Wolfgang Hillger's research while affiliated with German Aerospace Center (DLR) and other places

... However, the alternative reception using air-coupled ultrasound features three major advantages, i.e. lower costs and a broader spatial measuring range [16]- [18]. In addition, the reception using ultrasonic phased arrays [19], [20] and spatial filtering provides directional information, allowing a more precise defect localization [21] and the scanning of multiple areas in parallel [2]. ...

... Generally, non-destructive testing (NDT) is adopted to timely identify the damage in composite structures, and prevention measures are taken accordingly to prolong their service life. Some of the NDT methods frequently used to determine the health state of the composite structures include visual inspection, acoustic emission, ultrasonic testing, optical testing, thermographic testing, shearography testing, and electromagnetic testing [12,[14][15][16][17][18][19]. The accuracy of NDE techniques largely depends on the operator's expertise, and the ability to interpret results with domain knowledge. ...

... The simulation model is capable of computing the LW propagation within the considered panels as well as their interactions with artificially inserted defects (blind hole, delamination and crack). The study was divided in two parts: The first part [1] , concentrated on the description of the modelling process with special focus on underlying assumptions, applied boundary conditions as well as principal model capabilities. In addition, the dispersion characteristics of the component were numerically determined. ...

... [14][15][16][17][18][19][20][21][22] Others are more focusing on specific topics of the field, such as on ferroelectret materials, 23 on theoretical models and numerical simulations, 24, 25 on measurement techniques, [26][27][28] on applications in general, 29,30 in health monitoring, 31 in enery harvesting, [32][33][34][35] and in air-coupled ultrasonic non-destructive testing. 36,37 In this article, we review and discuss the up-to-date and progress trend in the field of ferroelectret research by covering a broad range of topics, including developments in ferroelectret materials, charging mechanisms and charging methods of high efficiency, fundamental and applicaion-relavent aspects of the piezoelectric activity, and representative applications. We discuss mostly results obtained at University of Potsdam on the research and development of ferroelectrets. ...

... The LCT used to receive Lamb waves was pasted on the upper side of the CFRP plate, and the receiving elements of the LCT were numbered from Puc1 to Puc8. The shape, velocity, and amplitudes of Lamb waves were significantly affected by the ply orientation of CFRP [38]. The delamination damage quantification law obtained from CFRP with specific ply orientation did not apply to the delamination damage quantification in CFRP under other ply orientations. ...

... It consists of seven plies of different thicknesses composed into a laminate of the form [(0/90) f / + 45 / − 45 /(0/90) f ] S (outer fabrics: 0.4 mm, center fabric: 0.2 mm and unidirectional layers: 0.25 mm). The dispersion characteristics of the plate are determined by an air-coupled ultrasonic technique in the work of Schmidt et al. [3] for several directions. For the 0°-direction, the dispersion curves are given for the fundamental Lamb wave modes, i.e., A 0 and S 0 , in Figure 1 for both phase-and group wave velocities. ...

... 14. The velocity of the nodes on the retro-reflective layer is measured with scanning a laser vibrometer to evaluate the wave properties [22,12]. The flight velocity of the propagated waves are calculated in reduced-size numerical models and compared with experimental results . ...

... A proper optimization of test parameters allowed performing a one-sided procedure, and an additional evaluation of the frequency spectra provided more detailed information about the internal structure of the part, including the visualization of inserts and lightning protection foils. Also, an additional mathematical analysis highlighted the detected defects [7]. ...

... However, due to the excitation of Lamb wave by the PZT at lower frequencies, the overlapping of the transmitted S 0 mode, A 0 mode, and mode-converted modes were observed. 31 Hence, among the various ultrasonic transducers, air-coupled ultrasonic transducer can generate and detect Lamb waves in acoustically matching composites and can be quickly traversed across a large surface area. These factors led us to investigate how air-coupled ultrasonics can aid the development of quick Lamb wave (S 0 mode)-based inspection method for the application of delamination detection in T-joint. ...

... In this regard, particular questions to be answered are whether and (if yes) how plate wave anisotropy (and accompanying energy flow parameters) depends on frequency. The answers are of primary importance, e.g., in plate wave application for structural health monitoring (SHM) of composite components where the plate waves are used for collecting and conveying information in various directions over a large area of an anisotropic material [14,15]. ...