246 research outputs found
Sensor network optimization for damage detection on aluminium stiffened helicopter panels
Health and Usage Monitoring Systems (HUMS) has received considerable
attention from the helicopter community in recent years with the declared aim to increase
flight safety, increase mission reliability, extend duration of life limited components and of
course reduce the maintenance costs. The latter is about 25 per cent of the direct operating
cost of the helicopter, thus playing an important role especially in the case of the ageing
aircrafts. In particular, with respect to helicopter fuselages, only some attempts were carried
out to monitor directly on-line the damage accumulation and propagation during life. In this
field, and in particular in the military applications, an integrated and reliable system for
monitoring the damage in the fuselage and for evaluating the time inspections and remaining
life (prognosis) is missing. However, because of the presence of many vibratory loads, the
diagnosis of helicopter structures is very critical. From one hand, a very large number of
sensors would be needed for a robust appreciation of the structural health, from the other hand
the industrialization of the product brings the need for a low impact over the existing
structures, or toward a reduction in the allowed amount of sensors. As a result, comes the
importance for an optimization of the sensor network, with the aim to find out the regions
inside the structure which are the most sensible to a damage and at the same time robust to
noise. The aim of the present work is to define a methodology for optimising the sensors
position inside an helicopter fuselage panel in order to obtain the best compromise between
the simplicity and the robustness of a sensor network. In particular, a Finite Element (FE)
model will be used to create a database of various damages inside the structure, thus
consequently optimising the network sensitivity to any damage. The evaluation of the
network performances is provided when some realistic noise [1,2] is added to the FE
calculation
A method for determining the distribution of carbon nanotubes in nanocomposites by electric conductivity
Carbon nanotube (CNT) polymer nanocomposites are one of the most promising materials due to their remarkable mechanical properties as well as the electrical conductivity, which offers the capability of monitoring the deformation and damage of composite structures by measuring the related conductivity variations. However, quantifying the distribution of CNTs inside the material remains a challenge with respects to both experimental and numerical works. In the current study, the electrical conductivity was used to determine the microstructure of CNT-reinforced polymer. By introducing a modified parameter related to the polar angle of CNTs, the mechanical properties as well as the electrical conductivity change with respect to deformation of nanocomposites can be replicated. After validation by experimental data from the multi-walled CNT/polymer nanocompo sites under tensile loading, the capability of the current method was then studied for composites with various weight fractions of nanotubes. (C) 2022 The Authors. Published by Elsevier B.V
Particle filter-based damage prognosis using online feature fusion and selection
Damage prognosis generally resorts to damage quantification functions and evolution models to quantify the current damage state and to predict the future states and the remaining useful life (RUL). The former typically consists of a function describing the relationship between the damage state and a statistical feature extracted from the measured signals, thus the prognostic performance will strongly depend on the selection of a proper feature. Given the best feature may vary for different specimens or even at each time instant for the same specimen during damage progression, such selection is a challenging task but has received little investigation so far. In this context, this paper proposes a particle filter-based damage prognosis framework, which involves an online feature fusion and selection scheme. A prognostic model is considered for each feature, with a multivariate process equation, formulated using both a damage degradation function and a bias parameter, and a measurement equation linking the damage state and that feature considering a data-driven model and the bias. One PF is used to estimate the damage state, its evolution parameters, and the bias for each model. Then, at each step, the feature with the smallest estimated bias is selected as the best feature providing the most likely state vectors and is used to select the most likely samples of the damage state and growth parameters for predicting the RUL and for calculating the prior at the next step. The proposed prognostic framework is demonstrated by an experimental study, where an aluminum lug structure subject to fatigue crack growth is monitored by a Lamb wave measurement system
Particle filter-based delamination shape prediction in composites subjected to fatigue loading
Modeling generic size features of delamination, such as area or length, has long been considered in the literature for damage prognosis in composites through specific models describing damage state evolution with load cycles or time. However, the delamination shape has never been considered, despite that it holds important information for damage diagnosis and prognosis, including the delamination area, its center, and perimeter, useful for structural safety evaluation. In this context, this paper develops a novel particle filter (PF)-based framework for delamination shape prediction. To this end, the delamination image is discretized by a mesh, where control points are defined as intersections between the grid lines and the perimeter of the delamination. A parametric data-driven function maps each point position as a function of the load cycles and is initially fitted on a sample test. Then, a PF is independently implemented for each node whereby to predict their future positions along the mesh lines, thus allowing delamination shape progression estimates. The new framework is demonstrated with reference to experimental tests of fatigue delamination growth in composite panels with ultrasonics C-scan monitoring
On the performance of a cointegration-based approach for novelty detection in realistic fatigue crack growth scenarios
Confounding influences, such as operational and environmental variations, represent a limitation to the implementation of Structural Health Monitoring (SHM) systems in real structures, potentially leading to damage misclassifications. In this framework, this study considers cointegration as a state of the art method for data normalisation in fatigue crack propagation scenarios, where monitoring is performed by a distributed network of strain sensors. Specifically, the work is aimed at demonstrating the effectiveness of cointegration on real engineering data in a new context, where the damage is continuously growing. Cointegration is applied at first in a controlled scenario consisting of a numerical strain simulation by means of a finite element model, modified in order to take realistic temperature fluctuations and sensor noise into account. Afterwards, detrending and anomaly detection performances are verified in two different experimental programmes on realistic aeronautical structures subjected to fatigue crack growth, including a full-scale fatigue test on a helicopter tail boom. Strain measurements are taken from a network of Fibre Bragg Grating (FBG) sensors, known to be extremely sensitive to temperature variations, hence delivering challenging scenarios for cointegration testing. Results are shown to be in good agreement with the experimental evidence, with the cointegration algorithm capable of detecting the onset of damage propagation within a 4 mm increment from a baseline condition
Searching for supergiant fast X-ray transients with Swift
Supergiant fast X-ray transients (SFXTs) are high mass X-ray binaries (HMXBs)
hosting a neutron star and an OB supergiant companion. We examine the available
Swift data, as well as other new or archival/serendipitous data, on three
sources: IGR J17407-2808, 2XMM J185114.3-000004, and IGR J18175-2419, whose
X-ray characteristics qualify them as candidate SFXT, in order to explore their
properties and test whether they are consistent with an SFXT nature. As IGR
J17407-2808 and 2XMM J185114.3-000004 triggered the Burst Alert Telescope on
board Swift, the Swift data allow us to provide their first arcsecond
localisations, leading to an unequivocal identification of the source CXOU
J174042.0-280724 as the soft X-ray counterpart of IGR J17407-2808, as well as
their first broadband spectra, which can be fit with models generally
describing accreting neutron stars in HMXBs. While still lacking optical
spectroscopy to assess the spectral type of the companion, we propose 2XMM
J185114.3-000004 as a very strong SFXT candidate. The nature of IGR J17407-2808
remains, instead, more uncertain. Its broad band properties cannot exclude that
the emission originates from either a HMXB (and in that case, a SFXT) or, more
likely, a low mass X-ray binary. Finally, based on the deep non-detection in
our XRT monitoring campaign and a careful reanalysis of the original Integral
data in which the discovery of the source was first reported, we show that IGR
J18175-2419 is likely a spurious detection.Comment: Accepted for publication in Astronomy and Astrophysics. 12 pages, 11
figures, 6 table
On a meta-learning population-based approach to damage prognosis
The current work studies the application of population-based structural health monitoring (PBSHM) to the problem of damage prognosis. Two methods are proposed for population-informed damage prognosis and they are evaluated according to their performance using an experimental dataset. The first method is an attempt to define a functional subspace, which includes the potential behaviour of members of the population subjected to the phenomenon of damage evolution. The second approach is a meta-learning method, the deep kernel transfer (DKT) method, which seeks to exploit information from a population in order to enhance the predictive performance of a Gaussian process. The predictive capabilities of the two methods are tested in an experimental crack-growth problem. The results reveal that the two methods are properly informed by the population to make predictions about new structures and show potential in dealing with the problem of damage evolution, which is a problem of imbalanced and difficult-to-acquire data
Redshift Limits of BL Lacertae Objects from Optical Spectroscopy
Context: BL Lacertae objects have been the targets for numerous recent
multiwavelength campaigns, continuum spectral variability studies, and
theoretical spectral and variability modeling. A meaningful interpretation of
the results of such studies requires a reliable knowledge of the objects'
redshifts; however, the redshifts for many are still unknown or uncertain.
Aims: Therefore, we hope to determine or constrain the redshifts of six BL Lac
objects with unknown or poorly known redshifts. Methods: Observations were made
of these objects with the MDM 2.4 m Hiltner telescope. Although no spectral
features were detected, and thus no redshifts could be measured, lower redshift
limits were assigned to the objects based on the expected equivalent widths of
absorption features in their host galaxies. Redshifts were also estimated for
some objects by assuming the host galaxies are standard candles and using host
galaxy apparent magnitudes taken from the literature. Results: The commonly
used redshift of for 1219+285 is almost certainly wrong, while the
redshifts of the other objects studied remain undetermined.Comment: 4 pages, 2 figures. Accepted by A&A Research Note
The Palermo Swift-BAT hard X-ray catalogue III. Results after 54 months of sky survey
We present the Second Palermo Swift-BAT hard X-ray catalogue obtained by
analysing data acquired in the first 54 months of the Swift mission. Using our
software dedicated to the analysis of data from coded mask telescopes, we
analysed the BAT survey data in three energy bands (15-30 keV, 15-70 keV,
15-150 keV), obtaining a list of 1256 detections above a significance threshold
of 4.8 standard deviations. The identification of the source counterparts is
pursued using two strategies: the analysis of field observations of soft X-ray
instruments and cross-correlation of our catalogue with source databases.The
survey covers 50% of the sky to a 15--150 keV flux limit of 1.0 x 10^-11 erg
s^-1 cm^-2 and 9.2 x 10^-12 erg s^-1 cm^-2 for |b| 10
degrees, respectively. The Second Palermo Swift-BAT hard X-ray catalogue
includes 1079 (86%) hard X-ray sources with an associated counterpart (26 with
a double association and 2 with a triple association) and 177 BAT excesses
(14%) that still lack a counterpart. The distribution of the BAT sources among
the different object classes consists of 19% Galactic sources, 57%
extragalactic sources, and 10% sources with a counterpart at softer energies
whose nature has not yet been determined. About half of the BAT associated
sources lack a counterpart in the ROSAT catalogues. This suggests that either
moderate or strong absorption may be preventing their detection in the ROSAT
energy band. The comparison of our BAT catalogue with the Fermi Large Area
Telescope First Source Catalogue identifies 59 BAT/Fermi correspondences: 48
blazars, 3 Seyfert galaxies, 1 interacting galaxy, 3 high mass X-ray binaries,
and 4 pulsars/supernova remnants. This small number of correspondences
indicates that different populations make the sky shine in these two different
energy bands
The Troublesome Broadband Evolution of GRB 061126: Does a Grey Burst Imply Grey Dust?
We report on observations of a gamma-ray burst (GRB 061126) with an extremely
bright (R ~ 12 mag at peak) early-time optical afterglow. The optical afterglow
is already fading as a power law 22 seconds after the trigger, with no
detectable prompt contribution in our first exposure, which was coincident with
a large prompt-emission gamma-ray pulse. The optical--infrared photometric
spectral energy distribution is an excellent fit to a power law, but it
exhibits a moderate red-to-blue evolution in the spectral index at about 500 s
after the burst. This color change is contemporaneous with a switch from a
relatively fast decay to slower decay. The rapidly decaying early afterglow is
broadly consistent with synchrotron emission from a reverse shock, but a bright
forward-shock component predicted by the intermediate- to late-time X-ray
observations under the assumptions of standard afterglow models is not
observed. Indeed, despite its remarkable early-time brightness, this burst
would qualify as a dark burst at later times on the basis of its nearly flat
optical-to-X-ray spectral index. Our photometric spectral energy distribution
provides no evidence of host-galaxy extinction, requiring either large
quantities of grey dust in the host system (at redshift 1.1588 +/- 0.0006,
based upon our late-time Keck spectroscopy) or separate physical origins for
the X-ray and optical afterglows.Comment: Revised version submitted to ApJ. Contains significantly expanded
discussion, an additional figure, and numerous other change
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