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Acoustic sensors monitor single carbon fibre tears to boost hydrogen … – Professional Engineering

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Professional Engineering
A new sensor-based structural monitoring system will enable continuous surveillance of hydrogen pressure tanks, its creators have said, potentially improving the safety of hydrogen vehicles.
Designed to reveal internal damage that visual inspections cannot detect, the system is in development as part of the HyMon research project at the Fraunhofer Institute for Structural Durability and System Reliability (LBF) in Darmstadt, Germany, and its partners.
The latest fuel cell vehicles carry hydrogen in gaseous form in pressurised tanks, up to 700 bar. Regular maintenance is mandatory, but a tank inspection that is required every two years only consists of an external visual inspection, a Fraunhofer announcement said. 
The tanks, made of fibre-reinforced composites (FRC) for low mass, need to maintain integrity after the recurring stresses caused by refuelling and withdrawal of hydrogen, or in the event of damage, such as from a collision.
The visual inspections currently specified to check for external damage cannot check tanks’ integrity however, the Fraunhofer team said. The researchers are developing the structural health monitoring (SHM) system to tackle the issue. 
The work is focused on acoustic emission sensors. If a single carbon fibre tears in the pressure tank, a sound wave is generated and travels through the fibres. The sensors detect this high-frequency sound wave, allowing them to determine the number of broken fibres. 
“Special load cases, such as rear-end collisions, can damage local areas of the tanks, causing a lot of fibres to break in a very short space of time,” said researcher Johannes Käsgen. “The measurement signals are processed by evaluation electronics to provide information about the health status of the tank.” 
The algorithms and methods for detecting fibre breaks are being developed at Fraunhofer LBF. These include sound wave frequency analysis. “If the rate of fibre breakage suddenly increases, this is an indication that the hydrogen tank is at the end of its useful life,” Käsgen said. 
One purpose of the SHM system is to provide data for service and repair. “To give an example, our technology provides inspectors from the German technical inspection associations (TÜV) with objective information about the stresses on the tank after an accident, enabling them to decide objectively whether it can be reused or needs to be replaced,” said Käsgen, a scientist at Fraunhofer LBF. The systems could also help reduce maintenance costs and ensure that tanks are used safely throughout their entire service lives. 
Fibre-optic strain sensors are also being integrated into the tanks, enabling continuous or periodic automated monitoring of strains. Unlike conventional strain sensors, the glass fibres are particularly suitable for monitoring carbon fibre-reinforced plastics due to their resilience to high material strains and load cycles. 
Measurement data from the strain sensors is first used to verify the calculation models of the pressure tanks, then to provide insight into how the material behaviour changes throughout the tanks’ service life.
The research teams are investigating how many sensors are required for structural monitoring, where they need to be positioned, and which adhesives are most suitable for attaching them to the hydrogen tank. A test vehicle will be fitted with sensors and onboard structural monitoring, before validation with a virtual crash and a real-life test setup. 
The partners aim to develop the complete system into a standard status monitoring system for the future.
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