Identificação de dano baseada na totalidade da informação espectral: aplicações

O tema da identificação de dano (deteção e localização) em estruturas através de métodos não destrutivos recebeu, ao longo das últimas décadas, um interesse acrescido em resultado de um significativo número de estruturas monitorizadas, tais como pontes ou edifícios históricos de elevado interesse patrimonial. Vários métodos de identificação de dano têm sido sugeridos com base em alterações dos parâmetros dinâmicos (e.g. frequências naturais ou modos de vibração), sempre que a matriz de rigidez ou de massa seja alterada no sistema. Contudo, o problema da identificação de dano é complexo, não havendo, até ao momento, um método ou um conjunto de métodos que resolva o problema. O presente artigo apresenta um método de identificação baseado na utilização da totalidade da informação espetral dentro de uma gama de frequências de interesse, obtida por ensaios de vibração ambiente (sem medir as excitações). O método é aplicado a três casos de estudo. Os resultados demostram que o método baseado na utilização da totalidade da informação espectral poderá ser um importante contributo para a identificação do dano em estruturas

Damage identification and seismic vulnerability assessment of a historic masonry chimney

The present paper deals with the dynamic characterisation of a historical masonry chimney aimed at identifying the structural damage and assessing its seismic performance. The structure was severely damaged by a lightning accident and in-depth repair works were executed to re-instate its sound configuration. The case study is fully detailed, including the aspects of survey, inspection, diagnosis, and evolution of the dynamic properties of the system throughout the structural intervention. Considering the explicit dependence of the power spectral densities of measured nodal processes on their frequency content, a spectrum-driven algorithm is used to detect and locate the damage. The paper shows that the eigenparameters obtained from the decomposition of the response power spectrum matrix are sensitive to system's changes caused by evolutionary damage scenarios, thereby resulting excellent indicators for assessing both the presence and position of structural vulnerabilities. The results are compared with the ones from other modal-based damage identification methods and the strengths/limitations of the tools currently available in literature are extensively discussed. Finally, based on the crack pattern surveyed before the repair works, the weakest links of the chimney are identified and the most meaningful collapse mechanisms are analysed to verify the seismic capacity of the structure. According to the results of the kinematic analysis, the chimney does withstand the maximum site peak ground acceleration.info:eu-repo/semantics/publishedVersio

Structural monitoring and damage identification on a masonry chimney by a spectral-based identification technique

The present work deals with the damage identification of a historical masonry chimney located in Guimarães (Portugal), including a detailed survey, inspection and diagnosis. The chimney was object of a continuous monitoring campaign carried out to catch the evolution of the modal parameters and evaluate the success of the rehabilitation works planned after a lightning accident. Based on the dynamic features extracted from the OMA data, a damage identification analysis was performed making use of different damage identification techniques. Considering the explicit dependence of output-only power spectral densities on frequency contents, a spectral-based identification method was used to detect the damage. Finally, an appropriate localization index was defined combining evolutionary complex eigenvectors obtained from the decomposition of the power spectral density matrix. The results allow to conclude that the spectral-based dynamic identification method is a non-destructive tool able to capture the global behavior of a structure and may reveal itself of great help for exploring damage at an early stage in historical constructions

Broad N2H+ emission towards the protostellar shock L1157-B1

We present the first detection of N2H+ towards a low-mass protostellar outflow, namely the L1157-B1 shock, at about 0.1 pc from the protostellar cocoon. The detection was obtained with the IRAM 30-m antenna. We observed emission at 93 GHz due to the J = 1-0 hyperfine lines. The analysis of the emission coupled with the HIFI CHESS multiline CO observations leads to the conclusion that the observed N2H+(1-0) line originates from the dense (> 10^5 cm-3) gas associated with the large (20-25 arcsec) cavities opened by the protostellar wind. We find a N2H+ column density of few 10^12 cm-2 corresponding to an abundance of (2-8) 10^-9. The N2H+ abundance can be matched by a model of quiescent gas evolved for more than 10^4 yr, i.e. for more than the shock kinematical age (about 2000 yr). Modelling of C-shocks confirms that the abundance of N2H+ is not increased by the passage of the shock. In summary, N2H+ is a fossil record of the pre-shock gas, formed when the density of the gas was around 10^4 cm-3, and then further compressed and accelerated by the shock.Comment: ApJ, in pres

H_{2} adsorption on multiwalled carbon nanotubes at low temperatures and low pressures

We present an experimental study on H_{2} adsorption on multiwalled carbon nanotubes (MWCNTs) at low temperatures (12–30 K) and low pressures (2×10^{-5}  Torr) using the temperature programmed desorption technique. Our results show that the molecular hydrogen uptake increases nearly exponentially from 6×10^{-9}  wt. % at 24.5 K to 2×10^{-7}  wt. % at 12.5 K and that the desorption kinetics is of the first order. Comparative measurements indicate that MWCNTs have an adsorption capacity about two orders higher than that of activated carbon (charcoal) making them a possible candidate as hydrogen cryosorber for eventual applications in accelerators and synchrotrons

A spectrum-driven damage Identification technique: application and validation through the numerical simulation of the Z24 Bridge

The present paper focuses on a damage identification method based on the use of the second order spectral properties of the nodal response processes. The explicit dependence on the frequency content of the outputs power spectral densities makes them suitable for damage detection and localization. The well-known case study of the Z24 Bridge in Switzerland is chosen to apply and further investigate this technique with the aim of validating its reliability. Numerical simulations of the dynamic response of the structure subjected to different types of excitation are carried out to assess the variability of the spectrum-driven method with respect to both type and position of the excitation sources. The simulated data obtained from random vibrations, impulse, ramp and shaking forces, allowed to build the power spectrum matrix from which the main eigenparameters of reference and damage scenarios are extracted. Afterwards, complex eigenvectors and real eigenvalues are properly weighed and combined and a damage index based on the difference between spectral modes is computed to pinpoint the damage. Finally, a group of vibration-based damage identification methods are selected from the literature to compare the results obtained and to evaluate the performance of the spectral index.The authors would like to express their sincere gratitude to Prof. Dr. Guido De Roeck for sharing his information about the Z24 Bridge. The first author would also like to acknowledge the Italian Ministry of Education, Universities and Research (MIUR) for the Ph.D. scholarship provided

A case of double cystic esophageal duplication in VACTERL syndrome: the first case report and a review of the literature

BackgroundAn esophageal duplication cyst (EDC) is a rare malformation resulting from the embryonic foregut. VACTERL syndrome is a genetic disorder affecting many systems of the human body. We report the first case of VACTERL syndrome associated to asymptomatic double EDC.Case reportA girl with anorectal malformation and rectovestibular fistula, kidney malformation, and various vertebral defects came to our attention at the time of birth. VACTERL disease was diagnosed. She underwent Peña anoplasty at 4 months of life without complications. MRI was conducted at the age of 2. It accidentally showed a double esophageal duplication (12 mm × 35 mm × 10 mm) at the D7–D9 level. We planned a thoracoscopy; previous intraoperative esophagogastroduodenoscopy showed an external compression of the native esophagus. Two duplicated esophageal lesions were removed. The patient made an uneventful recovery and was completely asymptomatic at long-term follow-up.ConclusionsVACTERL syndrome is still a not well-defined disease. Based on the current literature, this is the first case of a double esophageal duplication in a patient affected by VACTERL syndrome. According to us, the thoracoscopic approach of esophageal duplications can be followed by experts. Complete surgical excision is possible even if the cyst shares a common muscular wall with the esophagus. For this reason, we suggest to close the muscular wall by a simple interrupted suture

Spectral-based damage identification technique on an earthen mock-up construction tested on a shaking table

Conservation of ancient built heritage plays a leading role for modern societies. Knowledge about ancient building methods, essentially based on the use of natural materials such as earth, stone and wood, is fundamental to plan interventions aimed at preserving the architectural heritage. Due to the growing research on sustainable technologies, the interest in structural systems built using natural materials has been rising more and more. Taking an earthen mock-up construction as model, the paper focuses on the dynamic behaviour of such a system tested on a shaking table. Detailed descriptions of the model, its mechanical features, the seismic test performed and the damage pattern obtained are first presented. Then, the dynamic identification of the structure during damage occurrence is performed through the decomposition of the power spectral density matrix. Damage evolution and localization are also analyzed by an index based on the complex eigenvectors estimated from the matrix. Finally, comparisons between experimental and analytical results are addressed