Detection of Leakages on Composite High Performance Structure Parts
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With thermography it is possible to record temperatures on parts not only selectively but also on a global basis. In the autoclave laboratory unit of the German Aerospace Center DLR the implementation of an integrated thermographic system enables a higher information density. The data collected can be used to improve the process monitoring as well as for detecting leakages on composite parts.
Resource-conserving processes are a central research field of the 21st century against the background of climate and environmental protection. Energy and cost-effectiveness analyzes show the need for sustainable and quality-assured processes for complex and voluminous structural components for the aerospace industry made of composite materials. The goal of the German Aerospace Center in Stade is to improve the manufacturing processes of these components before and during curing in the autoclave. For this purpose, the autoclave of the research center was developed according to a new concept, which is characterized by online process quality assurance, through sensor technology. A special focus is on the recording of the temperature, since this is a decisive parameter for the curing process. The temperature sensors currently used in the industry are firmly anchored in the tools, with the disadvantage that they can only absorb local temperatures at the respective positions. The aim of the project OnQA — Online Quality Assurance in Autoclave Processing is to eliminate this problem by using a thermographic system within the autoclave and to enable an extended, global temperature measurement during curing in the pressure chamber   .
Thermography inside Autoclave
The ambient conditions in the autoclave provide the boundary conditions for the implementation of an integrated infrared camera system. The final system must withstand temperatures of up to 250 °C and pressures up to 10 bar over a process length of up to 10 h. Due to the parameters in the autoclave, it is necessary to take certain safety precautions to achieve an unrestricted function of the thermographic camera. Conventional camera systems are not designed to withstand ambient conditions in the autoclave, so it is necessary to integrate the entire system into a water-cooled pressure vessel. With this configuration and active cooling it is possible to use the camera for several hours during the curing process in the autoclave.
The autoclave process plays an important role in the production of fiber composite materials. If a leakage occurs during the process in the vacuum bagging of the component, a severe impact of the component quality or even scrapping of the component is to be expected. Such damage entails high costs and a delay in the following working sequences.
In order to prevent an impact on the component, various measures are taken in the production in order to detect and eliminate leakage before the autoclave cycle. The most important methods are the measurement of pressure loss and the scanning of the structure with an acoustic leak detector. However, these measures do not always lead to success, are very imprecise and involve a great deal of time. A further challenge is the larger and more complex geometry of the components, as a result of which the demand for new detection methods is steadily increasing.
With the thermographic system inside the autoclave, which is available at the DLR Stade, it is also possible to use the technology during the curing process. This allows the process to be monitored in real-time, allowing for possible intervention in the process and the elimination of any leaks. In the co-operation project “Leak detection in the autoclave” between Airbus Operations and the DLR, a series of experiments in the research autoclave was carried out from 2014 to 2015. The first series of experiments was used to define the limits of the visibility of leakages in relation to temperature, pressure and heating rate. Furthermore the influences of disturbances such as reflections, camera housing and autoclave atmosphere have been investigated. For this purpose, tests were carried out on small, flat vacuum bagging which were equipped with different leak types and sizes. With this experimental set-up, various autoclave cycles were processed in which the parameters such as pressure and temperature were first tested individually and afterwards combined. The results show that all leakages are, as expected, initially visible as cold spots. If there is a temperature rise in the air temperature in the autoclave by pressure build-up or heat-up, the leaks are shown as hot spots. During the holding phases of the pressure or the temperature, the leaks are no longer recognizable as hot spots for a short moment before they appear again as cold spots after the temperature has been adjusted. The influence of the disturbances in the autoclave is minimal and has no impact on leakage detection. In the second test series, more complex vacuum bagging with foil pleats and stiffening elements were examined and processed during a curing cycle of 10 bar and 180 °C for four hours. This test showed that leakages are visible, as long as they are not covered by foil wrinkles. The same applies to leaks in the area of the stiffening elements. The thermographic camera can only detect the temperature at the top of the film, which overlays the leakage below. As the stiffening elements and the wrinkles are directly exposed to the hot air flow of the autoclave, they heat up faster than the rest of the component, thus superimposing the hot spot of the leak.
The control of an actively heatable tool requires knowledge of the temperature distribution and the degree of curing.
Selected Fields of Application
In the conventional autoclave process, the component and the tool are heated indirectly via the heating of the air. High thermal mass of the tool, poor heat transfer from the autoclave air to the component, as well as disturbances of the autoclave flow through the tool lead to uneven heating of the component. The result is an inhomogeneous progress of the curing degree, which leads to residual stresses and distortion of the component. Long waiting times are often necessary in order to ensure that even at the coldest point of the component the temperature required for complete curing has been reached.
In the technology marketing project “Active flow front manipulation” as well as in the project ProTec NSR (production technologies for the New Short Range), concepts are investigated, such as the use of the data from the thermographic system in the autoclave for the flow front detection in order to control the infusion process.
It has been found that the output data of the thermographic sytem in the autoclave can be used to control the curing process of infusion components. In addition to the complete impregnation of the fibers by the resin, which can be detected by means of thermography as a progressive flow front, the main consideration here is the observation of exothermic reactions of the resin. They represent additional, undesirable heat sources, which can lead to an inhomogeneous temperature distribution. An analysis of the temperature measurement data and the return of this information into the autoclave control allow a targeted adjustment of the heating and holding phase and thus an online quality monitoring of the infusion part .
The use of a thermographic camera can significantly increase the information density compared to the state of the art.
Summary and Outlook
The investigations of the German Aerospace Center DLR shows that the use of a thermographic camera within the autoclave can significantly increase the information density compared to the prior state of the art. As a result, a continuous surface quality monitoring of the component over the entire autoclave process can be achieved.
In the field of leakage detection, it was shown that it is also possible to detect leakages on vacuum bagging inside the autoclave with the integrated thermographic system. The limits of visibility were shown at high temperatures and under foil folds. The Project ADeLe (Advanced Detection of Leaks) has been set up together with Airbus, in order to look for new technologies to detect leakages under these critical circumstances.
We would like to thank the project sponsors and partners who made the realization of the thermographic system as part of the project OnQA possible. We thank the project partner and the employees of Airbus Operations GmbH Stade for the support of the project “leakage detection in the autoclave”. We would also like to thank all of our former and current student staff who contributed to the project so far: Christian Mendig, Serkan Koltuk, Ali Emre Özkan, Farbod Firoozi, Bahattin Celik, Sergio Cala, Lukas Gross, Matthias Janzen, Philip Zapp and Sebastian Reichert.
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