities numerically for any constitutive model that can be written in rate form and for any suitable integrator of such a model. We further present a concise way to supply the output of the sensitivity analysis to the designing engineer. Our theoretical investigations are illustrated with element tests and with a typical geotechnical application. Copyright © 2005 John Wiley & Sons, Ltd.     

Seismology of the solar atmosphere

We describe a new instrument for seismically probing the properties of the Sun's lower atmosphere, and present some first results from an observational campaign carried out at the geographic South Pole during the austral summer of 2002/2003. A preliminary analysis of the data (simultaneous, high-cadence observations of the velocity signals from the photosphere and low chromosphere) shows that the well-known suppression of acoustic power in regions of strong magnetic field, and enhancement of high-frequency power around active regions (acoustic halos), are both consistent with a spreading out of the magnetic field lines with increasing height in the atmosphere. The data have also revealed some unexpected wave behavior. First, evanescent-like waves are found at frequencies substantially above the acoustic cut-off frequency in regions of intermediate magnetic field. Second, upward- and downward-propagating waves are detected in areas of strong magnetic field such as sunspots and plage: even at frequencies below the acoustic cut-off frequency. Third, the wave behavior in regions of strong magnetic field can change over periods of a few hours from propagating to evanescent. While we have no concrete explanation for the first two results, the latter result opens up the question of whether sound waves are involved in short-term events such as flares or CME's.     

Computation of X-Ray Blazar Light Curves Using RHD Simulations

We present the results of a two-dimensional relativistic hydrodynamic simulation of collisions of dense shells of matter moving within a uniform jet. The non-thermal synchrotron radiation produced by the relativistic electrons injected at shocks is computed following their temporal and spatial evolution. We test different parameterizations of the shock acceleration process and compute the corresponding X-ray light curves. A time lag between hard and soft X-ray radiation is found. The collision has an efficiency of few times 10^?3 in converting kinetic energy into radiation.     

Comparison of two small-strain concepts: ISA and intergranular strain applied to barodesy

Acta Geotechnica - The intergranular strain concept (IGS) and intergranular strain anisotropy formulation (ISA) are state of the art extensions to describe small-strain effects. The main...