TY - JOUR A1 - Bente, Klaas A1 - Rus, J. A1 - Mooshofer, H. A1 - Gaal, Mate A1 - Grosse, C.U. T1 - Broadband air-coupled ultrasound emitter and receiver enable simultaneous measurement of thickness and speed of sound in solids JF - Sensors N2 - 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. KW - Thermoacoustic emitter KW - Optical microphone KW - Air-coupled ultrasound KW - Local resonance KW - Thickness measurement KW - Thickness resonance PY - 2023 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-569533 DO - https://doi.org/10.3390/s23031379 VL - 23 IS - 3 SP - 1379 PB - MDPI AN - OPUS4-56953 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Shapovalov, O. A1 - Heckel, Thomas A1 - Gaal, Mate A1 - Weiß, S. T1 - External Acoustical Damping on a Metallic Angle Wedge in a High Temperature Resistant Ultrasonic Probe JF - Acoustics Australia N2 - Ultrasonic probes for high-temperature applications are provided with metallic wedges, which can withstand the contact with the high temperature of the inspected structure. The ultrasonic signal travels within the wedge and gets reflected from its boundaries, causing interference signals called “ghost echoes”. The current work presents an investigation of the additional damping effect provided by porous sintered metal plates applied onto the surface of the wedge. In particular, the study evaluates the effect of damping plate thickness on the interference signal level at different transmission frequencies. Damping plates made of sintered metal SIKA-R 15 AX were attached to a wedge prototype made of steel 1.4301. The study revealed, that the most effective thickness of damping plates in the selected frequency interval of 1 to 4 MHz is equal to 4 mm. The evaluation of the interference signal has shown that the application of such damping plates to the wedge surface contributes to an additional attenuation of an interference signal of 10 to 30 dB after 500 μs of signal propagation. KW - Ultrasonic testing KW - High temperature KW - Acoustical damping KW - Angle wedge KW - Ultrasonic probe PY - 2022 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-553920 DO - https://doi.org/10.1007/s40857-022-00270-9 SN - 0814-6039 SP - 1 EP - 11 PB - Springer AN - OPUS4-55392 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Essig, W. A1 - Bernhardt, Y. A1 - Döring, D. A1 - Solodov, I. A1 - Gautzsch, T. A1 - Gaal, Mate A1 - Hufschläger, Daniel A1 - Sommerhuber, R. A1 - Marhenke, T. A1 - Hasener, J. A1 - Szewieczek, A. A1 - Hillger, W. T1 - Air-coupled ultrasound - emerging NDT method JF - ZfP-Zeitung N2 - This paper deals with the state of the art of air-coupled ultrasonic testing. KW - Air-coupled KW - Ultrasonic testing KW - Transducers PY - 2021 SN - 1616-069X VL - 173 SP - 32 EP - 43 PB - Deutsche Gesellschaft für Zerstörungsfreie Prüfung (DGZfP) CY - Berlin AN - OPUS4-52231 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Rus, J. A1 - Gustschin, A. A1 - Mooshofer, H. A1 - Grager, J.-C. A1 - Bente, Klaas A1 - Gaal, Mate A1 - Pfeiffer, F. A1 - Grosse, C. U. T1 - Qualitative comparison of non-destructive methods for inspection of carbon fiber-reinforced polymer laminates JF - Journal of Composite Materials N2 - 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 ultra-sound 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 thermoa- coustic 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. KW - Carbon fiber-reinforced polymer KW - Air-coupled ultrasound KW - Optically excited lock-in thermography KW - X-ray micro-computed tomography KW - X-ray dark-field radiography PY - 2020 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-509473 DO - https://doi.org/10.1177/0021998320931162 SN - 0021-9983 VL - 54 IS - 27 SP - 4325 EP - 4337 PB - SAGE AN - OPUS4-50947 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Tiitta, M. A1 - Tiitta, V. A1 - Gaal, Mate A1 - Heikkinen, J. A1 - Lappalainen, R. A1 - Tomppo, L. T1 - Air-coupled ultrasound detection of natural defects in wood using ferroelectret and piezoelectric sensors JF - Wood Science and Technology N2 - Air-coupled ultrasound was used for assessing natural defects in wood boards by through-transmission scanning measurements. Gas matrix piezoelectric (GMP) and ferroelectret (FE) transducers were studied. The study also included tests with additional bias voltage with the ferroelectret receivers. Signal analyses, analyses of the measurement dynamics and statistical analyses of the signal parameters were conducted. After the measurement series, the samples were cut from the measurement regions and the defects were analyzed visually from the cross sections. The ultrasound responses were compared with the results of the visual examination of the cross sections. With the additional bias voltage, the ferroelectret measurement showed increased signal-to-noise ratio, which is especially important for air-coupled measurement of high-attenuation materials like wood. When comparing the defect response of GMP and FE sensors, it was found that FE sensors had more sensitive dynamic range, resulting from better s/n ratio and short response pulse. Classification test was made to test the possibility of detecting defects in sound wood. Machine learning methods including decision trees, k-nearest neighbor and support vector machine were used. The classification accuracy varied between 72 and 77% in the tests. All the tested machine learning methods could be used efficiently for the classification. KW - Air-coupled transducers KW - Wood KW - Ultrasound KW - Ultrasonic imaging KW - Ferroelectret KW - Machine learning PY - 2020 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-509503 DO - https://doi.org/10.1007/s00226-020-01189-y SP - 1 EP - 14 PB - Springer AN - OPUS4-50950 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Vössing, Konrad A1 - Gaal, Mate A1 - Niederleithinger, Ernst T1 - Imaging wood defects using air coupled ferroelectret ultrasonic transducers in reflection mode JF - Construction and Building Materials N2 - Air-coupled ultrasound (ACU) is used to detect defects in wood panels without physically contacting the sample and with a quick scanning rate. Transducers made of cellular polypropylene (PP) with a high signal-to-noise ratio are quite suitable for ACU testing of wood. The extremely low modulus of elasticity and low density of PP transducers results in a small difference in acoustic impedance for the Transmission of ultrasonic waves between the transducer and air, allowing new areas of application to become possible. To demonstrate the suitability of the reflection technique, measurements are shown on the three samples Multiplex, LVL, and MDF and compared with transmission measurements. Due to this acoustic barrier and the use of cellular PP transducers, an exact detection of delamination and cavities is possible in wood panels up to 40 mm thick. If only one side of an object is accessible and the depth of the defect is of interest, the reflection technique is preferred at the expense of a reduced measuring accuracy and Penetration depth. KW - Wood KW - Ultrasound KW - Air coupled KW - Defects PY - 2020 DO - https://doi.org/10.1016/j.conbuildmat.2020.118032 VL - 241 SP - 118032, 1 EP - 8 PB - Elsevier Ltd. AN - OPUS4-50235 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Gaal, Mate A1 - Caldeira, Rui A1 - Bartusch, Jürgen A1 - Schadow, Florian A1 - Vössing, Konrad A1 - Kupnik, M. T1 - Air-coupled ultrasonic ferroelectret receiver with additional bias voltage JF - IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control N2 - High sensitivity is an important requirement for air-coupled ultrasonic sensors applied to materials testing. With a lower acoustic impedance than any piezoelectric material, charged cellular polypropylene (PP) offers better matching to air with a similar piezoelectric coefficient. The piezoelectric properties of charged cellular PP originate from their polarization, creating permanent internal voltage. The sensitivity of the sensor can be increased by applying additional dc bias voltage, as it has been done already for transmitters. This work presents the first ultrasonic sensor based on charged cellular PP including a high-voltage module providing dc bias voltage up to 2 kV. This bias voltage led to an increase in the signal-to-noise ratio of up to 15 ± 1 dB. The measurement of the received signal depending on the applied bias voltage is proposed as a new method of determining the internal voltage of ferroelectrets. The sensor combined with a cellular PP transmitter was applied to nondestructive testing of a rotor blade segment and glued-laminated timber, enabling imaging of the internal structure of these specimens with a thickness around 4 cm. KW - Acoustic sensors KW - Ferroelectret KW - Nondestructive testing KW - Ultrasonic imaging KW - Ultrasonic transducers PY - 2019 DO - https://doi.org/10.1109/TUFFC.2019.2925666 SN - 0885-3010 SN - 1525-8955 VL - 66 IS - 10 SP - 1600 EP - 1605 PB - IEEE AN - OPUS4-49131 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Kotschate, Daniel A1 - Gaal, Mate A1 - Kersten, H. T1 - Acoustic emission by self-organising effects of micro-hollow cathode discharges JF - Applied Physics Letters N2 - We designed micro-hollow cathode discharge prototypes under atmospheric pressure and investi-gated their acoustic characteristics. For the acoustic model of the discharge, we correlated the self-organisation effect of the current density distribution with the ideal model of an acoustic membrane. For validation of the obtained model, sound particle velocity spectroscopy was used to detect and analyse the acoustic emission experimentally. The results have shown a behaviour similar to the ideal acoustic membrane. Therefore, the acoustic excitation is decomposable into its eigenfrequencies and predictable. The model was unified utilising the gas exhaust velocity caused by the electrohydrodynamic force. The results may allow a contactless prediction of the current density distribution by measuring the acoustic emission or using the micro-discharge as a tunable acoustic source for specific applications as well. KW - Micro hollow cathode discharge KW - Atmospheric pressure plasma KW - Gas discharges KW - Plasma acoustics PY - 2018 DO - https://doi.org/10.1063/1.5024459 SN - 0003-6951 VL - 112 IS - 15 SP - Article 154102, 1 EP - 4 PB - AIP Publishing AN - OPUS4-44659 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Vössing, Konrad A1 - Gaal, Mate A1 - Niederleithinger, Ernst T1 - Air‑coupled ferroelectret ultrasonic transducers for nondestructive testing of wood‑based materials JF - Wood Science and Technology N2 - Air-coupled ultrasound (ACU) is used in through transmission to detect delamination, rot, and cracks in wood without altering the structure permanently. Novel ferroelectret transducers with a high signal-to-noise ratio enable high-precision structure recognition. Transducers made of cellular polypropylene are quite suitable for ACU testing due to their extremely low Young’s modulus and low density resulting in a favorable acoustic impedance for the transmission of ultrasonic waves between the transducer and air. Thus, structures with great dimensions, with a thickness of up to 300 mm and material densities below 500 kg/m3, can be inspected. Promising results were obtained under laboratory conditions with frequencies ranging from 90 to 200 kHz. The advantage of ACU transducers is that they do not equire contact to the sample; they are accurate and cost-effective. Ultrasonic quality assurance for wood is an important avenue to increase the acceptance of wooden structures and toward sustainability in civil engineering in general. KW - Ultrasound KW - Wood KW - Defect KW - Air-coupled PY - 2018 DO - https://doi.org/10.1007/s00226-018-1052-8 VL - 52 IS - 6 SP - 1527 EP - 1538 PB - Springer AN - OPUS4-46653 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Gaal, Mate A1 - Bovtun, Viktor A1 - Stark, Wolfgang A1 - Erhard, Anton A1 - Yakymenko, Y. A1 - Kreutzbruck, Marc T1 - Viscoelastic properties of cellular polypropylene ferroelectrets JF - Journal of Applied Physics N2 - Viscoelastic properties of cellular polypropylene ferroelectrets (PP FEs) were studied at low frequencies (0.3–33 Hz) by dynamic mechanical analysis and at high frequencies (250 kHz) by laser Doppler vibrometry. Relaxation behavior of the in-plane Young’s modulus (Y´ 11~1500 MPa at room temperature) was observed and attributed to the viscoelastic response of polypropylene matrix. The out-of-plane Young’s modulus is very small (Y´33≈0.1 MPa) at low frequencies, frequency- and stress-dependent, evidencing nonlinear viscoelastic response of PP FEs. The highfrequency mechanical response of PP FEs is shown to be linear viscoelastic with Y´33≈0.8 MPa. It is described by thickness vibration mode and modeled as a damped harmonic oscillator with one degree of freedom. Frequency dependence of Y*33 in the large dynamic strain regime is described by the broad Cole-Cole relaxation with a mean frequency in kHz range attributed to the Dynamics of the air flow between partially closed air-filled voids in PP FEs. Switching-off the relaxation contribution causes dynamic crossover from the nonlinear viscoelastic regime at low frequencies to the linear viscoelastic regime at high frequencies. In the small strain regime, contribution of the air flow seems to be insignificant and the power-law response, attributed to the mechanics of polypropylene cell walls and closed air voids, dominates in a broad frequency range. Mechanical Relaxation caused by the air flow mechanism takes place in the sound and ultrasound frequency range (10 Hz–1MHz) and, therefore, should be taken into account in ultrasonic applications of the PP FEs deal with strong exciting or receiving signals. KW - Viscoelastic KW - Cellular polypropylene KW - Ferroelectret PY - 2016 DO - https://doi.org/10.1063/1.4944798 SN - 0021-8979 SN - 1089-7550 SN - 1520-8850 VL - 119 SP - Article Number: 125101 PB - AIP Publishing AN - OPUS4-35956 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -