## 2 Materials and Methods  
The discussion of long-term durability issues for SHM and CM with AE and GUW, hence, focusses on (1) the couplant materials and sensor mounting devices or methods, (2) transducers or sensors, (3) the electronic equipment for signal transmission, data acquisition, analysis and storage and (4) the software for their operation. Transducers or sensors for AE and GUW can be classified by the physical principles producing the signals. A major class of AE and GUW transducers uses the piezoelectric and inverse piezoelectric effect, respectively [33]. These transducers consist of piezoelectric elements with specific size and shapes, with electrodes providing electric contact, and usually packaged in some type of casing. Piezoelectric material is often lead-zirconate-titanate (PZT) [34; 35], except for applications at elevated temperatures, i.e., near or above the so-called Curie temperature of PZT [36; 37]. Due to environmental and health concerns, however, PZT may mandatorily have to be replaced by lead-free alternatives soon [38]. The shapes of the piezoelectric elements and transducers, respectively, comprise, e.g., disks [39] or wafers [40; 41]; thin, planar piezoelectric fiber composites, e.g., so-called Active Fiber Composites (AFC) [42; 43] or Macro Fiber Composites (MFC) [44; 45]; various piezoelectric composites made with piezoelectric powders or particles [35]; piezoceramic thin films [46]; or piezoelectric polymers often also in the form of thin films. The latter are mainly used in high-frequency UT devices, e.g., for medical ultrasound [47; 48]. Transducers for air-coupled ultrasound, i.e., contact-less signal excitation and/or recording, are also made from piezoelectric materials [49; 50; 51]. Micromachined transducers for UT were reviewed by, e.g., [52] and micro-electro-mechanical-systems (MEMS) Acoustic Emission (AE) sensors by, e.g., [53]. These consist of thin film piezoelectric components and may offer advantages in terms of small size and power efficiency, which are both important for the integration of such devices. A second class of AE and GUW sensors uses optical fibers, either made of glass [54] or polymers [55; 56; 57], typically with Fiber Bragg Gratings (FBG) for sensing [58; 59]. Recently, there were further materials developments for improving fiber optics performance [60]. Complex, multifunctional optical fibers can also be manufactured [61]. Optical fibers sensors for GUW require a separate excitation mechanism or device for generating the elastic waves propagating in the test objects. Advantages of fiber optics sensors include the following (cite): \"_Fiber-optic ultrasonic sensors have several merits, such as broadband response, high sensitivity, disturbance resistance, and good reusability..._\" [62] and \"_An important advantage of FOS_ [Fiber Optical Sensor] _over conventional piezoelectric sensors is their high temperature resistance. Temperature resistant FBGs for AE sensing have been of interest to the industry for a variety of applications_\" [59]. Fiber optics sensors, different from PZT-tranducers or other piezo-transducers, are not sensitive to electromagnetic interference, and this is useful for applications in environments with strong or oscillating electromagnetic fields. Even switching on neon lights or machines or equipment drawing high currents has been observed to create noise signals in PZT-transducers during AE measurements. However, there are also disadvantages of fiber optics (cite), \"_Their durability during handling and cost are two important disadvantages. Moreover, cladding modes that are observed during or after the fabrication process, adversely influence successful damage detection. These modes permit light to oscillate into the cladding from the fibre core, particularly in single-mode fibres where the light is strongly attenuated into the cladding..._\". Further disadvantages include the size and mass of fiber optics measurement equipment, often mounted on heavy optical tables for vibration damping and stability.