13,705 research outputs found
Correlation regimes in fluctuations of fatigue crack growth
This paper investigates correlation properties of fluctuations in fatigue
crack growth of polycrystalline materials, such as ductile alloys, that are
commonly encountered in structures and machinery components of complex
electromechanical systems. The model of crack damage measure indicates that the
fluctuations of fatigue crack growth are characterized by strong correlation
patterns within short time scales and are uncorrelated for larger time scales.
The two correlation regimes suggest that the 7075-T6 aluminum alloy, analyzed
in this paper, is characterized by a micro-structure which is responsible for
an intermittent correlated dynamics of fatigue crack growth within a certain
scale. The constitutive equations of the damage measure are built upon the
physics of fracture mechanics and are substantiated by Karhunen-Lo\`{e}ve
decomposition of fatigue test data. Statistical orthogonality of the estimated
damage measure and the resulting estimation error is demonstrated in a Hilbert
space setting.Comment: 30 pages, 8 figures, to appear in Physica
Towards Efficient Maximum Likelihood Estimation of LPV-SS Models
How to efficiently identify multiple-input multiple-output (MIMO) linear
parameter-varying (LPV) discrete-time state-space (SS) models with affine
dependence on the scheduling variable still remains an open question, as
identification methods proposed in the literature suffer heavily from the curse
of dimensionality and/or depend on over-restrictive approximations of the
measured signal behaviors. However, obtaining an SS model of the targeted
system is crucial for many LPV control synthesis methods, as these synthesis
tools are almost exclusively formulated for the aforementioned representation
of the system dynamics. Therefore, in this paper, we tackle the problem by
combining state-of-the-art LPV input-output (IO) identification methods with an
LPV-IO to LPV-SS realization scheme and a maximum likelihood refinement step.
The resulting modular LPV-SS identification approach achieves statical
efficiency with a relatively low computational load. The method contains the
following three steps: 1) estimation of the Markov coefficient sequence of the
underlying system using correlation analysis or Bayesian impulse response
estimation, then 2) LPV-SS realization of the estimated coefficients by using a
basis reduced Ho-Kalman method, and 3) refinement of the LPV-SS model estimate
from a maximum-likelihood point of view by a gradient-based or an
expectation-maximization optimization methodology. The effectiveness of the
full identification scheme is demonstrated by a Monte Carlo study where our
proposed method is compared to existing schemes for identifying a MIMO LPV
system
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