Many existing offshore wind farms (OWF) in European waters are coming to the end of their original design life in the next decade. Life extension offers the potential to continue their operation and to contribute to renewable energy generation and thus sustainability targets. Typically, these offshore wind turbines were constructed with a monopile embedded in sandy soil as the most efficient support structure in low to medium water depths. Fatigue due to variable wind and wave loading and corrosion due to the maritime conditions are the typical damage mechanisms that need to be considered. Critical weld locations below the mudline experience high and varying bending loads but are inaccessible for manual or visual inspection.

The feasibility of employing guided ultrasonic waves to detect corrosion and fatigue defects in the inaccessible, submerged part of the monopiles was investigated from Finite Element (FE) simulations and experiments on a laboratory scaled prototype. A prototype was manufactured from welded sections to approximate the monopile of a 5 MW offshore wind turbine on a scale of 1:25 with the correct ratios of length, diameter, and wall thickness. The tank was filled with 1.2 m of sand and 0.8 m of water from the bottom (saturated sand).

Experiments were conducted employing the T(0,1) torsional guided wave mode excited at the top end above the water level to monitor the wave propagation and sensitivity for defects. The effects of embedment in water and sand on the attenuation of the guided waves were quantified and compared to theoretical predictions to ascertain that the complete length of the monopile can be monitored. An artificial defect (notch) approximately 0.25 m below the mudline was introduced and increased in severity in stages and the sensitivity for the detection of defects at a circumferential weld verified.

FE calculations using the ABAQUS software were conducted to aid the selection of suitable guided wave modes and excitation frequencies, and the guided wave propagation across the multiple welded parts with thickness changes simulated. Good agreement with the experimental results was achieved. The guided ultrasonic wave data could be combined with load and stress predictions to facilitate fatigue reliability analysis based on Structural Health Monitoring (SHM).

Guided ultrasonic waves enable the structural health monitoring of thin-walled structures. For optimal damage detection sensitivity, selecting the frequency and excitation signal is crucial. A broadband structural identification using frequency response functions allows subsequent evaluation of different virtual excitation signals and frequencies, significantly reducing the experimental effort. The signal reconstruction uses virtual excitation signals within the investigated frequency range to generate corresponding response signals. This paper presents a study on signal reconstruction accuracy to evaluate the validity of applying frequency response functions in guided ultrasonic wave applications.

A standard laboratory structural health monitoring setup consisting of a signal generator, high-voltage amplifiers, piezoelectric transducers, an aluminum specimen, and a digital oscilloscope is used. Different broadband excitation signals, i.e., noise and sweep signals, are employed to determine the frequency response functions in a frequency range up to 500 kHz. To evaluate the signal reproducibility, an error measure according to Sprague and Geers (Sprague and Geers 2004), to assess amplitude, phase, and comprehensive errors separately, is applied. A second evaluation approach is followed to account for a more structural health monitoring applicable feature, the time-of-flight.

In a frequency range up to 500 kHz, the signal reproducibility error is above 5% for signals reconstructed using the frequency response function determined by noise excitation. When using the sweep excitation to identify the frequency response function, the signal reproducibility error exceeds 5% only above 250 kHz. The time-of-flight is reconstructed with higher accuracy in both approaches.

In conclusion, this work shows that the type of excitation signal for frequency response function determination and the threshold values used to evaluate the required reconstruction accuracy must be carefully selected and tailored to the specific application. The numerical values in this study are not to be generalized. They apply only to this specific setup, but they demonstrate that verifying the underlying assumptions is crucial for guided ultrasonic wave-based structural health monitoring systems that employ frequency response functions.

Structural degradation in rail heat exchangers—especially corrosion-driven wall thinning—can lead to leakage, unplanned downtime, and elevated life-cycle costs. While SHM is established in many domains, monitoring compact, fluid-filled heat exchangers under flow and temperature variations remains underexplored. This work investigates an integrated approach that combines guided ultrasonic waves for damage detection with inline refractometry for coolant state monitoring to support condition-based maintenance of rail cooling systems. The objectives are to demonstrate reliable detection and trending of corrosion-like wall thinning under realistic hydraulic and thermal conditions, to assess scalability to larger units, and to establish coolant refractive-index sensing as a complementary observable for future data fusion.

Laboratory heat exchangers were instrumented with surface-bonded piezoelectric transducers forming a sparse actuator–sensor network. Units were circulated with a 50:50 Antifrogen–water mixture and exposed to temperature profiles from −20 to +70 °C in a climatic chamber. Corrosion-like defects were introduced stepwise by electro-etching at defined locations. A path-wise damage index based on baseline subtraction and normalized cross-correlation was computed and single- and multiple-damage scenarios were evaluated. Scalability was investigated on a larger prototype instrumented with 16 transducers. In parallel, an inline refractive-index sensor was integrated via a pressure-tight, additively manufactured (SLS) bypass flow cell; automated spectral-peak extraction and temperature compensation enabled conversion to refractive index and mixture concentration. On a larger prototype, sufficient signal quality was achieved despite increased attenuation, indicating feasibility for scale-up. Therefore, coupled tests on a full-scale system in an industrial test facility were conducted.

The results evidence the feasibility of guided-wave SHM for detecting and trending corrosion in flowing heat exchangers under realistic thermal loads and establish inline refractometry as a complementary modality for coolant-state observables. The approach supports condition-based maintenance of rail cooling systems with potential to reduce maintenance costs and unplanned outages.

The demand for Non-Destructive Evaluation (NDE) of piled structures has increased as it has become evident that the grout, which bonds the pile to the rest of the structure, is degrading faster than its intended service life. Monitoring the condition of the grout layer is, therefore, essential for assessing the grout integrity. Traditional NDT methods are often ineffective at inspecting the grout layer. In this context, Guided Ultrasonic Waves (GUWs) provide an effective alternative because they can travel over long distances, and penetrate into the grout region, which conventional NDT techniques are often unable to reach. This study investigates the application of GUWs for defect detection in a scaled multilayer foundation model representative of offshore structures. The grout was acoustically characterized to determine its complex stiffness moduli, which were used to compute the dispersion curves needed for guided-wave generation and reception. The S1 and A1 Lamb wave modes were generated using a 121-element 2D matrix phased array, allowing 360° beamformed excitation and imaging. Despite the high damping of the grout and the complexity of the structure, guided wave signals successfully identified internal defects and boundaries, confirming the feasibility of GUWs for NDE in multilayer grout–steel systems.

Corrosion-induced degradation remains a major challenge for maintaining the structural integrity of industrial components, particularly in high-risk sectors such as oil and gas, marine engineering, and energy. The problem is especially critical for naval and ship structures, which are exposed to harsh environmental conditions for the majority of their service life. The standard method for assessing corrosion relies on point-by-point ultrasonic thickness measurements; however, this approach is time-consuming and provides only limited insight into the overall condition of a structure. This motivates the development of alternative techniques capable of assessing larger areas within a single measurement.

This paper presents several recent advances in evaluating corrosion severity in plate-like structures using guided ultrasonic waves (GUWs). We emphasize the statistical characterization of thickness variations in corroded plates and introduce dedicated algorithms capable of estimating key statistical parameters, including mean thickness and standard deviation, thereby providing a quantitative description of corrosion-induced geometric changes. Multiple approaches are discussed, including methods based on waveform asymmetry and asymmetry indices, wave velocity variations, and changes in signal energy caused by corrosion damage. The advantages and limitations of each approach are analyzed.

The proposed methodology integrates theoretical modeling, experimental studies, and numerical simulations carried out in Abaqus/Explicit, where random-field techniques are used to reproduce the stochastic nature of corrosion-induced surface roughness. Experimental validation was performed on steel plates subjected to accelerated corrosion processes in a dedicated corrosion tank that provided conditions approximating real marine environments (i.e., circulating saltwater, elevated temperature, and controlled oxygen concentration).

The results confirm the effectiveness of the proposed methods in accurately characterizing the geometry of corroded plates, even in cases of complex surface degradation. These techniques have the potential to enhance nondestructive evaluation of structural components and offer a promising alternative to conventional ultrasonic thickness gauging.