Showing papers on "Photoelasticity published in 1995"

Optical Methods of Engineering Analysis

Abstract: Introduction 1. Light and interference 2. Classical interferometry 3. Photoelasticity theory 4. Basic applied photoelasticity 5. Photoelasticity methods and applications 6. Geometrical moire theory 7. In-plane motion and strain measurement 8. Moire mapping of slope, contour and displacement 9. Diffraction and Fourier optics 10. Moire with diffraction and Fourier optical processing 11. Procedures of moire analysis with optical processing 12. Principles of moire interferometry 13. A moire interferometer 14. Experimental methods in moire interferometry 15. Holographic interferometry theory 16. Holographic interferometry methods 17. Laser speckle and combinations of speckle fields 18. Speckle photography 19. Speckle correlation interferometry 20. Electronic speckle pattern interferometry 21. Phase shifting to improve interferometry.

Towards RGB photoelasticity: Full-field automated photoelasticity in white light

Abstract: In this paper a new full-field method for the automatic analysis of isochromatic fringes in white light is presented. The method, named RGB photoelasticity, eliminates the typical drawbacks of the classical approach to photoelasticity in white light which requires a subjective analysis of colors and an experienced analyst to acquire and interpret the results.

Automated photoelasticity in white light: Influence of quarter-wave plates

Abstract: This paper deals with the influence of optical retarders on the isochromatic fringes obtained by automated white light photoelasticity using methods such as those based on spectral content analysis (SCA) and on primary colour (red, green and blue) analysis. In the following the light intensity equations of dark- and light-field polariscopes with both crossed and parallel optical retarders are reviewed. In particular, it is shown that the retardance error of the quarter-wave plates produces an attenuation of the maximum intensity in dark field and an increase of minimum intensity in light field. Experimental evidence of the influence of optical retarders is also shown.

Photoelastic effects in Nd:YAG rod and slab lasers

Abstract: The thermally induced stress birefringence in YAG rods and slabs has been analysed theoretically. After carrying out the transformation of the fourth-rank photoelastic and piezo-optic tensors from the crystal lattice coordinate system to the laboratory coordinate system, it is possible to calculate easily the modified indicatrix for any stress distribution. The calculations show that the birefringence in a zigzag slab, in general, depends on the angle under which the slab is cut from the boule.

Analysis of the birefringence properties of optical fibers made by a preform deformation technique

Abstract: The birefringence properties of polarization maintaining fibers manufactured through preform deformation are analyzed by use of the finite element method. From the calculated stress distribution in the fiber, the refractive index tensor change /spl Delta/n is obtained from the elastooptic effect. A vector H-field solution using the finite element method with adaptive remeshing and pre- and post-processing has been used to enable a very accurate solution for the birefringence of the fiber to be obtained. >

Automatic analysis of isochromatic and isoclinic fringes in photoelasticity using phase measuring techniques

Abstract: Photoelasticity is one of the oldest and remains one of the most useful means of optically analysing the way in which a structure reacts under load This is because it is still the only means by which internal surfaces and interfaces can be studied and it measures the most relevant parameter in fatigue life prediction, shear stress, directly This paper presents two new methods that provide a complete solution for the automatic fringe analysis of photoelastic data Previous solutions have not provided a complete full-field analysis of both fringe sets (isochromatic and isoclinic) over a number of fringes with an automatic measurement of absolute magnitude This paper reviews previous approaches and discusses the aims toward which this work has been directed Specifically, two new algorithms are presented Firstly, the combination of phase maps at three wavelengths in order to remove the isochromatic ambiguity at every pi turning point that would occur in a single phase map is discussed Additionally, this algorithm allows the automatic measurement of absolute isochromatic magnitude, provided that the neutral axis is within the field of view Secondly, a new method is proposed that removes the areas of 2 pi ambiguity in an isoclinic phase map by cross correlating regions of low modulation at two different wavelengths The algorithms are demonstrated by applying them to the analysis of a simple calibration disc The disc was manufactured as a full-scale three-dimensional model in araldite (Ciba-Geigy CT200), from which a central slice was taken after the sample had been stress-frozen

Plotting of fringe contours from finite element results

Abstract: A novel and simple approach to plot fringe contours from finite element results that simulates the fringe thickness variation as observed in experiments is presented. The procedure does not require solution of any non-linear equation. Shape functions are used as interpolation functions to plot fringe contours from the nodal values of fringe orders. The various fringe contours one usually comes across in stress analysis are also reviewed. The program code in Turbo-Pascal employing the new algorithm is given. Stress and displacement contours at the tip of a crack in an SEN (single-edge-notched) specimen are plotted using the code developed. The fringe contours plotted correspond to those observed in experiments such as photoelasticity, holography and Moire.

Distributed-force recovery for a planar photoelastic tactile sensor

Abstract: In this paper, an efficient force-recovery algorithm is presented for a novel planar photoelastic tactile transducer. A distributed-force profile input to the tactile sensor developed in our laboratory generates stress in the photoelastic layer of the transducer, making it birefringent. Circularly polarized light input to the transducer is elliptically polarized at the output due to phase-lead created in the stressed photoelastic layer. The algorithm presented recovers the phase-lead distribution and correlates it to the input force profile. Since this distribution is a linear function of the force profile, the solution of the inverse-tactile problem is significantly simplified. The paper presents the analytical basis of the algorithm as well as the numerical technique used in its solution.

Polycarbonate for frozen stress photoelasticity

Abstract: The possibility of using polycarbonate in the frozen stress photoelastic technique is investigated. The results obtained from strip specimens made of laminated and extruded polycarbonate and from plates confirm this possibility.

Stress Measurement of Transparent Materials by Polarized Laser

Abstract: Optical birefringence is measured by a high-frequency modulation method using a photoelastic modulator and a polarized laser. This measurement has the high sensitivity required to measure very small birefringence produced by stress. By this measuring method, the magnitudes of the differences in principal stresses and their directions are obtained directly and quantitatively. The photoelastic properties of glass, polymethyl methacrylate (PMMA) and two kinds of epoxy resins are measured with the equipment. The distributions of optical birefringence corresponding to stress distributions are measured for glass plates bonded with epoxy resin. The residual stress caused by the bonding process is measured. This stress measuring method is applicable to detection of the stress distribution of transparent solid materials.

On network descriptions of mechanical and optical properties of rubbers

Abstract: The non-Gaussian full network model for rubber elasticity is reformulated in a more efficient and more micromechanics-motived manner. Based on such a full network description, a so-called full network model for rubber photoelasticity is proposed, by introducing directional polarizabilities into the individual links of the idealized randomly jointed chain. This optical theory can be used to study the optical properties or birefringence-strain behaviour of rubbers in arbitrary three-dimensional deformation states. Detailed comparisons with two approximate models, namely the classical three-chain model and an eight-chain model for rubber photoelasticity, are provided for different types of deformation. The predicted numerical results are compared with experimental data found in the literature.

Photoelastic stress analysis in resin bonded bridge design.

Abstract: The aim of this investigation was to determine the stress magnitudes and directions in resin bonded bridges. 3D models (2.5 x life size) were prepared from photoelastic materials that represented the ratio of the elastic moduli of enamel and metal. The models were loaded during a stress freezing cycle and sectioned to view in a transmission polariscope. The fringes observed, indicated that the stress magnitudes concentrated at the proximo-lingual line angles to the pontic. The stress trajectories were plotted to show the pattern of stress direction. The stress factors which may contribute to debonding of these designs were analysed. It is suggested that the use of vertical grooves at the proximolingual line angles to the edentulous area in the preparation should reduce the stresses and hence improve the long term prognosis.

Derivation of phase in computer-aided photoelasticity.

Abstract: Phase-shifting techniques have been successfully implemented to yield the phase from sinusoidally modulated intensity fields in interferometry. However, when the intensity fields are that of squared sinusoidal modulation, as in the case of photoelasticity, implementation of existing phase-shift operations will result in error. An operation is outlined to derive faultlessly the phase in computer-aided photoelasticity.

Simple process for building large homogeneous adaptable retarders made from polymeric materials.

Abstract: A process for building large, homogeneous, adaptable retarders easily and at low cost is proposed and analyzed. This method is based on the properties of high polymers to present variable birefringence as a function of applied stresses and on the possibility of freezing these stresses inside the material by a thermal process. Various geometries for the applied forces make obtaining a large range of birefringence profiles possible. In the process that we describe composed bending leads to a linear birefringence profile. The superimposition of two pieces with identical profiles with opposite directions gives homogeneous constant retardation. This retardation can be adjusted by a relative displacement between the pieces. A precision of better than 1% over large areas (more than 3 cm in diameter) for a quarter-wave value has been obtained. The correct choice of material makes many applications possible with a large range of wavelengths.

Scanning Stress Measurement Method by Laser Photoelasticity : 1st Report: Stress Measurement by Synthesized Method

Abstract: Optical birefringence is measured by a high-frequency modulation method using a photoelastic modulator and polarized laser. A modified synthesized method was developed and applied for the measurement of stress distributions of a glass plate. The distributions of the differences of principal stresses and their directions were obtained directly by this method. The stress distributions under tensile load of a pulled rectangular glass plate with a hole at its center were measured. The results of stress distributions agreed with the analytical results. It was confirmed experimentally that the spatial resolution of stress measurement was the same as the diameter of the laser light. The stress at many points could be obtained quickly by the synthesized method and scanning stress distribution measurement was realized.

The effect of eccentric loading on the stress distribution in thread roots

Abstract: Three-dimensional photoelasticity has been used to examine the effect of eccentric loading on the stress distribution along the helix of the roots of threaded fasteners. In the investigation the ratios of axial to bending stress were similar to those found commonly in engineering components. The results showed that, as the level of eccentricity increased, the maximum stress in helix of the thread root did not change significantly, but there was an increase in the length of helix which experienced high stress. This will lead to an increase in the probability of crack initiation and propagation in the presence of eccentric loading.

Polymer materials in practical uses of photoelasticity

Abstract: A brief review of photoelastic materials comprising primarily high-sensitivity polymers and their mechanical and optical properties is presented. This review is followed by a discussion of their advantages, disadvantages, and the ranges of their applicability. The main selecting criteria of proper materials for photoelastic studies are listed and the relations between the chemical structure of polymers and their photoelastic and mechanical properties are discussed. The calibration methods are briefly described.

Phase-lead reconstruction of a photoelastic tactile sensor

Abstract: In this paper, a novel tactile photoelastic transducer for normal forces is presented. When a normal input force profile is applied to the tranduction medium, stress is generated in the photoelastic layer making it birefringent. Consequently, circularly-polarized input light becomes elliptically polarized at the output due to the introduction of a phase-lead distribution. If a circular-reflection polaridoscope is used, the output light-intensity is a circular function of the total phase-lead distribution. The first part of the paper describes the forward analysis of the transducer using finite-element analysis to determine the stress distribution in the transducer. Then, the phase-lead distribution is determined using the theory of photoelasticity. The second part of the paper describes a technique for the recovery of the phase-lead distribution from the ideal noise-free light-intensity distribution. Also, a verification method is proposed to determine whether a recovered phase-lead distribution is the correct one or not. In the third part of the paper, we consider the nonideal situation, where the light-intensity distribution is no longer noise-free. Quantization errors added to the detected light-intensity distribution are also considered. Recovering the phase-lead distribution under noisy conditions constitutes an ill-posed problem. An algorithm that accurately and effectively determines the phase-lead distribution from a noisy light-intensity distribution is presented. The inverse-tactile problem is solved using an optimization function.

Photoelastic analysis using automated polariscopes

Abstract: Two methods of automating photoelastic analysis are described. The first technique is based on phase stepping. A CCD camera and digitizer are used to record six images of intensity corresponding to six orientations of output quarter waveplate and polariser. These are combined mathematically to give full field images of the isochomatic and isoclinic paramaters of the specimen under study. The full field image is composed of 256x256 pixel points, each with an isochromatic fringe value. An alternative technique is proposed based on the point-by-point method of spectral contents analysis. The method used the CCD camera as a 256x256 pixel diode array and eight narrow band filters for the isolation of wavelengths in the range 450 to 750nm. Normalised intensity values from a circular polariscope are used to represent the spectrum of light for each pixel and comparison with the theoretical spectrum using a minimisation procedure produces the absolute retardation. Combination of the retardation and the normalised intensity spectra from the plan polariscope yields the isoclinic parameter. Both simple and complex components are analysed using both automated systems. Results compared well with both manual and theoretical analyses of the components.

Stresses in Faulted Tunnel Models by Photoelasticity and Adaptive Finite Element

Abstract: Research efforts in this area continue to investigate the development of a proper technique to analyze the stresses in the Ghost Dance fault and the effect of the fault on the stability of drifts in the proposed repository. Results from two parallel techniques are being compared to each other - Photoelastic models and Finite Element (FE) models. The Photoelastic plexiglass model (88.89 mm thick & 256.1 mm long and wide) has two adjacent spare openings (57.95 mm long and wide) and a central round opening (57.95 mm diameter) placed at a clear distance approximately equal to its diameter from the square openings. The vertical loading on top of the model is 2269 N (500 lb.). Saw cuts (0.5388 mm wide), representing a fault, are being propagated from the tunnels outward with stress measurements taken at predefined locations, as the saw cuts increase in length. The FE model duplicates exactly the Photoelastic models. The adaptive mesh generation method is used to refine the FE grid at every step of the analysis. This nonlinear interactive computational techniques uses various uses various percent tolerance errors in the convergence of stress values as a measure in ending the iterative process.

Dynamic photoelasticity using a laser scanner

Abstract: Over the years, several techniques have been developed to record dynamic fringe patterns in photoelasticity. Due to the whole field recording capability, the photoelastic technique continues to be of interest to the experimental mechanics community. A low-cost dynamic recording system employing an out-dated fax scanner is presented in this article. Details of the optical and electronic functions are discussed. The feasibility of the system is demonstrated by recording dynamic fringe patterns off a simply supported beam subjected to impact loading. Further modifications to the set-up to improve the quality of the fringe patterns are discussed briefly.