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Due to the large variety of possible flaws (e.g. air inclusions, delaminations, gluing errors, impurities, kissing bonds) different methods than the established measurement methods of metal bonds have to be used for the inspection of composite laminates. Depending on the type of discontinuity the requirements of the inspection task are very diverse and time consuming. Due to the differences in the lateral and axial dimensions of defects and the attenuation varieties it is necessary to measure as broad-banded as possible. Typical band-widths as implemented in contact testing methods cannot be used in air-coupled ultrasonic testing at the moment. We describe an efficient implementation of such an inspection with significantly improved results compared to conventional systems. It is based on a new multi-element air ultrasonic transducer. Each of the elements has different frequencies. The overlapping sound beams of the individual elements allow generating a broad-band signal which can be additionally steered inside the material as it is well-known from phased-array UT. The new transducer increases the bandwidth considerably by an in-phase simultaneous activation of all elements. In order to operate such an ultrasonic transducer a special sender-receiver electronic is necessary. As the individual transducer elements have to be activated simultaneously the system must have at least as many channels as the ultrasonic transducer. Additionally all channels with different frequencies have to be activated with a very high time accuracy relative to each other. The evaluation takes place under consideration of the dispersive properties of the inspected test objects with respect to the interdependency with ultrasound. Frequency-dependent properties like attenuation as well as geometry characteristics such as scattering behavior at interfaces cause a change in the measured ultrasonic signal. Accordingly, it is possible to achieve new contrast mechanisms. An increase of the effective bandwidth by combining a multi-element air-coupled ultrasonic transducer and a multi-channel measurement system allow gaining more information of the sample in only one measurement cycle. This accelerates the measurement of different frequencies drastically and eliminates uncertainties during the alignment of transducers with different frequencies.

Data compression technologies such as MP3 are extensively used for audio technologies. There are two objectives: to maintain the quality of the original sound and two compress the data as much as possible. The degree of compression depends strongly from the signal character. The information of pure harmonic signals can be much better compressed than by considering noisy signals. The ultrasound produced by processes in industry is used for maintenance purposes since some decades. On the one hand there are many signals with noisy character. Examples are leakage noise, jet noise, the noise of friction in bearings and gears. On the other hand there are signals with dominantly repeating impulses such as bearing faults (cracks or spalled areas) or arcing during electrical discharge. For both types of data different compressing rates has to be found. Traditionally, ultrasound data for maintenance are evaluated by means of narrow band techniques such as heterodyning the ultrasound signal in the audible range, thereby a data reduction can be achieved. In order to avoid the loss of real physical information, an ultrasound broadband technology has been introduced in order to consider the complete frequency range leading to a considerably increased data rate. Long-term recordings of service data would generate a huge amount of data on short time scales. Furthermore, the broadband technologies requires new approaches to make the signal audible. A vocoder technique has been modified in this contribution in order to compress the ultrasound data in real-time to a bandwidth which is compatible to the listening capabilities of testing persons. The spectra of compressed data can be used for the interpretation of measuring data. The degree of loss with respect to the information content has been estimated in comparison to the original uncompressed signal. Furthermore, ultrasound data – original and the compressed audible data – has been taken by applying pattern recognition techniques based on Cellular Neural Networks which will be exemplified by means of maintenance related data. Authors: Peter Holstein, Andreas Tharandt, Christian Probst (SONOTEC Ultraschallsensorik Halle GmbH) Ronald Tetzlaff, Steffen Seitz, Jens Müller (Institute of Circuits and Systems, TU Dresden

Non-contact air-coupled ultrasonic testing has grown in importance during the last years especially due to the application of new composite materials. The main advantage of waiving couplants like water is set against the disadvantage of high coupling losses, which make the interpretation of testing results more complicated. Therefore improvements in the testing system and its components are necessary. A promising approach is the application of the phased array technique to the contact-free ultrasonic testing. By using probes with multiple elements and a multi-channel instrument it is possible to adapt the ultrasonic sound field to the application as well as improving testing processes. This technique is introduced and has proved its performance in industry and medicine since decades. However, only a few systems are available for the use of non-contact testing. One reason is, that the design of non-contact probes differs from conventional probes. An adaption of the concepts of phased array probes to the design of such probes is necessary. The novel SONOSCAN CF400 series includes a piezocomposite transducer of Ø20 mm. The mean frequency is 400 kHz. In the standard type the transducer is curved, leading to a focal distance of 50 mm. Typically these transducers are used in transmission technique, which gives an optimum distance of transmitter and receiver of 100 mm. A shorter focal distance cannot be achieved by a stronger bending of the transducer due to material limits. Based on this probe the SONOSCAN CF400 3E type with the same transducer dimension has been developed. The electrode was constructed as an annular array with 3 equal-sized elements. By applying an appropriate delay law the focal distance has been adjusted to 10 mm. The focal width has been measured with 4 mm. The corresponding sound field is shown in Figure 1. The advantage of electronic focussing phased array probes against conventional probes can be found in applications, where the distance between transmitter and receiver is limited by external restrictions. Because of interference in the near field of the probe, the presentation of scans using conventional probes will include artefacts. Phased array probes will avoid these.

The automotive industry profits of many fully automated processes during its fabrication stages. Most of the processes are performed by robots just like all typical welding processes. To achieve quality requirements testing is necessary. It is recommended that a radiation and contact-free nondestructive testing method is applicable. This ensures a maximum testing speed and a minimum of necessary safety provisions, preparations or post-processing of the test objects. Two special testing scenarios will be discussed using single-sided air-coupled ultrasonic testing. A main advantage is the contactless measurement without the application of any oil or water using high-power ultrasonic pulses. In addition to common transmission mode testing, we present a pitch-catch-technique using Lamb-waves. The influence of geometry effects and the importance of high bandwidth will be discussed. The first test specimen are edge-to-edge laser-welded metal sheets as used for instance in automotive production processes. The goal of a better quality assurance is the immediate control of the welding process. Accordingly, for future testing systems it is recommended to implement the work tool and the testing sensor into the welding robot. The aim of the testing is to monitor the complete performance of the welding and to record interruptions of the weld seam or any reduction of the laser energy. Any discontinuity within the sound path leads to changes of the ultrasonic signal. Due to the fact that the non-welded plates should have no defects, changes of the signal indicate directly a change of the weld seam. The second test samples are adhesive bonds found in composite material structures such as combinations of metal sheets and carbon fiber reinforced plastics (CFRP). The mechanical stability, lifetime and performance of such composite structures strongly depends on the quality of the bond. Air-coupled ultrasonic testing is a promising nondestructive method for the characterization of these bonds. In transmission mode the absence of glue acts as an additional interface within the test object which strongly reduces the transmitted ultrasound wave. Glue application errors can be detected easily.

The development of piezo-composites enables new possibilities for ultrasound transducers even in the frequency range of air-coupled ultrasound. In contrast to bulk ceramics as transducer material, the piezo-composite vibrates in the thickness mode. Thus, the diameter of the transducer is independent of the resonance frequency. This advantage opens up the field of air-coupled ultrasound testing (ACU) above 300 kHz. Additionally, the large area and the reduced lateral coupling of the piezo-composite material allow multi-channel electrode design for sound field forming. We present a new design of ultrasound transducers with three annular electrodes in combination with a fully controllable three channel ultrasound testing system. The sound field is characterized with the ball reflector method in comparison to a standard single electrode transducer. An improvement of the lateral resolution is observed which is verified by scanning a test specimen with blind holes of different sizes as well as lightweight materials with imperfect bonding and delamination. Beside the smaller sound field diameter the near field length could be reduced from originally 70 to 25 mm which leads to an increase of intensity due to lesser losses in the air section. We could also show that the use of a three-electrodes receiver in combination with a multi-channel system (transmitter and receiver) leads to an additional increase of the resolution. Furthermore, we developed a new approach to single-sided air-coupling measurement by means of multi-receiver method. This technique directly provides spatially resolved imaging of subsurface structures of lightweight materials. In contrast to other techniques, for example lamb waves or SAFT, no additional data processing is necessary. This leads to direct imaging of defects in intrinsically complex samples, as expected e.g. for honeycomb structures.

Air tightness testing has importance in industrial environments where in compressed air systems leaks account for a high percentage of energy loss. These losses can effectively be reduced by locating and repairing of leaks. Furthermore, tightness is a criterion for the quality of different kinds of seals and joints. For leak detection and tightness testing acoustic methods (passive and active) can be applied. Variations of the technology enable access to different problems. The methods for leak detection can be graded up by the estimation of leak sizes and energetic losses. Similarly, slot size and length of tightness leaks are responsible for energetic losses in buildings (window and door seals) or for dysfunctions in technical systems such as cabins, heat exchangers and others. Leaks can be found easily by means of traditional ultrasound leak detectors which operate in a narrow frequency band (e.g. 40 kHz). However, this simple technology is inadequate for quantitative estimations of leak and tightness losses. Therefore, new theoretically based approaches have been developed and experimentally exemplified. Sound will be created by passing of air through leaks (for high and low pressure as well). Compressed air exits an orifice generating a turbulent jet, which emits a broadband sound. Acoustic leak finder devices can localize these leaks. However, quantification of their size remains a crucial task (with respect to energy loss and priority of repair). From the point of view of aero-acoustical theory a quantification of fluid parameters of leak jets can only be achieved by evaluation of a broad frequency band (20 kHz to 100 kHz). A new testing procedure and evaluation algorithm for finding and quantification of leaks will be presented. Tightness of many technical systems can be tested by means of actively transmitted (ultra-)sound. Again, quantification remains a demanding task. The sound field strongly depends on depth, width and geometry of an orifice and is affected by diffraction, reflection, transmission and interference. The method is discussed with respect to the quantification of leak size. It is shown which demands are to be met on the sound source and the generated acoustic fields.

Accurate time-delay estimations are a prerequisite for the estimation of geometric dimensions by means of ultrasound pulse-echo methods. Vice versa, highly resolved sound velocity data can provide accurate material parameters when the geometric data is available. Typical applications are the estimation of the wall thickness or of Young’s modulus. The estimation can be demanding when materials are heterogeneous or highly attenuating. Typical material classes are wood and fibre reinforced plastics composite. Additional challenges are induced by the anisotropy of the material and of the acoustic parameters. Caused by the high degree of attenuation, low frequency excitation will be needed in many practical applications which results in a lowered resolution. This is caused by longer excitation pulses and broader echoes. Therefore, improved excitation techniques and signal processing algorithms will be proposed in order to enhance the accuracy and resolution of time-delay measurements. The improvements will be achieved by the application of broad-band transducers, modulated (coded) excitation pulses and wavelet-correlation analyses. Technology and algorithms will be exemplified for wood (Pernambuco, used for the bows of string instruments) and composite samples. It will be demonstrated that the method enables an improved access to time-delay based information for heterogeneous materials.