Showing papers on "Photoelasticity published in 1991"

Prediction of Dynamic Contact Loads in Granular Assemblies

Abstract: An experimental-numerical hybrid technique has been developed to predict the intergranular contact load trasfer in granular media subjected to explosive loading. The granular media were simulated by assemblies of circular disks in contact

Combining photoelasticity and finite-element methods for stress analysis using least squares

Abstract: Photoelastic data are combined with the finite-element method for stress solutions over regions partially bounded by free surfaces and axes of symmetry. Least-squares solutions are obtained without presumed values of applied forces at element nodes and without isoclinic data. Varied example problems are used to compare the results to independent photoelastic and finite-element solutions and to theoretical stress values.

Polarization apparatus for interference microscopy and macroscopy of isotropic transparent objects

Abstract: The various instruments to be described are based on the following principle: a wave front which has been deformed by optical path difference variations in the object is doubled into two identical wave fronts by means of a birefringent system. The interference phenomena between these two wave fronts bring out the path difference variations in the object. By this means the transparent object is made visible. The birefringent systems employed are mainly of the Savart polariscope and Wollaston prism types. They can be equally well used in microscopy or macroscopy. These instruments can be used, by means of adequate modifications, without any collimation. In the latter case they are extremely luminous. They are of particular interest in microscopy where they permit the measure of refractive indexes and of thicknesses of small details in preparations. We shall describe a very simple differential observation method which places in evidence the gradient of the optical path. The sensitivity of the method depends on the width of the object. It is of the order of λ/50 for an object 1 mm wide. This sensitivity can therefore be very large for very small objects.

Contact stress distributions in a turbine disc dovetail type joint ‐ a comparison of photoelastic and finite element results

Abstract: Two dimensional photoelastic frozen stress techniques were used to investigate the stress distributions of an axially loaded dovetail joint as is found in the design of some turbine blade-disc fixings. The internal stress distributions were obtained by the shear difference method and were compared to results obtained by using the finite element method. It was found that steep principal stress gradients were present immediately below the contact surfaces in the zone adjacent to the fillet radius of the dovetail fixing, which could be an important consideration in the fatigue strength of the joint. The finite element model used gave the same trends of stress as the photoelastic model, predicted higher peak principal stresses and did not exhibit the same detailed variations of stress distribution.

Accurate measurement of thin-polymeric-films index variations: application to elasto-optic effect and to photochromism

Abstract: Attenuated total reflection (ATR) is commonly used for measuring small variations of the index of refraction of thin polymeric films. We show that a careful analysis of reflectivity dips, due to the excitation of guided modes in the film, allows a precise measurement of very small variations of the anisotropic index of refraction and of the thickness of the film. This is very useful for studying a wide variety of physical phenomena such as electro-optic effects, piezoelectricity, electrostriction, mechanical and thermal strains inducing elasto-optic effects (i.e., photoelasticity), and photochromism. In this paper we particularly emphasize the measurement of some components of the photoelastic tensor and of the index variations associated with cis-trans photoisomerization of azo dyes which have been clearly demonstrated in doped polymeric films. Kinetics of cis-trans optical pumping and of relaxation in darkness have been studied. It is noticeable that the layered configuration of samples and the guided waves excited by ATR are quite similar to the structure and to the propagation conditions of integrated optics devices.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Polarization light source device

Abstract: PURPOSE:To efficiently convert light radiated from a light source lamp into linearly polarized light and to emit the light by forming an angle of 45 deg. with the main cross section of a lambda/4 wavelength plate and the polarization plane of lenearly polarized light reflected from a polariscope. CONSTITUTION:The angle formed by the main cross section of the lambda/4 wavelength plate and the polarization plane of a linearly polarized light reflected from the polariscope 15 is 45 deg.. After non-polarized light emitted from a light source device passes through the lambda/4 wavelength plate 14, the linearly polarized light is extracted by the polariscope 15. Linearly polarized light intersecting orthogonally with this linearly polarized light is reflected in the light incident direction and its opposite direction by the polariscope 15, and again passes through the lambda/4 wavelength plate to be circularly polarized. This circularly polarized light is reflected by a reflector in the light source device to be circularly polarized in the opposite direction. This light passes again through the lambda/4 wavelength plate 14 to be linearly polarized. At this time, the polarization plane of the linearly polarized light is intersected orthogonally with the polarization plane obtained when the light is reflected by the polariscope 15, therefore the light is emitted in the same direction and with the same plarization plane as those of the linearly polarized light extracted first after the light is again made incident on the polariscope 15. Since all the non-polarized light emitted from the light source device is converted into the linearly polarized light and used, the coefficient of utilization is increased.

The influence of photoelasticity on the self-diffraction of light in cubic photorefractive crystals

Abstract: We present a theory of the influence of photoelasticity and optical activity on the two-beam coupling in cubic photorefractive crystals rotated around the [110] axis. The two beam coupling experiments in BTO fit the theory closely for both linearly polarised and circular polarised interacting beams.

Influence of the photoelasticity on self-diffraction of light in electrooptic crystals

Abstract: Theoretical and experimental investigations were made of the orientational dependence of energy exchange in the case of a two-wave interaction in a cubic photorefractive crystal of Bi12SiO20. It was found that in the case of noncentrosymmetric crystals the theory agrees well with the experimental results if an allowance is made for the contributions of the piezoelectric and photoelastic effects to self-diffraction.

Interaction effect of two arbitrary oriented cracks-Part III

Abstract: The elastosatic problem solved in this paper is of an isotropic homogeneous infinite plate, with two arbitrarily oriented cracks of different lengths, subjected to uniform uniaxial tension at infinity. The problem is formulated in the complex plane using the Kolossoff-Muskhelishvili stress functions and further the Schwarz's alternating method is used to solve the problem of the doubly connected region. The mode I and mode II stress intensity factors at all the four crack tips for various crack length ratios, crack angles and crack spacings are found, and are in good agreement with those obtained by other research workers. The fracture angles at the four crack tips are evaluated using the strain energy density theory and maximum tangential stress theory. The minimum strain energy density factor is also found at all the tips.

Determining the three individual stress components from measured isochromatic fringes

Abstract: This paper combines measured isochromatic information, complex stress functions and numerical concepts into a new and effective hybrid method of stress analysis. The technique simultaneously smooths the measured isochromatic data, provides accurate boundary information, and separates the isochromatic information into normal and shear stresses at nonboundary locations. No additional experimental data such as the isoclinics are needed. The technique is illustrated experimentally by application to a tensile plate containing a hole.

Stress intensity determination by an experimental−numerical hybrid technique

Abstract: Stress intensity factors are obtained from displacement-prescribed boundary conditions on a subregion containing the crack. The subregion, whose boundary can be any arbitrary contour within the physical structure, is analysed numerically using structural finite elements. Nodal boundary displacements of this subregion are obtained photomechanically using a smoothing finite element representation. The technique requires little experimental information and is unaffected by rigid body motions. It is numerically stable and accurate - even in the presence of significant experimental scatter - and the measured data need not come from the immediate vicinity of the crack. The method's practicality is demonstrated by determining the stress intensity factors in a compact tension specimen from moire-measured displacements at locations away from the crack tip.

A Two-dimensional Stress Analysis of Butt Adhesive Joints Having a Circular Hole Defect in the Adhesive Subjected to External Bending Moments

Abstract: This paper deals with a two-dimensional stress analysis of butt adhesive joints, with a circular hole defect in the adhesive, subjected to external bending moments. The analysis was done using the two-dimensional theory of elasticity in order to examine the strength of the joints. It was assumed that the adherends were rigid and the adhesive was replaced with a finite strip including a hole defect. The effects of the location and size of a hole defect on the stress distributions around the hole and at the interfaces were obtained by numerical calculations. In addition, the singular stress near the edge of the interface was obtained. For verification, photoelastic experiments were performed. The analytical results were fairly consistent with the experimental results. It was seen that the principal stress around a hole becomes larger with a certain shift toward the free boundary. It was also seen that the stress concentration became larger with an increase of the size of the hole.

Two-Dimensional Crack Inclusion Interaction Effects: Analysis and Experiments

Abstract: The plane interaction between a crack and a rigid elliptical inclusion is investigated. In particular, the effects of the orientation and aspect ratio of the inclusion are considered. Analytically, the solution for the interaction of a dislocation with an inclusion is used as Green’s function for the problem. The crack problem is then cast in the form of a set of integral equations with Cauchy singularities. These are solved numerically by the use of piecewise quadratic polynomials to approximate the unknown dislocation density along the crack. From this density the stress intensity at the crack tip can be determined. To model the situation experimentally photoelastic specimens with various elliptical inclusions are tested in uniaxial tension. The stress intensity factors are evaluated from the isochromatic fringe patterns and compared to those predicted numerically.

High strain rate testing of materials - a fully validated test calibration by a hybrid numerical/experimental technique

Abstract: Some of the conclusions of an extensive experimental and numerical study of the double-notch shear (DNS) test for the determination of material behaviour at very high rates of strain are described. Ultra-high speed photography, dynamic photoelasticity and the finite element method of numerical analysis are used. The DNS test is calibrated and conclusions are drawn on the direction of future high strain rate material testing.

Photoelastic studies on rubber-to-rubber joints

Abstract: Photoelasticity is a method which yields information on the principal stress difference and orientation in a composite structure. Various problems associated with this technique, especially those concerning the fundamental relationship between the fringe order and the stress, have yet to be investigated. A few studies of this relationship in a universal stress state have been presented, particularly in the field of rubber-to-metal and rubber-to-fabric composites, but no evaluations have so far been made in the field of rubber-to-rubber joints. Applying the photoelastic method, we report our observations on the stress distribution in a natural rubber to natural rubber joint subjected to uniaxial tension. A comparison between the results of experimental photoelastic studies and the corresponding computer modelling has been illustrated. The theoretical displacement pattern of the angular joints of bi-rubber part has also been highlighted.

Simulation and observation of the images of dislocations in (100) silicon using infrared piezobirefringence

Abstract: A general algorithm for the simulation of dislocation images in cubic single‐crystal infrared transparent semiconductor samples using infrared piezobirefringence is presented in this paper. A data bank has been created for the simulated images of arbitrarily oriented dislocations. These images are functions of the sample orientation, the orientation of the dislocation line with respect to the crystal 〈100〉 axes, the orientation of the Burgers vector with respect to the dislocation line, and the polarization angle of the incident light. A dark‐field plane polariscope has been constructed to observe the dislocation images experimentally. In principle, a sample having any arbitrary dislocation can be placed under the polariscope and the experimental images obtained for varying polarization angles of the incident light. Then an image matching procedure between the experimentally observed and the simulated images stored in the data bank can yield the information about the dislocations present in the sample. This technique offers a superior speed advantage over the more conventional defect characterization technique of x‐ray topography. Some experimental results and their match with the theoretical ones are also presented.

Coupled Finite Element Method of Photoelasticity

Abstract: In this paper, the coupled finite element method ofphotoelasticity is presented. This method is that for a pair ofcoupled systems, if the dispersed solution of one of some systems(model body) is given, the dispersed dual solution of othercorresponding system(prototype body) can be solved by means ofthe coupled finite element method. The calculated values showthat the accuracy of dual solutions found in coupled finiteelement method is high. The coupled finite element method is anew method and has important significance in the theory and

Photoelastic sensors for automatic control system of dam safety

Abstract: Photoelastic sensor with fringe image transfer by an image-guide to a computer for stress analysis was constructed for those applications where the place of measurements is hard to reach and electrical instruments are not permissible. Two types of sensors are presented: one, disc-like, for uniaxial stress observations and the second one, called a hollow glass cylinder, with axially bored stress concentrator for biaxial stress measurements. Sensor is a central cylinder glass cemented in a cubical concrete block. The sensor is connected to autocollimating polariscope, which allows the sensor to be lit and transport an isochrome image to digital image processor card. The numerical analysis of the fringes images is done in many ways. The original programs for the analysis are presented.

On a Hybrid Method to Analyse Viscoelastic Problems

Abstract: To analyze problems of plane stress states when the material shows time-depending response, i.e. linear viscoelastic response, experimental procedures can be applied similar to photoelasticity. However, not only the mechanical, but also the optical rheological effects must be considered. Beside the isochromatic fringe pattern, the isoclinics are to be recorded over time. Obviously, recording the isoclinics is difficult and the reason of unavoidable incertainty and inaccuracy. On the other hand, the two different fringe patterns are to be evaluated separately by means of automatic digital image processing and the data to combine in order to determine the components of the stress-tensor according to algorithms, which are not appropriate to computer calculations. To avoid the difficulties, in order to obtain more reliable results and to save time for recording and evaluating the optical phenomena as well as computer capacity, a procedure will be described, which enables the principal stress components to be calculated, although the principal directions of the stress tensor, i.e. the isoclinics, are still unknown.