Damage Assessment of Structures VII

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Authors: Andrzej Świercz, Przemysław Kołakowski, Jan Holnicki-Szulc
Authors: Federico Bosia, Nicola Maria Pugno, Giuseppe Lacidogna, Alberto Carpinteri
Abstract: In this contribution some characteristics and predictive capabilities are discussed of a recently introduced model for damage progression and energy release, in view of modelling Acoustic Emission. The specimen is discretized in a network of connected springs, similar to a Fibre Bundle Model approach, with the spring intrinsic strengths statistically distributed according to a Weibull distribution. Rigorous energy balance considerations allow the determination of the dissipated energy due to crack surface formation and kinetic energy propagation. Based on results of simulations, the macroscopic behaviour emerging from different choices at “mesoscopic” level is discussed, in particular the relevance of model parameters such as the distribution of spring cross sections, Weibull modulus values, and discretization parameters in determining results like stressstrain curves and energy scaling versus time or specimen size.
Authors: Majid R. Ayatollahi, M.R.M. Aliha, S. Rahmanian
Abstract: A new crack specimen called the diagonally loaded square plate (DLSP) containing an angled central crack is analyzed using the finite element method. The stress intensity factors and the T-stress are calculated for the new specimen in the full range from pure mode I to pure mode II. Unlike the conventional center cracked specimen under uniaxial loading, the improved specimen is able to provide pure mode II. It is shown that the effect of non-singular stress component T in the DLSP specimen is not ignorable relative to the singular stress terms, particularly for mode II dominated loading conditions.
Authors: X.B. Wang
Abstract: For many quasi-brittle materials (such as rock, ceramic and concrete) in pure bending state, the material on the tensile side will fail firstly since the compressive strength can be ten times the tensile strength. After tensile strain localization zone is initiated in the midspan of the beam, its propagation direction will be perpendicular to the neutral axis. In the paper, using nonlocal theory or gradient-dependent plasticity, the distributions of local plastic tensile strain and local damage variable in tensile strain localization zone of a pure bending beam are analyzed theoretically. The evolutions of the maximum local plastic tensile strain, the maximum local damage variable and the bending moment with tensile stress acting on the tensile side are presented through examples. The distributions of local plastic tensile strain and local damage variable in tensile strain localization zone are highly nonuniform due to microstructural effect. When the maximum bending moment is reached, the maximum local damage variable is proportional to the ratio of elastic modulus to elastoplastic modulus, while the maximum local plastic tensile strain is inversely proportional to elastic modulus and elastoplastic modulus. For quasi-brittle materials, the elastoplastic modulus that is a constitutive parameter equal to the absolute value of the slope of tensile stress-tensile strain curve in strain-softening stage is much higher. The present theoretical results mean that the precursors to failure are less apparent for extremely brittle materials.
Authors: N.M.M. Maia, A.M.R. Ribeiro, M. Fontul, D. Montalvão, R.P.C. Sampaio
Abstract: The Detection and Relative Damage Quantification Indicator (DRQ) was presented previously as a reliable damage detection indicator when used with Operational Deflection Shapes (ODS). The DRQ was computed from the Response Vector Assurance Criterion (RVAC) between the damaged and the initial ODS and the resulting value proved to be a good indicator of the presence of damage. The use of the ODS implies that the loads applied to the structure with and without damage are either known or, at least, the same. If the forces are not deterministic but still ergodic, the power spectrum could be used to evaluate the ODS, but still the above conditions hold, in a statistical sense. When a structure is subjected to ambient excitation, those conditions can hardly be assured. The loads may vary quite significantly and the ODS changes may be due to those changes instead of the presence of damage. To avoid this handicap, the authors explore here the use of the Transmissibility functions. If properly defined, the Transmissibility is invariant with respect to the amplitude of the loads. Since the Displacement Transmissibility is load invariant, a picked set of responses can be measured in service and used to predict another set; the result will then be correlated to the actual values using the RVAC and the DRQ will be computed. Numerical and experimental examples illustrate the proposed technique.
Authors: M. Klinikov, Claus Peter Fritzen
Abstract: For purposes of monitoring and damage prognosis it is important to know the external loads which act on a structure. The knowledge of these loads enables us to make an assessment of damage after extreme events and updated forecasts of the remaining life-time. In many practical applications it is not possible to measure the forces e.g. resulting from wind loads or traffic directly. Therefore, these forces are determined indirectly from dynamic measurements. In this contribution, an updated overview of available time domain load reconstruction methods is presented. An attempt of highlighting the main advantages and disadvantages of different approaches, which are used in engineering is done. The importance of sensors type as well as their locations is considered for each approach. Finally, the methods applicability to real structures, where the online reconstruction plays an important role, is discussed.
Authors: Joanna Iwaniec
Abstract: Vast majority of industrial mechanical systems do not behave in linear way around any operating point and work under non-measurable operational loads. Identification of such systems can’t be carried out by means of the classical nonlinear system identification methods. The paper concerns the method combining restoring force, boundary perturbation and direct parameter estimation techniques that, on the contrary to classical methods, requires neither input measurement nor linear behaviour of the considered system around an operating point. There are also presented results of method application to parameter identification of machine shaft flexible support.
Authors: Jerome Antoni, David Hanson, Bob Randall
Abstract: An underlying assumption of many operational modal analysis techniques is that the excitation is evenly distributed over the system, i.e. the inputs are spatially white, and is constant with frequency, i.e. frequentially white. This paper investigates the use of cyclostationarity, in combination with the Frequency Domain Subspace identification technique, to relax these constraints. Such a technique is suitable for application on systems which are excited by at least one cyclostationary input with a unique cyclic frequency, such as an internal combustion engine in a car or locomotive. The cyclostationary properties of this input are employed to reduce a multiple-inputmultiple- output system to a single-input-multiple-output system by extracting the component of each response measurement which is attributable to the cyclostationary input alone. The system modal properties; the resonances, damping and mode shapes, are then identified using the frequency domain subspace algorithm. The effectiveness of the technique is demonstrated through experiments on a laboratory test rig and a passenger train, and compared with results obtained using the knowledge of the inputs.
Authors: Carlo Rainieri, Giovanni Fabbrocino, E. Cosenza
Abstract: The aim of structural health monitoring for civil structures is not only detection of sudden or progressive damages but also monitoring their performance under operational conditions or under some particular environmental issues such as earthquakes. Seismic protection of buildings at risk can be reached increasing the knowledge of the structural behavior of existing constructions. This circumstance points out the opportunity of monitoring the performance of civil structures over their operational lives. The present paper deals with automated Structural Health Monitoring (SHM) technologies adopted for the School of Engineering Main Building at the University of Naples “Federico II”. In particular, the attention is focused on the development of an automated procedure based on the Operational Modal Analysis (OMA) that must ensure the continuous monitoring and extraction of the modal parameters of the building. Some numerical examples are then discussed in order to point out effectiveness of the algorithm and relevant issues that need to be improved.
Authors: Massimo Olivero, Guido Perrone, Alberto Vallan, Silvio Abrate
Abstract: In the following we present a low-cost optical system for cracks evolution monitoring. The transducer principle is based on the variation of light transmitted between two facing fibers with their axial distance, exploiting the unique light-collecting capabilities of large-core polymer optical fibers. The characterization shows that the working range of the transducer is up to 3.5cm, with a resolution of 10$m and a repeatability of 5$m. With our current custom-developed control unit, up to four transducers can be arranged to provide a multi-axial displacement sensor or to simultaneously monitor four points for distributed sensing. A closed-loop light modulation/detection scheme is implemented to reduce the environmental noise sensitivity. The control unit is interfaced to a PC via USB port or via GSM/GPRS modem to automatically send periodical reports of the measurements and to issue warnings in case of displacements above a given threshold. Extensive tests in an environmental chamber have been carried out in order to extract calibration curves and to compensate for day-night and summer-winter temperature fluctuations.

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