1,721,053 research outputs found
Groves R.M., Couper M.P. — Nonresponse in Household Interview Survey
No full textRiandey Benoît. Groves R.M., Couper M.P. — Nonresponse in Household Interview Survey. In: Population, 55ᵉ année, n°2, 2000. p. 408
Acoustic emission monitoring of fatigue damage progression in composite ship structures: A constitutively-informed approach to acoustic emission source classification
No full textAE-based structural integrity monitoring of composite structures potentially offers the means to gain knowledge on in-service damage accumulation, as well as mitigation of some of the risks associated with in-service damage. However, validation of any identified classes of AE events with regard to their underlying source mechanics is generally limited to data-centred metrics in existing literature. The approach in this project applies a mechanics-informed perspective on AE event classification by using the fatigue cycle phase as hit labels and validating this labelling with the stiffness degradation measured using DIC. A promising correlation is found between this labelling and the coupon stiffness degradation.Aerospace EngineeringAerospace Structures and Material
Is the future of aircraft maintenance with automated NDT or SHM?
No full textStructural Integrity & Composite
The Influence of the Indoor Air Quality of the Microclimate Box on the Material Degradation of Historic Objects
No full textThe paper gives an assessment of the Indoor Air Quality (IAQ) within a microclimate box and its influence on the material degradation. IAQ is concerned with maintaining acceptable temperature, humidity and oxygen levels and low pollutant levels in the rooms, cabinets or other enclosed spaces, for humans to live and work, while meeting legal guidelines. In the microclimate box it is concerned with maintaining an acceptable environment for the conservation of historic objects.As a reference we should first consider the composition of outside air. This is contains approximately 78% nitrogen, 21% oxygen and 1% argon, and varying amounts of water vapour depending on local climate. Further it contains smaller quantities carbon dioxide, sulphur dioxide, nitrous oxides and small particles depending on local pollution levels. Temperature varies by season, time of day and location. Historically records have been kept of some of these parameters of more than 100 years as part of weather monitoring and prediction. The indoor climate in a room in a house, office or museum is modified by the building construction, heating and cooling, and by occupants and objects within the building. An unheated building is buffered from the outdoor climate by the insulation effect of the building itself, but may contain localised microclimates due to airflow (draughts) and solar radiation through the windows. Heating and cooling may be passive, by natural convection and conduction, or active airflow of heated or cooled air. Occupants and objects within the building interact with the Indoor Air Quality, for example breathing in oxygen and expelling carbon dioxide, or outgassing of volatile materials, such as paints.An IAQ engineer has a range of instruments to monitor the indoor climate. Temperature and relative humidity (RH) can be measured using electronic sensors and if necessary logged at regular intervals over a longer period. Airflows from a heating and ventilation air conditioning (HVAC) system can be measured with an anemometer. Several anemometer types exist and they can measure air velocity and if the aperture size is known, air volume per hour. Air volume per hour allows the calculation of room air changes per hour, which is related to the dilution of pollutants in the air by the HVAC system. Examples of pollutants which can be measured include carbon monoxide (CO), carbon dioxide (CO2) and particulates e.g. PM10s, bacteria, moulds and yeasts. Gaseous particulates, such as CO and CO2 follow an exponential dilution curve due to the air changes and particulates follow a greater that exponential dilution curve as settling of particles also plays a role.In a sealed microclimate, these air changes are not present, so the buffering effect is enhanced, but the active dilution of pollutants is not present. Published studies have focussed mostly on the buffering effect, while some recent studies, for example at the Metropolitan Museum of Art, measure the gaseous pollutants present. Further there is a lot of information in the literature about the influence of pollutants in outside air causing an enhanced rate of material degradation on historic buildings. This study will bring together these sources of information to assess how the air within microclimate boxes could cause an enhanced rate of chemical degradation of historic objects.Structural Integrity & Composite
Development of permanent composite overwrap repairs for steel pipelines, incorporating Structural Health Monitoring
No full textA good structural health monitoring system on a composite repair can improve the understanding of the life of such a repair. Currently, overwrap repairs are considered to be temporary in nature due to the unknown life expectancy. Using finite element modelling, a model was created of a pipe with a repaired defect and a composite overwrap. This model was then validated by testing pipe specimens that have the same defect and the same repair. In these models and test specimens an initial (100%) repaired damage is present which was increased to a 150% initial defect. A comparison between the 100% and 150% shows the ability of detecting the increase in both the model and the specimen for axial and hoop strain. Further damage is researched by implementing delaminations at the edges (ingress points for water/corrosion) and in between the edge and the initial repaired defect. The conducted tests revealed strain transfer problems on a (composite) surface containing voids/defects. The main issue however is that damage (and damage growth) is quantitatively detected in both the model and the test specimens. Other tests included the comparison of adhesives that are used to apply the optical sensors to the pipe. This in order to rule out any errors in measurements between the surface of the composite repair and the fibre Bragg grating itself. The result was that there is no difference between adhesives in the conducted fatigue test. Also some material characterization was attempted but these results are not reliable due to the voided nature of the used cured composite. The other aspects of a structural health monitoring system are of course the system used to detect strains. A close look at the available optical systems, such as Brillouin and Rayleigh scattering, shows that fibre Bragg gratings are the most suitable in this project. Deeper investigation into critical and less critical parameters shows that the design of such a sensor system is not trivial. Consideration to coatings, temperature, fatigue limits, optical losses, cost etc. has to be given as these can influence the final resulting sensor system. The harsh environment the sensors operate in has to be researched in more depth in following reports since they are critical to the performance.Design and production of composite structuresAerospace Structures and MaterialsAerospace Engineerin
Characterising delamination growth in composites under dynamic loading using infrared thermography: An experimental approach to use an infrared camera to monitor delamination growth in composite test samples
No full textWith an ever increasing demand for energy, the renewable energy sector is gaining a lot of importance. Among the various renewable energy sources, wind energy is becoming more attractive compared to its counterparts (solar, biomass, etc.). Some of the main advantages of wind energy are faster payback time and that power generation is possible both during the day and night. The wind energy industry is constantly aiming at larger size rotors for increased power generation and these larger rotor blades demand stronger and more durable materials. Currently, composite materials are extensively used for wind turbine blades. With a large heterogeneity in composite materials and complications in manufacturing processes, defects in composites are inevitable. A variety of defects and damages can be seen in wind turbine blades. Composites with such defects and damage undergo a significant loss in their mechanical properties. For the use of composites as a structural material, a good knowledge of possible defects and their behaviour should be understood. To do so, the behaviour of defects under different load conditions should be assessed. Many techniques such as ultrasound scans, X-ray radiography, etc. are available to detect the defects in composites, whereas studying the damage growth in a composite is still a challenging process. In this research, a method has been developed to use thermography for characterising the delamination under dynamic loading. To demonstrate this method, a test sample with double shear configuration (DSC) and an initial delamination consisting of a Polytetrafluoroethylene (PTFE) insert was developed. The test sample was tested under fatigue loading and an infrared (IR) camera was used to monitor its thermal response and the delamination growth during loading. The data from the thermal camera was processed in two steps, firstly, fast Fourier transform (FFT) was used to transform the raw data from time domain to frequency domain. In the second step, FFT thermographs were further processed using an image segmentation algorithm. Here, the thermal plots are segmented to separate the delaminated and un-delaminated areas. By computing the number of pixels in the delaminated region, the area of delamination was obtained at each cycle and was plotted against the cycles to failure. The strain energy was computed with the help of force and displacement data from the test machine. Such signals allowed computing of the fatigue propagation curves and an understanding of the fatigue behaviour of the test samples. The results from this research were promising as the delamination behaviour reported using this method was in good accordance with a reference visual inspection method. The quantifiable output from this method can be a good starting point to study delamination experimentally and computationally. In future, this technique could be extended to different test types that cannot be quantitatively analysed using the conventional testing methods. The research was also presented at the European Conference on Composite Materials ECCM17, Munich, Germany.Aerospace EngineeringAerospace Structures & MaterialsStructural Integrity & Composite
Optical characterisation of complex structures, from engineering composites to bio-materials; abstract
No full textAerospace Structures and MaterialsAerospace Engineerin
Advanced signal processing techniques for fibre-optic structural health monitoring
No full textFibre optic sensors can measure a range of physics and chemical parameters. Some of the more common fibre optic sensors are the fibre Bragg grating (FBG), the long period grating (LPG), the Fabry-Pérot Interferometer (FPI) and various distributed fibre optic sensors based on optical time-domain reflectometry (OTDR) and optical frequency domain reflectometry (OFDR). Each of these sensor types utilises different interrogator hardware and signal processing software. The goals of this research are to develop new algorithms for multi-parameter sensing and to improve the sensitivity and resolution of fibre optic sensing by developing new approaches. This is done by stepping back from current algorithms, and considering what additional information is expected to be present in and can be extracted from the signal. Recent publications have shown that advanced signal processing techniques can be used for bend sensing, for damage type classification and to improve the spatial resolution of the sensing. Structural health monitoring requires the measurement of different structural parameters to determine the health of a structure. A commonly used definition of structural health monitoring is “SHM is the integration of sensing and possibly also actuation devices to allow the loading and damaging conditions of a structure to be recorded, analysed, localized, and predicted in a way that non-destructive testing (NDT) becomes an integral part of the structure and a material”. From this definition four levels of structural heath monitoring are defined: (1) mechanical and environmental load monitoring, (2) identification and location of damage, (3) damage quantification, and (4) prognosis of residual life. The paper will explore how advanced signal processing techniques can drive the development of multi-parameter sensing with fibre optics, and can lead to the goal of integrated fibre optic sensing system for structural health monitoring applications.Structural Integrity & Composite
Perspectives on Structural Health Monitoring of Composite Civil Aircraft
No full textSafe and cost effective operation are the highest priorities for civil aircraft. Considering that many events that can occur during normal aircraft operation which cause a reduction in the residual strength of the structure, a rigid adherence to inspection and maintenance schedules and timely repair of damage is required. Since Structural Health Monitoring (SHM) has the capability to investigate critical areas of the aircraft structure, it is potentially applicable to a wide range of current civil aircraft, including general aviation, business jets and large passenger aircraft. Although SHM could be extended to the complete aircraft structure in the future, in the shorter term it is more practical to consider the most critical structural elements/assemblies, both for reasons of cost and the time, especially in certification, leading to more standardised procedures in future. On a technical level SHM should be addressed by identifying critical structural elements/assemblies such as: blade stiffened skin panels, sandwich panels, hat stiffened skin panels, multi-rib and multi-spar structures, welded , mechanically-fastened and co-cured joints. For each of these, expected failure modes are assessed and used to determine the expected damage types. Damage sizes which post a risk to the structural integrity of the aircraft are then matched with SHM technologies which have suitable damage detection capability. This paper proposes the use of metrics to quantify the effectiveness and efficiency of the SHM system according to the six most important elements of SHM: Damage event detection, Damage event localization, Damage type detection, Damage extent detection, Damage effect estimation, and Damage prognosis. SHM Technologies are a combination of non-destructive testing techniques, developed further for in-situ monitoring, and new technologies. The techniques considered to have the most potential for SHM of composite aircraft are Acoustic emission, Guided Lamb wave sensing and Fibre optic sensing. SHM comprises part of the Smart Materials and Structures Concept that will be the basis for future Smart and Efficient Aircraft with lightweight structures, on-board monitoring, health diagnosis and adaptive structures. In this concept, optimal sensor positioning, distributed communication networks and algorithms, miniaturisation and energy harvesting are also considered.Structural Integrity & Composite
3.12 Inspection and Monitoring of Composite Aircraft Structures
No full textThe new generation of civil aircraft, including the Airbus A380 and the Boeing 787, are composed of composite materials for their primary aircraft structures. This presents a challenge for airline maintenance, repair, and overhaul (MRO) operations in both the short- and long-term. In the short-term, new technologies are needed for maintenance, monitoring, and repair techniques to cope with accidental damage that aircraft suffer throughout their lifetime. The short-term challenge also extends to manufacturing facilities and the need for quality checks of composite components, together with substructures assembled using adhesive bonding, co-curing and thermoplastic welding. As these A380 and B787 aircraft age, they will be subject to normal wear and tear and will undergo C and D checks to maintain airworthiness. At this stage of their lifetime it will be important to perform efficient and cost-effective maintenance and repair techniques to reduce lifetime costs. This chapter first describes composites materials and structures, together with an overview of expected failure modes. It will then provide an overview of current and emerging nondestructive testing (NDT) and structural health monitoring (SHM) technologies and describe some of the challenges in inspection and monitoring of composite aircraft structures.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.Structural Integrity & Composite
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