Repair patch monitoring with embedded optical sensors by a residual strain release technique

Malte Frövel, Javier Sanmillan, José Maroto, José María Pintado, Iddo Kressel, Jonathan Balter, Nachum Gorbatov, Moshe Tur

Research output: Contribution to conferencePaperpeer-review

1 Scopus citations


Repair of metal structures with carbon fibre reinforced plastic (CFRP) patches holds a high potential for aeronautic structures. Bonded repairs with CFRP offer the advantage within others to repair in places where a standard riveted repair is not possible due to space limitations or geometrical conditions. It is of vital importance that a bonded repair patch will not peel off during service. Monitoring the patches provides a tool to detect a debond in the repair patch before the debond becomes a critical issue. A monitoring technique has been studied that enables to detect a patch peel off using fiber optic Bragg grating sensors (FBGSs) embedded in the CFRP-patch. Temperature compensation has been introduced to enable the application of this technique in a wide temperature range. The technique allows the monitoring of the patch on ground after a defined number of flight cycles. The viability of the monitoring principle was studied and demonstrated on seven specimens made of typical aeronautic aluminum with bonded CFRP patches that are instrumented with two or four embedded FBGS. The results of the tests show that the embedded sensors can indeed sense the debonding of a bonded CFRP patch used to repair an aluminum structure. The sensitivity of the sensors is sufficiently high to detect a crack front that is still 10 mm far from the sensor. The temperature effect on the embedded sensors that monitors the crack front can be compensated by a second sensor that is far from the sensor front. A robust pre-cured sensor mat design was developed and manufactured. This sensing mat was made using three fibers each with four FBGSs, embedded between two glass fabric plies, providing repeatable, easy and robust sensor integration. Test structures with monitored composite repair patches were designed and manufactured to demonstrate the performance of the sensing mat during fatigue tests under wide temperature range. The change in readings of a low spatial resolution sensor net, in response to the growth of critical defects is significant enough to make a clear indication of patch deterioration with a reduced chance for misinterpretation. It is, therefore, hoped, that the use of such a system will allow a significant increase in the regular inspection intervals of bonded repairs without compromising safety.

Original languageEnglish
Number of pages8
StatePublished - 2014
Event7th European Workshop on Structural Health Monitoring, EWSHM 2014 - Nantes, France
Duration: 8 Jul 201411 Jul 2014


Conference7th European Workshop on Structural Health Monitoring, EWSHM 2014


  • Bonded composite patch repair
  • FBGS
  • Metalic aircraft structures
  • SHM
  • Strain relese


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