Load-bearing components made of composite laminates of several centimetres in thickness, for example those used in wind turbine blades, are frequently used in the energy sector. These components are usually tested using conventional ultrasound techniques. A typical approach to increase the energy penetration depth is testing with lower frequencies. This leads to a decrease in sensitivity and consequently to reduced detectability of small defects compared to higher frequencies, especially for defects close to the surface. Another possibility is to use high excitation voltage or gain to improve penetration, but this also leads to a much more pronounced initial pulse with saturated or clipped A-scans, resulting in a loss of information. Consequently, the defects close to the surface are often indistinguishable to the initial pulse and are not detected. In comparison to conventional ultrasonic testing, the total focusing method (TFM) shows higher resolution of near-surface defects using the same frequencies. The TFM can be adapted to anisotropic media by consideration of the direction-dependent wave propagation. Therefore, sound paths not perpendicular to the surface, which show less clipping, can be used for imaging. In this paper, approaches for improving the detectability of defects close to the surface in carbon fibre-reinforced plastic (CFRP) and aluminium using full matrix capture (FMC) and the TFM are discussed. As a result, defects in CFRP with a depth of 0.9 mm and above can be detected. The presented methods also improve the signal-to-noise ratio (SNR) of near-surface defects in the TFM reconstructions up to 4 dB. The first approach filters the FMC pulses in the wavenumber-frequency domain, which reduces the aforementioned disturbances in the time-domain signals and thus improves the detectability of near-surface defects. The second approach is based on a maximum angle in the reconstruction step, which reduces the entries of the information matrix based on location. This procedure is similar to taking the directivity function of each array element into account. Therefore, only time signals with a high signal-to-noise ratio are considered.

Air-coupled ultrasound sensors have advantages over contact ultrasound sensors when a sample should not become contaminated or influenced by the couplant or the measurement has to be a fast and automated inline process. Thereby, air-coupled transducers must emit high-energy pulses due to the low air-to-solid power transmission ratios (10−3 to 10−8). Currently used resonant transducers trade bandwidth—a prerequisite for material parameter analysis—against pulse energy. Here we show that a combination of a non-resonant ultrasound emitter and a non-resonant detector enables the generation and detection of pulses that are both high in amplitude (130 dB) and bandwidth (2 µs pulse width). We further show an initial application: the detection of reflections inside of a carbon fiber reinforced plastic plate with thicknesses between 1.7 mm and 10 mm. As the sensors work contact-free, the time of flight and the period of the in-plate reflections are independent parameters. Hence, a variation of ultrasound velocity is distinguishable from a variation of plate thickness and both properties are determined simultaneously. The sensor combination is likely to find numerous industrial applications necessitating high automation capacity and opens possibilities for air-coupled, single-side ultrasonic inspection.

The use of pulse-compression in ultrasonic non-destructive testing has assured, in various applications, a significant improvement in the signal-to-noise ratio. In this work, the technique is combined with linear phased array to improve the sensitivity and resolution in the ultrasonic imaging of highly attenuating and scattering materials. A series of tests were conducted on a 60 mm thick carbon fiber reinforced polymer benchmark sample with known defects using a custom-made pulse-compression-based phased array system. Sector scan and total focusing method images of the sample were obtained with the developed system and were compared with those reconstructed by using a commercial pulse-echo phased array system. While an almost identical sensitivity was found in the near field, the pulse-compression-based system surpassed the standard one in the far-field producing a more accurate imaging of the deepest defects and of the backwall of the sample.

The article “Contextual Application of Pulse-Compression and Multi-frequency Distance-Gain Size Analysis”.

Large rotor forged parts, which are usually one of the most critical components in land-based turbines and generators for power generation, require a complex volumetric test for a sufficient service life. This is usually performed manually or automatically with ultrasound. New requirements, designs and materials require more sensitive testing. This can be achieved by SAFT, also called ultrasound computer tomography. SAFT is based on the Synthetic Aperture Radar (SAR) and has been further developed by several universities. The introduction of SAFT in the volume production of large forged parts was achieved by the introduction of the quantitative SAFT developed by Siemens, also called AVG or DGS-SAFT, which allows an evaluation of each voxel in units of a replacement reflector, and by an acceleration that allows the reconstruction of the complete volume of a large forged component, which could be obtained when the SAFT test was introduced into volume production. The challenges for level 2/3 reviewers are discussed, such as volume-corrected display of results, handling of large amounts of data, focusing of displays, amplitude representation in units of a replacement reflector and handling of the software. Furthermore, it is shown how displays are represented by SAFT, how the detection limit can be determined in the case of quantitative SAFT, and which artifacts can occur during series testing with SAFT.

In the rapidly expanding composite industry, novel inspection methods have been developed in recent years. Particularly promising for air-coupled testing are cellular polypropylene transducers which offer better impedance matching to air than piezoelectric transducers. Furthermore, broadband transmitters (laser-induced ultrasound and thermoacoustic emitters) and receivers (optical microphones) have opened a completely new chapter for advanced contact-free ultrasound inspection. X-ray dark-field radiography offers a different approach to detect porosity and microcracks, employing small angle X-ray scattering. These innovative ultrasonic and radiographic alternatives were evaluated in comparison with well-established inspection techniques. We applied thirteen different non-destructive methods to inspect the same specimen (a carbon fiber-reinforced polymer laminate with induced impact damage): air-coupled ultrasound testing (using piezoelectric transducers, broadband optical microphones, cellular polypropylene transducers, and a thermoacoustic emitter), laser-induced ultrasound testing, ultrasonic immersion testing, phased array ultrasonic testing, optically excited lock-in thermography, and X-ray radiography (projectional absorption and dark-field, tomosynthesis, and micro-computed tomography). The inspection methods were qualitatively characterized by comparing the scan results. The conclusions are advantageous for a decision on the optimal method for certain testing constraints.

In the field of ultrasonic testing there are two key questions: Which defects can be found and – in the case indications are found – do they restrict the use of the part? Regarding both questions, the prerequisite is a method for defect sizing. Over the last decades sizing methods were established like DGS (Distance Gain Size) or DAC (Distance Amplitude Correction) for defects smaller than the beam profile. Those methods utilize the echo amplitude and provide results which are proportional to the defect area. However, those approximations are only accurate for defects larger than one wavelength even that experience shows it can be applied for slightly smaller defects. With the progress of material technology and ultrasonic inspection the need to detect and size smaller defects is growing. Therefore, both for flat bottom holes and disc shaped reflectors the usability for small defects needs to be checked. In this publication, it is investigated how to correctly size small defects below one wavelength. Utilizing a grid-based simulation method the echo signals of cylinder and disc shaped reflectors of various sizes are calculated. By properly choosing the simulation method and grid it is ensured that all physical wave modes are included in the simulation and that the discretization error is negligible. A good correspondence between the simulation and classical defect sizing for defects larger than one wavelength is found. In the region between one quarter of a wavelength and one wavelength resonance effects are found, which result in classical defect sizing methods giving conservative results. In the region below one quarter of a wavelength classical DGS and DAC sizing leads to undersizing. This is discussed in detail and a formula for defect sizing is derived, which is applicable to small as well as large defects.

Air-coupled ultrasound (ACU) is increasingly used for automated and contactless inspection of large-scale composite structures as well as for non-destructive testing (NDT) of water-sensitive or porous materials. The major challenge to overcome using ACU in NDT is the enormous loss of ultrasonic energy at each solid-air interface caused by the high acoustic impedance mismatch. Resonant low-frequency piezoceramic transducers are specially designed to achieve high sound pressure levels. For an expanded use of this technique, however, the spatial resolution needs to be increased. Recent studies of our collaborative research group demonstrated the successful application of a resonance-free, highly sensitive receiver that uses a Fabry-Perot etalon instead of piezoceramic materials or membranes. However, to reach the full potential of this broadband small-aperture optical microphone, novel transmitter concepts have to be developed and evaluated for advanced NDT applications. Different types of transmitter were tested in combination with the optical microphone acting as receiver and they were compared to conventional piezoceramic transducers in through-transmission mode. Monolithic carbon fiber-reinforced plastics (CFRP) and CFRP sandwich structures containing different defect types were inspected. Presented results are processed as C-scan images and further evaluated for spatial resolution, signal-to-noise ratio and sensitivity of each measurement setup. Novel transmitter concepts, such as ferroelectret and thermoacoustic emitters, show promising findings with a considerably improved time and spatial resolution for ACU-NDT.

Structural composite components of several centimeters thickness are increasingly used in the energy industry. Consequently, the demand for more reliable and sensitive nondestructive testing techniques for such structures is also rising. In the case of ultrasonic testing, lower test frequencies are often used to temper prominent effects like scattering or absorption in thicker polymer matrix composites. This facilitates the inspection but also reduces the sensitivity to small reflectors. Novel array reconstruction algorithms such as the total focusing method (TFM) counteract this issue. Recent studies of our group demonstrated that modifications of the TFM are necessary to reconstruct full matrix capture (FMC) acquired data for carbon fiberreinforced polymers (CFRP). First, constructive superposition of reflector signals is achieved by considering the anisotropic sound velocity within the material. Second, the noticeably smaller divergence angle of a single-element transmission, compared to that in isotropic metals, should be taken into account. As a result, the signal-to-noise ratio (SNR) in the ultrasonic image and the detectability in the near-subsurface region (close to the transducer) is increased. This paper describes the successful application of an adapted TFM algorithm to image small artificial defects in a 20 mm thick CFRP laminate. Of particular concern is the evaluation of its imaging performance to reconstruct side-drilled holes (1.0 mm and 3.0 mm in diameter) located at different depths. The SNR and the spatial resolution of the reconstructed images are evaluated and optimized. Ultrasonic imaging artifacts that occur in the TFM images are discussed. This contribution also addresses the challenge of inspecting thick and highly attenuative composites using the total focusing method. To this end, two similar arrangements of side-drilled holes for aluminum and CFRP were examined and their TFM results compared.

Forgings, being usually one of the most critical components especially in power generation machinery, require intensive volumetric inspection to guarantee a sufficient lifetime. This is usually accomplished by manual or automated ultrasonic testing. The authors are reporting about a game changer in ultrasonic testing: Ultrasonic Computed Tomography uses analytics (i.e., a mathematical algorithm) to reconstruct the volume (In fact it uses a linearized diffraction tomographic approach for the solution of the inverse problem). This does not only allow to display indications spatially and visually correct in the 3D volume, but also improves the signal to noise ratio significantly, allowing an increase of sensitivity by up to an order of magnitude. The method is based on the Synthetic Aperture Focusing Technique (SAFT). The applied software is a brand‐new implementation of SAFT with a strong focus for a large scale industrial application: the complete 2D as well as 3D reconstruction of ultrasonic inspections of heavy rotor forgings. This paper shows the working principle of the method along with the first results and computation times. Ultrasonic Computed Tomography is also awarded by the Werner von Siemens Award as one of the Top 15 ingenuity programs.

Sizing of indications and the determination of the inspection sensitivity are core tasks in ultrasonic testing. They ensure that critical inhomogenities can be detected and assessed. For the Synthetic Aperture Focussing Technique (SAFT) the sizing of indications is currently performed by evaluating the spatial extent in the reconstruction result, i.e. counting the number of voxels belonging to an indication. This limits sizing to the resolution of SAFT, which is in the order of one wavelength. For smaller defects si zing is not possible. Moreover, no information about the sensitivity of the inspection method is provided. Compared to classical UT inspection SAFT significantly improves the signal-to-noise ratio (SNR), regarding stochastic noise and grain noise. However, this improvement of the sensitivity cannot be utilized adequately without a met hod for determination of the sensitivity and for sizing small indications. Similar to classic UT there is a second option to gain information about indications: The amplitude sum, which incorporates the echo amplitude, the angle dependent scattering characteristics, the probe parameters and the specimen geometry. In this paper we show that the amplitude sum is suitable for sizing of small indications and for the determination of sensitivity. The relation between the amplitude sum and the size of indications is explained, and it is shown, that the location of indications and the shape of the object to inspect must be considered. Based on these results a method for sizing of small indications using SAFT is developed which translates the amplitude sum of each voxel in an equivalent reflector size, similar to the Distance Gain Size method (DGS) in classical UT. This sizing method completes SAFT, growing it from an imaging tool to a full-fledged quantitative measurement technique.

Within the frame of the publicly cofinanced “MAIzfp” project, a highly promising ultrasonic data acquisition technique, termed Full Matrix Capture (FMC ), is used in this paper to detect real defects in carbon fiber-reinforced plastics (CFRP) (i.e. impact damages). With this approach, a full raw data set of ti me domain signals (A-Scans) from every transmitter-receiver pair in an array probe is individ ually collected, stored and subsequently reconstructed as an image. The most comm on reconstruction algorithm for FMC data is the Total Focusing Method (TFM) in which all individually collected signals are time-shifted and combined to synthesize a focus at every voxel in the resulting image. This algorithm assumes a point source model where spherical ultrasonic waves are emitted at a particular propagation velocity. These assumptions may apply in particular test situations, but not when conducting FMC on CFRPs . Especially, directional sound velocity variations within these composites have to be considered in the image reconstruction procedur e. This paper describes the essential requirements (e.g. influence of the beam directivity) to successfully apply Full Matrix Capture for the testing o f anisotropic and inhomogeneous CFRP materials. Of particular concern is the modification of the TFM algorithm in order to incorporate the anisotropic sound velocity data of CFRP. Therefore, we present two methods to obtain an angular sound velocity profile. First, the experimental Backwall Reflection Method and second, a for w rd modeling approach. FMC measurements on impact damages are performed with a standard phased array controller using a sound velocity corrected TFM algo rithm.

Ultrasonic nondestructive testing of steel forgings aims at the detection and classification of material inhomogeneities to ensure the components fitness for use. Due to the high price and safety critical nature of large forgings for turbo-machinery, there is great interest in the application of imaging algorithms to inspection data. However, small flaw indications that cannot be sufficiently resolved have to be characterized using amplitude-based quantification. One such method is the distance gain size method, which converts the maximum echo amplitudes into the diameters of penny-shaped equivalent size reflectors. The approach presented in this contribution combines the synthetic aperture focusing technique (SAFT) with an iterative inversion scheme to locate and quantify small flaws in a more reliable way. Ultrasonic inspection data obtained in a pulse-echo configuration are reconstructed by means of an Synthetic Focusing Technique (SAFT). From the reconstructed data, the amount and approximate location of small flaws are extracted. These predetermined positions, along with the constrained defect model of a penny-shaped crack, provide the initial parametrization for an elastodynamic simulation based on the Kirchhoff approximation. The identification of the optimal parameter set is achieved through an iteratively regularized Gauss-Newton method. By testing the characterization method on a series of flat-bottom holes under laboratory conditions, we demonstrate that the procedure is applicable over a wide range of defect sizes. To show suitability for large forging inspection, we additionally evaluate the inspection data of a large generator shaft forging of 0.6-m diameter.

SAFT (Synthetic Aperture Focusing Technique) has substantial benefits over conventional UT and becomes increasingly popular due to the computational power available nowadays. In addition to more precise defect localization, also better separation of group indications and the improvement in grain induced (as well as electronically induced) signal-to-noise ratio (SNR) are important factors for utilizing SAFT. For practical application, it is not only important to ensure sufficient detection sensitivity but also to develop an economic solution. Besides system costs the inspection duration (respectively the throughput) is the key parameter to be optimized. Hence, the question for the optimum inspection grid size arises. Usually the scan grid for SAFT inspection is chosen empirically as this question has not been investigated sufficiently yet. The criterion that the sound beams shall cover the volume to inspect at least n-times – which is used for conventional ultrasonic inspection – is not helpful for SAFT. Of course, at least a single coverage has to be demanded, but tests have shown that this is not sufficient usually. Within this contribution the influence of the inspection grid is investigated in detail. The connection between the choice of the inspection grid and the appearance of artifacts is shown. By specifying a limit for the maximum tolerable artifact amplitude a criterion for the choice of the scan grid can be derived. The influence of specific inspection parameter, such as frequency, sound velocity, beam angle, and beam divergence is investigated systematically and it is derived, how the inspection grid can be optimized for SAFT inspection.

Zusammenfassung Große zylindrische Stahlprüflinge werden mittels der Methode der finiten Differenzen im Zeitbereich (engl. finite differences in time domain, FDTD) simulativ untersucht. Dabei werden Pitch-Catch-Messanordnungen verwendet. Es werden zwei Bildgebungsansätze vorgestellt: ersterer basiert auf dem Imaging Principle nach Claerbout, letzterer basiert auf gradientenbasierter Optimierung eines Zielfunktionals.