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The Scaled Boundary Finite Element Method (SBFEM) for prismatic structures is an efficient method for the simulation of acoustic behavior. Hence a further development of the method is of great interest. The wave propagation can be calculated for isotropic and anisotropic materials in solids. As for many applications the acoustic behavior in fluids and the behavior in case of fluid-structure interaction (FSI) is subject of research, the implementation of a fluid model in SBFEM for prismatic structures is needed. In case of FSI the coupling between fluid and solid domains can be performed without additional effort when describing both domains in the same variables. Hence a displacement-based fluid description is used. As the discretized formulation leads to spurious modes, a penalty method to suppress the unphysical behavior is chosen. To validate the derived model a comparison with analytical solutions of purely fluid domains is made. As to verify that in case of FSI the model shows the right behavior, dispersion curves of water-filled pipes are calculated and compared to results obtained with Comsol.
Processing ultrasonic data by coda wave interferometry to monitor load tests of concrete beams
(2018)
Ultrasonic transmission measurements have been used for decades to monitor concrete elements, mostly on a laboratory scale. Recently, coda wave interferometry (CWI), a technique adapted from seismology, was introduced to civil engineering experiments. It can be used to reveal subtle changes in concrete laboratory samples and even large structural elements without having a transducer directly at the place where the change is taking place. Here, several load tests until failure on large posttensioned concrete beams have been monitored using networks of embedded transducers. To detect subtle effects at the beginning of the experiments and cope with severe changes due to cracking close to failure, the coda wave interferometry procedures had to be modified to an adapted step-wise approach. Using this methodology, we were able to monitor stress distribution and localize large cracks by a relatively simple technique. Implementation of this approach on selected real structures might help to make decisions in infrastructure asset management.
Post-tensioning systems provide safe and efficient construction solutions for long span bridges. Despite the improved grouting practices over the past decade, existing post-tensioning systems may have significant amount of grout defects, which could lead to corrosion of the strands. Condition assessment of post-tensioning systems is necessary to allow bridge owners to take timely, proactive actions to mitigate or prevent further Deterioration and unanticipated tendon failures. A detailed experimental study conducted to assess the performance of nondestructive evaluation techniques in detecting grout defects within internal tendons is presented herein.
Nondestructive evaluation techniques that include Ground Penetrating Radar, Impact Echo, Ultrasonic Tomography, and Ultrasonic Echo are evaluated in terms of detecting the location and severity of fabricated grout defects in a full-scale post-tensioned U-girder mock-up specimen. While Ground Penetrating Radar can identify the location and profile of the internal tendons, particularly metal ducts due to strong reflections, this method did not provide any information about the defect conditions within the tendon. Both Impact Echo and Ultrasonic Echo techniques are effective in identifying the Location of grout defects, but could not differentiate between water, void, or compromised grout conditions. The study clearly demonstrates the need for NDE techniques that are applicable to anchorage regions, and that are capable of estimating the severity and nature of grout defects in internal tendons.
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-Pérot 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.
Air-coupled ultrasonic testing (ACUT) has experienced rapid growth within the last years. It is especially well suited to inspection of lightweight structures consisting of composite materials and adhesive joints. Uniform coupling and easy maintenance are its advantages compared to contact technique. However, the impedance mismatch between the transducer and air poses a major challenge to the development of ACUT transducers. Commercially available air-coupled transducers consist of a piezocomposite material and matching layers. Their fabrication is difficult in handling and their signal-to-noise ratio sometimes not sufficient for various testing requirements. However, there are several innovative approaches using other materials and other physical principles to transmit and receive an ultrasonic pulse. We present a review of the latest advances in research on air-coupled transducers for non-destructive testing, including previously unpublished results. We recognize two major directions as most promising: ferroelectrets and thermoacoustic transducers. Ferroelectrets are charged cellular polymers exhibiting piezoelectric properties. Their small acoustic impedance is matched to air better than matching layers applied in conventional air-coupled transducers. Applying bias voltage to a ferroelectret receiver is the latest development in this field, which increased the received signal by 12 to 15 dB. Thermoacoustic transducers use heat to initiate an ultrasonic wave, acting as transmitters. The working principle is known from nature as thunder and lightning: thermal energy of an electrically heated material, which can also be air, is converted into acoustic energy. Some thermoacoustic transmitters consist of a conductive layer with a thickness in the nanometer range deposited on a solid substrate. Another possibility is to use an electric spark. For the first time, measurements of the sound field of an electric spark up to 500 kHz were performed. Thermoacoustic transducers enable excitation of extremely broadband pulses while producing high pressure levels, which opens new possibilities for advanced signal processing.
Matrix phased array probes are becoming more prominently used in industrial applications. The main drawbacks, using probes incorporating a very large number of transducer elements, are needed for an appropriate cabling and an ultrasonic device offering many parallel channels. Matrix arrays designed for extended functionality feature at least 64 or more elements. Typical arrangements are square matrices, e.g., 8 by 8 or 11 by 11 or rectangular matrixes, e.g., 8 by 16 or 10 by 12 to fit a 128-channel phased array system. In some phased array systems, the number of simultaneous active elements is limited to a certain number, e.g., 32 or 64. Those setups do not allow running the probe with all elements active, which may cause a significant change in the directivity pattern of the resulting sound beam. When only a subset of elements can be used during a single acquisition, different strategies may be applied to collect enough data for rebuilding the missing information from the echo signal. Omission of certain elements may be one approach, overlay of subsequent shots with different active areas may be another one. This paper presents the influence of a decreased number of active elements on the sound field and their distribution on the array. Solutions using subsets with different element activity patterns on matrix arrays and their advantages and disadvantages concerning the sound field are evaluated using semi-analytical simulation tools. Sound field criteria are discussed, which are significant for non-destructive testing results and for the system setup.
A composite material is a combination of two or more materials with very different mechanical, thermal and electrical properties. The various forms of composite materials, due to their high material properties, are widely used as structural materials in the aviation, space, marine, automobile, and sports industries. However, some defects like voids, delamination, or inhomogeneous fiber distribution that form during the fabricating processes of composites can seriously affect the mechanical properties of the composite material. In this study, several imaging NDE techniques such as: thermography, high frequency eddy current, ultrasonic, x-ray radiography, x-ray laminography, and high resolution x-ray CT were conducted to characterize the microstructure of carbon fiber composites. Then, a 3D analysis was implemented by the destructive technique of serial sectioning for the same sample tested by the NDE methods. To better analyze the results of this work and extract a clear volume image for all features and defects contained in the composite material, an intensive comparison was conducted among hundreds of 3D-NDE and multi serial sections’ scan images showing the microstructure variation.
Among the photothermal methods, full-field thermal imaging is used to characterize materials, to determine thicknesses of layers, or to find inhomogeneities such as voids or cracks. The use of classical light sources such as flash lamps (impulse heating) or halogen lamps (modulated heating) led to a variety of nondestructive testing methods, in particular, lock-in and flash-thermography. In vertical-cavity surface-emitting lasers (VCSELs), laser light is emitted perpendicularly to the surface with a symmetrical beam profile. Due to the vertical structure, they can be arranged in large arrays of many thousands of individual lasers, which allows power scaling into the kilowatt range. Recently, a high-power yet very compact version of such a VCSEL-array became available that offers both the fast timing behavior of a laser as well as the large illumination area of a lamp. Moreover, it allows a spatial and temporal control of the heating because individual parts of the array can be controlled arbitrarily in frequency, amplitude, and phase. In conjunction with a fast infrared camera, such structured heating opens up a field of novel thermal imaging and testing methods. As a first demonstration of this approach, we chose a testing problem very challenging to conventional thermal infrared testing: The detection of very thin subsurface defects perpendicularly oriented to the surface of metallic samples. First, we generate destructively interfering thermal wave fields, which are then affected by the presence of defects within their reach. It turned out that this technique allows highly sensitive detection of subsurface defects down to depths in excess of the usual thermographic rule of thumb, with no need for a reference or surface preparation.
Acoustic emission testing
(2018)