Researchers at the Fraunhofer Institute for Structural Durability and System Reliability (LBF) are working on a technology for the automated monitoring of high-pressure storage systems. The new technology is intended to enable continuous monitoring of pressurised containers. This should enable greater safety at lower costs than conventional maintenance methods.
The aim of the research is to enable continuous monitoring of the pressure vessels in fuel cell vehicles by means of on-board structural monitoring. To this end, sensors and evaluation electronics are used to enable evaluation on the basis of structural health monitoring (SHM) data.
In addition to utilising SHM data for repairs and servicing, the technology will also be used in the event of accidents. The permanent monitoring should also enable targeted measures for safe vehicle recovery during rescue operations.
Fraunhofer LBF is working closely with other partners as part of the project. Initially, a methodology for applying fibre-optic and piezoelectric sensors to fibre composite pressure vessels will be further developed. The fibre-optic strain sensors will be able to measure the material stresses in the fibre laminate. The piezoelectric sensors are used to record the ultrasonic noises that occur when the matrix or individual fibres of the laminate are damaged as a result of impingement.
Damage detection through ultrasonic noise
Using material samples, the damage to the material is first artificially generated in the laboratory and recorded with the sensors. These are then classified on the basis of the signal signatures. The developed sensor and application technology is transferred to the application on FRP high-pressure tanks and validated in burst, fatigue and impact tests.
The development of a signal analysis method based on the acoustic emission method to differentiate between various material-typical damage mechanisms and a calculation method for the fatigue behaviour of the fibre composite are key priorities at Fraunhofer LBF. Finally, based on its existing experience in the field of vehicle operation measurement, the institute provides support in integrating the container monitoring system into the vehicle's electrical, electronic and information technology environment.
Assessment to date
Overall, the automated metrological monitoring of pressurised containers promises an increase in the level of safety. At the same time, high costs resulting from the unnecessary replacement of tanks that are still functional should be avoided. In addition, the sensor technology enables cost-effective and efficient monitoring of production quality. Accompanying monitoring can provide more information during product development. This can also lead to a reduction in development and testing times.
As highly stressed safety components, pressurised containers for storage are a core element of hydrogen drive systems. Tanks made of fibre-reinforced plastics have a significantly lower mass than pure metal tanks with the same working pressure. This makes them attractive for use in the mobility and transport sector. They are designed for a service life of up to 20 years at operating pressures of 200 to 1000 bar.
And what is the current procedure? According to the institute, a purely external assessment is carried out every two years. However, this prescribed inspection hardly allows any conclusions to be drawn about the condition inside the thick-walled fibre composite. On the one hand, "overly" conservative complaints, which may result in the unnecessary replacement of tanks that are actually still functional, could result in high repair costs. On the other hand, the incorrect assessment of a defective tank as "safe" must be ruled out under all circumstances.
