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Autonomous systems thermographic NDT of composite structures
Transient thermography is a method used successfully in the evaluation of composite materials and aerospace structures. It has the capacity to deliver both qualitative and quantitative results about hidden defects or features in a composite structure. Aircraft must undergo routine maintenance – inspection to check for any critical damage and thus to ensure its safety. This work aims to address the challenge of NDT automated inspection and improve the defects’ detection by suggesting an autonomous thermographic imaging approach using a UAV (Unmanned Aerial Vehicle) active thermographic system. The concept of active thermography is discussed and presented in the inspection of aircraft CFRP panels along with the mission planning for aerial inspection using the UAV for real time inspection. Results indicate that the suggested approach could significantly reduce the inspection time, cost, and workload, whilst potentially increase the probability of detection of defects on aircraft composites
The use of pulse-compression thermography for detecting defects in paintings
Interest in the conservation of paintings grows year by year. Their periodic inspection is essential for their conservation over the time. Thermographic non-destructive inspection is one technique useful for paintings, but it is essential to be able to detect buried defects while minimising the level of thermal stimulus. This paper describes a pulse-compression infrared thermography technique whereby defect detection is optimized while minimising the rise in temperature. To accomplish this task, LED lamps driven by a coded waveform based on a linear frequency modulated chirp signal have been used on paintings on both a wooden panel and a canvas layer. These specimens contained artificially fabricated defects. Although the physical condition of each painting was different, the experimental results show that the proposed signal processing procedure is able to detect defects using a low temperature contrast
Electron-electron interactions and two-dimensional - two-dimensional tunneling
We derive and evaluate expressions for the dc tunneling conductance between
interacting two-dimensional electron systems at non-zero temperature. The
possibility of using the dependence of the tunneling conductance on voltage and
temperature to determine the temperature-dependent electron-electron scattering
rate at the Fermi energy is discussed. The finite electronic lifetime produced
by electron-electron interactions is calculated as a function of temperature
for quasiparticles near the Fermi circle. Vertex corrections to the random
phase approximation substantially increase the electronic scattering rate. Our
results are in an excellent quantitative agreement with experiment.Comment: Revtex style, 21 pages and 8 postscript figures in a separate file;
Phys. Rev. B (in press
Scaling theory of the Mott-Hubbard metal-insulator transition in one dimension
We use the Bethe ansatz equations to calculate the charge stiffness of the one-dimensional
repulsive-interaction Hubbard model for electron densities close to the Mott
insulating value of one electron per site (), where is the ground
state energy, is the circumference of the system (assumed to have periodic
boundary conditions), and is the magnetic flux
enclosed. We obtain an exact result for the asymptotic form of
as at , which defines and yields an analytic expression for
the correlation length in the Mott insulating phase of the model as a
function of the on-site repulsion . In the vicinity of the zero temperature
critical point U=0, , we show that the charge stiffness has the
hyperscaling form , where and is a universal scaling function which we calculate. The
physical significance of in the metallic phase of the model is that it
defines the characteristic size of the charge-carrying solitons, or {\em
holons}. We construct an explicit mapping for arbitrary and of the holons onto weakly interacting spinless fermions, and use this
mapping to obtain an asymptotically exact expression for the low temperature
thermopower near the metal-insulator transition, which is a generalization to
arbitrary of a result previously obtained using a weak- coupling
approximation, and implies hole-like transport for .Comment: 34 pages, REVTEX (5 figures by request
FRP-to-masonry bond durability assessment with infrared thermography method
The bond behavior between FRP composites and masonry substrate plays an important role
in the performance of externally bonded reinforced masonry structures. Therefore, monitoring
the bond quality during the application and subsequent service life of a structure is of crucial
importance for execution control and structural health monitoring. The bond quality can change
during the service life of the structure due to environmental conditions. Local detachments may
occur at the FRP/substrate interface, affecting the bond performance to a large extent. Therefore, the use of expedite and efficient non-destructive techniques for assessment of the bond quality
and monitoring FRP delamination is of much interest.
Active infrared thermography (IR) technique was used in this study for assessing the bond
quality in environmentally degraded FRP-strengthened masonry elements. The applicability and
accuracy of the adopted method was initially validated by localization and size quantification of
artificially embedded defects in FRP-strengthened brick specimens. Then, the method was used
for investigating the appearance and progression of FRP delaminations due to environmental
conditions. GFRP-strengthened brick specimens were exposed to accelerated hygrothermal
ageing tests and inspected periodically with the IR camera. The results showed environmental
exposure may produce large progressive FRP delaminations.Fundação para a Ciência e Tecnologi
Saturation of electrical resistivity
Resistivity saturation is observed in many metallic systems with a large
resistivity, i.e., when the resistivity has reached a critical value, its
further increase with temperature is substantially reduced. This typically
happens when the apparent mean free path is comparable to the interatomic
separations - the Ioffe-Regel condition. Recently, several exceptions to this
rule have been found. Here, we review experimental results and early theories
of resistivity saturation. We then describe more recent theoretical work,
addressing cases both where the Ioffe-Regel condition is satisfied and where it
is violated. In particular we show how the (semiclassical) Ioffe-Regel
condition can be derived quantum-mechanically under certain assumptions about
the system and why these assumptions are violated for high-Tc cuprates and
alkali-doped fullerides.Comment: 16 pages, RevTeX, 15 eps figures, additional material available at
http://www.mpi-stuttgart.mpg.de/andersen/saturation
Quantitative estimates of unique continuation for parabolic equations, determination of unknown time-varying boundaries and optimal stability estimates
In this paper we will review the main results concerning the issue of
stability for the determination unknown boundary portion of a thermic
conducting body from Cauchy data for parabolic equations. We give detailed and
selfcontained proofs. We prove that such problems are severely ill-posed in the
sense that under a priori regularity assumptions on the unknown boundaries, up
to any finite order of differentiability, the continuous dependence of unknown
boundary from the measured data is, at best, of logarithmic type