Browsing by Author "Post, Daniel"

Now showing 1 - 14 of 14
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    Compact Grating Interferometer For Producing Photoresist Gratings with Incoherent-Light
    Post, Daniel; Patorski, K.; Ning, P. (Optical Society of America, 1987)
    An achromatic interferometer was developed to produce 1200-lines/mm crossed-line photoresist gratings with a mercury arc light source. It is a compact reflection system of outstanding stability. Alignment procedures are described. The most stringent requirement, coplanar alignment of two folding gratings, was accomplished with the aid of a Twyman-Green interferometer. The grating interferometer produced crossed-line photoresist gratings with first-order diffraction efficiency exceeding 20%.
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    Compressive strength and behavior of 8H C3000/PMR15 woven composite material
    Mirzadeh, Farshad (Virginia Polytechnic Institute and State University, 1988)
    Center-notched and unnotched specimens cut from Celion 3000/PMR15 woven composite panels with 60% fiber volume fraction were tested under quasi-static compressive load to failure at room temperature. Micrographic evidence clearly identifies the mode of compressive failure as fiber kinking. Each fiber in the kink fractures because of a combination of compressive and shear stresses. A post failure mechanism follows the local fiber bundle failures, which completely deforms the material by large cracks. ln center notched specimens, fiber kinks start from the notch and propagate to some distance from the notch before the post failure takes place. The effect of bundle interactions on stresses and strains was clearly distinguished by comparing the results of the finite element analysis of a bundle surrounded by other plies to the results of the Moire interferometry on the edge of a laminate. A model was introduced which incorporated the micromechanical geometry as well as the constituent properties to predict the notched and unnotched compressive strengths of the woven material. For notched strength predictions, the Average Stress Criterion was used, and the characteristic distance was found to be a function of laminate thickness. Predicted notched and unnotched strengths correlate very well with the experimental results.
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    Determination of thermal strains in the neighborhood of a bimaterial interface
    Wood, Judy D. (Virginia Tech, 1992)
    An experimental analysis was conducted on a bimaterial plate of steel and brass, subjected to a uniform temperature change. The steel and brass portions of the plate were joined along a common edge with a nearly zero thickness medium. Whole-field, in-plane displacement measurements U and V were made by means of high-sensitivity moiré interferometry. The corresponding distributions of stresses, σx, σy, and τxy were determined for the free surface. Near the interface, and along its length, the largest stresses were σy, acting perpendicular to the interface. The σy peaks occurred very close to the interface, but not on it. These peak values were tensile in the steel and compressive in the brass. The transition between these opposite peak stresses featured an extremely strong gradient in a 50 µm interface zone. The distribution was akin to that of a stress singularity, but the stresses reached finite peak values in the physical experiment. Even larger stresses were found near the corner, where the interface of the two materials intersects the free edge. Both macroscopic and microscopic moiré interferometry were required to determine the stress distribution.
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    Displacement-Fields (U,W) Obtained Simultaneously By Moiré Interferometry
    Basehore, M. L.; Post, Daniel (Optical Society of America, 1982-11-01)
    A high-frequency phase grating on a specimen surface is illuminated symmetrically by two oblique beams. The diffracted beams emerge with wave front warpages that define both the in-plane U and out-of-plane W displacement fields. Contour maps of these wave fronts, with added carrier fringes, are obtained as a single photographic record. They are manipulated by moiré and optical filtering steps to yield whole-field fringe patterns of U and W. Sensitivities of 0.833 _m/fringe (32.8 _in./fringe) for in-plane displacements and 0.132 _m/fringe (5.2 _m./fringe) for out-of-plane displacements were demonstrated. Since data acquisition is experimentally simple, dynamic as well as static analyses are applicable.
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    An experimental and numerical analysis of the exit flow in a slit die for polymer melts
    Read, Michael David (Virginia Polytechnic Institute and State University, 1986)
    A slit die has been constructed to use both flow birefringence and direct pressure measurements to study the extrapolated exit pressure (Px) and the exit pressure theory used to evaluate the magnitude of the primary normal stress difference (N1) from the value of the exit pressure. Flow birefringence is used to directly assess the principal assumptions in the exit pressure theory and to evaluate the magnitude of Px from an expression derived from the macroscopic momentum balance equation. The effect of stress field rearrangement upstream of the die exit plane on the value of the exit pressure was then evaluated using flow birefringence data. The effect of stress field rearrangement was also shown to affect the pressure drop ΔP/ΔL in the exit region of the die and the pressure distribution from the centerline of the slit to the die wall. To complement the experimental investigation, a mixed penalty method finite element simulation of the die swell problem was performed using the White-Metzner and upper-convected Maxwell constitutive equations. The flow birefringence experiments were performed for a polystyrene (Styron 678), LDPE (NPE 952), and HDPE (LY600-00) melts for the following shear rate (γ̇) and wall shear stress (σw) 0.05 ≤ γ̇w ≤ 3.2 s⁻¹ and 4.84 ≤ σw ≤ 16.4 KPa. It was found that the flow in the die exit region is not a unidirectional shear flow, which is direct violation of the assumptions in the exit pressure theory. Normal stresses generated by an elongational flow field were observed along the slit centerline and in the region adjacent to the die walls. Also, shear stress contributions due to stress field rearrangement evaluated using an expression obtained from a macroscopic momentum balance, comprise over 50% of the magnitude of the calculated exit pressure. The numerically calculated stress field was in good agreement with the results of the flow birefringence results. Convergence for the numerical technique was limited to Deborah numbers of 0.61 for the White-Metzner model and 0.75 for the upper-convected Maxwell constitutive equation.
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    An experimental study of frictional phenomena around the pin joints of plates using moire interferometry
    Joh, Duksung (Virginia Polytechnic Institute and State University, 1986)
    Although contact problems with friction have received considerable attention in recent years, analytical as well as experimental limitations have tended to obscure some of their essential features. All the experimental techniques employed in the past lacked either required sensitivity or adequate spatial resolution for local measurements of deformation near the contact surface. Further, most techniques also did not allow the use of prototype material which is crucial for investigation of contact stress problems with friction. In the present study, a relatively new experimental method, which has been developed at VPI & SU, is employed: high-sensitivity moiré interferometry. Using a clearance pin-joint model made of prototype structural material, Aluminum 7071-T6, studies on frictional phenomena between the pin and plate are conducted to provide a comprehensive treatment of the following subjects: slip-stick phenomena, variation of contact zone, distribution of frictional force at the contact region, effects of frictional shear stress on stress concentration, and identification of slip amplitude. Experimental techniques and algorithms of analysis necessary for the research are further developed. The results showed a strong influence of friction, including significant differences in the load-increasing and load-decreasing phases.
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    Fabrication Of Holographic Gratings Using A Moving Point-Source
    Post, Daniel; McKelvie, J.; Tu, M. R.; Dai, F. L. (Optical Society of America, 1989-02-01)
    In the manufacture of holographic gratings using coherent light there is an associated problem of optical noise, the conventional remedies for which involve various restrictions. In this work a moving point source is adopted and analysis indicates that if the path is a circle several of these restrictions are removed. The scheme is implemented and the results are highly satisfactory with high-efficiency gratings being produced at frequencies up to 2400 lines/mm, in sizes up to 65 mm square. Significant operational advantages also accrue in relation to alignment procedures and light utilization.
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    Grating Speckle Method For In-Plane Displacement Measurement
    Tu, M. R.; McKelvie, J.; Dai, F. L.; Post, Daniel (Optical Society of America, 1989-12-01)
    A high sensitivity technique combining lensless speckle photography and a grid technique is proposed and demonstrated. A high frequency grid is applied to the specimen surface and contact specklegrams are made in white light before and after loading, in the double exposure mode. Subsequent optical processing yields the various components of in-plane displacement. The sensitivity can be chosen equivalent to the grid frequency or multiples thereof. The method is simple and gives low-noise fringe patterns with high contrast. It is suitable for application in the mechanical laboratory and field environments, because vibration isolation precautions are not necessary.
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    Interlaminar Deformation at a Hole in Laminated Composites: A Detailed Experimental Investigation Using Moire Interferometry
    Mollenhauer, David Hilton (Virginia Tech, 1997-08-20)
    The deformation on cylindrical surfaces of holes in tensile loaded laminated composite specimens was measured using new moire interferometry techniques. These new techniques were developed and evaluated using a 7075-T6 aluminum control specimen. Grating replication techniques were developed for replicating high quality diffraction gratings onto the cylindrical surfaces of holes. Replicas of the cylindrical specimen gratings (undeformed and deformed) were fabricated onto circular steel sectors. Narrow angular regions of these sector gratings were directly evaluated in a moire interferometer. This moire interferometry approach eliminated potential sources of error associated with other moire interferometry approaches. Two composite tensile specimens, fabricated from IM7/5250-4 pre-preg with ply layups of [0₄/90₄]3s and [+30₂/-30₂/90₄]3s, were examined using the newly developed moire interferometry techniques. Circumferential and thickness direction displacement fringe patterns (each 3 degrees wide) were assembled into 90 degrees wide mosaics around the hole periphery for both composite specimens. Distributions of strain were calculated with high confidence on a sub-ply basis at select angular locations. Measured strain behavior was complex and displayed ply-by-ply trends. Large ply related variations in the circumferential strain were observed at certain angular locations around the periphery of the holes in both composites. Extremely large ply-by-ply variations of the shear strain were also documented in both composites. Peak values of shear strain approached 30 times the applied far-field axial strain. Post-loaded viscoelastic shearing strains were recorded that were associated with the regions of large load-induced shearing strains. Large ply-grouping related variations in the thickness direction strain were observed in the [+30₂/-30₂/90₄]3s specimen. An important large-scale trend was observed where the thickness direction strain tended to be more tensile near the outside faces of the laminate than near the mid-ply region. The measured strains were compared with the three-dimensional analysis technique known as Spline Variational Elastic Laminate Technology (SVELT), resulting in a very close match and corroborating the usefulness of SVELT.
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    Interlaminar deformations on the cylindrical surface of a hole in laminated composites: an experimental study
    Boeman, Raymond G. (Virginia Tech, 1990-12-17)
    Free-edge effects in composite laminates were studied experimentally. Strains were determined and compared on a ply-by-ply basis for the curved edges of a hole in thick composite panels and along the straight free-edge of the panels. The experimental technique of moire interferometry was extended to take measurements of in-plane deformations on singly-curved surfaces. A replication scheme was developed to produce high-frequency diffraction gratings on singly-curved surfaces. Two different techniques were developed to interrogate specimen gratings on 25.4 mm (1 in.) diameter holes. Eight thick composite laminates from three material systems were tested in uniaxial compression on a screw-driven testing machine. Interlaminar deformations were measured at the straight free-edge on four of the specimens. Strain distributions on the straight free-edge were compared with FEM results for two specimens. Good agreement was obtained for one specimen while poor agreement was obtained for the other.
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    Moire Interferometry Near The Theoretical Limit
    Weissman, Eric M.; Post, Daniel (Optical Society of America, 1982-01-01)
    The theoretical upper limit of moire interferometry is approached as the reference grating pitch approaches _/2 and its frequency approaches 2/_. This work demonstrates the method at 97.6% of the theoretical limit. A virtual reference grating of 4000 lines/mm (101,600 lines/in.) was used in conjunction with a phase type reflection grating of half of that frequency on the specimen. Sensitivity was 0.25 _m/fringe (9.8 _in./fringe). In-plane displacement fringes of excellent definition were obtained throughout the 76 _ 51-mm (3 _ 2-in.) field of view. They were very closely packed, exhibiting a maximum fringe density of 24 fringes/mm (610 fringes/in.). Effectiveness of moire interferometry near the theoretical limit was proved.
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    Moiré Interferometry with White-Light
    Post, Daniel (Optical Society of America, 1979-11-01)
    While high-sensitivity moiré interferometry requires monochromatic light, an auxiliary compensator grating removes the requirement. Experimental verification utilized white light for moiré interference of sensitivity 0.833 μm/fringe. Configurations readily amenable to compensation include moiré fringe multiplication by even factors and the minimum-deviation arrangement. Chromatic compensation is effective with virtual reference gratings as well as real reference gratings; in both cases the moiré interference system becomes achromatic. Compensation provides the special quality of nearly zero phase difference—an actual range of ±π—between interfering beams. Potential applications include use of electrical spark light sources for high-sensitivity dynamic moiré photography.
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    Real-Time Optical Differentiation For Moire Interferometry
    Patorski, K.; Post, Daniel; Czarnek, R.; Guo, Y. F. (Optical Society of America, 1987)
    Real-time methods for differentiation of in-plane displacement fields produced by the moire interferometry technique are introduced. Two approaches are developed: (1) optical shearing of displacement patterns and (2) superposition of two lateral shear interferograms of wavefronts from 2 diffraction orders of the specimen grating. Coherence problems are circumvented by polarization effects. In both cases additive-type moire fringes give the map of displacement derivatives of the object under load. The issue of carrier patterns and extraneous fractional fringe order is clarified. Experimental verification of the principle is presented.
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    Small displacement measurement in ultrasound: quantitative optical noncontacting detection methods
    Sarrafzadeh-Khoee, Adel (Virginia Polytechnic Institute and State University, 1986)
    In this study the description and development of intensity-based laser interferometric techniques for the detection and measurement of ultrasonic stress waves and their small displacement amplitudes is presented. The dynamic displacement sensitive interferometers described in the following chapters allow the quantitative point-by-point measurement of both in-plane and out-of-plane components of surface displacement motion. These uniquely developed interferometric sensors are: 1) an optical system design for the detection of the surface acoustic wave (Rayleigh wave). The technique is based on the Fourier analysis of coherent light and diffraction imaging properties of an illuminated grating; 2) the design and construction of a two-beam unequal-path laser interferometer for the measurement of out-of-plane surface displacement of ultrasonic waves; 3) extension of a flexible fiber optic probing device which is optically coupled to the test arm of the above two-beam interferometer. This permits scanning of the test surface which may be at some distance from the main optical system components; 4) the design and construction of a laser speckle interferometer for retro-reflective diffusing surfaces in which the in-plane displacements of the ultrasonic wave are interrogated. The inherent advantages of these newly designed optical configurations in terms of their greater simplicity, feasibility, and sensitivity over the conventional counterparts (classical/speckle laser interferometers) are explained. The function-response limitations of these interferometric sensors on lateral displacement resolution, on upper and lower-bound displacement sensitivity (dynamic range), on high-frequency bandwidth probing capability, on low-frequency environmentally associated noise disturbance, and on specularly reflective or diffusively retro-reflective specimen surface preparation are also mentioned. Finally, in a series of experimental observations, the application of a couple of these acoustic sensors in pulsed-excitation ultrasonic Specifically, the optically testing methods is cited. detected ultrasonic signals revealing the true nature of the various surface displacement modes of vibration are presented.