## 3 Long-Term Durability of Sensor Coupling and Mounting and of the Measurement Equipment
### Long-Term Sensor Coupling
#### 3.1.3 Examples of Long-Term Coupling for AE Monitoring
The AE bridge data never exceeded the combined criteria defined from the separate fracture tests, indicating that no fractures had occurred in the bridge structures during monitoring. Of course, this holds for the given sensitivity level of the AE system as installed. There is a system troubleshooting timeline table in Appendix D of the document [93]. This shows system power problems (low battery power) and modern problems as main troubles. A fourth example, for a reinforced concrete bridge, is an in situ monitoring operation of a reinforced concrete slab on a sixty-year-old bridge with a combination of AE, UT, strain gauges and thermocouples [94,95]. The aim was to detect fatigue damage initiation under normal service loads, which, of course, had increased since the design and construction of the bridge. The monitoring system with strain gauges and thermocouples was installed in 2016 and operated at least until 2019 (publication date). The AE system installed in November 2018 had 32 channels, of which 24 were connected with 150 kHz resonant sensors (type PK15l) with an integrated preamplifier of 26 bB gain; the threshold was set at 30 dB. Remote monitoring was performed with a 4G signal transmission system. Finite element simulations helped to select sensor positions near the highest loads of the concrete slab. Traffic noise was shown to be eliminated by a front-end filter set to let pass AE hits between 40 and 1000 counts and the waveforms with counts between 5 and 10,000 and an amplitude between 40 and 1000 dB. A time filter was also defined, namely Peak Definition Time (PDT) 200 us, Hit Definition Time (HDT) 800 us and Hit Lockout Time (HLT) 1000 ms. The monitoring duration was 18 months. The Ph.D. thesis published recently provides detailed analysis and results [95]. The main conclusion is that there was no indication of fatigue problems in the concrete as well as in steel rebars. A fifth example is from an acoustic monitoring operation of a North Sea well with hydrophones (frequency range from 10 kHz to 100 kHz and 200,000 samples per second). There were two consecutive campaigns: one of four months in the summer of 2011 [96] and the second for seven months from Fall of 2011 to Spring 2012, both in a harsh environment. The measurement system was designed for an acquisition duration of about ten months. Again, no problems with sensor performance or data acquisition were explicitly noted. The cited examples clearly illustrate that for long-term AE monitoring experiments reported in the literature, there is scant explicit, detailed and quantitative information on long-term performance at best. Lack of published specific details on technical problems with the sensors or the measurement chain, however, does not constitute proof that no problems occurred or were not observed. An example of long-term sensor bonding from the authors' laboratory is an MFC element adhesively bonded on the landing wheel gear of a real glider plane [97]. This device was mounted with a standard Araldite(r) adhesive and used with a wireless electro-mechanical impedance (EMI) measurement setup that fitted into the wheel bay of the plane. Even though no measurements were taken during flight ever, laboratory investigations on identical replicas of the landing wheel gear showed that fine, hardly visible cracks induced by mechanical loading could be identified by changes in the 20-25 kHz range of the EMI-spectrum (recorded in 200 Hz steps). MFC was mounted in 2010 and is still bonded to the gear after a total of 240 flight hours and 120 takeoffs and landings until the spring of 2021 (the glider plane is kept at an airfield and flying in Switzerland). Repeating the EMI-measurements after more than ten years, however, is still pending. There are a few examples of long-term infrastructure monitoring over a period of almost 25 years now. An Empa publication in 2019 summarized bridge monitoring experience up to 23 years [98], but the measurement systems are still in place and operational.