## 3 Long-Term Durability of Sensor Coupling and Mounting and of the Measurement Equipment
### Durability of Sensors or Transducers under Service Loads and Environmental Exposure  
Ambient temperature and its variation are likely the most important environmental in-service exposures affecting transducer performance--if packaged into a suitable case--and durability of coupling agents. Transducers may reach service lives of thirty and more years as shown, e.g., by sensors in the authors' laboratory manufactured in the early 1990s. Of course, using sensors in a service environment that is sufficiently stable and moderate, e.g., laboratory with controlled climate is advantageous. Piezoelectric materials such as PZT for signal detection do have an upper operating temperature limit. This is caused by the fact that the piezo-effect disappears due to a phase transition above the Curie temperature (Tc). In practice, the maximum operating temperature has to be set sufficiently below Tc. In a student project at Empa, an MFC element was repeatedly bonded and debonded to a GFRP beam with a thermoplastic adhesive applied by a heat gun. The thermoplastic adhesive was briefly heated to a temperature between about 110 \({}^{\circ}\)C and 120 \({}^{\circ}\)C for application on the GFRP. Consecutive measurements, in this case with EMI, indicated a decrease in MFC sensitivity with increasing numbers of bonding-debonding cycles, even though the temperature was below Tc [99]. This is likely due to thermally induced depolarization of the PZT. An investigation of thermomechanical loading effects on MFC in [100] established a theoretical fatigue model that fitted experimental data of the degradation of the piezoelectric coupling coefficients fairly well. This model further allowed for predictions of stiffness and strength degradation of the MFC under thermomechanical loads. Whether higher-temperature cure of thermoset adhesives (e.g., epoxies) might induce analogous changes in sensitivity of coupled PZT-transducers is not known. If the thermoset bonding is applied only once and not repeatedly, the effect of the cure on sensitivity is expected to be limited, otherwise room-temperature cure adhesives can be used [101]. In principle, transducers showing thermal depolarization effects for whatever reason during us could possibly be re-polarized by applying a suitable poling voltage at elevated temperature for a defined duration [102]. For AFC, the poling voltages varied between about 2.5 and 3 kV per mm distance between the interdigitated electrodes and re-poling would likely require similar voltages. AFC electrode distance varied between 0.7 and 1.3 mm, and poling temperature varied between 40 \({}^{\circ}\)C and 80 \({}^{\circ}\)C (clearly below Tc of the PZT fibers and below Tg of the epoxy matrix of the AFC). Such a procedure has never been applied to commercial transducers to the best knowledge of the author. Depending on their design, voltages significantly exceeding 4 kV might have to be applied, and this could induce electric breakdown (as it did in several AFC elements for poling voltages above about 3 kV/mm). For trials on commercial transducers, information on which combination of temperature, time and poling voltage is appropriate for re-poling would have to be obtained from the manufacturer. For AE and GUW measurements at \"high\" temperatures, the following approaches have been attemped: (1) design and optimize piezoelectric materials with higher Tc for sensors operating at higher temperatures (see, e.g., [36; 37; 103]); (2) use of waveguides between the surface of the test object and the sensor (these, however, do affect the signals that are transmitted; see, e.g., [67]); and (3) use of alternative physical principles for sensor and measurement. The latter comprise, e.g., fiber optics either integrated or surface bonded [104]; special AE sensors made with optical fibers [72]; or contactless methods, e.g., laser interferometry [43], vibrometry [63] or air-coupled ultrasound for GUW [50; 51; 105]; and electric methods, e.g., electrical conductivity for CFRP [106; 107] or dielectric methods [108].