Wolfgang R. Habel

Fiber Bragg grating sensors for monitoring of wind turbine blades

Fiber Bragg grating sensor arrays can be used to monitor the mechanical behavior of rotor blades of wind turbines. In order to investigate how stable and reliably work such sensors, different fiber Bragg gratings were embedded into textilereinforced composite. Long-term temperature and tensile (fatigue) tests have been carried out with composite structure specimens. This paper reports on selected results of the tests and discusses important reliability aspects with regard to the fiber design and the fiber material.

Influence of concrete and alkaline solutions on different surfaces of optical fibres for sensors

Primarily coated optical fibers for non-destructive monitoring of structures were embedded in cement mortar bodies and, in a separate test series, exposed to concrete-specific chemical attacks. Microstructure studies after four weeks revealed the relative resistance of acrylate-coated and flourine polymer-coated fibers. Polyimide-coated fibers showed serious changes in the coatings after the exposure.

Strain measurements in reinforced concrete walls during the hydration reaction by means of embedded fibre interferometers

In a reinforced concrete wall the deformation during concrete hardening was measured by means of embedded extrinsic fibre-Fabry-Perot-interferometers. The sensors were specifically modified in order to provide a self-calibration cycle and to ensure the functional efficiency under adverse conditions at the building site. The installation was done in the walls cage of reinforcement before its concreting. The measurement was carried out automatically over a period of 35 days. The measuring results are very satisfactory and give a resolution of 0.1 micrometers /m.

How to reliably measure composite-embedded fibre Bragg grating sensors influenced by transverse and point-wise deformations?

Embedded conventional fibre Bragg grating (FBG) sensors with acrylate coating have been chosen for monitoring novel textile-reinforced composite materials with a complicated wavy structure as a result of reinforcing textures. Problems of monitoring this type of material occur due to the harsh fabrication technology of the composite and its complicated structure. Embedded fibre sensors are exposed to the influence of transverse and point-wise deformations. To what extent will these effects influence the measurement signal of a sensor? Temperature cycles, tension and vibration tests were carried out to answer this question. Transverse and point-wise deformation and micromechanical indentation tests on non-embedded FBG sensors were carried out to clarify the reliability of recorded signals.

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.

Fiber optic pH sensor for early detection of danger of corrosion in steel-reinforced concrete structures

Corrosion in steel-reinforced concrete structures is a critical issue. Corrosion appears if the pH value of the concrete matrix decreases due to deterioration of the calcium hydroxide layer on the steel surface. At present, several reliable systems for determination of chemical parameters in aggressive environments are available on the market, but can not be used for long-term monitoring of pH in concrete structures. This paper describes the development of a fiber optic chemical sensor for this purpose. Particular attention is paid to the requirements on such a sensing system. Usually applied methods of fiber optical chemical sensing were investigated and compared by using several pH-sensitive materials. Based on these results, a functional pH sensor has been configured. It shows good response behavior and works under strongly alkaline conditions for one year. Therefore, it represents a promising sensor type for in-situ long-term monitoring in concrete structures. Further work is in progress to test such sensors on-site under real application conditions, e.g. in ground anchors.

Acoustic data of cross linked polyethylene (XLPE) and cured liquid silicone rubber (LSR) by means of ultrasonic and low frequency DMTA

Partial discharges may cause damage to electrical insulation of high voltage equipment. They initiate elastic waves in the insulating material, e.g. in the stress cone of an outdoor termination. Localisation of the origin of such elastic waves can help to predict serious damaging processes in the electrical insulation. In order to measure and evaluate the wave propagation effects in typical multilayered elastomeric structures, knowledge of the material properties is required. The propagating velocity and the attenuation of longitudinal waves are important parameters. Values for these quantities found in the literature were not appropriate. Therefore, for cross-linked polyethylene (XLPE) and cured liquid silicone rubber (LSR), the longitudinal wave velocity and the attenuation were evaluated in the temperature interval from -20°C to 50°C and in the frequency range from 200 kHz to 600 kHz using a two-sample ultrasound technique. The loss factor was determined from these measured quantities. Additionally, low frequency Dynamic Mechanical Thermal Analysis (DMTA) was applied to investigate LSR and XLPE in a temperature interval between -100 and 50°C and to check qualitatively the ultrasound data.

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.

Standards and guidelines: Could they enhance user confidence in fiber sensor technology?

Different experience from fibre sensor applications on-site has revealed the need of guidelines for developers, manufacturers, suppliers as well as users. Although best knowledge and well-founded experience are available in the fibre sensor community, a lack of technical instructions for all persons involved can lead to unreliable measurement results or even damage of sensing components. In contrast, manufacturing and application procedures according to generally binding technical rules will ensure long-term stable sensor systems and reliable measurement results. These rules have to include definitions of particular terms to describe the sensor and sensor system characteristics as well as methods to validate and use sensors properly. Because FBG sensors are the most popular sensing techniques in very different fields, important aspects to structure a set of technical recommendations will be discussed, especially for the use of FBG sensors in experimental stress analysis and for structure monitoring.

Sensors in Adaptronics

Chapter 7 deals with sensors that can be applied advantageously in adaptronic structures. After explaining the connection between possible sensor solutions and the demands of adaptronics, the basic principles of fibre optic sensors and piezoelectric sensors are described. Diverse sensor variants and applications are presented with special attention on the integration of sensors in structures.