This paper studies the detection of barely visible impact damage in composite plates with guided waves (GW) based SHM systems and focuses on how this detection is affected by the degradation of the sensors potentially induced by the exposition to thermal cycling. Two frequencies are investigated to test both fundamental Lamb waves modes (A0 at 25 kHz and S0 at 200 kHz). Two different types of sensors are tested (commonly used piezoelectric PZT transducers and Fiber Bragg Grating (FBG) sensors) which provide very similar results in term of GW measurements. Sensors degradation is monitored through diagnostic measurements based on Electromechanical Impedance (EMI) measurements and Laser Doppler Vibrometry (LDV) scans for PZT and measurements of the reflection spectrums for the FBG. As degradation of an SHM system can also comes with evolutions of the bonding layers, two different bonding methods were tested: bonding with a secondary adhesive and cobonding on composite surface during the curing process.

The effects of the damage and the sensors degradation due to the thermal cycling are compared through their influence on a commonly used damage index (DI) based on cross-correlation with the baseline signals. Cobonded sensors showed a very good stability to the thermal cycling exposition for both the "diagnostic" measurements and the GW signals, which allows a good detection of the impact damage above 7.5J of impact energy. On the contrary, for bonded sensors, significant degradations were detected mainly with the appearance of debondings under some of the PZT discs which lead to DI values up to the values corresponding to a 10J impact damage that would therefore lead to false detection. However, those degradations were easily identifiable with the diagnostic measurements of the corresponding sensors, therefore those GW acquisitions could be flagged as faulty to avoid a false detection. These results will likely strongly depend on the choice of the DI and therefore other common DI formulations should be tested and presented at the conference.