Tobias Gaul

Pipes made of glass fiber reinforced plastic (GFRP) are used in a wide range of applications. The combination of high strength and rigidity with low weight and excellent corrosion and chemical resistance makes them superior to conventional plastic or metal pipes in many areas of application. They are characterized by their chemical resistance to a wide range of acids and salts, lack of electrical conductivity and good tightness. Due to their properties, they are primarily used in drinking water and sewage pipes, in the chemical industry, as pressure pipes and downhole tubing and casings in geothermal applications. Matrix cracking and, to a lesser extent, delamination are considered to be the main reason for the failure of GFRP pipes. This involves the detachment of laminate layers or the liner from the structural composite, which can lead to leaks or reduced load-bearing capacity and sudden failure. Matrix cracking and delamination are primarily caused by existing operating loads. GFRP pipes are primarily tested using visual inspection, thermography, ultrasound or acoustic emission testing. Apart from acoustic emission testing, it is not possible to test or monitor large areas. Therefore, guided waves are another promising testing method where waves propagate along the pipe and are guided by the inner and outer surfaces. Compared to ultrasonic waves, guided waves are less strongly attenuated and can propagate several meters through pipes which enable monitoring large areas. In this paper, guided waves were used to distinguish between matrix cracked/delaminated and intact pipe segments. During a pressure test, pipes were pressurized to burst, and matrix cracked/delaminated areas occurred as a result. Measurements were then taken on five damaged pipe segments and, with five intact pipes serving as a control group. Statistical methods were implemented to identify the damaged pipes and to distinguish from the control group of intact pipes. A network of piezoelectric sensors on the pipes was used to send and receive guided waves. Repeated application of the network to the pipes caused deliberate variability in the measurement data, which is used to evaluate the fault detection probability of the method. To compare the measurements with a model, the dispersion curve of the GFRP pipes was calculated in advance and the recorded measurement data was verified against it. With an adapted model for the material parameters, the combined damage mechanisms in the pipes are also simulated using the dispersion curve.

Hydrogen, as an energy carrier, is finding its way into more and more areas of application, such as on-board tanks for passenger cars, trucks and buses, high-pressure buffer storage at refueling stations, stationary and backup power systems, emerging rail/ship/offshore demonstrators, and industrial/laboratory gas supplies. Safe, available, and cost‑effective hydrogen storage demands monitoring solutions that operate continuously without disrupting service. We present an application‑driven Acousto‑Ultrasonic (AU) concept for Type IV pressure vessels that integrates a dense network of piezoelectric transducers into the outer composite structure via a low‑temperature secondary lamination process. The approach targets guided‑wave interrogation across the cylindrical wall to detect and localize damage relevant to hydrogen service while fitting existing manufacturing and maintenance workflows. A 9 L, 300 bar Type IV vessel (PET liner, carbon fiber overwrap) serves as the demonstrator. The sensing network comprises three circumferential rings of piezoelectric transducers with triangular spacing (typically 3–4 nodes per ring). Signal routing is realized on a thin, conformal multilayer flex PCB that is co‑bonded to the tank and fully encapsulated under glass‑fiber/epoxy cover plies, yielding a total buildup below ~1.5 mm and preserving aerodynamic and handling requirements. The piezoelectric sensors themselves are soldered to the PCB, ensuring a reliable connection. A single external interface—implemented as a low‑profile, keyed, magnetically latched pogo‑pin connector—enables rapid, tool‑less attachment of AU instrumentation while maintaining a sealed, snag‑resistant surface during operation. Process development focused on field viability: vacuum‑assisted hot‑bonding below 90°C avoids thermal over‑exposure of the polymer liner; surface preparation and layup sequences were optimized to minimize voids and ensure adhesion on curved substrates. Trials on representative coupons, metal cylinders, and composite tanks confirmed reliable conformity of 200–350 µm interconnects, robust anchoring of local steel backings for the removable connector, and repeatable lamination with minimal porosity when surfaces are abraded and vacuum levels ~130 mbar are maintained. The measurement concept uses a pitch–catch scheme—one actuator, all others as receivers—with comprehensive shielding and a star‑routed ground to enhance signal integrity over longer runs and reduce the number of required bulkhead connections. The primary impact of this work is twofold. First, it demonstrates a retrofit‑compatible path to deploy guided‑wave SHM on certified vessels without altering the original winding or requiring autoclave processes. Second, it provides a manufacturable blueprint—sensor placement rules, interconnect architecture, connectorization, and a validated lamination window—that scales to larger vessels and higher sensor densities. Ongoing testing includes AU baseline stability under pressure/temperature, defect detectability in the hoop regions, and durability. The results support condition‑based maintenance of hydrogen vessels, enabling reduced inspection downtime, expanded coverage of otherwise inaccessible regions, and improved fleet safety at manageable integration cost.

Monitoring infrastructure with acoustic methods and conventional spatially distributed sensor networks such as piezoelectric transducers is often limited to critical parts due to the high costs of instrumentation. With the new Distributed Acoustic Sensing (DAS) technology highly temporal and spatially sampled data can be collected by a single fiber optic cable over a long distance up to several kilometers. Critical problems associated with conventional sensor systems, such as electromagnetic shielding, explosion protection and robustness under extreme conditions, do not arise with this technology. . Nevertheless, challenges remain regarding adapted data processing and achieving comparable sensitivity as conventional systems especially for high frequency signals. This contribution deals with applications in harsh environments using the exemple of offshore applications and addresses the sensitivity of the measurements compared to classic non-destructive testing with acoustic emission. Multiple data processing and cable positioning strategies are considered to enhance sensitivity and shift the upper frequency limit.

In coal-fired power plants, the flue gas is transported through fibreglass-reinforced pipelines to cooling towers during the combustion of the respective raw material. These glass-fibre reinforced pipelines are usually located at great heights and can have very large diameters of several meters. Up to now, testing has been carried out as part of inspections during planned shutdowns and is associated with corresponding costs. The article is discussing the application of elastic wave measurement methods in hindsight of a 4-month field test at the power plants in Niederaussem and Neurath in Germany. The active measurement method, based on evaluation of guided elastic waves, was used to investigate the relationship between elastic material characteristics of the aged laminates and the phase velocities of guided waves. For this purpose, various sample panels were extracted from the cleaned gas channels and material parameters were determined by destructive testing methods afterwards. These material parameters were then compared with experimentally determined phase velocities of different wave modes, which were excited by piezoelectric transducers and recorded using a laser Doppler vibrometer. Furthermore, the influence of residual deposits in the gas channels on guided waves was also investigated as this would give an opportunity to detect liquid or solid residuals during operation non-destructively. Therefore, laboratory tests were conducted with varying filling levels to find suitable excitation frequencies and configurations. Based on the previous results a 4-month field test at Neurath was carried out, with additional focus on the influence of changing environmental conditions on the measurement results.

The operation efficiency and safety of pressure vessels in the oil and gas industry profits from an accurate knowledge about the inner filling distribution. However, an accurate and reliable estimation of the multi-phase height levels in such objects is a challenging task, especially when considering the high demands in practicability, robustness in harsh environments and safety regulations. Most common systems rely on impractical instrumentation, lack the ability to measure solid phases or require additional safety precautions due to their working principle. In this work, another possibility to determine height levels by attenuation tomography with guided elastic waves is proposed. The method uses a complete instrumentation on the outer vessel shell and is based on the energy conversion rates along the travel path of the guided waves. Noisy data and multiple measurements from sparsely distributed sensor networks are translated into filling levels with accuracies in the centimeter range by solving a constrained optimization problem. It was possible to simultaneously determine sand, water, and oil phases on a mock-up scale experiment, even for artificially created sand slopes. The accuracy was validated by artificial benchmarking for a horizontal vessel, giving references for constructing an affordable prototype system.

Nowadays, fiber-reinforced plastics are found in numerous industrial applications, such as aerospace, automotive engineering, railway, and naval engineering. These materials have high tensile and flexural strengths and nevertheless a low density at the same time. The use of fiber-reinforced plastics is particularly relevant in areas where large masses have to be moved and accelerated. However, testing and monitoring these structures is still a challenge caused by the different damage behavior compared to metal structures. Non-visible structural changes, such as delaminations and fiber-fractures, may cause local degradation and finally the failure of the components. In this work, active and passive ultrasonic methods based on guided elastic waves are investigated for their applicability to carbon fiber-reinforced structures. Therefore, tensile tests with cyclically increasing load are carried out on specimens with different fiber orientations until complete failure. The acoustic emissions in the specimen during the load are recorded. As a second technique, actively excited guided waves are transmitted and received during the rest periods between the measuring ramps. Different parameters are extracted from the measured data, which allow the monitoring of the specimen’s degradation. A comparison of the results of the active and passive method follows. Finally, a combination of both methods is carried out addressing issues like its informative value and its sensitivity.

A piezoelectric fiber patch (PFP) is a transducer type that is suitable for guided-wave-based structural health monitoring (SHM) due to its light, thin, and flexible characteristics. In our previous work, a PFP-based transducer design for selective excitation of the zero-order shear horizontal wave mode (SH0) was introduced (shear horizontal PFP (SHPFP)). In this work, two modified SH0 wave PFP transducer designs are proposed: the rounded corner design and the dual design. The degree of improvement is determined by a numerical simulation and the dual design—the design with the most promise—is experimentally realized. Laser Vibrometry measured the generated wave field, confirming the results from the simulation. The new designs can generate an almost pure SH0 wave. The dual design has a very strong directivity that is useful for several guided-wave-based SHM applications. The conclusions on the design’s properties as a transmitter are also valid for its properties as a sensor due to the reciprocity of piezoelectric transducers.