Papers by Keyword: Fibre Optic Sensor

Abstract: A hybrid piezoelectric (PZT)/fibre optic diagnostic system has been developed for damage detection in built up composite structures. The hybrid system uses PZT transducers to actuate the structure and fibre optic (FO) sensors to capture the propagating wave. The diagnostic system will then have the advantages of both PZT and FO sensors. The applicability of the system is then tested for detecting an artificial damage at a skin/stiffener interface of a thick composite structure. The response of the FO sensors is then compared to PZT sensors and presented.

Abstract: The development of optical fibre temperature indicator using epoxy glue as a detection membrane is presented. This study, investigates the effects of epoxy glue from the reaction of epoxy resin, bisphenol A (BPA) (80-05-7) and adhesive epichlorohydrine (ECH) (106-89-8) as a temperature indicator membrane. In this work the response of epoxy glue to excitation source 395 nm is tested and analyzed under cryogenic conditions. A fiber optic temperature sensor for detecting ambient temperature ranging from 15 °C to 80 °C has been examined. The epoxy glue fluoresce when excited with UV-blue light source. The intensity of the fluorescence of the material decreases when the epoxy glue is exposed to an environment of higher temperature. These decrease level of fluorescence signal has been used to indicate temperature. In this paper, the basic principle of operation, development process and emission response characteristics of this sensor are discussed.

Abstract: Fibre-optic sensors have advantages over existing electrical sensors in many strain and stress monitoring applications. However, bare optical fibres are fragile and packaging techniques must be developed before these sensors can be used more widely. One such method is the Smart Patch, in which the fibre Bragg grating is encapsulated between plies of glass-reinforced epoxy where rugged cables are anchored. This forms a flat flexible patch in which the fibre is protected from mechanical and environmental damage. However, it is important that the mechanical strength of the patch is not achieved at the expense of good strain transfer characteristics. To confirm this, fibre Bragg gratings with acrylate and polyimide coatings were embedded in a glass-epoxy patch that was bonded to an aluminium tensile specimen. An electrical strain gauge was also bonded to the specimen to provide a strain reference. Tests were carried out at different loading rates and at temperatures from -30°C to +80°C. There was good agreement between the fibre-optic sensors and the electrical strain gauge demonstrating that the patch performed in a practically identical manner to the conventional gauges. A second experiment on a representative part of ship structure demonstrated the versatility of the patch.

Abstract: A novel fibre optic sensor system has been developed for detection of acoustic emission. The sensor design was based on a 2×2 fused tapered optical fibre coupler configuration. Given the intended use of this sensor for the detection of acoustic emission in smart materials and structures, its lightweight, compatibility with composite material and immunity from electromagnetic interference are great advantages. Apart from the performance specification and reliability, the overall cost of the sensor system is a major factor in their adoption by end-users. The manufacturing of this sensor is low-cost and the signal output from the sensor system developed can be directly fed to the commercial AE acquisition system. The demonstration of acoustic emission detection was conducted using fibre coupler-based AE sensors in different materials and structures. The AE sources are various and depend on the different applications. In the current paper, we report on the use of the fibre optic AE sensor system for concrete damage monitoring. An experiment on damage detection on a composite plate with this fibre optical AE sensor is also reported in the paper.

Abstract: The measurement of changes in the properties of ultrasonic Lamb waves propagating through structural material has frequently been proposed as a method for the detection of damage. In this paper we describe work that uses optical fibre sensors to detect the Lamb waves and show that the directional properties of these sensors allow us to not only detect damage, but also to locate it. We look at two types of optical fibre sensor, a polarimetric sensor and the fibre Bragg grating. The polarimetric sensor measures the change in birefringence of a fibre caused by the pressure wave of the ultrasound acting upon it. This is an integrated sensor since the fibre length bonded to the sample needs to be greater than the ultrasonic wavelength in order to obtain the required sensitivity. The maximum sensitivity of this sensor is when the fibre is positioned normal to the direction of wave propagation. Fibre Bragg gratings are essentially point sensors since the grating length needs to be a fraction of the ultrasound wavelength to obtain maximum sensitivity. Ultrasound is detected mainly through the in-plane strain it produces and maximum sensitivity is therefore produced when the grating is aligned parallel to the direction of wave propagation. Holes drilled into sample plates can be detected using both type of sensor by examining the changes in either the transmitted Lamb wave or through detection of the reflections produced by the hole. The sensitivity of the technique is shown to be determined by the relative positions of the acoustic source, the hole and the sensor. If we use fibre Bragg gratings in a rosette configuration (i.e. 3 gratings forming an equilateral triangle) then the direction of the Lamb wave can be determined using the directional sensitivities of the gratings. Using two such rosettes allows us to calculate the source of the wave from the intersection of two of these directions. If the source of the wave is the hole (which acts as a passive source), then the location of that hole can be determined.