77 research outputs found
Development of a retrofit layer with an embedded array of piezoelectric sensors for transient pressure measurement in maritime applications
No full textMeasurement of transient pressure distribution on maritime structures is important for the assessment of the hydrodynamic loads applied. The commonly used pressure sensors are mostly bulky, need to be bolted to the structure, and/or only provide point-wise measurements. In this paper, an elastic matrix layer with a network of embedded piezoelectric sensors is proposed to address these issues. For experimental validation, a 400 × 400 × 5 mm epoxy layer is fabricated embedding 25 piezoelectric sensors on a square grid in accordance with Gauss-Lobatto-Legendre points. A finite element based inverse procedure is developed to reconstruct the pressure field from the electric potentials measured by the piezoelectric transducers. Feasibility of the concept is evaluated by measuring and reconstructing the pressure field generated by a travelling wave in a water tank. Sensitivity of the layer is also investigated through the experiments. The results indicate that the retrofit layer is capable of pressure field reconstruction, and that the presence of disturbances on the sensing surface does not affect the measurements in a notable way, while non-ideal conditions of the mounting can have a significant impact on the accuracy of the measurements. The results highlight the potential of the concept in pressure distribution measurements.</p
Wave Propagation in Thin-walled Composite Structures: Application to Structural Health Monitoring
No full textIn order for the increased use of fiber-reinforced composite structures to be financially feasible, employment of reliable and economical systems to detect damage and evaluate structural integrity is necessary. This task has traditionally been performed using off-line non-destructive testing (NDT) techniques. Safety enhancement programs and cost minimization schemes for repairs, however, have substantially increased the demand for real time integrity monitoring systems, i.e. structural health monitoring (SHM) systems, in the past few years. The real time feature imposes an additional constraint on SHM systems to be fast and computationally efficient. Among the existing approaches fulfilling these requirements, guided ultrasonic wave (GUW)-based methods are of particular interest, since they provide the possibility of finding small size defects, both at the surface and internal, and covering relatively large areas with reasonable hardware costs. Next to theses appealing features, there are certain complexities in utilizing GUWs for SHM of fiber-reinforced composites, that mainly arise from the multi-layer, anisotropic, and non-homogeneous nature of the material. In addition, the multi-mode character of GUWs further increases the complexity of the SHM problem in these materials. It is believed that computationally efficient methods for simulation of GUWs in composite structures can substantially contribute to the field of SHM. Such numerical tools do not only improve the understanding of the propagation of ultrasonic waves and their interaction with different damage types and boundary conditions, but can also make model-based damage identification techniques feasible in the context of on-line SHM. In this dissertation an improved framework for simulation of GUWs in composite structures is developed. The improvements are mainly brought about through the use of (i) physical constraints that reduces the dimensionality of the problem, (ii) improved approximation bases for spatial and temporal discretization of the governing equations, and (iii) efficient mathematical tools to enable the possibility of parallel computation. The formulated approach is a wavelet-based spectral finite element method (WSFEM), which offers the possibility of complete decoupling of the spatial and temporal discretization schemes, and results in parallel implementation of the temporal solution. Although the concept of the WSFEM was introduced a few years prior to this research, to the author's best knowledge, no general framework was proposed for dealing with 2D and 3D problems with inhomogeneity, anisotropy, geometrical complexity, and arbitrary boundary conditions. These issues are addressed in this dissertation in multiple steps as described below. 1- Improvement of the temporal discretization using compactly-supported wavelets, by computing the operators of the wavelet-Galerkin method over finite intervals, and demonstrating about 50% reduction in the number of sampling points, with the same accuracy, compared to the conventional wavelet-based approach. 2- Extension of the existing formulation of the 1D WSFEM based on an in-plane displacement field to 1D waveguides based on a 3D displacement field. In the 1D finite element formulation, spectral shape functions are employed which satisfy the governing equations, in which shear deformation and thickness contraction effects are also incorporated. The minimum number of elements for modeling 1D waveguides is used in this approach. 3- Formulation of a novel 2D WSFEM in which frequency-dependent basis functions are suggested for spatial discretization. Contrary to the conventional WSFEM, the presented scheme discretizes the spatial domain with 2D elements and does not require extra treatments for non-periodic boundary conditions. Superior properties of the formulation are shown in comparison with some time domain FEM schemes. 4- Generalization of the WSFEM and extension to 3D geometries. It is demonstrated that the standard spatial discretization schemes can be combined with the wavelet-Galerkin approach, to fully parallelize the temporal solution. A higher-order pseudo-spectral finite element method, i.e. spectral element method (SEM), is further adopted to attain spectral convergence properties over space and time. The developed WSFEM is subsequently employed in the passive time reversal (TR) method, which is a model-based approach for detection of load and damage location, and operates based on the time invariance of linear elastodynamic equations. It is shown that using the passive TR scheme, the problem of load and damage detection, which is essentially an inverse problem, can be solved in the form of a forward problem, thereby alleviating uniqueness and stability issues. A number of case studies and examples, numerical and experimental, are presented throughout this dissertation to better demonstrate the applicability of the proposed framework.Aerospace Structures and Computational MechanicsAerospace Engineerin
Condition Assessment of Low-Speed Slew Bearings in Offshore Applications Using Acoustic Emission Monitoring
No full textThis study presents an approach for the detection of evolving degradation in large-scale low-speed roller bearings by clustering of Acoustic Emission (AE) events, and its application to experimental degradation data. To acquire the latter, a purpose-built linear bearing, representative of a segment of a turret bearing, has been instrumented with multiple piezoelectric AE transducers in the frequency range between 40–580 kHz. Clustering based on cross-correlation has identified a number of significant clusters that are linked to the observed damage. The results suggest that condition monitoring based on AE waveform similarity clustering is suitable for detection and identification of degradation in a large-scale roller bearing.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Ship Hydromechanics and Structure
Condition Assessment of Highly-loaded Low-speed Bearings Using Acoustic Emission Monitoring: A Feasibility Study
No full textIn the present study, feasibility of implementing an acoustic emission (AE) system for condition monitoring of highly-loaded and low-speed roller bearings has been quantitatively investigated. To evaluate the transmission of damage-induced AE signals inside the bearing and their detectability on the accessible surfaces, stress wave propagation through the complex geometry and interfaces of a representative offshore bearing has been studied experimentally. The results suggest that in the investigated frequency ranges, the AE signals that pass through the bearing rollers and their interface with the inner and outer raceways can potentially remain of sufficient strength to be detected for condition assessment of these bearings.Accepted Author ManuscriptShip and Offshore StructuresShip Hydromechanics and Structure
Numerical and Experimental Study of Acoustic Emission Source Signal Reconstruction in Fibre-Reinforced Composite Panels
No full textThe recording and processing of acoustic emissions can be used to identify and localise damage mechanisms occurring in engineering structures. In plate-like structures, acoustic emissions propagate through the structure as guided waves. With a measurement location away from the source location, dispersion effects in the guided wave distort the acoustic emission signal. The distortion of the original signal hampers identification of damage mechanisms. This research describes and assesses a method to reconstruct the original acoustic emission signal using dispersion compensation. Simulations and experiments are performed involving thick glass-fibre reinforced plastic laminates. The signal reconstruction on the simulated data gives a reasonable representation of the simulated signal at the location of interest. In the experimental case, similarity slightly degrades. Deviation in arrival time between original measurement and reconstruction is attributed to a possible discrepancy in material properties in reality versus the properties used in the reconstruction.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Aerospace Structures & Computational MechanicsShip Hydromechanics and Structure
Before Taliban. Genealogies of the Afghan Jihad. University of California Press, 2002, 354 p.
No full textAccording to the author, this is a book about the Marxists and their enemies. His objective is to contextualize the history of the present in the history of the past, in order to evaluate what happened in Afghanistan. To attain this objective the author tries to frame his own exercise in historical understanding in genealogical terms. He believes that friendship, authority, love, and even enmity are volatile until they have been transmitted into genealogical form, which traditionally is the a..
Serum Level of Metalloproteinase in Patients Infected with Helicobacter Pylori in Ardabil
No full textBackground & Objectives: Matrix Metalloproteinases (MMPs), a member of photolytic enzyme family, degrade the extra cellular matrix. MMPs have very important roles in physiological and pathological processes. It has been reported that MMPs concentration increase in malignancies such as stomach, breast, colon, lung, head and neck cancers. Infection with Helicobacter pylori is risk factor for gastric cancer and may increase the serum level of MMP-9. The aim of this study was to measure the concentration of MMP-9 in sera of patients infected with H. pylori.
Methods: In a descriptive-experimental study, apparent healthy individuals who were refereed for stool and blood tests were randomly selected and their stools and sera samples were collected. A questionnaire containing age, sex, smoking and special diseases in family and type of their diseases was filled for every volunteer. The sera collected immediately after blood sampling and stored in -70oC until used. The concentration of MMP-9 was assessed using ELISA. Stool samples were used for detection of H. pylori antigen.
Results: H. pylori positive, negative and equivocal volunteers were 52.38%, 29.76% and 17.85%, respectively. Differences between MMP-9 concentration and H. pylori negative and positive groups were not significant (p =0.25). A significant increase in MMP-9 concentration was found in sera from drivers in compare with housewives.
Conclusion: Although there was an increase in serum concentration of MMP-9 in H. pylori infected person, differences between H. pylori negative and positive groups were not significant. The raised concentration of MMP-9 in drivers may arise from difference in their health condition in compare with the housewives. This study suggests that MMP-9 level in serum may increase before initiation of gastric cancer in H. pylori infected individuals
Feasibility evaluation for non-contact ultrasonic inspection of mooring chains through marine growth
No full textMooring chains are subject to dynamic loads and corrosion. A challenging aspect in assessing the integrity of mooring chains is the presence of marine growth. Marine growth removal is expensive, environmentally damaging, and includes the risk of restarting corrosion process. The focus of this research is on the feasibility of mooring chains assessment in the presence of marine growth using active and passive ultrasound techniques. The approach includes experimental and numerical investigations for fatigue and corrosion damage. In the experimental part, specimens with and without marine growth have been designed, manufactured, and subjected to a series of experiments to determine the transmission characteristics of ultrasound waves through marine growth and its interaction with damage. Fatigue crack signals were simulated on the specimens using Hus-Nelson source. For the investigated compositions, the signal amplitude drop due to the presence of marine growth. Accelerated corrosion experiments were conducted to reproduce corrosion-induced acoustic emissions. Corrosion-induced signals were successfully detected and localized. Active ultrasound experiments were conducted using guided waves. These waves were successfully excited and measured in the test sample with naturally-cultured marine growth. The investigation suggests that guided waves can have a high potential for identification of fatigue cracks in the mooring chains. These experiments were complemented with numerical simulation of the setup. For the sake of computational efficiency and accuracy, a higher-order spectral finite element method was used. Simulations were further extended to investigate and confirm the possibility of detection of surfacebreaking cracks in the test sample.Marine Technolog
Feasibility evaluation of a non-destructive estimation of material properties of FRC structures using ultrasonic guided waves
No full textVariations in the production process of fiber-reinforced composite (FRC)materials often influence the material properties of the end product more substantially than their metallic counterparts. It can be valuable for the structural performance and reliability to have a better estimation of the properties of the FRC after production. Detecting these material properties is often performed by intrusive or destructive testing, which are not desirable for in-situ applications. Samples are commonly taken from the material for mechanical testing and material characterization in the laboratory. Feasibility of a non-destructive in-situ assessment method based on ultrasonic guided waves for estimation of the material properties of FRCs, e.g. stiffness in different directions, is investigated in this research. A portable and easy-to- apply measurement system is proposed that can also be less sensitive to the environmental conditions. Point-contact transducers containing a piezoelectric material are utilized for measuring guided waves that propagate through the material. The dispersion characteristics, i.e. phase speed and group speed, of these guided waves are dependent on the layup of the fibers, thickness, and ply properties. Information about the group speed from multiple directions are extracted and compared with the group speeds obtained using a semi-analytical method. A genetic algorithm (GA) is implemented to minimize the difference between the measured and modeled group speeds and obtain the ply properties. The methodology was applied to five glass FRC laminates with different layups. In the numerically simulated case, the error in the estimated material properties turned out to be less than 12%. The coefficient of variation in the experimental results of stiffness values was also less than 20%. The results suggest that the proposed combination of point contact transducers, analysis of group speeds, and GA optimization procedure can potentially form a viable approach for in-situ assessment of material properties of FRCs.Marine Technology | Ship Hydromechanic
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