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 $D_{\rm c} = (L/2) d^2 E_0/d\Phi_{\rm c}^2|_{\Phi_{\rm c}=0}$ of the one-dimensional repulsive-interaction Hubbard model for electron densities close to the Mott insulating value of one electron per site ($n=1$), where $E_0$ is the ground state energy, $L$ is the circumference of the system (assumed to have periodic boundary conditions), and $(\hbar c/e)\Phi_{\rm c}$ is the magnetic flux enclosed. We obtain an exact result for the asymptotic form of $D_{\rm c}(L)$ as $L\to \infty$ at $n=1$, which defines and yields an analytic expression for the correlation length $\xi$ in the Mott insulating phase of the model as a function of the on-site repulsion $U$. In the vicinity of the zero temperature critical point U=0, $n=1$, we show that the charge stiffness has the hyperscaling form $D_{\rm c}(n,L,U)=Y_+(\xi \delta, \xi/L)$, where $\delta =|1-n|$ and $Y_+$ is a universal scaling function which we calculate. The physical significance of $\xi$ 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 $U$ and $\xi \delta \ll 1$ 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 $U$ of a result previously obtained using a weak- coupling approximation, and implies hole-like transport for $0<1-n\ll\xi^{-1}$.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