Home HomeG. Busse, Universitaet Stuttgart (Germany)
Non destructive evaluation (NDE) of metals is well establish since many decades, techniques based on X-ays, ultrasonics, and eddy currents are familiar to all people involved in quality control. Polymers initially intended to be a cheap replacement for metal have found their way into applications where their high specific strength is Important. As many of these applications are safety-relevant, one is highly interested In NDE of polymer materials, However, as the physical properties of these materials differ significantly from metals, it is obvious that some methods can be applied to polymers and not to metals, and vice versa. This paper deals with the potential of new methods and shows which method provides which kind of information, its advantages and drawbacks. Remote detection of delaminations, impact damage, fibre orientations, and curing processes will be highlighted for glass fibre (GFRP) and carbon fibre reinforced polymer materials (CFRP). The methods to be presented include modulated heat transport, microwaves, ultrasonics and vibrometry.
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Masato Abe, Iwate University (Japan)
Kenji Sasaki, Iwate University (Japan)
This paper describes a method to estimate, using several sensors, the position of a crack in a concrete block, most of which is buried in the ground. An array of sensors is attached on the surface of the concrete block, and a vibration pulse is forced using a small hammer. Since the forced vibration reflects at the position of a crack, the position will be easily estimated by picking up the reflection wave with an array of sensor using a beamforming technique if only the reflections from the crack can be detected. However, there were many reflections from the boundaries such as side walls and the bottom of the concrete block, and the surface wave, whose magnitude is much greater than that of the necessary reflection wave from the crack. Therefore, conventional beam forming technique does not offer a good result. By the proposed method, not only the effect of the surface wave but also that of the reflection waves from the side walls are decreased, and the necessary reflections (primary and secondary waves) from a crack are extracted. The position of the crack is estimated using the extracted waves by a beamforming technique. We made three concrete blocks with different types of cracks and the positions of the crack were estimated well with the proposed method.
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Pavel M. Trivailo, Royal Melbourne Institute of Technology (Australia)
L.A. Plotnikova, Royal Melbourne Institute of Technology (Australia)
L.A. Wood, Royal Melbourne Institute of Technology (Australia)
For safety and economic reasons, it is important for many aerospace, maritime, mechanical and civil engineering structures to have reliable and efficient methods enabling damage identification and their health monitoring. The research investigates a promising new approach to damage detection and structural integrity assessment for aerospace, mechanical and civil engineering structures, which, in contrast to the existing techniques, enables the engineer to step outside the oclosed spaceoe of limited data available using current conventional approaches and to develop a detection strategy which provides fresh new data which, in principle, is simple to generate. The new method is based on existing frequency sensitivity techniques, but it is radically enhanced by the inclusion of data from "twin" structures, a novel concept in this context. Numerical investigations of the proposed method have yielded exceptionally encouraging results, and demonstrated the potential advantages of simplicity (hence lower costs), improved reliability and wider applicability when compared with current techniques. The feasibility of the method is demonstrated using a wide range of structural systems as examples: spring-mass systems; 2D and 3D complex truss structures; beam and frame models. Multiple damage sites have been successfully identified and quantified. The method has been shown to perform well. This technology can be especially efficient for large elastic truss and frame space structures.
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A. Salerno, Universitaet Stuttgart (Germany)
S. Danesi, Universitaet Stuttgart (Germany)
D. Wu, Universitaet Stuttgart (Germany)
S. Ritter, Universitaet Stuttgart (Germany)
G. Busse, Universitaet Stuttgart (Germany)
Early detection of defects is relevant for maintenance of components. This detection can be based on the way how the propagation of electromagnetic or acoustic waves are modified by the defect (indirect methods). From such results one derive the mechanical aspect of the defect. The influence of defects on the mechanical behaviour can be observed more directly by interferometric mean. Electronic speckle pattern interferometry (ESPI) displays the deformation pattern induced under load, e.g. thermal expansion under absorbed illumination. The presence of a defect modifies the overall deformation pattern, so one needs to compare patterns. If one could heat defects selectively, defects would reveal themselves by their Iocalized thermal expansion. Defects are characterized e.g. by stress concentration, lack of adhesion or cracks: all these influences enhance the local mechanical hysteresis. Therefore elastic waves are predominantly attenuated in such areas and mechanical energy is converted into heat with a consequent thermal expansion. To detect these deformations ESPl-measurements were performed after or while being exposed to ultrasonic excitation. Fringes (= levels of equal deformation) are observed around defects in various kind of materials, e.g. wood and carbon fiber reinforced material. This dark field method (which is similar to ultrasonic lockin thermography) should be suited to detect defects in a sensitive way independently of complicated whoie body-deformation.
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D. Geisse, Universitaet Stuttgart (Germany)
K. Nixdorf, Universitaet Stuttgart (Germany)
G. Busse, Universitaet Stuttgart (Germany)
Anew method is described that allows to monitor the curing process ofthin adhesive laminates in bonded metal joints. The method responds in a sensitive way todifferences in the properties ofthe adhesive polymer. The experimental setup consists of two ultrasonic transducers coupled to a metal bar specimen and supplied bya network analyzer. The resonance spectrum of the joint metal bar is interpreted in terms of additional twin resonance frequencies which arise in the curing process of the adhesive polymer. The curing function of the effective Youngs modulus and the spectral energy is used as a tool to calculate quality values to characterise slight differences between several adhesive polymers. It has been shown, that already the beginning of the curing process (first 20 minutes) provides sufficient information to recognize differences between the adhesive polymers. However, it is not possible to find such sensitive differences in curing at the final state. The capacity of the specimen is simultaneously measured to evaluate differences in the thickness of the glueline which influences the calculation of the effective Youngs modulus and the spectral energy, We find similarities in the elastic and the dielectric properties of the adhesive during the curing.
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Ch. Doettinger, Universitaet Stuttgart (Germany)
J. Stanullo, Universitaet Stuttgart (Germany)
Leonid M. Lyamshev, General Physics Institute, Russian Academy of Sciences (Russia)
G. Busse, Universitaet Stuttgart (Germany)
Vibrometry is basically an old technique allowing for defect detection in an empirical way. The use of piezosensors combined with fast data processing have made this technique even more efficient in terms of quantification and of frequency range. However, mechanical excitation of vibration together with are not always applicable: Samples may be wet or sticky, small to attach even a small sensor to them. In any case, piezoelectric detection hot or cold, or just too every attached sensor affects the dynamic behaviour (stiffness, mass, losses) of the inspected sample. The only rigorous solution to this problem is remote generation and remote detection of vibration. We report results of experiments performed with tunable continuous monofrequency excitation which was performed either acoustically (loudspeaker) or optically by an intensity modulated laser beam. In this case the absorption of periodically deposited energy results in periodical thermal expansion which drives the mechanical excitation. Detection was performed with an interferometric laser vibrometer coupled to a Iockin amplifier. The resolution achieved in these measurements was up to $10^8$.This kind of remote mechanical spectroscopy allowed for the characterisation of processes, e.g. drying of paint on polymers and wood, diffusion of humidity into these materials, and sintering of ceramics.
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Suzanne Thwaites, CSIRO National Measurement Laboratory (Australia)
Norman H. Clark, CSIRO National Measurement Laboratory (Australia)
A new technique for detecting faults in composite panels has been developed. It Is based on measurements of the local phase velocity of flexural waves in the frequency range 2-30 kHz. Since a broadband continuous excitationis used the method includes source configuratlons and software procedures designed to eliminate the effect of the reverberant field. An X-Y scanning version of the instrument has been built employing non-contact methods of excitation and detection and a handheld field version based on a notebook computer has also been constructed. The method has been used on multi-ply carbon fibre panels with various lay-ups to determine elements of the stiffness matrix and lay-up errors. Delarninations and core damage in Nomex honeycomb/carbon fibre skin sandwich panels of varying thicknesses have also been detected via scattering of the flexural waves off the defects, This has been confirmed by calculations involving a simple scattering model.
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