- Email: [email protected]
Photoacoustic resonance absorption spectra of Langmuir-Blodgett films
and one filter H... ,,(j) corresponding to the distributed attenuation. The backscattering is closely related to a.,(j) in the model for through transmission - it is just another side of the same process, ie another coefficient is needed, and the attenuation is similar to through transmission but with a whole range of x-values since the scatterers are distributed. The fact that the attenuation reduces the effect of backscattering in several ways, eg the downwards centre frequency shift of the attenuation cancels out part of the upwards shift from the backscattering process itself, makes backscatter less interesting for extracting information about the average grain size. For the second case, when there is a single large reflector echo dominating the signal, it is found that the material is better modelled by a set offilters: attenuation of the echo signal is the same as for through transmission, Ha,,(j), keeping in mind that x is the two-way distance, and backscattering is accounted for as an additive component H,,,,,(j)Hda,,(j), as described above. There is also target filtering, H,.,.(f), a description of the frequency response for the specific reflector target, including orientation. From a signal processing point of view, the interesting features of the large echo dominated pulse-echo case are the same as for through transmission, but there are additional aspects that make information extraction more complicated. To summarize the theoretical analysis: through transmission is the preferred measurement setup, since it should give good possibilities to extract information about the mean grain size without unnecessary complications. Pulse-echo dominated by a single echo is preferable to that dominated by backscatter, and it may be used when through transmission is not possible, if the signal-to-backscatter ratio is high enough. Backscattering gives a more complex and less easily analysed situation. Several spectrally based methods to classify specimens according to inclusion content (mean grain size) are studied, including methods for estimating the centre frequency shift utilizing autoregressive spectral estimation and homomorphic filtering. Application of neural networks for the classification is proposed and discussed in the context of spectral domain based methods. Measured ultrasonic signals from several different specimens of powder metallic materials are used to demonstrate that it is indeed possible to classify these specimens with respect to inclusions.
Anastassopou/os, A.A.*; Van Heme/rijck, D.t; De Wilde, w.P.t; Paipetis, S. * Defect characterization in carbon/epoxy plates via signal processing and pattern recognition analysis of ultrasonic A-scan signals *University of Patras, Dept. of Mechanical Engineering, Applied Mechanics Laboratory, PO Box 1134, Patras 26110, Greece tFree University Brussels, Faculty of Engineering, Dept. of Structural Analysis, Pleinlaan 2, B-I050 Brussels, Belgium Ultrasonics is a well established technique for non-destructive inspection of composites. Ultrasound waves are transmitted to the test piece and information on defects in the structural component is extracted from the amplitude variation of the reflected wave (pulse-echo method). Defect location in the x-y plane can be determined by C-scan testing. The time-gated echo amplitudes can be evaluated by several thresholds resulting in colour C-scan plots. By measuring the transient time between different echoes on the A-scan signal, the through-thickness position of defects in the test piece can be determined. It is generally known that small defects or defects running parallel to the sound beam or near-surface defects do not produce a distinctively separate peak in the time domain signal and are, therefore, difficult to detect by conventional methods. On the other hand, defects of different nature may produce echoes of the same amplitude and accordingly be represented by the same colour in conventional C-scan plots. Finally, no sufficient information is contained in the amplitude variation of the defect echo, if any, in order to determine the thickness of the defect.
In the present work the capabilities of the system are extended, by determining the nature and the thickness of defects in laminated composites. A 16-layer carbon/epoxy plate and two different materials (Teflon and paper) of five different thicknesses were used to create artificial defects in the plate. A complete set of the ten different defect forms was embedded between the second and third layers, in the middle of the plate and between layers 14 and 15 resulting in the 30 different defects in the plate. The ultrasonic signals are generated and received by a USIP 12 generator from Krautkramer. A 80486 computer equipped with a Sonotek STR 8100 AID board with a sampling frequency of 100 MHz is used to digitize the entire A-scan signal. In order to extract more information from the reflected wave, signal processing techniques are applied to the A-scan signals. Twenty different shots from each defect (the location of which was previously determined by C-scan) were taken. Transformation of the time domain signal to the frequency, auto and cross-correlation domains as well as to the deconvolution domain is performed by software. From each of the above domains signal characteristics (such as a number of peaks, peak amplitude, rise time etc) are further analysed. The most useful (according to their discrimination power) of those characteristics are found by means of Wilk's Lambda method and are used for further classification of the various defects. Typical examples from each defect class are presented as training samples to the classifier (supervised pattern recognition) and, according to this information, discrimination between the various defects and the different thicknesses of the same defect was successfully performed. The effect of different signal characteristics and classification methods on the results is discussed.
NDT & E International Volume 25 Number 4/5 1992
Shu-yi Zhang·; Yao-chun Shen*; Yue-song Jiang*; Rong Zhut
Photoacoustic resonauce alJsorptioD spectra of Langmuir-Blodgett films *Nanjing University, Institute of Acoustics, Nanjing 21008, China tSoutheast University, Dept. of Biomedical Engineering, Nanjing 210018, China It is well known that Langmuir-Blodgett (LB) films have potential applications in integrated optics and molecular electronics. Great efforts have concentrated on the study ofthe electro-optic properties ofLB films. In this paper, the resonance absorption spectra of LB multilayer films have been obtained by a photoacoustic (PA) technique. The new PA technique is based on resonant attenuated total reflection (ATR) spectroscopy, which has been successful\y developed for the nondestructive characterization of surfaces and thin overlayers, as well as multilayer heterostructures.
In the PA ATR spectroscopic studies, polymerized diacetylenes and manganese stearate multilayer films with different thicknesses are deposited onto silicon substrates by the LB dipping technique. The amphiphilic molecules in two neighbouring sublayers are arranged in head to head and tail to tail modes. A P-polarized argon ion laser beam with wavelength 488 nm modulated by an acousto-optic modulator is used to illuminate the sample. A hemicylinder prism coupler with refractive index 1.523 is used to make the optical beam with total internal reflection in the sample. Then the excited electromagnetic guidewave resonance modes in LB films will achieve a strong enhancement in sensitivity. The coupling is obtained by pressing the hemicylinder against the sample; since the surfaces in contact are not perfectly flat, an air gap is left, which is adjusted empirically to optimize the sharpness of the PA spectrum. A cylindrical PZT transducer with longitudinal vibration mode is bounded on the back of the silicon substrate to detect the PA signal. Then the output electric signal is provided to a lock-in amplifier via a preamplifier, and then to a recorder. In the experiment, the resonance PA absorption spectrum can be obtained by the measurement of the PA signal, either as a function of the wavelength of the incident laser beam at a fixed incidence angle fI (ie wavelength PA spectrum), or as a function of fI at a fixed .< (angular PA spectrum). We prefer to measure the angular spectrum, which simplifies the analysis of the results by avoiding dispersion effects. As the intensity of the optical beam is low, there is an absorption peak in each spectrum, which corresponds to the guidewave resonance mode in the LB films. The angle position of the peak increases with the thickness of the film. In order to explain the ~ngular PA spectra, an algorithm based on rigorous electromagnetic theory in layered media is developed. In the theory, the angle position of the absorption peak is a function of the parameters of the thickness and refractive index of the film as well as the thickness of the air gap. Therefore, the thickness and I or refractive index can be determined by fitting the theoretical results with those of the experiment. In our case, since the LB films are very thin, only one resonance mode, ie, one absorption peak, exists in each film. If the thickness of the film is known, the consistency between the theoretical angular PA spectrum and the experimental one can be obtained by the suitable selection of the refractive index. It can also be expected that, as the thickness of the LB film further increases, more absorption peaks may appear due to the increase of the number of the resonance modes. Then both thickness and refractive index can be easily determined simultaneously. However, we also find that as the pump laser power increases, additional peaks appear in the angle range from 0 to 60°, which may be related to the non-linearity of the LB films. Further studies are in progress. In addition, it is clear that, compared with the conventional ATR technique, PA ATR spectroscopy has the advantages of a simpler optical system and higher sensitivity. 0
NDT&E Rose, J.L.; Pilarski, A.; Ditri, J.J. Ultrasonic NDE guided mode selection principles Penn State University, Engineering Science and Mechanics Department, 114 Hallowell Building, University Park, PA 16802, USA Ultrasonic nondestructive evaluation has progressed rapidly from an art to a science over the last two decades. A countless number of ultrasonic NDE mode and frequency selection techniques are now available for carrying out experiments in ultrasonics. Each technique has its own generation, penetration, reception, resolution and sensitivity considerations along with specific cost benefits, advantages and disadvantages. Concepts are presented in this paper to optimize the approach to the various mode selection aspects of ultrasonic nondestructive testing. Modes and particular frequencies must be selected to optimize detection, location, sizing and classification potential for particular structural geometry and practical environmental boundary conditions. A generalized approach to mode selection is discussed here for tackling inspection problems in bonding, composite materials, wood, concrete, coatings and overall material and defect characterization. Problems must be treated from both a hulk wave and guided wave propagation point of view with emphasis on both theoretical and experimental elements. Selection of bulk longitudinal or shear wave propagation is traditionally straightforward but even bulk wave selection with the many new developments recently in the handling of waves in inhomogeneous and anisotropic media can present some interesting choices with respect to sensitivity and resolution for material and defect characterization analysis. Each different launch angle into the material produces a different problem with respect to the elements of wave propagation and sensitivity to material and defect analysis because of the
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Anastassopou/os, A.A.*; Van Heme/rijck, D.t; De Wilde, w.P.t; Paipetis, S. * Defect characterization in carbon/epoxy plates via signal processing and pattern recognition analysis of ultrasonic A-scan signals *University of Patras, Dept. of Mechanical Engineering, Applied Mechanics Laboratory, PO Box 1134, Patras 26110, Greece tFree University Brussels, Faculty of Engineering, Dept. of Structural Analysis, Pleinlaan 2, B-I050 Brussels, Belgium Ultrasonics is a well established technique for non-destructive inspection of composites. Ultrasound waves are transmitted to the test piece and information on defects in the structural component is extracted from the amplitude variation of the reflected wave (pulse-echo method). Defect location in the x-y plane can be determined by C-scan testing. The time-gated echo amplitudes can be evaluated by several thresholds resulting in colour C-scan plots. By measuring the transient time between different echoes on the A-scan signal, the through-thickness position of defects in the test piece can be determined. It is generally known that small defects or defects running parallel to the sound beam or near-surface defects do not produce a distinctively separate peak in the time domain signal and are, therefore, difficult to detect by conventional methods. On the other hand, defects of different nature may produce echoes of the same amplitude and accordingly be represented by the same colour in conventional C-scan plots. Finally, no sufficient information is contained in the amplitude variation of the defect echo, if any, in order to determine the thickness of the defect.
In the present work the capabilities of the system are extended, by determining the nature and the thickness of defects in laminated composites. A 16-layer carbon/epoxy plate and two different materials (Teflon and paper) of five different thicknesses were used to create artificial defects in the plate. A complete set of the ten different defect forms was embedded between the second and third layers, in the middle of the plate and between layers 14 and 15 resulting in the 30 different defects in the plate. The ultrasonic signals are generated and received by a USIP 12 generator from Krautkramer. A 80486 computer equipped with a Sonotek STR 8100 AID board with a sampling frequency of 100 MHz is used to digitize the entire A-scan signal. In order to extract more information from the reflected wave, signal processing techniques are applied to the A-scan signals. Twenty different shots from each defect (the location of which was previously determined by C-scan) were taken. Transformation of the time domain signal to the frequency, auto and cross-correlation domains as well as to the deconvolution domain is performed by software. From each of the above domains signal characteristics (such as a number of peaks, peak amplitude, rise time etc) are further analysed. The most useful (according to their discrimination power) of those characteristics are found by means of Wilk's Lambda method and are used for further classification of the various defects. Typical examples from each defect class are presented as training samples to the classifier (supervised pattern recognition) and, according to this information, discrimination between the various defects and the different thicknesses of the same defect was successfully performed. The effect of different signal characteristics and classification methods on the results is discussed.
NDT & E International Volume 25 Number 4/5 1992
Shu-yi Zhang·; Yao-chun Shen*; Yue-song Jiang*; Rong Zhut
Photoacoustic resonauce alJsorptioD spectra of Langmuir-Blodgett films *Nanjing University, Institute of Acoustics, Nanjing 21008, China tSoutheast University, Dept. of Biomedical Engineering, Nanjing 210018, China It is well known that Langmuir-Blodgett (LB) films have potential applications in integrated optics and molecular electronics. Great efforts have concentrated on the study ofthe electro-optic properties ofLB films. In this paper, the resonance absorption spectra of LB multilayer films have been obtained by a photoacoustic (PA) technique. The new PA technique is based on resonant attenuated total reflection (ATR) spectroscopy, which has been successful\y developed for the nondestructive characterization of surfaces and thin overlayers, as well as multilayer heterostructures.
In the PA ATR spectroscopic studies, polymerized diacetylenes and manganese stearate multilayer films with different thicknesses are deposited onto silicon substrates by the LB dipping technique. The amphiphilic molecules in two neighbouring sublayers are arranged in head to head and tail to tail modes. A P-polarized argon ion laser beam with wavelength 488 nm modulated by an acousto-optic modulator is used to illuminate the sample. A hemicylinder prism coupler with refractive index 1.523 is used to make the optical beam with total internal reflection in the sample. Then the excited electromagnetic guidewave resonance modes in LB films will achieve a strong enhancement in sensitivity. The coupling is obtained by pressing the hemicylinder against the sample; since the surfaces in contact are not perfectly flat, an air gap is left, which is adjusted empirically to optimize the sharpness of the PA spectrum. A cylindrical PZT transducer with longitudinal vibration mode is bounded on the back of the silicon substrate to detect the PA signal. Then the output electric signal is provided to a lock-in amplifier via a preamplifier, and then to a recorder. In the experiment, the resonance PA absorption spectrum can be obtained by the measurement of the PA signal, either as a function of the wavelength of the incident laser beam at a fixed incidence angle fI (ie wavelength PA spectrum), or as a function of fI at a fixed .< (angular PA spectrum). We prefer to measure the angular spectrum, which simplifies the analysis of the results by avoiding dispersion effects. As the intensity of the optical beam is low, there is an absorption peak in each spectrum, which corresponds to the guidewave resonance mode in the LB films. The angle position of the peak increases with the thickness of the film. In order to explain the ~ngular PA spectra, an algorithm based on rigorous electromagnetic theory in layered media is developed. In the theory, the angle position of the absorption peak is a function of the parameters of the thickness and refractive index of the film as well as the thickness of the air gap. Therefore, the thickness and I or refractive index can be determined by fitting the theoretical results with those of the experiment. In our case, since the LB films are very thin, only one resonance mode, ie, one absorption peak, exists in each film. If the thickness of the film is known, the consistency between the theoretical angular PA spectrum and the experimental one can be obtained by the suitable selection of the refractive index. It can also be expected that, as the thickness of the LB film further increases, more absorption peaks may appear due to the increase of the number of the resonance modes. Then both thickness and refractive index can be easily determined simultaneously. However, we also find that as the pump laser power increases, additional peaks appear in the angle range from 0 to 60°, which may be related to the non-linearity of the LB films. Further studies are in progress. In addition, it is clear that, compared with the conventional ATR technique, PA ATR spectroscopy has the advantages of a simpler optical system and higher sensitivity. 0
NDT&E Rose, J.L.; Pilarski, A.; Ditri, J.J. Ultrasonic NDE guided mode selection principles Penn State University, Engineering Science and Mechanics Department, 114 Hallowell Building, University Park, PA 16802, USA Ultrasonic nondestructive evaluation has progressed rapidly from an art to a science over the last two decades. A countless number of ultrasonic NDE mode and frequency selection techniques are now available for carrying out experiments in ultrasonics. Each technique has its own generation, penetration, reception, resolution and sensitivity considerations along with specific cost benefits, advantages and disadvantages. Concepts are presented in this paper to optimize the approach to the various mode selection aspects of ultrasonic nondestructive testing. Modes and particular frequencies must be selected to optimize detection, location, sizing and classification potential for particular structural geometry and practical environmental boundary conditions. A generalized approach to mode selection is discussed here for tackling inspection problems in bonding, composite materials, wood, concrete, coatings and overall material and defect characterization. Problems must be treated from both a hulk wave and guided wave propagation point of view with emphasis on both theoretical and experimental elements. Selection of bulk longitudinal or shear wave propagation is traditionally straightforward but even bulk wave selection with the many new developments recently in the handling of waves in inhomogeneous and anisotropic media can present some interesting choices with respect to sensitivity and resolution for material and defect characterization analysis. Each different launch angle into the material produces a different problem with respect to the elements of wave propagation and sensitivity to material and defect analysis because of the
223