## 3 Long-Term Durability of Sensor Coupling and Mounting and of the Measurement Equipment
### Long-Term Sensor Coupling  
#### 3.1.1 Surfaces to Which AE and GUW Transducers Are Coupled  
The coupling material or \"couplant\" shall provide the \"acoustic\" contact, i.e., allow the transmission of elastic waves with frequencies in the relevant range of AE signals or GUW from the test object to the sensor. Many transducers for AE or GUW have a ceramic contact plate; hence, one partner in the coupling often is a ceramic material. Some special transducers have a full metal (steel) case, e.g., those developed for use in so-called ATEX condition, i.e., potentially explosive environments [73; 74]. AFC and MFC are packaged between polyimide polymer films (trade name Kapton(r)). The main contact surface materials on the transducers' side, hence, are essentially ceramics, metals or polymers. Contact surfaces of test objects consist of a wider range of materials, e.g., metals, polymers, concrete, rock and sometimes coatings or combinations of different materials. The contact surfaces can be planar, uniaxially or biaxially curved and, for any overall shape, show varying degrees of corrugation or roughness on the smaller scales. The general aspects of contact surface preparation are noted, e.g., in [75] (cite): _\"The contacting surfaces should be cleaned and mechanically prepared. This will enhance the detection of the desired acoustic waves by assuring reliable coupling of the acoustic energy from the structure to the sensor. Preparation of these surfaces must be compatible with the constriction materials used in both the sensor and the structure. Possible losses in acoustic energy transmission caused by coatings such as paint, encapsulants, loose-mill scale, weld spatter, and oxides as well as losses due to surface curvature at the contact area must be considered\"_. These recommendations are fairly general, and one may ask, e.g., how \"compatible surfaces\" are defined. Surface curvature is noted, but surface roughness or corrugation on smaller scales are not. Usually, the couplant is expected to fill the small scale \"valleys\" on rough or corrugated surfaces, while the waveguides provide adaptation to the overall shape (e.g., uniaxial or biaxial curvature) of the test object. As mentioned above, AFC and MFC can conform directly to biaxially curved surfaces of the test object to some extent [68; 69]. In this case, the contact surface of the pipe was cleaned with acetone and slightly abraded with energy paper, and the AFC bonded with a thermoset epoxy adhesive. Other transducers with stiff casing essentially require the use of a waveguide providing a sufficiently flat and sufficiently large contact area on the side of the transducer and a suitably shaped contact surface on the side of the test object. In addition to adapting the sensor contact to the curvature and size of the surface on the test object, waveguides may also be required for tests objects operated, e.g., at elevated temperatures exceeding the operating temperature range of the transducers, or more generally in harsh or hostile conditions not suitable for transducer monitoring operations. In these cases, waveguide designs often are comparatively thin metallic rods with suitable contact surfaces at either end. Their length is determined by the requirement to place the sensor into an environment compatible with the operating limits of the transducers. With respect to the mounting device or mounting method, ref. [75] again notes several important points: _\"Mounting fixtures must be constructed so that they do not create extraneous Acoustic Emission or mask valid Acoustic Emission generated in the structure being monitored\"_ and further, _\"The mount must not contain any loose parts of particles\"_. The movement of transducers relative to the test object is a potential source of noise signals, and an important caveat, hence, is as follows: _\"Permanent mounting may require special techniques to prevent sensor movement caused by environmental changes\"_.