17,366 research outputs found
An interval finite element method for the analysis of structures with spatially varying uncertainties
Finite element analysis of linear-elastic structures with spatially varying uncertain properties is addressed within the framework of the interval model of uncertainty. Resorting to a recently proposed interval field model, the uncertain properties are expressed as the superposition of deterministic basis functions weighted by particular unitary intervals. An Interval Finite Element Method (IFEM) incorporating the interval field representation of uncertainties is formulated by applying an interval extension in conjunction with the standard energy approach. Uncertainty propagation analysis is performed by adopting a response surface approach which provides approximate explicit expressions of response bounds requiring only a few deterministic analyses. Then, the whole procedure is implemented in ABAQUS’ environment by coding User Subroutines and Python scripts.
2D plane stress and bending problems involving uncertain Young's modulus of the material are analyzed. The accuracy of the proposed IFEM as well as response variability under spatially dependent uncertainty are investigated
Spatially varying fuzzy multi-scale uncertainty propagation in unidirectional fibre reinforced composites
SN and SS are grateful for the support provided through the Lloyd’s Register Foundation Centre. The Foundation helps to protect life and property by supporting engineering-related education, public engagement and the application of research.Peer reviewedPostprin
An experimental investigation of the natural frequency statistics of a beam with spatially correlated random masses
Experimental investigations into the dynamic response of structures with material or geometrical random fields usually depend upon an initial characterization of this variability, with very little control over the statistics at its early manufacturing stage. This provides the need of a minimal number of samples to generate an ensemble of dynamic responses, making such experimental data scarcely found in the literature. In this work, a cantilever beam with small masses attached along its length according to a given discrete random field has an ensemble of natural frequencies measured for a number of correlation lengths. The results can be used to investigate the effects of the correlation length on the subsequent natural frequency statistics. The experimental results are compared with a wave approximation for flexural waves using a continuous random field for the mass density, in order to approximate the mass distribution. Issues concerning this approximation are discussed. In addition, results are also compared with a simple added mass approximation with assumed modes from a FE solution
Seismic fragility analysis of reinforced concrete bridges with chloride induced corrosion subjected to spatially varying ground motion
This paper studies the time-dependent seismic fragility of reinforced concrete bridges with chloride induced corrosion under spatially varying ground motions. The time-varying characteristic of the chloride corrosion current density and the uncertainties related to the structural, material and corrosion parameters are both considered in the probabilistic finite element modeling of the example RC bridge at different time steps during its life-cycle. Spatially varying ground motions at different bridge supports are stochastically simulated and used as inputs in the fragility analysis. Seismic fragility curves of the corroded RC bridge at different time steps are generated using the probabilistic seismic demand analysis (PSDA) method. Numerical results indicate that both chloride induced corrosion and ground motion spatial variations have a significant effect on the bridge structural seismic fragility. As compared to the intact bridge, the mean peak ground accelerations (PGAs) of the fragility curves of the RC bridge decrease by approximately 40% after 90 years since the initiation of corrosion. Moreover, the effect of ground motion spatial variations changes along with the process of chloride induced corrosion owing to the structural stiffness degradation. Neglecting seismic ground motion spatial variations may not lead to an accurate estimation of the lifetime seismic fragility of RC bridges with chloride induced corrosion
Decorrelation Times of Photospheric Fields and Flows
We use autocorrelation to investigate evolution in flow fields inferred by
applying Fourier Local Correlation Tracking (FLCT) to a sequence of
high-resolution (0.3 \arcsec), high-cadence ( min) line-of-sight
magnetograms of NOAA active region (AR) 10930 recorded by the Narrowband Filter
Imager (NFI) of the Solar Optical Telescope (SOT) aboard the {\em Hinode}
satellite over 12--13 December 2006. To baseline the timescales of flow
evolution, we also autocorrelated the magnetograms, at several spatial
binnings, to characterize the lifetimes of active region magnetic structures
versus spatial scale. Autocorrelation of flow maps can be used to optimize
tracking parameters, to understand tracking algorithms' susceptibility to
noise, and to estimate flow lifetimes. Tracking parameters varied include: time
interval between magnetogram pairs tracked, spatial binning applied
to the magnetograms, and windowing parameter used in FLCT. Flow
structures vary over a range of spatial and temporal scales (including
unresolved scales), so tracked flows represent a local average of the flow over
a particular range of space and time. We define flow lifetime to be the flow
decorrelation time, . For , tracking results represent
the average velocity over one or more flow lifetimes. We analyze lifetimes of
flow components, divergences, and curls as functions of magnetic field strength
and spatial scale. We find a significant trend of increasing lifetimes of flow
components, divergences, and curls with field strength, consistent with Lorentz
forces partially governing flows in the active photosphere, as well as strong
trends of increasing flow lifetime and decreasing magnitudes with increases in
both spatial scale and .Comment: 48 pages, 20 figures, submitted to the Astrophysical Journal;
full-resolution images in manuscript (8MB) at
http://solarmuri.ssl.berkeley.edu/~welsch/public/manuscripts/flow_lifetimes_v2.pd
On the dynamic analysis of engineering structures with high and low level random uncertainties
The ability to predict the effect of dimension and thickness variability on the dynamic
response of realistically uncertain engineering structures is examined in this thesis.
Initially, available methods for predicting key response statistics and probabilities, for
both low and high frequencies are examined to establish their strengths and limitations
for specified levels of random dimension variability. For low frequency applications,
the ability of Direct Integration (DI) and the First-Order Reliability Method (FORM) to
predict exceedance probability is examined. For high frequency applications, the ability
of the methods of Statistical Energy Analysis (SEA) and DI to predict the mean and
standard deviation of the energy response is examined.
The use of Extreme Value (EV) theory is included as a way to bound responses using
simulated or measured responses. The strengths and limitations of Monte Carlo
simulation methods are explored for both low and high frequency responses of
randomly uncertain structures using both simple mode superposition plate-structure
solutions and (commercially available) finite element solutions for coupled plate
structures.
To address, without the need to undertake expensive Monte Carlo simulation, the
problem of predicting response bounds for structures with varying levels of uncertainty,
a novel DI-EV method is developed and examined. It is tested first on a simple plate
structure, then on a coupled plate structure, with low-level and high-level random
dimension and thickness uncertainty. In addition, the method is compared with the
SEA-EV method.
The thesis shows that the results from the existing SEA-EV bounding approach gives
good bounds only at particular frequencies and mainly for low levels of dimension
variability. In contrast, the proposed DI-EV bounding approach compare extremely well
with Monte Carlo simulations, which is not only at all frequencies but also with both
low-level and high-level uncertainties, for simple and coupled plate structures with
dimension and thickness variation
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