Choose an application

• Reference Manager
• EndNote
• RefWorks (Direct export to RefWorks)

In this work, non-invasive measurement methods are used to solve problems in daily aircraft maintenance. The measurement methods used are based on the Acoustic Emission Technique, Infrared Thermography and Laser Doppler Vibrometry. The procedures and analysis methods are not entirely new, but they can, however, be considered new in the context of the described aircraft maintenance work. Therefore, it is possible to speak of innovative non-invasive measurement methods in aircraft maintenance. Two explicit questions that arise directly from aircraft maintenance in daily operations are addressed. A well-known problem in civil aviation is water accumulation in fuel tanks, which aircraft operators face daily. This water accumulation and the resulting ice that forms during flight can lead to costly and sometimes very dangerous situations. Therefore, efforts are underway to develop a reliable detection system to determine the remaining amount of accumulated ice after flight and during recovery operations. By using such a technology, it would be possible to increase the safety and efficiency of aircraft operations in this highly competitive market. This PhD manuscript discusses the use of the Acoustic Emission Technique to reliably and non-invasively monitor the melting of ice in fuel tanks. This technique is based in principle on the fact that a phase transition is often accompanied by stress relaxation, which can be used to monitor the process. Therefore, the melting of ice can essentially be monitored with AE without directly accessing the ice. The findings presented in this work can potentially lead to new technologies for ice detection, especially in remote areas that are not easily accessible with other techniques. The second application of novel inspection methods based on non-invasive measurement methods concerns the inspection of fasteners in aluminum joints. In the aerospace industry, such inspections are time-consuming and costly, but mandatory. Until today, manual methods use mainly the naked eye and do not allow the tracking of damaging behavior over time or objective comparison between different inspections. A digital inspection procedure addresses both shortcomings while leading to a significant reduction in inspection time. The aim is to compare three inspection methods based on measurements on a serviceable aircraft component. An essential part of this is the development of a new digital and automated inspection method based on in-plane heatwave thermography, which is based on the analysis of thermal disturbances caused by irregularities in the plate-like structure. Simultaneously, measurements are made using ultrasonic lock-in thermography and a scanning laser Doppler vibrometer. Benchmarking of all three methods is performed on an operational aircraft fuselage panel. The data presented confirm the feasibility of detecting damage and qualifying countersunk rivets and screws in aluminum fuselage panels using the methods discussed.

Choose an application

• Reference Manager
• EndNote
• RefWorks (Direct export to RefWorks)

During the history of the space sector, a tremendous amount of money has been involved. The best known and biggest agencies such as NASA and ESA invest huge sums of money to reach space with their missions. In the past, this was the only way, but nowadays smaller, lower budget companies with a single business man at the top, think of Elon Musk, are emerging. Their focus is to find a cheaper, and even more importantly, a more sustainable way for space travel. This changed the industry big time and re-usability became a must. A second evolution in the space sector is the increase of mission duration. Astronauts are preparing for long duration missions to the moon and Mars, where resupplying becomes increasingly difficult and a process of a long time. Therefore, it becomes more and more important to reduce the need of redundant parts and to increase the lifetime of space structures. This can be, although partially, achieved by researching and tackling issues regarding contamination. The term contamination includes all unwanted matter affecting or degrading the performance and/or lifetime of the spacecraft. It can result from manufacture on Earth, through deposition on surfaces or due to a process called outgassing, where gas is released that was previously contained in materials. Another source is the human crew, who breathe and sweat, but also for whom a habitable environment with a sufficient humidity level has to be provided. These contaminants can induce a whole range of issues, from the growth of dangerous microorganisms to the degradation of structural materials and subsystems. They might actually endanger the whole mission. A lot of preventive measures are already in place to avoid contamination as much as possible, e.g. strict clean room environments on Earth, criteria for material selection, active air filtering onboard etc. However, it seems that, to some extend, contamination might be unavoidable. Therefore, in addition to an elaborated description of the situation in nowadays space operations, the proposition in this thesis is to construct a tool kit with ready to go sensors that astronauts can implement themselves, named the “space sensor kit”. This way they can perform research on the different types of contamination, but they can also implement them in areas they think as crucial. Contaminant sensitive areas in spacecraft can change throughout its lifetime and crew members will have the best knowledge of the structure and will be most capable of making well considered choices. The space sensor kit would include three versatile sensors based on different physical phenomena, with their own specific advantages, that can be implemented during operations in space.

Choose an application

• Reference Manager
• EndNote
• RefWorks (Direct export to RefWorks)

The aim of this investigation is to determine whether the acoustic response of a melting ice droplet can be reliably monitored in a reproducible fashion for different icing scenarios. The end-goal is to optimize an acoustic emission (AE)-based ice sensing technology that can detect ice accumulation inside an aircraft fuel tank or on the surface of an aircraft wing or a wind turbine blade. This was achieved by acoustically monitoring the melting of ice droplets adhering to an aluminium substrate and evaluating the influence of a variety of parameters. When no influencing factors were implemented, a strong variability in the acoustic response was observed. This was most likely the result of frost acoustic signals. In addition, by testing the influence of droplet size and ice porosity, it was shown that larger droplets would not necessarily lead to more AE because they had more air inclusions that could scatter sound waves. Similarly, porous ice had a much less intensive acoustic response than non-porous ice. In addition, impurities arising from kerosene will decrease the acoustic intensity. This could be due to the presence of supercooled liquid domains within the ice droplet that decrease the amount of potential AE sources. Finally, purposely accelerating freezing by playing on the surface temperature will not affect the resulting acoustic behaviour of a melting droplet. From this study, a first set of boundary conditions to a repeatable and reliable detectability was set but further spectral, thermodynamical and microscopical analysis are required for a more in-depth understanding of those boundary conditions. Repeating these experiments with much larger sample sizes for more representative results is of high importance.

Choose an application

• Reference Manager
• EndNote
• RefWorks (Direct export to RefWorks)

Underwater sea drones are used for both mapping the seabed and in the search of foreign objects (ships or plane wrecks) on the seabed. Current drone sonars receive unwanted surface reflected sound waves, interfering with acoustic waves that contain useful information. With the current sonars, the unwanted waves are programmed out, although resulting in information loss. The accuracy of the sonars can thus be improved by placing an acoustic damping material on top of the sonar, resulting in a cleaner input. These sonars function at low frequencies in order to search the sub-seabed, while most known underwater damping materials are only capable dampers in higher frequency ranges. Therefore, the scope of this thesis is to design a material that functions in the low frequency range of 0.5 to 10 kHz, damping up to 90 % of underwater acoustic waves. Based on the literature, a polyurethane (PU) matrix mixed with 1 vol% carbon fibre (CF) inclusions with a total thickness of 2-3 cm was selected. This composite was characterized via modelling, showing that adding CF to a pure PU matrix resulted in enhanced sound attenuation. A decrease in PU porosity from 5 % to 1 % air porosity was modelled to increase acoustic performance under high hydrostatic pressure, as attenuation increased from 6 to 14 dB at 300 m (5 kHz). In addition to modelling, experimental characterisation was conducted using two water based test set-ups. Adding CF via shear mixing, to a pure PU matrix, in a sample of 3 cm thickness increased the attenuation by 8.5 dB at 1 kHz. Attenuation was optimal when the CFs were dispersed via shear mixing, resulting in CF clusters, potentially hindering the acoustic wave propagation, and when the PU components were degassed. The achieved dispersion of the CF was characterized, both micro- and macroscopically, with the macroscopic dispersion of a sonicated sample showing that an area of 97.5 % contained CF, while in the shear mixed sample the dispersion was less homogeneous at 83 %. A CF of type 'IM' with a higher Young's modulus and aspect ratio was found to result in an attenuation increase from 4.5 dB to 8.5 dB (1 kHz). To conclude, a degassed PU matrix with 1 vol% CF inclusions of type 'IM' randomly dispersed using only shear mixing was found to significantly increase acoustic low frequency attenuation, which can be used to successfully improve the accuracy of future underwater sonar drones.

Choose an application

• Reference Manager
• EndNote
• RefWorks (Direct export to RefWorks)

In a world where efficiency and therefore weight reduction are becoming more important, composite materials can offer a solution. Fibre reinforced composites are materials that consist of at least two different materials, one fibre phase and a matrix phase. Besides studying the properties of these individual materials it is important to study how those materials interact with each other. This thesis starts with determining the compatibility of different fibre matrix combinations at room temperature. A frequently used method to study the compatibility is based on the surface energy components. These are determined by contact angle measurements of fibres and polymer films with different test liquids at room temperature. One important parameter to judge the compatibility is the work of adhesion. At room temperature this one is calculated based on the acid-base theory. The second part of this thesis tried to develop a new method to determine the work of adhesion of a system. Instead of calculating the work of adhesion at room temperature this method aimed to determine it at high temperature. This method was based on determining the contact angle between one fibre and a droplet. It was easier to accurately measure the inflection angle of the droplet then measure the contact angle directly. Therefore this method measured the inflection angle at high temperature. This method delivered some unexpected results. Even in an inert environment the droplet volume of polypropylene droplet was decreasing. This influenced the contact angle measurements. When the volume decreased the contact angle measured was the receding contact angle and not the advancing since the embedded length was decreasing. To finalise this thesis tried to measure the interfacial shear strength(IFSS) using a microdroplet pull out test equipped with acoustic emission. Even though the setup was similar to previous research using this technique this method didn’t have useable results. The IFSS was calculate using the shear-lag model as well as the ‘alternative’ Zhandarov model. The results from the Zhandarov model suggest that the interphase already had some failure before testing since they were not constant for larger embedded lengths.