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Acoustic emission from modulated laser beam welding of materials

JOURNAL OF LASER APPLICATIONS 1996年第4期8卷 205-210页

作者： Gu, HP Duley, WW Guelph‐Waterloo Program for Graduate Work in Physics University of Waterloo Waterloo Ontario Canada N2L 3G1

Acoustic emission during the laser welding of metals with a modulated CO2 laser beam has been investigated. Sharp resonances at many frequencies in the acoustic emission spectra have been observed. This was most noticeable when the average laser power was high enough to produce full penetration welds whose acoustic emission at high harmonics with frequencies that overlap with bands of vibrational frequencies corresponding to eigenmodes of the keyhole are greatly enhanced. Lower harmonics were not clearly observed. However, in partial penetration welding, many harmonic components were observed. These results are not affected by a surface coating of the metal as in the galvanized steel sheet sample. This observation indicates that the keyhole and its surrounding liquid layer act as a frequency selective amplifier for pressure fluctuations induced by changes in the interaction of the laser radiation with the walls of the keyhole. When a continuous wave (CW) beam is used, random fluctuations with frequencies within these allowed bands are amplified to produce a spectrum consisting of a large number of overlapping but discrete frequency components. When the laser beam is modulated, a forced response is elicited resulting in a much simplified emission spectrum.

关键词： laser welding acoustic monitoring

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Reflected-light, photoluminescence and OBIC imaging of solar cells using a confocal scanning laser MACROscope/microscope

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SOLAR ENERGY MATERIALS AND SOLAR CELLS 1996年第4期44卷 439-450页

作者： Ribes, AC Damaskinos, S Tiedge, HF Dixon, AE Brodie, DE Guelph-Waterloo Program for Graduate Work in Physics Waterloo Campus University of Waterloo Waterloo Ont. Canada N2L 3G1

This paper describes a confocal scanning beam MACROscope/Microscope which can image specimens up to 7x7 cm in size using reflected light, photoluminescence and optical beam induced current. The MACROscope provides a 10 mu m spot size at various wavelengths and generates 512 x 512 pixel images in less than 5 s. When used in combination with a conventional confocal scanning laser microscope sub-micron spot sizes become possible providing resolutions as high as 0.25 mu m laterally and 0.5 mu m axially in reflected light. The main function of this imaging system is to spatially resolve any defects within solar cells and similar devices. Several reflected-light, photoluminescence and OBIC images of CdS/CuInSe2 and CdZnS/CuInSe2 thin film solar cells are presented.

关键词： confocal macroscope OBIC photoluminescence

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Direct use of variational principles as an approximation technique in classical mechanics

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AMERICAN JOURNAL OF physics 1996年第9期64卷 1177-1184页

作者： Gray, CG Karl, G Novikov, VA Guelph‐Waterloo Program for Graduate Work in Physics and Department of Physics University of Guelph Guelph Ontario N1G 2W1 Canada

The Hamilton least action principle, a reformulated Maupertuis least action principle, and their reciprocals, are shown to be useful as direct methods for approximate solutions of dynamics problems. We discuss applications to trajectories of all types, i.e., periodic, quasiperiodic, chaotic, scattering, and arbitrary segments of arbitrary trajectories. The analogy with the standard technique used in quantum mechanics is very striking, especially in one of the reformulations (extremization of the mean energy), and in the calculational procedure (Rayleigh-Ritz type). (C) 1996 American Association of physics Teachers.

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Spherical particle size determination by analytical inversion of the UV-visible NIR extinction spectrum

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APPLIED OPTICS 1996年第1期35卷 193-197页

作者： Wang, JH Hallett, FR []The authors are with the Guelph–Waterloo Program for Graduate Work in Physics Department of Physics University of Guelph Guelph Ontario N1G 2W1 Canada

An analytic inversion method, based on the anomalous diffraction approximation for nonabsorbing spherical particles, was developed to retrieve the size distribution from the optical turbidity or extinction spectrum. This method makes use of a differential Fourier cosine transform approach and provides a simple and fast inversion by means of fast Fourier transform and the Savitzky-Golay filter. The applicability of this algorithm was tested on the extinction data generated by the Mie solution. The effects of noise, modality, band limits, and data set size were analyzed by comparison with simulated data. This method can be used to reconstruct the original monomodal and bimodal distributions from 10% noise-corrupted data. The peak position and ratio of peak heights can be recovered with 10% or less deviation. The experiments with latex spheres showed that the inversion result from this method compares favorably with that from the dynamic light scattering measurement. (C) 1996 Optical Society of America

关键词： particle size measurement turbidity optical characterization inverse problem and light scattering

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Monte Carlo simulations of randomly branched polymers with annealed and quenched branching structures

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Physical Review E 1996年第6期53卷 6238-6238页

作者： Shi-Min Cui Zheng Yu Chen Guelph-Waterloo Program for Graduate Work in Physics and Department of Physics University of Waterloo Waterloo Ontario Canada N2L 3G1

We present three-dimensional off-lattice Monte Carlo simulations on randomly branched polymers with annealed and quenched branching structures when monomers interact with each other through excluded volume interactions. The values of the critical exponent for the mean-square radius of gyration are estimated on the basis of scaling analysis as νa=0.49±0.01 (annealed) and νq=0.45±0.01 (quenched), which suggest that annealed and quenched randomly branched polymers belong to different universality classes. We further confirm the existence of a scaling parameter that was deduced earlier by the perturbation theory and the Flory-type argument. © 1996 The American Physical Society.

关键词： Polymers

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A statistical approach to acoustic monitoring of laser welding

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JOURNAL OF physics D-APPLIED physics 1996年第3期29卷 556-560页

作者： Gu, HP Duley, WW Guelph-Waterloo Program for Graduate Work in Physics University of Waterloo Waterloo Ontario Canada N2L 3G1

Acoustic emission at frequencies between 20 Hz and 20 kHz during CO2 laser welding of steel contains important diagnostic information related to weld morphology, depth of penetration and the heat-affected zone. We have recorded acoustic spectra under a variety of welding conditions with mild and galvanized steel sheet and find that a well-defined spectrum is obtained under conditions that lead to an optimization of these weld characteristics. A statistical approach is developed which yields a predictive measure of weld quality, based on a comparison between real-time acoustic spectra, and that obtained under optimized conditions. This technique is applied to an analysis of bead on plate, lap and butt laser welds in mild and galvanized steel.

关键词： Laser beam welding

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Holographic inferometry of isotherms during laser drilling of fused quartz

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JOURNAL OF physics D-APPLIED physics 1996年第5期29卷 1140-1145页

作者： Olfert, M Duley, WW Guelph - Waterloo Program for Graduate Work in Physics University of Waterloo Waterloo Ontario Canada N2L 2G1

The propagation of heat into fused quartz during drilling with CO2 laser radiation had been studied using differential holographic interferometry. This technique yields the temporal evolution of the thermally induced optical path difference in a cross section through the sample in a plane which contains the longitudinal axis of the drill hole. With the assumption of cylindrical symmetry, a portion of the temperature field in the sample has been obtained from these optical path length differences and is compared to the results of theoretical models for heat transfer from a high-aspect-ratio laser-heated drill hole. Isotherms derived in this way have been obtained during CO2 laser drilling of fused quartz in normal gravity and under the reduced gravity conditions of the NASA KC 135 aircraft under parabolic flight conditions. Heat transfer from a pre-drilled hole absorbing laser radiation has also been studied to evaluate the possible effect of material removal on heat transfer. Comparisons of these thermal profiles with those generated by numerical modelling of heat transfer from a laser-heated drill differences. Possible sources of these differences measured isotherms are discussed.

关键词： Laser applications

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Resonant acoustic emission during laser welding of metals

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JOURNAL OF physics D-APPLIED physics 1996年第3期29卷 550-555页

作者： Gu, HP Duley, WW Guelph-Waterloo Program for Graduate Work in Physics University of Waterloo Waterloo Ontario Canada N2L 3G1

We observed sharp resonances at certain frequencies in the acoustic and optical emission from metals during laser welding with a modulated CO2 laser beam. Strong emission is observed at high-order harmonics of the laser modulation frequency. Harmonics whose frequencies overlap with bands of vibrational frequencies corresponding to eigenmodes of the keyhole are greatly enhanced, relative to other harmonics whose frequencies lie outside these bands. The keyhole and its surrounding liquid layer act as a frequency selective amplifier for pressure fluctuations induced by changes in the interaction of laser radiation with the walls of the keyhole. When a CW beam is used, random fluctuations with frequencies within these allowed bands are amplified to produce a spectrum consisting of a large number of overlapping but discrete frequency components. When the laser beam is modulated, a forced response is elicited resulting in a much simplified emission spectrum. These results offer some new insights into the physical mechanisms that convert laser radiation to heat in laser welding, drilling and cutting applications.

关键词： Acoustic emissions

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The decomposition of hydrogenated amorphous carbon: A connection with polycyclic aromatic hydrocarbon molecules

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ASTROPHYSICAL JOURNAL 1996年第2期472卷 L123-&页

作者： Scott, A Duley, WW Guelph-Waterloo Program for Graduate Work in Physics University of Waterloo Waterloo ON N2L 3G1 Canada

We have produced thin solid films of hydrogenated amorphous carbon (HAC) by vapor deposition in vacuum and have investigated the structure and infrared spectra of these materials under conditions in which they are at the point of decomposition. We find that the IR absorption spectrum of HAC under these conditions contains all the major spectral components seen in emission in nebulae. In addition, high-resolution scanning tunneling microscopy of HAC prior to decomposition reveals protographitic islands with dimensions similar to 1-5 nm showing that polycyclic aromatic hydrocarbon (PAH)-like molecular groups evolve from HAC. These experimental results suggest for the first time how PAHs and HAC may be related and how PAH molecules could evolve from HAC in regions of high excitation. The material produced as HAC decomposes is a very low density carbonaceous ''aerogel'' consisting primarily of these aromatic protographitic clusters in a weakly connected friable network. These protographitic clusters may be the source of interstellar graphite grains.

关键词： dust, extinction infrared, ISM, lines and bands ISM, abundances ISM, molecules

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Metastability of freely suspended liquid-crystal films

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Physical Review E 1996年第3期53卷 2466-2466页

作者： Y. Martinez A. M. Somoza L. Mederos D. E. Sullivan Department of Physics and Guelph-Waterloo Program for Graduate Work in Physics University of Guelph Guelph Ontario Canada N1G 2W1

We investigate the conditions required for the existence of smectic-A liquid-crystal films freely suspended in vapor. This work is based on a molecular density-functional theory developed in earlier studies of wetting and layering transitions of smectic-A films at liquid-solid and liquid-vapor interfaces. It is emphasized that all freely suspended films are metastable with respect to either the absence of the film or the formation of the bulk smectic phase, depending on the relative stability of the bulk vapor and smectic phases. Films containing different numbers of layers correspond to local minima of the grand canonical potential, and are ‘‘stabilized’’ relative to each other by the presence of sufficiently large meta- stability barriers. The disappearance of these barriers corresponds physically to the ‘‘rupturing’’ of the films. It is demonstrated that surface enhancement of smectic ordering does not play an essential role in the existence of freely suspended films, but may influence the order of disappearance of the layer-layer metastability barriers on changing temperature. We relate these findings to the results of a recent experimental study of ‘‘layer thinning transitions’’ in freely suspended films. © 1996 The American Physical Society.

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