Showing papers on "Photoelasticity published in 1969"

Studies in photoplasticity

Abstract: Summary This paper describes an extension of the “photoelastic” method to the plastic state. Using thin models of cellulose nitrate, factors of stress concentration and stress distributions were determined for several basic cases. The extension is made possible by the establishment of a suitable stress-optic law, and the employment of the shear-difference method. The results obtained are in substantial agreement with theory and experiment. The method is at present limited to plane stress problems in the elastoplastic state without unloading.

An automated process for three-dimensional photoelastic analysis: A fully automated process for obtaining and processing data from slices of a stress-frozen three-dimensional photoelastic model is described

Abstract: The use of two monochromatic light sources to obtain two isochromatic patterns permits a fully automated shear-difference analysis of the stress distribution in a three-dimensional photoelastic model. A development is given of the logic necessary to process the data taken from an automated polariscope as well as the design details of the polariscope.

A Numerical Technique Of Data Retrieval From Moire Or Photoelastic Patterns

Abstract: It is shown that the light intensity distribution in the image plane of an optical system producing a moire patterns the relevant information is the phase angle. The above described analysis forms the basis of a method for data retrieval. The intensity of light distribution on a pattern is recorded in the form of density changes in a photographic film negative. The changes of density are then retrieved by a light sensor. If one analyzes the density record, one can see that the trace is not only modulated by the relevant quantities but also by various other processes. Furthermore the effect of the nonlinear characteristic curve of the recording film is to generate for each narrow-band componet of the signal, an infinite number of narrow-band high order components of the fundamental argument. To eliminate the effect of unwanted componets, of the fundamental or the effect of unwanted comonents, the digitalized vision of the signal is filtered by numerical narrow-band pass filters in-quadrature. The computer program gives the strains in the case of moire patterns and relative retardations in the case of photoelasticity. The given examples of application show that the proposed data retrieving and data processing technique increases both the sensitivity and the accuracy of the moire and photoelasticity. This result is of great practical value particularly in the moire method since it extends its applicability to the ranges of interest of practically all engineering materials.© (1969) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Long Cylindrical Tube Subjected to Two Diametrically Opposite Loads

Abstract: A theoretical study is made of the problem of a long, thick cylindrical tube subjected to two equal and diametrically opposite normal loads. The stress state is analysed by the three-dimensional theory of elasticity, with the Papkovich and Neuber stress-function approach. Numerical results for stresses and radial displacement are obtained to show the nature of stress concentration in the neighbourhood of the load. Some related experiments using the photoelasticity technique are also accomplished. Favourable correlation is shown between the theory and the test results.

Analysis of scattered-light methods in photoelasticity

Abstract: The use of scattered light for nondestructive analysis of general photoelastic models is becoming quite common and, during the last few years, several methods have appeared in the literature to determine the directions of the secondary principal stresses and their differences at any general point. Among the methods suggested, some are mentioned as “exact” and some “approximate”. Even the exact methods have limitations in their applicability for a generally stressed model. The present discussion attempts to bring out the inaccuracies involved in the various exact methods. Also, a few modifications to improve the accuracy and a new method called the Mini-max method are proposed.

Measurement of "Shear" photoelastic constants in lithium niobate

Abstract: A technique is presented for measuring the "shear" photoelastic constants of lithium niobate by Bragg scattering from acoustic shear waves at moderate frequencies and at small Bragg angles. Values of p 66 =0.048, p 14 =0.056, p 44 =0.062, and p 41 =0.050 are obtained, the first two agreeing well with the results of other workers.

Final Report for Phase II

Abstract: : A summary is given of research concerned with the experimental investigation of the stresses developed in a solid fuel rocket motor when subjected to different loading conditions Also, a detailed report is given of a study involving scattered light photoelasticity used to determine the stress distribution in a three dimensional, solid propellant rocket motor The model was a case bonded solid propellant with a star grain internal boundary Stresses were induced by both internal pressure and a uniform temperature change The uniform temperature change simulated the shrinkage stresses imposed on actual propellants in storage The pressure load simulated part of the loads developed in firing a rocket The model material was polyurethane rubber which is similar to the binder material in actual rockets The model construction and data analysis are discussed in detail, and the results are presented in graphical form (Author)

An automatic system for scattered-light photoelasticity

Abstract: This paper describes the recent development of an automatic data-collecting-and-interpreting system for a dual-observation method in scattered-light photoelasticity.

Graphical methods for determining the resulting photoelastic effect of compound states of stress

Abstract: The aim of this work is to give a simplified representation of an interesting subject, namely, the calculation of the photoelastic effect behind a system in which the principal stresses or secondary principal stresses rotate about the direction of the propagation of light. The methods are described in a popular form for understanding the essential features and the necessary graphical operations for solving these complicated problems. Further, the relations between the different methods are shown. Thej-circle technique is improved in order to simplify the operations and a new possibility in applying Wulff's grid is introduced in photoelasticity. The graphical tools can then be applied as so-called “rotation rules.” Some examples related to recent papers which present theoretical results or new methods are given in order to study the function of these “rotation rules” and to recognize the power of these methods.

On the fidelity of high-speed photographic systems for dynamic photoelasticity

Abstract: A general theory has been developed which predicts the influence of the characteristics of a high-speed recording system on the fidelity of a dynamic photoelastic-fringe pattern. The general theory was applied to recording systems currently employed in dynamic photoelasticity which include the high-speed framing camera, the spark-gap camera and the Q-spoiled laser system. Equations for a nondimensional form of the exposure as a function of normalized position have been developed for each of these three recording systems. From these equations, criteria to measure the fidelity of the recording systems were established. The methods developed are applied to the spark-gap system and examples of resolution limits, time shift and distortion are illustrated.

Photoelastic Study of Beams on Elastic Foundations

Abstract: The bending of beams supported by an elastic medium is analyzed using photoelasticity. Four models were considered: (1) A model representing a beam of infinite length with a concentrated control load; (2) a finite beam with a central load; (3) a finite beam with a load at the quarter point; and (4) a finite beam with two concentrated loads at the two quarter points. The bending moments, shears and contact pressures were obtained and compared with corresponding values for beams resting on a Winkler-type foundation. Several alternative ways of determining the pressure distribution at the surface of contact are presented. The analysis includes the stress distribution in the elastic medium.

Transient stress concentration at an elliptical discontinuity

Abstract: : The purpose of the investigation was to determine experimentally the dynamic-stress concentration factors in a strut loaded by high-velocity impact and containing a symmetrically located elliptical discontinuity. The photoelastic technique was used and the conventional light source with its many inherent problems was replaced with a modulated ruby laser. (Author)

Development of experimental stress analysis methods to determine stresses and strains in solid propellant grains. stress distribution around a circular bar, with flat and spherical ends, embedded in a matrix in a triaxial stress field,

Abstract: : The report deals with the stress and strain distribution around a circular bar embedded in a matrix and subjected to a mechanically applied triaxial load The three-dimensional 'freezing' technique of photoelasticity was used The results obtained correspond to the case of a rigid circular bar embedded in an homogeneous, elastic matrix Two models were analyzed; in one the axis of the circular bar was parallel to the direction of the loading and in the other the axis of the circular bar was normal to the direction of loading By combining the results obtained independently for these two cases, stress and strain distribution around the circular bar for various ratios of triaxial load can be obtained (Author)

Stress Fields Around Multiple Inclusions

Abstract: : The stress fields around multiple inclusions have been calculated by using the finite-element method, and observed by the photoelastic technique Discontinuous fibrous composites are simulated by glass-epoxy specimens The distributions of the fiber stress and the interfacial shearing stress are found to differ significantly from those calculated by the shear lag analysis The perturbation effect of fiber stress, the concept of typical region, and the boundary conditions around the fibers are discussed (Author)

Stress and finite-strain analysis of a circular ring under diametral compression

Abstract: Using a moire, large-strain analysis method, a complete solution is shown in this paper of the fields of strain and stress for a circular ring subjected to diametral compression between two flat platens. The isotheticsu andv, obtained using 1000-lines-per-inch gratings, were differentiated photographically by the shifting technique (moire-of-moire) to determine ∂u/∂x, ∂v/∂y, ∂u/∂y and ∂v/∂x. Using the exact finite strain-displacement relationship, the Eulerian strains ɛ x E , ɛ y E and γ xy E were computed. From these, the principal Eulerian strains were obtained. These results were verified with the isochromatics obtained from a large-deformation photoelasticity analysis. The ring was made of a polyurethane rubber which exhibits a linear relationship between natural strain and a newly introduced concept of “natural stress”. The Eulerian strains were converted to natural strains, and from these natural stresses were computed using the newly developed concept. Results are presented graphically for the whole field of the ring.

Stress concentrations in a rectangular plate with circular perforations along its two bonded edges and subjected to restrained shrinkage

Abstract: Two-dimensional photoelasticity is used for minimising stress concentration associated with a fillet present at the end of a solid propellant rocket grain subjected to restrained shrinkage. For this purpose, successive holes were added along the bonded interface of case and propellant, but no decrease was obtained. The photoelastic method used illustrates two uncommon techniques of interest to experimental stress analysts: (1) restrained shrinkage loading and (2) a parametric stress-analysis solution obtained with a single model.

An investigation of residual stresses in tempered glass plates of aircraft windshields.

Abstract: A scattered-light photoelastic method that determines nondestructively the distribution of residual stresses across the thickness in tempered glass plates of aircraft windshields is presented. This paper begins with an introduction to scattered-light photoelasticity and a discussion of the state of stress in tempered glass plates. Then, the theory of the new method is developed. Apparatus and techniques are described. Experiments were made on an aircraft windshield sample having a three-layer (glass-vinyl-glass, thermally tempered) sandwich structure. Photographs of photoelastic fringe pattern in scattered light were taken and the distribution of residual stresses across the thickness of the glass layer was obtained. The accuracy of the method is established from the fact that a good agreement exists between results of different experiments and that the condition of equilibrium is satisfied.

A contribution to the application of photoelasticity to the micromechanics of composite materials

Abstract: Photoelastic-model methods can prove advantageous for the investigation of microstresses in composite materials. Some two-dimensional investigations of this type are discussed and the extension of this work into three dimensions is considered. It is suggested that more than one approach to the three-dimensional problem may be practicable, and special attention is paid to obtaining the maximum amount of information from a sandwiched polariscope by means of light-intensity measurements. A cold-casting technique for the fabrication of composite models is also described.

Application of micromechanics to joints and cutouts

Abstract: : The stress distribution in loop and bolt joints made of composite materials is studied using analytical and experimental methods. Also, the stress concentration in orthotropic composite sheets with a circular hole is studied. The numerical technique to evaluate the stresses and displacements is described. Simple design formulas to compute the stresses at critical regions are developed. A procedure of structural design is delineated and applied to specific cases. Experimental results obtained by testing composite joints are reported. Stress distribution tests were performed using the photoelastic technique. Failure location and ultimate loads for several composite joints are shown. This report contains the analysis of the stress concentration that appears around a circular hole in an orthotropic composite sheet under simple tension and simple shear loadings.

Stress distribution in a stator vane under bending

Abstract: The stress distribution on the surface of a stator vane loaded statically as a cantilever has been obtained by means of brittle coatings, strain gauges and three-dimensional photoelasticity. A metal prototype was available for the determinations and from this an epoxy model was made by casting. The combination of methods used suggests the most efficient way of stress analyzing this type of structure.

The sandwich technique in photoelasticity

Abstract: A methyl methacrylate composition of exactly zero birefringence has been developed and the technique of its application to the sandwich method with a birefringent layer of Araldite is described in detail with special consideration of the problems of cementing the two different materials. Tensile and bending tests on bars with stress concentrations demonstrate the validity of the method.

Measurements of Stress Waves in Solids by Dynamc Photoelasticity

Abstract: The technique of dynamic stress-birefringence has been applied in two unique ways to a study of stress waves in solids. Observations of both longitudinal and shear waves, induced in Plexiglas by a pulsed beam of high-energy electrons, have been made with a polariscope operating in the visible. Stress waves induced in silicon by an exploding wire facility were studied with the polariscope in the infrared. The theories of thermoelasticity and birefringence are reviewed briefly and solutions describing stress induced by transient energy depositions are obtained. These solutions are compared to experimental results obtained with a flash x-ray generator.

Application of two-dimensional photoelasticity to the study of acceleration stresses under quasi-static conditions

Abstract: A laboratory method has been developed to simulate the state of stress in various structures under rapid-acceleration loading conditions. This method subjects a two-dimensional photoelastic model to impact loading, using a commercially purchased drop tester. The particular approach selected for studying this dynamic situation was a static technique utilizing a 4×5 studio camera and a high-intensity Xenon-light source. The light was flashed by the maximum pulse from an accelerator mounted on the model. This static treatment provided a significant improvement in the optical quality of the fringe patterns over that obtained by dynamic high-speed photographic methods used in earlier impact studies. It permitted evaluation of the reproducibility of the loading system and associated photographic synchronization and triggering circuits, since double exposures could be taken of separate individual tests of the same loading condition with little degradation of the optical fringe data. A composite picture made of four separate photographs of a series of individual tests, with the camera viewing a different portion of the sample in each picture, showed continuity of fringe pattern and highlighted the regions of critical stress within the model.