A prototype sensor tape was developed consisting of a polyamide optical sensor embedded in a glass fibre - epoxy prepreg tape. The following issues were addressed: 1. Producibility 2. Handling, sensor survivability 3. Strength and stiffness 4. Bonding compatibility with the composite structure of the pressure vessel This technique was investigated as an alternative to a polyamide optical sensor integrated into a glass-PPS tape. The main advantage of using this type of tape compared to a glass-PPS tape is the very good bonding compatibility with the carbon-epoxy structure of the composite high-pressure tank. No additional adhesive is needed. This saves time and costs. This type of sensor tape can be used for any composite structure using an epoxy resin matrix. After placing the tape with the embedded sensor it can be co-cured, giving a very good connection of the embedded sensor to the composite structure, ensuring reliable measurements. Thus, this sensor tape can be used for structural health monitoring in composite marine, transport and aerospace structures. At this stage the prototype sensor tape must be further developed to improve producibility and achieve a low-cost end product.
The SOFO Reading Unit is computerized equipment that measures all SOFO sensors. It includes an optical source, a mobile mirror, a photo-detector and the related electronics. These components are contained within a case designed to withstand the harsh environment of a civil engineering construction site. The unit is powered either by the internal battery or by an AC source. An external portable PC controls the Reading Unit allowing to initiate the measurements and store the obtained results for further analysis. A single Reading Unit allows the measurements of an unlimited number of sensors. Optical switching units are used to automatically multiplex different sensors. SOFO V units are available in single channel configuration used for manual measurements, with an integrated switching having unit up to 12 channels, or with external units having up to 100 channels each. Internal and external units can be cascaded to reach even a greater number of channels. A standard serial link connects the SOFO unit with the control PC, while the SOFObus port is used to connect external switching units and external data acquisition devices to the SOFO Reading Unit. The SOFO V Reading Unit contains a data logged that can be programmed to automatically read SOFO sensors and external data acquisition devices and store the measurements in its memory. The memory can hold thousands of measurements that can be later downloaded into the SOFO SDB database. A modem (standard, radio, cellular, cable, fibre optic or Ethernet) can be connected to the SOFO unit to allow remote measurement, check the unit's status, or download data from the SOFOV memory. The SOFO Dynamic reading unit allows measuring SOFO sensors at high frequencies. One reading unit can be used to demodulate up to 8 channels. Multiple units can be combined when higher channel counts are needed. The SOFO Dynamic reading unit can be used in conjunction with SOFO sensors. SOFO Dynamic compatible sensors can be measured with both the SOFO Dynamic (dynamic measurements) and the SOFO V reading unit (static and long-term measurements, reduced range). Most standard SOFO sensors can be measured by the SOFO Dynamic reading unit using an external custom -made compensator. The SOFO Dynamic reading unit is based on a heterodyne low-coherence interferometer operating at 1550nm. The optical signal is phase modulated by the demodulation interferometer. After detection, the reading unit tracks the phase modulation introduced by the sensors and converts it into a displacement. The resulting deformation is available in analogue form on the analogue outputs or in digital form on the USB connection that can be used to transfer the measurements directly to a PC for storage and further analysis. The measurements are relative and the zero point is lost on power off, but can be recalibrated using the SOFO V reading unit. Application domains: Measurements of dynamic deformations of structures under, dynamic loads such as traffic, wind, seismic, waves, Evaluation of dynamic amplification factors, System identification through modal analysis (ambient and forced vibrations). Determination of mode-shapes and modal curvatures. Damage detection through changes in modal parameters, stiffness and damping factors, Weight-in-motion through structural response
A method has been developed for measuring strain using optical fibre sensors that is independent of optical power levels over a wide range of values. This is important for the strain measurement of gas tanks since pressurisation of the tanks produces changes in the fibre attenuation. It could also have applications in other strain measurement applications, since changing power levels due to ageing of components or external environmental factors is always a very strong possibility. The sensors can be used in either transmissive or reflective mode, which makes them easier to implement into a practical system. Optical switching to interrogate a number of sensors sequentially has also proved to be effective. The technology used for the interrogation system is based on electronics operating at similar frequencies to those used in the mobile phone industry and therefore offers the possibility of development for mass production at an economical cost
Final sensor system to be embedded during tank manufacturing: polyimide OF integrated in a PPS-glass tape
Composite materials, in general, are manufactured in form of filaments, tapes or sheets, while optical fibre sensors are to be embedded within the structure, depending on the structural layers that have to be monitored. Improper embedding of the sensors may be a source of delaminating that causes a significant decrease of mechanical properties. Sensor can also be installed on the surface of the structure, and in this case the optical fibre has to be protected against environmental influences. On the other hand, if the sensor is designed to monitor strain or deformation, it is necessary to guarantee a good bonding between the optical fibre and the composite. Finally, for an industrial deployment of fibre optic sensors in this domain, it is necessary to package the sensors in a way that makes them as easy to handle as other components used for composite production. The solution in found in pre-packaging the measurement optical fibre in a thin composite tape, that can than be embedded or surface mounted on the composite structure. The tape gives to the optical fibre necessary protection against an accidental damaging during handling and installation. The fibre-reinforced composite tape with integrated optical fibre is called SMARTape. The SMARTape sensing performance was laboratory tested. In addition mechanical, microscopic and fatigue tests were performed. The results confirmed the same sensing performance as in case of standard SOFO sensor and excellent mechanical (robust, resistant, elastic, with no fatigue), thermal (temperature range from -40°C to +300°C) and chemical performance (resistant to aggressive environments). Sensing solutions based on the SMARTape concept are now commercially offered by SMARTEC. In particular SMARTape sensors compatible with the SOFO and DiTeSt systems have been developed, introduced and have found the first applications outside the ZEM project.
Demonstrator vehicle: Multiple Bi-power with three 48 litres all composite tanks and embedded sensors
A demonstrator vehicle has been set up: - FIAT Multipla 1.6, 16v with Bipower engine - three all composite tanks are housed beneath the floor and can hold a maximum of 144 litres of fuel storage (three 48 litres size), enough for a 440km range - each tank is sensorised with four fibre optic sensor and one thermocouple - two pressure gauge to measure pressure during refilling and working - the SOFO interrogation unit is placed in the trunk - a laptop connected to SOFO unit is place inside vehicle close to the driver.
From the structural point of view it is interesting to monitor two principal directions of strain, i.e. longitudinal and circumferential direction. Longitudinal direction was monitored using the sensors parallel to longitudinal axis of the tank, while for circumferential direction helicoidally shaped sensors were used. After testing it was found that using a single set of sensors installed over an helix covering the full length and half circumference would give the best results and simplify installation. An installation procedure using prefabricated gals' fibre mats containing the sensing tapes was developed and successfully transferred to ULLIT for industrial production of the instrumented tanks.
Today, the method to control and re-qualify CNG cylinders in service is a hydrostatic test at 1.5 working pressure, which means hydraulic test at 300 bar. An alternative method "visual inspection" is used today to avoid dismantling the cylinders from the vehicle. Both methods, hydraulic test and visual inspection do not give any information about the behaviour of the pressure vessel. We just learn that the cylinder withstand the test pressure! The use of optic fibre sensors embedded on the cylinder provides a permanent information, either at each refuelling or at inspection that the cylinder has no deformation capable to reach to a failure. During the first hydraulic test, just after manufacturing, the cylinder's deformation is registered and compared to the nominal deformation. Then a threshold deformation is determined; below the cylinder is considered as safe; above the cylinder must be re-inspected. During the ZEM program, any overpressure, artificial damages as cuts, flaws, shocks, impacts, delamination have been detected by optic sensor. This method to monitoring high-pressure cylinders will be a reliable system for controlling of future 700 bar hydrogen tanks.
Methodology on fibre optic sensor embedding in high-pressure composite gas tank using sensor tapes in pre-fabricated fabric sheet
A simple, low-cost and robust method of integrating an optical sensor system in a composite high-pressure gas tank was developed using a pre-fabricated sheet with embedded sensors. The impact on the production process is minimal, no extra manufacturing equipment or machinery is needed. Extensive prototype testing has been performed showing: - 100% survivability of integration of the sensors in the composite structure of the tank, - Very good adhesion of sensors to the composite structure - Damage detection possible of relative small (harmless) damage This method is especially fit for integrating a optical sensor system in filament wound structures, such as pressure vessels. The main target markets are Natural Gas and Hydrogen high-pressure tanks for the automotive and (public) transport sector. The pre-fabricated sensor sheet adds no significant weight. The added cost to the tank is 10-20%, making it commercially feasible to use this method for manufacturing tanks with a structural monitoring system.
The tow-preg filament winding production process can be applied as an alternative to wet-filament winding. Research and production testing have shown the following advantages over wet filament winding: 1. Higher production speeds 2. Better laminate quality with lower weight 3. Healthier production environment 4. Less impact on environment Cost analyses have shown that the process can be made competitive with wet-filament winding for large production quantities of carbon-epoxy pressure vessels. Other potential users that are targeted are filament wound carbon-epoxy drive shafts for the marine industry and filament wound carbon-epoxy refuelling booms, for the aerospace industry. The tow-preg production process is applicable for any filament wound product, which requires a high quality laminate in combination with fast production. The initial material costs are a first barrier that must be overcome, but as the prices of carbon fibres themselves continue to increase, the tow-preg production process becomes even more lucrative. At this stage the production process development is ready for direct application in production.
In case of damage, an increase of flexibility of the tanks is expected. In addition a global deformation of the tank is expected - bending, ovalization, etc. Another particularly important aspect to be monitored is the symmetry of the tank behaviour. The strain field in tank is expected to be symmetrical with respect to the plane perpendicular to the axis of the tank and crossing the middle of the cylindrical part, and rotationally symmetrical with respect to axis itself. In case of damage, the strain field is expected to loose the symmetry. To detect the damage, the behaviour of the tank is assessed at three levels: 1. In case of damage the tanks is expected to become more flexible; therefore, the slope of the strain-pressure diagram is expected to increase 2. In case of damage the tanks is expected to deform (bend, ovalize, etc.); as a consequence, axially symmetrical pairs of sensors will measure different values of strain 3. In case of damage the strain field in the tank is expected to loose the symmetry; hence, the coefficients of linear correlation between the sensors will change. The optimised sensor layout was demonstrated to detect cutting and impact damages that significantly reduce the burst pressure. The defect can be detected at pressure well below the burst pressure, thus allowing an early response in case of dangerous damages.