This paper deals with the problem of damage detection using output-only vibration measurements under changing environmental conditions. Two types of features are extracted from the measurements: eigenproperties of the structure using an automated stochastic subspace identification procedure and peak indicators computed on the Fourier transform of modal filters. The effects of environment are treated using factor analysis and damage is detected using statistical process control with the multivariate Shewhart-T control charts.A numerical example of a bridge subject to environmental changes and damage is presented. The sensitivity of the damage detection procedure to noise on the measurements, environment and damage is studied. An estimation of the computational time needed to extract the different features is given, and a table is provided to summarize the advantages and drawbacks of each of the features studied. r
SUMMARYFor high wave numbers, the Helmholtz equation su!ers the so-called &pollution e!ect'. This e!ect is directly related to the dispersion. A method to measure the dispersion on any numerical method related to the classical Galerkin FEM is presented. This method does not require to compute the numerical solution of the problem and is extremely fast. Numerical results on the classical Galerkin FEM (p-method) is compared to modi"ed methods presented in the literature. A study of the in#uence of the topology triangles is also carried out. The e$ciency of the di!erent methods is compared. The numerical results in two of the mesh and for square elements show that the high order elements control the dispersion well. The most e!ective modi"ed method is the QSFEM [1, 2] but it is also very complicated in the general setting. The residual-free bubble [3,4] is e!ective in one dimension but not in higher dimensions. The least-square method [1,5] approach lowers the dispersion but relatively little. The results for triangular meshes show that the best topology is the &criss-cross' pattern.
This paper focuses on the modelling of structures equipped with Macro Fiber Composite (MFC) transducers. Based on the uniform field method under the plane stress assumption, we derive analytical mixing rules in order to evaluate equivalent properties for d 31 and d 33 MFC transducers. In particular, mixing rules are derived for the longitudinal and transverse piezoelectric coefficients of MFCs. These mixing rules are validated using finite element computations and experimental results available from the literature.
This paper reports on a six-axis vibration isolator for space applications. It is divided into three parts. The first part recalls the principles of active isolation and summarizes the main theoretical results for multiple-axis decentralized control based on force feedback. The second part discusses the technology and describes the evolution of the design over the five years of this project. The third part is devoted to the identification of the transmissibility matrix and the performance evaluation. Zero-gravity tests in parabolic flight are reported. The isolator is proved efficient in a frequency band between 5 Hz and 400 Hz, with a maximum attenuation of-40 dB between 50 Hz and 200 Hz.
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