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Monitoring system for delamination detection - Qualification of structural health monitoring (SHM) systems

: Lehmann, M.; Büter, A.; Frankenstein, B.; Hentschel, D.; Brunner, B.

Fulltext (PDF; ) Online resource (2006), 10 pp.
ISSN: 1435-4934
Journal Article, Electronic Publication
Fraunhofer LBF ()

Traditional vibration-based monitoring techniques provide general information about the investigated structure by identifying and analyzing specific resonance modes. Using low frequencies, they are limited to the identification of large defects, and in the precision of their localisation as well. The paper shows that vibration monitoring of essential structure parts can be significantly enhanced by using elastic waves in the kHz frequency range generated by piezo fibre composite-modules. The mentioned active modules can be embedded within a structure due to their low thickness and show a significantly different sensitivity in directions parallel or perpendicular to the fibre direction. Having a shorter wave length, these guided waves are more sensitive to smaller defects. Theoretical foundations for describing their propagation in various structures and the interaction with defects are reviewed. On the basis of this analysis, methodical concepts for active and passive monitoring systems have been developed which employ acoustic signature analysis and acoustic emission techniques. Implemented in an early-warning system, they can raise an alarm long before any critical damage occurs.
Acoustic monitoring techniques require technical solutions that eliminate noise as much as possible. To meet this requirement, sensor near evaluation of the signals can contribute substantially. A micro system for signal evaluation has been developed which is based on modular components complying with the Match-X platform technology. This solution contains components for analog preprocessing of acoustic signals, their digitization, algorithms for data reduction, and digital communication. The core component is a digital signal processor. The acoustic monitoring concept using guided waves has been tested in combination with GFRP (glass fibre reinforced plastic) structures. Test specimens were damaged with mechanical impacts and were then tested. Application examples show how the monitoring system reveals delaminations.
Before a new technology like the above presented Monitoring System can be introduced using existing production processes several conditions have to be met. For the reliability of any Structural Health Monitoring Concept with integrated systems appropriate quality management must be maintained in the production process of the fibrecomposite-structure to ensure the proper position, cable routing and function of the modules after curing of the fibre composite material. Necessary modifications in the process have to be economically justifiable.
The cable routing and the sensor of an embedded SHM-system must not have a negative influence on the structural integrity of the component to be monitored. The most important factor here is the possible generation of delaminations by the embedded modules or cables. The reliability and durability of the sensor under cyclic loads is another critical factor which is investigated. A degradation of the sensor signal in connection with fatigue has to be accounted for when interpreting the signals. Otherwise the degraded signal could appear as a defect in the GFRP-structure. By considering these aspects early in the development a safe and reliable Structural Health Monitoring System can be developed.