The use of shear horizontal (SH) waves in nondestructive evaluation (NDE) and structural health monitoring (SHM) has significant advantages compared to the application of other wave modes. SH waves show no mode conversion while propagating parallel to the testing surface which leads to clear and well interpretable signals. Moreover, the leaky wave emission into fluid media is suppressed so that the loss of energy is small and the operating distance is correspondingly long. In plate-like structures the lowest-order SH mode (SH0) is free of dispersion and is therefore perfectly suited for long-range applications. With electromagnetic transducers (e.g. EMATs) SH waves can be directly generated and detected inside the material. However EMATs are relatively large and restricted to conductive materials. Conventional piezoelectric transducers have to be coupled by glue or highly viscous fluids in order to transmit shear forces. Moreover, they need a large backside seismic mass for the effective insertion of the shear forces. Both types of transducers are therefore not suitable for SHM applications. Piezoelectric Fibre Patches (PFPs) are well known from their adaptronic applications. However, they can also be used for the generation and detection of symmetric (S) and antisymmetric (A) Lamb waves in the frame of SHM with guided elastic waves. So far the excitation of SH modes with PFPs is very ineffective and only based on edge effects as a by-product of S- and A-mode excitation. In the present work a new concept for a specific SH transducer based on PFP technology is presented. In this type of sensor two separate fibre layers with converse polarization are tilted by ±45° relative to the electrode fingers. Numerical 3-D simulations based on the Elastodynamic Finite Integration Technique (EFIT) show that with this new PFP design SH mode generation and detection is much more effective than with the hitherto existing PFP concepts.

Ultrasonic immersion testing with conventional focused and unfocused transducers represents one of the most important measurement set-ups in nondestructive testing. In order to optimize the relevant UT configuration for specific materials and flaws a detailed understanding of the interaction between ultrasonic waves and specimen is essential. This includes the excited wave field in water, reflection and transmission at the surface, mode conversion between pressure and shear waves, shortening of the focal length in solids, interaction with the microstructure of the material, interaction with defects, reflections at the back wall including multiple reflections between top surface and back wall as well as the propagation path back to the transducer. Most of these aspects are in general guided by wave physics. This is of particular importance if the dominant wavelengths of the ultrasonic pulse are comparable to the lateral size of the flaw and/or the grain size of the microstructure. In the present work we present a numerical wave-physical modeling platform for ultrasonic immersion testing based on the Elastodynamic Finite Integration Technique (EFIT). Besides the characteristics of the transducer like diameter, focal length, frequency, pulse form, tilting angle etc., the platform is able to model the most important flaw geometries like cracks, notches, flat bottom holes, voids, inclusions, delaminations etc. Moreover, complex polycrystalline microstructures based on random isometric and textured Voronoi tessellations can be included and their frequency-dependent influence on the wave field can be studied. Numerical results of typical immersion testing applications are presented in terms of time-resolved wave front animations and pulse-echo A-Scans. These findings are briefly discussed and compared to experimental data.

This study presents a developed High Frequency Eddy Current testing system for special issues in surface analysis. Using the system allows characterization of wet conductive coatings on different substrates. The system is based on Eddy Current principles and comprises a sensor for non-contact coupling to wet coatings, as well as software based on multi-frequency algorithms for analyzing coating parameters, - with changes during the drying time, such as thickness and conductivity. The system can also be used to characterize the drying behavior of wet conductive lacquers deposited on carbon-fiber-reinforced plastic substrate and other substrates with low or non-electrical conductivity. A special application of the proposed system is to characterize the lightning protection of airplanes during its application. Due to the growing usage of carbon fiber composites in the aircraft industry, the necessity to protect against the damage caused by a lightning strike increases. Instead of copper-wire networks that are located on the surface of these materials, wet conductive coatings are being used now as an alternative. Conductivity of the wet layers should be homogeneous along the coated area in order to avoid a critical local current density in case of lightning strike. Therefore, the conductivity as well as the thickness of the wet conductive layers have to be controlled during the application and can be provided by usage of the developed Eddy Current system. Additionally, the developed High-Frequency Eddy Current System for analyzing wet conductive coatings can be implemented in many industrial applications where carbon-fiber reinforced plastic materials become increasingly important, such as automotive, wind power, aircraft industry, etc.

The wheelsets of the railway rolling stock are periodically inspected by non-destructive testing to ensure the safety in the passenger and freight transportation. A first generation of automatic test benches is applied since more than a decade for periodic in-service inspection of solid axles. These test benches using the ultrasonic phased array technique to detect transverse cracks at the surface of the axles of mounted wheelsets at an early stage. We present a new generation of these test benches using new phased array devices. The phased array devices are characterised by a large number of channels, a high signal-to-noise ratio and small dimensions. Hence multiple devices can be placed close to the ultrasonic probes, so that interferences, caused by long transducer cables, can be reduced. Furthermore the probes can operate in parallel and so the inspection time can be shortened. Adapted phased array probes allow the inspection of coated and thick coated axles with high sensitivity. Hence the preparation of the surface by sandblasting and the re-coating of the axles are not required. The new database-supported software controls the inspection process and allows a fast analysing and reporting of the inspection data by a clear and easy operable user interface. The costumers expect a long service life, low service costs and a high availability of the new test benches.

Resistance spot welding is an established joining technology e,g, in the automotive industry, in frame-and-body construction and in sheet metal forming. It is characterized by a high cost effectiveness and process reliability. The quality of spot weldings can be determined by destructive and non-destructive testing methods as well as by an indirect analysis of the process parameters. In contrast to a fuzzy parameter analysis NDT techniques allow for a direct quantitative evaluation of the spot welding. Cost-intensive destructive sampling tests can be reduced to a minimum. For this purpose several commercial and mobile ultrasonic NDT systems are available on the market. Most of them offer only a very limited C-Scan image resolution of the spot welding. With scanning acoustic microscopy (SAM) it is possible to obtain high-resolution B- and C-Scans and perform an additional quantitative analysis of HF A-Scans. This allows for a sophisticated analysis of the weld nugget. Therefore, SAM can be used as quantitative reference and calibration tool for commercial testing systems. In the present work the results of ultrasonic microscopy are compared with a commercial US testing system based on a matrix sensor. In this context a novel spectral evaluation technique with successive image correlation analysis is applied. It is further demonstrated that with high-frequency ultrasound it is possible to determine not only the lateral dimension (diameter) of the nugget but also its approximate thickness. The latter is obtained by analyzing the effective ultrasound attenuation caused by the interaction with the modified grain structure inside the nugget.

Nowadays most common ultrasonic transducers measure only one scalar quantity instead of an elastic displacement field. In the case of Lamb wave propagation, this lack of information is critical under conventional imaging conditions. For baseline based and baseline free crack detection will lead to incorrect localization and classification due to different dispersive phase velocities for each propagating mode. The presented approach provides an heuristic operator to identify and select certain Lamb modes in single component datasets measured by piezoelectric transducers. This allows for the suppression of undesired side–modes. A specific pre–processing step gives us the possibility to use single component imaging procedures like conventional Kirchhoff–, Fresnel– and Reverse-Time–Migration in the terms of physics in a more precise manner.

Reliability of piezoelectric transducers depends largely on internal adhesive layers connecting the active piezoelectric elements with matching layers and damping bodies. Direct sputtering of ZnO and AlN layers eliminates the presence of the adhesives and the resulting high frequency transducers (frequencies greater than 100 MHz) are very reliable. Unfortunately, the thickness of the layers is limited by the sputter process and thus, transducers with frequencies below 100 MHz are difficult to produce. To overcome this limitation and close the gap between classical transducers and thin layer transducers, we propose the deposition of AlN using a CVD process with a much higher deposition rate. It will be shown that AlN layers can be deposited on silicon and on cemented carbide substrates. AlN displays piezoelectric properties only when textured in certain grain orientations. Pole figures obtained for the deposited layers, as well as the SEM images of the films cross-sections indicate presence of strong texture in desired orientation within the film. The calculated texture coefficient varies from 5 to 8 depending on the substrate and the film thickness. We have also characterized the surface of the deposited films by use of atomic force microscopy. The topography images reveal regularly shaped crystallites with diameter in the range of single micrometers. Local piezoelectric properties were tested on nanoscale by use of piezo-mode AFM. The acquired piezo-mode images confirm local piezoactivity of the sample. The standard method to prove of the piezoelectricity is the determination of the piezoelectric constants. Additional to that, we apply the piezo-mode AFM technique, where we can map the piezoelectric coupling locally and highly resolved. This information provides hints to the structure/property-relation and therefore delivers starting points for property improvements.