Nadine Kusche

Validation and qualification of surface-applied fibre optic strain sensors using application-independent optical techniques

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 sensors 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.

How Reliably do Fiber Bragg Grating Patches Perform as Strain Sensors

In Germany, the first guideline for the use of fiber Bragg grating strain sensors, “Optical Strain Sensor based on Fiber Bragg Grating” (Berlin, Germany: Beuth-Verlag, 2010), has been developed by the GESA guideline group of VDI, “The Association of German Engineers” and published by Beuth-Verlag. This guideline provides the basic specifications of this sensor type and the sensor characteristics, which have to be known for a reliable sensor performance. In conformity to this guideline, experimental investigations on the strain transfer characteristics of fiber Bragg grating patches have been carried out. A comparison between patches and resistance strain gauges during tensile tests and combined temperature and tensile loading was carried out. The evaluated strain gauge factor and the temperature sensitivity of the strain gauge factor have been compared to the manufacturers data. The overall performance of the patches has been evaluated. The experimental investigations showed that there are considerable disagreements between the manufacturers specifications and the observed characteristics.

Fibre-optic strain sensors are making the leap from lab to industrial use—reliability and validation as a precondition for standards

Currently, fibre-optic sensors (FOSs) are commonly used if special requirements make the application of electrical sensors impossible, or economic benefit is promised. The scientific background of FOS technology is well developed; however, there are still some restrictions with respect to long-term reliable use. For widespread practical use, sensor products must be manufactured, characterized and validated according to standards. Guidelines on how to apply sensors and evaluate their operation on-site including special facilities to evaluate applied sensors are needed. This paper will focus on important aspects, such as when FOSs may be used under real practical conditions, and will present validation methodologies to evaluate the overall quality of the sensor systems function. It will also indicate the lack of knowledge and methods to be elaborated to promote the use of FOS.

Experimental qualification by extensive evaluation of fibre optic strain sensors

Fibre optic strain sensors used in practical applications have to provide reliable measurements. Therefore, the applied sensor and the sensor systems must be validated experimentally. This can be achieved with facilities which use physically independent measurement systems in order to avoid the influences caused by the application of a reference sensor. This paper describes the testing methods of the specially developed validation facility KALFOS for the qualification and evaluation of surface-applied strain sensors. For reliable sensor results, the performance of fibre optic strain patches with and without FBG under combined thermal and mechanical loading was investigated. Additionally, the strain gauge factor of the fibre optic strain patches with FBG was determined experimentally and compared to the specified strain gauge factor. These results will be the basis for the development of guidelines and standards concerning the application of the sensors.

Delamination-diagnosis-method for adhesively surface-applied FBG strain sensors

Delamination growth and debonding of surface-applied fiber Bragg grating (FBG) strain sensors lead to spectral deformation of the reflected spectrum. The FBG spectrum shape is sensitive to the partial debonding of the sensor and therefore this information can be used to describe delamination growth. The developed analytical method to determine degree and direction of debonding and the additional experimental investigations show a possible way of qualifying and evaluating the interface performance of adhesively applied sensors.

The role of experimental validation in achieving reliable measurement data with applied FBG strain sensors

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 sensors 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.

Field examples for optical fibre sensor condition diagnostics based on distributed fibre optic strain sensing

Fibre optic sensors for monitoring in safety-relevant structures have to be validated in order to proof their reliability under typical structural load conditions. The reliable use of optical fibre sensors depends strongly on an appropriate and qualitative application. Diagnostics of the physical condition of embedded and surface-applied fibre optic strain sensors are demonstrated on field examples. Distributed strain measurement based on Rayleigh backscattering is used to determine breakage of the fibre, interface adhesion problems and to identify application related strain transfer mechanisms.

Experimental qualification and validation of fibre optic strain sensors

Strain sensors used in practical applications must provide reliable measurement data. To achieve this, sensor systems must be validated by using experimental facilities that enable physically independent statements about the performance of the sensor components. The paper describes qualification and validation procedures using a special facility to qualify surface-applied strain sensors and to achieve reliable sensor results. Based on examples concerning fibre optic strain sensor patches with and without FBG sensors, the determination of the strain gauge factor also under combined thermal and mechanical loading will be presented. These results are the basis for development of guidelines and standards.

Calibration facility for quality certification of surface-attached fiber optic and electrical strain sensors

Strain measurement in structures with the purpose of long-term structural health monitoring must provide reliable information about the structures behavior over the whole period of use. The user must be sure that installed sensors are validated and work to the utmost satisfaction. For this purpose, sensor systems are tested using special facilities. Because it is not easy to characterize the strain transfer quality from the host structure into surface-applied strain sensors, a unique testing facility has been developed. Originally developed for fiber Bragg grating based sensors, the KALFOS facility (= calibration of fiber optic sensors) can also be used for electrical strain sensors. Calibration measurements are referenced by unbiased Digital Image Correlation (DIC) and Electronic Speckle Pattern Interferometer (ESPI) methods. The strain transfer behavior can experimentally be analyzed and investigated under combined thermal and mechanical loading conditions and allows revealing weaknesses in commonly used attachment methodologies. The deformation of all members (particularly the coating/substrate - adhesive combination) in the sensing area is physically independently gained and recorded. Results achieved allow precise description of the strain transfer function, validation of the longterm strain sensor characteristics, matching of specific measurement requirements with environmental conditions, and, moreover, the verification of standards for use of strain sensors.