## 3 Long-Term Durability of Sensor Coupling and Mounting and of the Measurement Equipment
### Long-Term Sensor Coupling
#### 3.1.2. Couplants and Bonding Agents  
The ASTM standard guide for mounting piezoelectric Acoustic Emission sensors [75] provides general recommendations for the couplants used in compression mounts or for bonding agents for permanent bonding of the sensors (cite): \"_The type of couplant or bonding agent should be selected with appropriate consideration for the effects of the environment (for example, temperature, pressure, composition of gas, or liquid environment) on the couplant and the constraints of the application. It should be chemically compatible with the structure and not be a possible cause of corrosion. In some cases, it may be a requirement that the couplant be completely removable from the surface after examination_\". For fiber-reinforced polymer composite parts, in particular components or structures that are adhesively bonded for aerospace or other applications, silicone contaminations on the surface are not allowed [82]. Therefore, silicone or silicone-containing couplants shall not be used in these cases. On the other hand, a silicone-free vacuum grease used as sensor couplant on CFRP at the authors' laboratory still left visible stains on the surface that could not be removed. The question of whether these might also have an effect on the quality of adhesive bonds, however, is not answered. The AE standard [75] provides further details on the choice of couplants (cite): \"_Testing has shown that in most cases, when working at frequencies below 500 kHz, most couplants will suffice. However, due to potential loss of high frequency (HF) spectra when working above 500 kHz, a low viscosity couplant or rigid bond, relative to sensor motion response, is recommended. Additionally, when spectral response above 500 kHz is needed, it is recommended that FFT be performed to verify adequacy of HF response_\" and \"_The thickness of the couplant may alter the effective sensitivity of the sensor. The thinnest practical layer of continuous couplant is usually the best. Care should be taken that there are no entrapped voids in the couplant. Unewitness, such as a taper from one side of the sensor to the other, can also reduce sensitivity or produce an unwanted directionality in the sensor response_\". The thickness of the couplant layer has to exceed the level of surface roughness or corrugation of the test object, and this provides a lower bound. Furthermore, a question can be asked with respect to the extent the couplant has to be \"void-free\" and how this could be quantitatively assessed, whatever the required limits are. It is not known to which extent specific couplants really penetrate to the \"bottom\" of rough or corrugated surfaces from macroscales to nanoscales and, thus, provide a sufficiently homogeneous contact layer. Surface preparation, type of couplant, its viscosity and wetting behavior on the material of the test object, the procedure for applying it and possibly further treatment, ambient environmental and variable service conditions may all play a role. There are additional recommendations for the choice of couplants and their application in [83], a Standard Practice on Acoustic Emission Monitoring During Resistance Spot-Welding, e.g., (cite) \"_For sustained monitoring, such as on-line AE examination or control of each nugget, the sensor should be permanently mounted using an epoxy adhesive or a similar material. A preamplifier is usually positioned near the sensor_\". This is one example explicitly suggesting permanent adhesive bonding for coupling sensors for sustained, i.e., longer, monitoring. In cases where such a permanent bonding of the AE sensors is used, a general recommendation in [75] states the following (cite) \"_When bonding agent are used, the possibility of damaging either the sensor or the surface of the structure during sensor removal must be considered_\".