Advances in Grinding and Abrasive Technology XIV

Vols. 359-360

Vols. 359-360

Progresses in Fracture and Strength of Materials and Structures

Vols. 353-358

Vols. 353-358

Innovation in Ceramic Science and Engineering

Vol. 352

Vol. 352

Composite Materials V

Vol. 351

Vol. 351

Electroceramics in Japan X

Vol. 350

Vol. 350

Advances in Fracture and Damage Mechanics VI

Vols. 348-349

Vols. 348-349

Damage Assessment of Structures VII

Vol. 347

Vol. 347

The Mechanical Behavior of Materials X

Vols. 345-346

Vols. 345-346

Sheet Metal 2007

Vol. 344

Vol. 344

Advanced Biomaterials VII

Vols. 342-343

Vols. 342-343

Engineering Plasticity and Its Applications

Vols. 340-341

Vols. 340-341

Progress of Precision Engineering and Nano Technology

Vol. 339

Vol. 339

High-Performance Ceramics IV

Vols. 336-338

Vols. 336-338

# Damage Assessment of Structures VII

文章题目 页数

摘要: 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.

435

摘要: 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.

441

摘要: 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.

447

摘要: 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.

455

摘要: 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.

461

摘要: 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.

467

摘要: 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.

473

摘要: 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.

479

摘要: 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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