In my editorial entitled "The Review Process," which appeared in the September/October 1985 issue of Optical Engineering, I discussed the important role that referees play in determining the fate of manuscripts submitted to technical journals. In the present editorial I describe the procedures we employ in selecting referees and some of the difficulties we encounter in making the decision to publish or reject a manuscript.
Following the special issue of March 1988 we continue our survey of photomechanics in this issue. One of the major activities in solid mechanics today is the study of fracture. We investigate the mechanism under which a structure fails catastrophically due to the presence of a crack. Photo mechanics has made significant contributions to the basic understanding of fracture.
Applications of the method of reflected caustics to the measurement of the J integral in ductile materials are reviewed. It is demonstrated, both numerically and experimentally, that the conditions for accurate interpretation of caustics on the basis of plane stress small scale yielding analyses are often overrestrictive. To overcome these restrictions, we used a three-dimensional, elastic-plastic finite-element calculation to analyze caustics formed by reflection of light from a particular test specimen. Experimental measurements on the same specimen confirm the numerically obtained results. The out-of-plane surface displacements, measured experimentally by interferometry, are in excellent agreement with the corresponding numerical results. In addition, the experimentally obtained caustics agree well with the numerically generated caustics. The excellent agreement between experiment and calculations demonstrates the accuracy of the numerical model and establishes confidence in the interpretation of caustics in the presence of both extensive plasticity and three dimensionality. The analysis of caustics as based on the three-dimensional calculation is applied to the direct optical measurement of the time history of the J integral in a dynamically loaded specimen. The specimen was loaded in a drop weight tower, and the caustics were photographed with a high speed camera.
The optical method of caustics for a blunt crack (or keyhole crack) is analyzed. The stress field near the keyhole crack is expressed by the Williams-Irwin series expansion including higher order singular terms than the usual r-1/2 singular term. The coefficients of the higher order singular terms are determined by the boundary collocation method for K1 field dominance at far field. The numerical and experimental caustics show good agreement in shape and size. This analysis concludes that the effect of bluntness is significant only within the distance four times the notch root radius from the notch root. If the initial curve of a caustic lies within the distance, the stress intensity factor is highly overestimated by the caustic formula of a sharp crack.
This paper describes a methodology that uses the frozen stress method of photoelasticity, with appropriate algorithms, for extracting the field strengths of the three local modes of crack tip deformation (K1, K2, K3) from a cracked photoelastic model. A part-circular crack in a large plate and a circum-ferentially cracked torsion bar are used to illustrate the application of the method. The accuracy of the results is estimated.
This paper examines the experimental u1, u2, and u3 displacements obtained in the vicinity of a plastically deformed crack tip in different work hardening materials in the background of the Hutchinson-Rice-Rosengren (HRR) field equations. It is shown that the two-dimensional plane stress solution breaks down in the immediate vicinity of the crack tip because of the three-dimensional nature of the deformations due to finite plate thickness. This "inner limit" seems to vary from 0.75 to 1.5 times the thickness with directional dependence. Some light is also shed on the "outer limit" of HRR equations, beyond which the theory ceases to be valid under the influence of the surrounding elastic field or the physical boundary of the plate or a combination of both.
Uniaxial compressive creep loads were applied to quasi-isotropic composite laminates at room temperature. The x and y displacement fields were measured using moire interferometry. Moire fringe patterns were recorded photographically; these photographic records were subsequently digitized and reduced to in-plane strain levels numerically. Strain contours were plotted, providing a whole-field representation of in-plane strain distribution. Slight viscoelastic behavior was observed over the time period considered, except in one test in which time-dependent delamination failures were observed.
The technique of half-fringe photoelasticity was used to study the applicability of thin plate theory for cantilever rectangular plates subjected to transverse loading. The results are presented and compared with those obtained from the finite-element method for three cases, including plates with and without discontinuities.
To date, powerful lasers such as argon or ruby lasers have been used in most applications of particle image velocimetry. We present an analysis of the ability of this technique when a low power (5 mW He-Ne) laser is used. As a practical example we apply it to the Rayleigh-Benard convective flow, where velocities ranging up to several mm/s have been measured with about 100µJ energy pulses. The parameters that allow us to obtain good quality photographs with such low light power are analyzed. Several examples of pictures taken from different flow planes at different Rayleigh numbers are shown. The data reduction is made by fringe analysis. The digital image of the Young's fringes is recorded with a videocamera at each point of the photograph and is then processed with the 1-D averaging technique. This technique is the fastest but has the drawback of needing an external operator to select the fringe direction. To avoid this problem, we implement an algorithm whose practical performance is analyzed here. Another improvement proposed for this technique is the pedestal removal from the signal by digital high pass filtering. In this way, automatic detection of the first peak in the Fourier transform of the 1-D averaged signal is reached more easily. Thus, full automation of the 1-D averaging technique is obtained. The spatial filtering technique is also used. Several examples of whole-field velocity maps obtained from the photographs are shown, allowing a quick analysis of the velocity field. Finally, results obtained from postprocessing of the velocity measurements are shown. They include velocity, vorticity, and streamline maps. Isovelocity contour maps are also generated with the computer from the velocity field to compare with the maps obtained from the spatial filtering technique.
By shifting the discrete Fourier spectrum of the image of a deformed grating, we obtain the "complex moire pattern," from which strain distribution is given as the derivatives of the phases of the complex moire fringes. The analysis is completely automated by digital image processing. All of the laborious and subjective procedures required in the conventional analysis such as fringe sign determination, fringe ordering, and fringe interpolation are thus eliminated, permitting objective, fast, and accurate analysis. Some applications for rubber plates are shown.
A folded cavity laser resonator operating in the TEMoo mode has been built and tested. The new oscillator configuration leads to an increase in efficiency and to better line narrowing due to the increased number of passes through the laser rod and tuning elements, respectively. The modification is shown to lead to cavity ruggedization.
Six optical technologies are evaluated for use in a full-duplex telecommunications link between geostationary INTELSAT satellites. The six technologies are based upon the carbon dioxide gas laser, the neodymium-doped yttrium aluminum garnet (Nd:YAG) solid-state laser, the indium gallium arsenide phosphide (InGaAsP) semiconductor diode laser, and the gallium aluminum arsenide (GaAlAs) semiconductor diode laser (three systems). The salient features of the various technologies are described, and their communications performances are analyzed to determine antenna diameter requirements for various modulation formats. The GaAlAs systems, with their small size, high electrical-to-optical conversion efficiency, direct modulation capability, wave-length selectability, and high reliability are found to be promising technologies for the full-duplex geostationary intersatellite link.
A simple and accurate phase-tracking measurement technique for multichannel Bragg cells is presented. The procedure is carried out by illuminating two adjacent channels at a time and then comparing the relative positions of the diffraction fringe patterns in the Fourier plane to obtain the relative phase distribution. The application of this technique is demonstrated using a five-channel Bragg cell.
A technique for measuring the emissivity of single, nominally 50-µm-sized tungsten particles at elevated temperatures is demonstrated. Single particles are electrodynamically suspended in the cavity of a cw Nd:YAG laser and subsequently heated by the laser beam. The light emitted by the heated particles is focused onto the slit of a spectrometer. The detector used is an intensified gatable diode array that allows for the collection of time-resolved spectral data. Spectral radiance is deduced by calibrating the system against a standard lamp. Assuming that the relationship between emissivity and wavelength is a smooth function, the particle temperature and emissivity are determined by a curve-fitting procedure involving Planck's spectral radiation law. Experimental demonstration and error estimates of the technique are presented.
Digital tomosynthesis involves tomographic motion about a single plane during which separate radiographs are acquired at equal angular increments. The resulting series of images can be shifted and recombined to yield planes in focus at any depth in the patient. Previous work has used image intensifiers for acquisition. However, conventional image intensifiers suffer from a limited field of view and from problems arising from the curved image input. The present work, which uses digitized 14 x 17 in. films, eliminates these problems in order to explore the potential for high resolution tomosynthesis of the chest. The method utilizes linear tomographic motion about one plane, acquiring films every 2.5° over a 50° arc. These 21 films are digitized with a laser scanner, registered to ±1 pixel accuracy, shifted, and added in order to generate images in any desired plane. These tomosynthesized images are then displayed on a 23-in. 1000-line monitor. Work has been directed specifically at chest imaging. A tomography phantom, an anthropomorphic chest phantom, and a dog have been imaged. In addition to the synthesis of tomographic planes, thin slice images with differential tissue emphasis have been accomplished. These tomograms in digital format readily lend themselves to image processing.
A review of ultrafast techniques for characterizing wide modu-lation bandwidth semiconductor lasers is presented. The origin of nonlinear gain effects and their influence on the modulation bandwidth are investigated. The most recent experimental results and trends are discussed.
A mathematical model for direct coupling an InSb photodiode to a silicon charge-coupled device is constructed and then compared with experimental data. The model is used to characterize the coupling scheme for linearity between light intensity and output signal. A figure of merit for the range of linear response is also proposed.