Acoustic Emission (AE) and Guided Ultrasonic Waves (GUWs) are non-destructive testing (NDT) methods in several industrial sectors for, e.g., proof testing and periodic inspection of pressure vessels, storage tanks, pipes or pipelines and leak or corrosion detection. In materials research, AE and GUW are useful for characterizing damage accumulation and microscopic damage mechanisms. AE and GUW also show potential for long-term Structural Health and Condition Monitoring (SHM and CM). With increasing computational power, even online monitoring of industrial manufacturing processes has become feasible. Combined with Artificial Intelligence (AI) for analysis this may soon allow for efficient, automated online process control. AI also plays a role in predictive maintenance and cost optimization. Long-term SHM, CM and process control require sensor integration together with data acquisition equipment and possibly data analysis. This raises the question of the long-term durability of all components of the measurement system. So far, only scant quantitative data are available. This paper presents and discusses selected aspects of the long-term durability of sensor behavior, sensor coupling and measurement hardware and software. The aim is to identify research and development needs for reliable, cost-effective, long-term SHM and CM with AE and GUW under combined mechanical and environmental service loads.

Extending the service life of ageing infrastructure, transportation structures, and processing and manufacturing plants in an era of limited resources has spurred extensive research and development in structural health monitoring systems and their integration. Even though piezoelectric transducers are not the only sensor technology for SHM, they are widely used for data acquisition from, e.g., wave-based or vibrational non-destructive test methods such as ultrasonic guided waves, acoustic emission, electromechanical impedance, vibration monitoring or modal analysis, but also provide electric power via local energy harvesting for equipment operation. Operational environments include mechanical loads, e.g., stress induced deformations and vibrations, but also stochastic events, such as impact of foreign objects, temperature and humidity changes (e.g., daily and seasonal or process-dependent), and electromagnetic interference. All operator actions, correct or erroneous, as well as unintentional interference by unauthorized people, vandalism, or even cyber-attacks, may affect the performance of the transducers. In nuclear power plants, as well as in aerospace, structures and health monitoring systems are exposed to high-energy electromagnetic or particle radiation or (micro-)meteorite impact. Even if environmental effects are not detrimental for the transducers, they may induce large amounts of non-relevant signals, i.e., coming from sources not related to changes in structural integrity. Selected issues discussed comprise the durability of piezoelectric transducers, and of their coupling and mounting, but also detection and elimination of non-relevant signals and signal de-noising. For long-term service, developing concepts for maintenance and repair, or designing robust or redundant SHM systems, are of importance for the reliable long-term operation of transducers for structural health monitoring.

Automated tape placement with in-situ consolidation (ATPisc) is a layer-wise manufacturing process in which the achievement of proper interlayer bonding constitutes one of the most challenging aspects. In the present study, unidirectional carbon fiber reinforced thermoplastic laminates were produced following different manufacturing protocols using ATPisc. The interlayer bonding of the laminates produced was characterized by mode I fatigue fracture tests with double cantilever beam (DCB) specimens. Independent of the manufacturing approach, the laminates exhibited multiple cracking during DCB testing, which could not be evaluated simply following standard methods. Thus, various data analysis methodologies from literature were applied for the quantitative assessment of the fracture behavior of the laminate. The examination of the evolution of the damage parameter $\phi$ and the effective flexural modulus throughout testing enabled a better understanding of the damage accumulation. The Hartman-Schijve based approach was revealed to be a convenient method to present fatigue crack growth curves of laminates with multiple delaminations. Moreover, a preliminary attempt was made to employ a ‘zero-fiber bridging’ methodology to eliminate the effect of additional damage processes on the fatigue crack growth that resulted in large-scale, partially massive fiber bridging.

Quasi-static or cyclic loading of an artificial starter crack in unidirectionally fibre-reinforced composite test coupons yields fracture mechanics data—the toughness or strain-energy release rate (labelled G)—for characterising delamination initiation and propagation. Thus far, the reproducibility of these tests is typically between 10 and 20%. However, differences in the size and possibly the shape, but also in the fibre lay-up, between test coupons and components or structures raise additional questions: Is G from a coupon test a suitable parameter for describing the behaviour of delaminations in composite structures? Can planar, two-dimensional, delamination propagation in composite plates or shells be properly predicted from essentially one-dimensional propagation in coupons? How does fibre bridging in unidirectionally reinforced test coupons relate to delamination propagation in multidirectional lay-ups of components and structures? How can multiple, localised delaminations—often created by impact in composite structures—and their interaction under service loads with constant or variable amplitudes be accounted for? Does planar delamination propagation depend on laminate thickness, thickness variation or the overall shape of the structure? How does exposure to different, variable service environments affect delamination initiation and propagation? Is the microscopic and mesoscopic morphology of FRP composite structures sufficiently understood for accurate predictive modelling and simulation of delamination behaviour? This contribution will examine selected issues and discuss the consequences for test development and analysis. The discussion indicates that current coupon testing and analysis are unlikely to provide the data for reliable long-term predictions of delamination behaviour in FRP composite structures. The attempts to make the building block design methodology for composite structures more efficient via combinations of experiments and related modelling look promising, but models require input data with low scatter and, even more importantly, insight into the physics of the microscopic damage processes yielding delamination initiation and propagation.

The approach to cyclic fatigue fracture testing of metals and alloys with the data presentation as "Paris-Graph" developed by Paul Paris and co-workers has proven immensely useful for structural engineering. It is, therefore, understandable that the later development of fatigue fracture test methods for fiber-reinforced polymer (FRP) composites followed this route also. However, recently questions have been raised by several researchers that require looking at fatigue fracture of FRP composites in more detail in attempts at using the full potential of this class of materials in structural applications. One of these questions is how the apparent analogy in the shape of the curves between Paris-Graphs of metals, alloys and FRP composites relates to the material specific, physical damage mechanisms. For FRP composites, the investigation of these mechanisms has a long history, but only recently, there seems to be a real advance using sophisticated pattern recognition of acoustic emission waveforms with complementary methods such as multi-physics simulations or high-resolution X-ray computed tomography. Comparing, e.g., crack sizes on the order of tens of micrometers occurring on time-scales of microseconds or less with visually observed delamination propagation, it becomes clear that the data analysis for the Paris-graph is averaging over orders of magnitudes in both, length and time-scales and this inevitably involves some scatter. Another question raised concerns the apparently larger scatter in Paris-Graphs obtained for FRP composites compared to those of metals or alloys. Again, a full understanding of the sources of this scatter has not been reached yet, but likely, the complex morphology and microscopic damage mechanisms of FRP composites play a role. There are further effects from mesoscopic or large-scale fiber bridging in fatigue fracture due to special fiber lay-up used in testing. For structural design with FRP composites, it is not sufficient to select the material performing "best" in the fatigue fracture tests, but it is essential to have safe, reliable, but on the other hand realistic design limits. Scatter in the data and how much of that has to be taken into account first requires understanding the extrinsic (test related) and intrinsic (material related) scatter sources. The concept of a fatigue threshold is also important for fracture-based structural design, since there are indications that threshold values for FRP composites may be quite low.