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Organisationseinheit der BAM
Eingeladener Vortrag
- nein (6)
Fiber optic strain sensors are increasingly used and sensor Systems are provided with specifications. Even if the performance is well specified, the strain characteristics of the sensor, strain transfer factor, mechanical stability under thermal influences, the performance of applied strain sensors can seriously differ from virgin sensor’s the performance. Therefore the contribution considers validation issues to come to reliable strain measurements and how to validate strain measurements of applied sensors. A new validation facility will be presented.
Surface-applied fibre optic strain sensors were investigated using a unique validation facility equipped with application-independent optical reference systems. First, different adhesives for the sensor's application were analysed regarding their material properties. Measurements resulting from conventional measurement techniques, such as thermo-mechanical analysis and dynamic mechanical analysis, were compared with measurements resulting from digital image correlation, which has the advantage of being a non-contact technique. Second, fibre optic strain sensors were applied to test specimens with the selected adhesives. Their strain-transfer mechanism was analysed in comparison with conventional strain gauges. Relative movements between the applied sensor and the test specimen were visualized easily using optical reference methods, digital image correlation and electronic speckle pattern interferometry. Conventional strain gauges showed limited opportunities for an objective strain-transfer analysis because they are also affected by application conditions.
This work presents a further development of the methods of simultaneous determination of temperature and axial strain using a single fibre Bragg grating (FBG). The reflected spectrum of composite-embedded FBG sensors has been analyzed in order to separate temperature and load effects. We found out that during the curing process of the laminates, a superstructure has been introduced on the FBG. Analyzing this effect, a temperature and strain separation was implemented by simply calculating and comparing the integrated intensity of the spectral response signal. A mathematical four-parametric model has been developed to calculate the reflected spectrum of a superstructured fibre Bragg grating. The mathematically achieved data has been evaluated with experimentally determined data from fibre Bragg gratings embedded in glass-fibre reinforced composite materials.
Fiber-optic strain sensors are increasingly used in very different technical fields. Sensors are provided with specifications defined by the manufacturer or ascertained by the interested user. In some cases, sensor specification is not sufficiently validated and must therefore additionally be validated in a laboratory, primarily to ensure reliable measurement information over the intended period of operation. Even if the performance of delivered sensor is well specified, the sensor's strain characteristics and the performance of an applied sensor can significantly differ from the virgin sensor's performance. In this case, applied sensors do not provide full reliability and lead sometimes to uncertain measurement results. This contribution will therefore focus on the role of validation in avoiding a decrease or even deterioration of the sensor function of applied sensors. Experimental validation - not only modelling - is very important, however, before experimental investigations are planned knowledge about key issues and problems that influence the measurement results must be available. Few aspects to be considered and investigated will be discussed. Selected experimental facilities to reveal weaknesses in the sensor function will be described; an outlook to open questions is given.
Fibre-optic sensors need to be more established in the sensor market. Their advantages have unquestionably been verified by numerous demonstrations. However, there are some open questions leading now and then to restraints in the user's community. The paper dicusses examples where fibre-optic sensors provide outstanding knowledge about the structure's behaviour, but application is often challenging. Requirements are listed and open questions to be solved are discussed. Short outlook to standards useful for better design, characterization and application is given. Standards are the basis for establiching SHM systems, especially for safety-critical structural diagnostics.
Reliable measurement results with fibre optic sensors require profound knowledge of physical mode of function, of materials used and appropriate application. There is often serious discrepancy between use in laboratory environment and on site. The paper identifies important aspects to be considered and shows ways to transfer innovations onto practical use.
Reliability analysis of fibre optic sensors for monitoring of industrial facilities; keynote lecture
(2013)
Fiber optic sensors have gained increasing importance in recent years and are well established in many areas of industrial applications. In this paper, we introduce a concept of a self-diagnostic fiber optic sensor. The presented sensor is to resolve the problems of embedded fiber optic sensors in complex structures and to enable the validation under operational conditions. For this purpose, different magnetostrictive coated fiber optic sensors were developed and various experiments were performed to verify their mode of operation and to determine the respective reproducibility. The measuring principle is illustrated by obtained experimental results, which showed a change in wavelength from 1 pm at a magnetic field strength change of 0.25 mT. In addition, the temperature characteristics of the implemented magnetostrictive sensor were analyzed and an experimental factor of 1.5 compared to a reference fiber optic sensor was determined.