SUMMARY There is a continuous need for the monitoring of engineering structures, especially when scheduled inspections are difficult to perform or if they are too expensive. The present paper gives some results on the performance of sensors on the basis of percolation thresholds that are able to detect leakage of diverse harmful liquids, such as water, hydraulic liquids, kerosene and jet engine oils. The focus here lies on sensor systems that give semi-quantitative, fuse-like information on the presence of harmful liquids, which indirectly gives evidence on the presence of structural damage. The common underlying principle is the interruption of percolation conductivity when the respective sensing material is exposed to harmful liquids; that is, the electrical conductivity is essentially lost by sorption and swelling of a dedicated matrix material. The temperature dependence and response times give valuable information on the usability of the sensor in the respective environments. The sensor can be manufactured as a cord sensor or as self-adhesive patches where the functional layer is deposited by screen printing. Copyright © 2013 John Wiley & Sons, Ltd.
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