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Advanced ultrasonic sensors for large area structural testing
VI '93
abstracts~~~~
The following are abstracts of papers relating to nondestructive testing and evaluation due to be presented at the Ultrasonics International '93 conference, from 6-8 July 1993 in Vienna, Austria. Acoustic microscopy Frestel, N.; Liot, A.; Tretout, H.; De Mol, R. Scanning acoustic microscopy for quantitative analysis of superplastic forming diffusion bonding titanium aerospace components Dassault Aviation CTNTFjEND, 78 Quai Marcel Dassault, 92552 Saint-Cloud Cedex 300, France The need to improve the performance of aircraft by reducing the cost and weight of aerospace metallic structures has led to increased interest in superplastic forming and diffusion bonding titanium alloy processes. The production of heavily loaded flying SPF lOB components for Dassault Aviation's fighters has required quantitative characterization of diffusion bonds. The particular problems which arise in SPF lOB processing of titanium alloys at the development stage can induce defects at the bond-line, which may be divided into voids of different sizes with gaps ranging from intimate contact disbond up to 20 11m. Ultrasonic techniques appeared to have the greatest potential for defect detection and sizing in diffusion bonds. Unfortunately the smallest defect size can be much less than the wavelength of conventional ultrasound,leading us to look for higher frequency equipment from 30 MHz up to 100 MHz and above, point focusing transducers, signal processing techniques and high resolution mechanical scanning rigs. Acoustic microscopy is used by Dassault Aviation to characterize diffusion bonded sheets of titanium alloys. We present here the use of high frequency ultrasound from an industrial point of view. the relevant parameters, the current detection limitations, statistical analysis of C-scans and the use of image processing techniques in order to quantify the percentage of welded grains. The work includes the development of an acoustic microscope, the evaluation of several types of transducers such as PZT, PVDF, and piezo-composite types, as well as the means for their metrology. Two kinds of parameters are measured: electrical and acoustic. Regarding acoustic parameters, unlike the metrology of classical transducers with frequencies lower than 30 MHz, characterization of high frequency transducers is complex. We will speak about the difficulties of obtaining representative results of the acoustic field in water or in a material such as titanium. Regarding electrical parameters, we analyse the shape of the reflected signal on a glass plate, the ratio of stationary waves and the frequency bandwidth. The effects of surface roughness (at the bond line and at the surface of the sample), which is probably the most important variable affecting the quality of diffusion welded joints and the detectivity of flaws. Indeed, we show that there is a loss of 8 dB between two surface roughnesses such as 0.811m and 3.8 11m, for a 100 MHz frequency inspection. So, any flaws may be hidden by the diffraction effect which is due to poor roughness and less energy arriving on the bond line. Flaw detectivity is also presented. The correlations obtained from the analysis of bond-line micrographs and C-scan recording have enabled us to define the current limitations of pulse echo ultrasonic testing. The results obtained for different bonds are discussed. Regarding influences of the water path between the transducer and the sample, we have quantified the frequency shift and the attenuation for several transducers with different nominal frequencies. Influences of this parameter on the acoustic beam are also discussed. Regarding high resolution mechanical scanning rigs, we show that it is very interesting for local expertise works to have a very small step of acquisition (ie 0.01 mm). Indeed, we manage to realize an image of the inspected bonded area, with a very good resolution of flaws. Correlations between local expertise works and the industrial step of acquisition will be presented, and also the correlation which permits us to determine the percentage of welded grains. At the present time, we can say that the amplitude of ultrasonic echoes versus percentage of welded grains seems to follow a Lorentz type law. But the results have to be confirmed with several correlations for different frequencies, surface roughnesses, amplification gain etc. The use of image processing techniques is more and more required when industry needs help for inspection diagnostics. So, we present our latest investigations in this field, and also the latest developments to inspect pieces with complex shape and varying thickness from 2 mm to 15 mm of titanium. In this case, we have to go through the first bond line of the piece. So, many difficulties in diagnostics appeared and the use of image processing techniques is again required.
NDE/NDT Guillois, F.*; Corbel, c.*; Royer, D.t; De Mol, R.t Autofocusing ultrasonic propagation in composite media based on laser generated ultrasound tDassault Aviation CTNFjEND, 78 Quai Marcel Dassault, 92552 Cedex 300 St. Cloud, France * Laboratoire Ondes et Acoustique, Universite Paris 7, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France
NDT & E International Volume 25 Number 4/5 1992
Among the nondestructive testing processes used in aeronautics a new contactless technique based on laser generated ultrasound has been applied to the inspection of composite materials. To study the propagation of ultrasound with a thermoelastic source in composite materials, two different laminated carbon epoxy lay ups have been made: unidirectional and cross-ply stacking. The experimental samples have a half-cylinder shape to enable half plan analysis. Only thermoelastic generation based on laser excitation has been considered, whereas contact piezoelectric transducers and an interferometric system were used for ultrasonic detection. After the recognition of all ultrasonic wave types through wavespeed analysis, the principal point of interest has been the angular directivity pattern of the thermoelastic source in composite media. The experimental results for both longitudinal and shear waves in cross-ply stacking have shown an important change of directivity pattern and a focusing effect observed for longitudinal waves. Interpretation of this phenomenon assumes that the auto-focusing concept takes into account the energy vector direction in the anisotropic plans. In addition, the influence of the resin film at the surface has been experimentally described and identified. This influence acts as a reinforcing of the longitudinal mode at the epicentre. A computed result confirmed that auto-focusing does exist in composite materials. These results demonstrate the pulsed laser capabilities to generate elastic waves simultaneously in laminated composite materials. We have shown that acoustic waves are strongly affected by physical parameter anisotropy. However, the mechanical anisotropy in plan of fibre alignment involves an auto-focusing effect in the far field.
Sribar, Rok; Sachse, Wolfgang An experimental investigation of lattice-type structures for AE source location and magnitude using smart signal processing Cornell University, Department of Theoretical and Applied Mechanics, Ithaca, New York 14853, USA This paper reports the results of the experimental investigation of acoustic emission (AE) source location and characterization in frames with rigid joints using an array of very small strain gauges and three different 'smart' signal processing algorithms. Determination of the AE source location and magnitude from the displacement field is an inverse problem. Although advances have been made over the past few years, the known solutions to elastic wave inverse problems are computationally too restrictive for practical applications. It is known that humans have superb sensory abilities to unravel a large number of difficult inverse problems with little effort. Here, we try to ilJlitate nature by applying smart, neural-like signal processing algorithms to solve the problems of the AE source location and magnitude determination. The signal processing algorithms employed here are: feed forward neural networks (FFNN) trained by a back-propagation algorithm, a multi-dimensional regression algorithm called an automatic modeller (AM), and a linear system called an auto-associative processor (AAP). The common feature ofthese algorithms is the use of a set of pre-processed, measured prototype signals to develop a system memory. The pre-processed signals used by the AM and AAP algorithms are teacher augmented, while in the case of the FFNN the pre-processed signals are non-linearly mapped to the information provided by the teacher. The system memory is subsequently employed to process experimental signals to determine the AE source location and its magnitude.
Djordjevic, Boro B. Advanced ultrasonic sensors for large area structural testing Martin Marietta Laboratories, 1450 S. Rolling Road, Baltimore, Maryland 21227, USA Ultrasonic nondestructive evaluation (NDE) of large structural components for critical aerospace applications requires the ability to rapidly scan surfaces ranging in size from several square meters to 100 or more square meters. Additionally, the structures are often bonded or contain composite or honeycomb components. To achieve quantitative and rapid ultrasonic examination of such materials, a scanning ultrasonic probe that retains fidelity of the ultrasonic signal must be utilized. Because of their size and construction, large structures cannot be immersed in water or other liquids for ultrasonic tests. At present, two significantly different test methodologies are utilized for ultrasonic inspection of such structures: high performance ultrasonic liquid jet probes and laser light remote stress-waves probes. This paper will present the engineering application and technical considerations for these methodologies. In ultrasonic examination of layered structures, low-amplitude noise and precise signal gating for C-scan presentation are key to the detection of critical flaw sizes. In addition, signal processing of ultrasonic rf waveforms often adds insight to structural integrity. The low-noise ultrasonic probe (LUPTM) and laser scanner system are designed to maintain maximum ultrasonic signal fidelity. The test results show that LUP probes bave better than 40 dB signal-to-noise capability
211
abstracts~~~~
The following are abstracts of papers relating to nondestructive testing and evaluation due to be presented at the Ultrasonics International '93 conference, from 6-8 July 1993 in Vienna, Austria. Acoustic microscopy Frestel, N.; Liot, A.; Tretout, H.; De Mol, R. Scanning acoustic microscopy for quantitative analysis of superplastic forming diffusion bonding titanium aerospace components Dassault Aviation CTNTFjEND, 78 Quai Marcel Dassault, 92552 Saint-Cloud Cedex 300, France The need to improve the performance of aircraft by reducing the cost and weight of aerospace metallic structures has led to increased interest in superplastic forming and diffusion bonding titanium alloy processes. The production of heavily loaded flying SPF lOB components for Dassault Aviation's fighters has required quantitative characterization of diffusion bonds. The particular problems which arise in SPF lOB processing of titanium alloys at the development stage can induce defects at the bond-line, which may be divided into voids of different sizes with gaps ranging from intimate contact disbond up to 20 11m. Ultrasonic techniques appeared to have the greatest potential for defect detection and sizing in diffusion bonds. Unfortunately the smallest defect size can be much less than the wavelength of conventional ultrasound,leading us to look for higher frequency equipment from 30 MHz up to 100 MHz and above, point focusing transducers, signal processing techniques and high resolution mechanical scanning rigs. Acoustic microscopy is used by Dassault Aviation to characterize diffusion bonded sheets of titanium alloys. We present here the use of high frequency ultrasound from an industrial point of view. the relevant parameters, the current detection limitations, statistical analysis of C-scans and the use of image processing techniques in order to quantify the percentage of welded grains. The work includes the development of an acoustic microscope, the evaluation of several types of transducers such as PZT, PVDF, and piezo-composite types, as well as the means for their metrology. Two kinds of parameters are measured: electrical and acoustic. Regarding acoustic parameters, unlike the metrology of classical transducers with frequencies lower than 30 MHz, characterization of high frequency transducers is complex. We will speak about the difficulties of obtaining representative results of the acoustic field in water or in a material such as titanium. Regarding electrical parameters, we analyse the shape of the reflected signal on a glass plate, the ratio of stationary waves and the frequency bandwidth. The effects of surface roughness (at the bond line and at the surface of the sample), which is probably the most important variable affecting the quality of diffusion welded joints and the detectivity of flaws. Indeed, we show that there is a loss of 8 dB between two surface roughnesses such as 0.811m and 3.8 11m, for a 100 MHz frequency inspection. So, any flaws may be hidden by the diffraction effect which is due to poor roughness and less energy arriving on the bond line. Flaw detectivity is also presented. The correlations obtained from the analysis of bond-line micrographs and C-scan recording have enabled us to define the current limitations of pulse echo ultrasonic testing. The results obtained for different bonds are discussed. Regarding influences of the water path between the transducer and the sample, we have quantified the frequency shift and the attenuation for several transducers with different nominal frequencies. Influences of this parameter on the acoustic beam are also discussed. Regarding high resolution mechanical scanning rigs, we show that it is very interesting for local expertise works to have a very small step of acquisition (ie 0.01 mm). Indeed, we manage to realize an image of the inspected bonded area, with a very good resolution of flaws. Correlations between local expertise works and the industrial step of acquisition will be presented, and also the correlation which permits us to determine the percentage of welded grains. At the present time, we can say that the amplitude of ultrasonic echoes versus percentage of welded grains seems to follow a Lorentz type law. But the results have to be confirmed with several correlations for different frequencies, surface roughnesses, amplification gain etc. The use of image processing techniques is more and more required when industry needs help for inspection diagnostics. So, we present our latest investigations in this field, and also the latest developments to inspect pieces with complex shape and varying thickness from 2 mm to 15 mm of titanium. In this case, we have to go through the first bond line of the piece. So, many difficulties in diagnostics appeared and the use of image processing techniques is again required.
NDE/NDT Guillois, F.*; Corbel, c.*; Royer, D.t; De Mol, R.t Autofocusing ultrasonic propagation in composite media based on laser generated ultrasound tDassault Aviation CTNFjEND, 78 Quai Marcel Dassault, 92552 Cedex 300 St. Cloud, France * Laboratoire Ondes et Acoustique, Universite Paris 7, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France
NDT & E International Volume 25 Number 4/5 1992
Among the nondestructive testing processes used in aeronautics a new contactless technique based on laser generated ultrasound has been applied to the inspection of composite materials. To study the propagation of ultrasound with a thermoelastic source in composite materials, two different laminated carbon epoxy lay ups have been made: unidirectional and cross-ply stacking. The experimental samples have a half-cylinder shape to enable half plan analysis. Only thermoelastic generation based on laser excitation has been considered, whereas contact piezoelectric transducers and an interferometric system were used for ultrasonic detection. After the recognition of all ultrasonic wave types through wavespeed analysis, the principal point of interest has been the angular directivity pattern of the thermoelastic source in composite media. The experimental results for both longitudinal and shear waves in cross-ply stacking have shown an important change of directivity pattern and a focusing effect observed for longitudinal waves. Interpretation of this phenomenon assumes that the auto-focusing concept takes into account the energy vector direction in the anisotropic plans. In addition, the influence of the resin film at the surface has been experimentally described and identified. This influence acts as a reinforcing of the longitudinal mode at the epicentre. A computed result confirmed that auto-focusing does exist in composite materials. These results demonstrate the pulsed laser capabilities to generate elastic waves simultaneously in laminated composite materials. We have shown that acoustic waves are strongly affected by physical parameter anisotropy. However, the mechanical anisotropy in plan of fibre alignment involves an auto-focusing effect in the far field.
Sribar, Rok; Sachse, Wolfgang An experimental investigation of lattice-type structures for AE source location and magnitude using smart signal processing Cornell University, Department of Theoretical and Applied Mechanics, Ithaca, New York 14853, USA This paper reports the results of the experimental investigation of acoustic emission (AE) source location and characterization in frames with rigid joints using an array of very small strain gauges and three different 'smart' signal processing algorithms. Determination of the AE source location and magnitude from the displacement field is an inverse problem. Although advances have been made over the past few years, the known solutions to elastic wave inverse problems are computationally too restrictive for practical applications. It is known that humans have superb sensory abilities to unravel a large number of difficult inverse problems with little effort. Here, we try to ilJlitate nature by applying smart, neural-like signal processing algorithms to solve the problems of the AE source location and magnitude determination. The signal processing algorithms employed here are: feed forward neural networks (FFNN) trained by a back-propagation algorithm, a multi-dimensional regression algorithm called an automatic modeller (AM), and a linear system called an auto-associative processor (AAP). The common feature ofthese algorithms is the use of a set of pre-processed, measured prototype signals to develop a system memory. The pre-processed signals used by the AM and AAP algorithms are teacher augmented, while in the case of the FFNN the pre-processed signals are non-linearly mapped to the information provided by the teacher. The system memory is subsequently employed to process experimental signals to determine the AE source location and its magnitude.
Djordjevic, Boro B. Advanced ultrasonic sensors for large area structural testing Martin Marietta Laboratories, 1450 S. Rolling Road, Baltimore, Maryland 21227, USA Ultrasonic nondestructive evaluation (NDE) of large structural components for critical aerospace applications requires the ability to rapidly scan surfaces ranging in size from several square meters to 100 or more square meters. Additionally, the structures are often bonded or contain composite or honeycomb components. To achieve quantitative and rapid ultrasonic examination of such materials, a scanning ultrasonic probe that retains fidelity of the ultrasonic signal must be utilized. Because of their size and construction, large structures cannot be immersed in water or other liquids for ultrasonic tests. At present, two significantly different test methodologies are utilized for ultrasonic inspection of such structures: high performance ultrasonic liquid jet probes and laser light remote stress-waves probes. This paper will present the engineering application and technical considerations for these methodologies. In ultrasonic examination of layered structures, low-amplitude noise and precise signal gating for C-scan presentation are key to the detection of critical flaw sizes. In addition, signal processing of ultrasonic rf waveforms often adds insight to structural integrity. The low-noise ultrasonic probe (LUPTM) and laser scanner system are designed to maintain maximum ultrasonic signal fidelity. The test results show that LUP probes bave better than 40 dB signal-to-noise capability
211