Digital beamforming methods in plate–like structures are widely exploited for Lamb waves-based damage imaging. Among them, the Delay and Sum (DAS) imaging technique is the most popular thanks to its low computational cost and ease of implementation. However, the imaging outputs are low–quality due to the high levels of side lobes and limited off-axis signal rejection, which leads to limited image resolution and contrast. Recently, the Delay Multiply and Sum (DMAS) beamforming has been applied to NDT field as a promising DAS alternative able to enhance the imaging reconstruction in terms of contrast and damage detectability. However, DMAS is still affected by high levels of artefacts. To tackle this aspect, literature offers a beamforming algorithm called Double Stage DMAS (DS–DMAS), firstly introduced in photoacoustic imaging and medical ultrasound imaging. In this paper, the DS–DMAS performance is analysed for Lamb waves inspection, in order to provide an exhaustive comparison between DAS, DMAS and DS–DMAS. As a further step, a filtering process addressed as Fresnel Zone Filtering (FZF) is used to restrict the beamforming partial sums in a physical way to the area around the scattering point. The proposed approach is an adaptation of a well–established technique in seismic data processing called Fresnel migration, able to suppress artefacts and enhance the quality of the imaging. The algorithms have been compared and characterized by exploiting an online free data set for guided waves inspection (http://openguidedwaves.de/) which collects piezo pitch–catch signals travelling through a quasi-isotropic Carbon Fiber Reinforced Plate (CFRP) at different actuated frequencies and damage positions.

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.

Corrosion is an irreversible form of damage to pipe materials, which leads to the degradation of their mechanical and chemical properties. Corrosion damage reduces the lifespan of materials and, in some cases, can even lead to catastrophic failures. Therefore, it is essential to detect corrosion damages and develop effective preventative measures to maintain the structural integrity of the pipelines. In recent times, ultrasonic guided wave testing techniques have been employed to detect and monitor corrosion damage as they are useful for scanning large areas and conducting tomographic analyses. The obtained guided wave signals are then analyzed using the Reconstruction Algorithm for Probabilistic Inspection of Damage (RAPID) to obtain imaging of the damages. However, conventional RAPID assigns the same signal difference coefficient to all reconstruction points along a sensing path, which limits its ability to capture localized signal variations. In addition, it lacks a mechanism for evaluating signal reliability, making it prone to false positives caused by noise. Thus, this paper proposes an improved RAPID-based algorithm by employing a local signal difference coefficient (LSDC) and reliability coefficient (RC) to ensure that damage is accurately detected. The LSDC is employed to enhance the sensitivity to damage by capturing localized signal variations, while the RC is developed to weigh the signal reliability. The experimental results demonstrate that the proposed method accurately predicts corrosion locations. It maintained a strong overlap between the predicted and the actual defect locations, with an overlapping rate between 73.02% and 79.43% throughout all cycles.

In order to reduce the CO2-emissions and to increase the energy efficiency, the operating temperatures of power plants will be increased up to 720°C. This demands for novel high-performance steels in the piping systems. Higher temperatures lead to a higher risk of damage and have a direct impact on the structure stability and the deposition structure. Adequately trusted results for the prediction of the residual service life of those high strength steels are not available so far. To overcome these problems the implementation of an online monitoring system in addition to periodic testing is needed. RWE operates the lignite power plant Neurath. All test and research activities have to be checked regarding their safety and have to be coordinated with the business operation of the plant. An extra bypass was established for this research and made the investigations independent from the power plant operating. In order to protect the actuators and sensors from the heat radiated from the pipe, waveguides were welded to the bypass. The data was evaluated regarding their dependencies on the environmental influences like temperature and correction algorithms were developed. Furthermore, damages were introduced into the pipe with diameters of 8 mm to 10 mm and successfully detected by the acoustic method.

Offshore foundations for wind turbines are expected to increase in significance over the coming years. Various wind parks are already producing energy and there are many more planned or currently being built. The maintenance of these offshore wind turbines is challenging due to the limited accessibility and expensive logistics. Currently, 25% of these structures have to be evaluated each year regarding their condition and stability. For this task, welded seams are the focus of interest. At the moment, they are visually inspected by divers; in the oil and gas industry, the alternating current field measurement (ACFM) technique is used as well. None of these are applicable on the inside of jackets, for example, due to safety issues. Visible inspection is often limited by the sight conditions and ACFM can only measure directly under the sensor. The Fraunhofer IKTS has therefore developed a transducer ring to be placed permanently on the foundations of offshore wind turbines. It is particularly well suited for the monitoring of jackets. The measurement device, CoMoSeam, has been successfully tested underwater. Furthermore, an artificially-initialised crack could be detected and located correctly. The investigation is realised by guided waves, which have a lower frequency range than the commonly used ultrasound techniques. The advantage lies in the reduced number of sensors compared to NDT ultrasound techniques, which also leads to a lower resolution. Nevertheless, the resolution is decreased but is still far better than that achieved by the currently used inspection methods. This paper presents the hardware used for building the sensor ring as well as the measurement technique. The lamination required to ensure that the equipment is waterproof is especially challenging due to the large diameters demanded for offshore platforms. To detect cracks correctly, even in harsh environments, sophisticated data processing is necessary to automatically eliminate all obviously incorrect data. The method is just being introduced in the regulations and will be adapted to a diver-free installation and operating regime.