Showing papers on "Photoelasticity published in 1971"

Photoelastic determination of stress-intensity factors

Abstract: The increasing number of analytical and numerical solutions for the crack-tip stress-intensity factor has greatly widened the scope of application of linear elastic fracture-mechanics technology. Experimental verification of a particular solution by elastic stress analysis is often a necessary supplement to provide the criteria for proper application to actual design problems. In this paper, it is shown that the photoelastic technique can be used to obtain rather good estimates of the stress-intensity factor for various specimen geometries and loading conditions. Treated are the following cases: wedge-opening load specimen, several notched rotating-disk configurations, and a notched pressure vessel. A sharp crack is simulated by a relatively narrow notch terminating in a root radius of 0.010 in or less. Stress distributions along the section of symmetry ahead of the notch tip are obtained using three-dimensional frozen-stress photoelasticity. The results are used to determine the stress-intensity factor, cK I , by three methods. Two of these are based on Irwin's expressions for the elastic stress field at the tip cf a crack, and the other is a result of Neuber's hyperbolic-notch analysis. Agreement, with available analytical solutions is good.

A new holographic interferometer for stress analysis

Abstract: In holographic interferometry with a photoelastic model, two families of fringes are generated simultaneously when the model is stressed. One family represents the isochromatic-fringe pattern normally associated with photoelasticity which yields the difference between the principal stresses. The other family represents the isopachic-fringe pattern associated with interferometry which yields the sum of the principal stresses. From these complementary patterns, the magnitudes of the principal stresses can readily be determined throughout the field of observation. Unfortunately, these fringe patterns are not completely independent but interact in such a way as to make interpretation difficult in critical regions of the model. A new system has been developed which readily permits simultaneous acquisition of these fringe patterns without their undesirable mutual interaction, as well as providing increased sensitivity.

Advancements in holographic photoelasticity: The interpretation of fringes in double-exposure holographic photoelasticity is discussed, and single- and double-exposure photoelastic holograms of stress waves taken with a ruby laser are presented

Abstract: The success of double-exposure holography as an interferometric technique for experimental stress analysis has lead to several recent publications dealing with the theoretical expression which describes the resulting photoelastic patterns. This paper describes the extension of current theory to include the effect of an intensity difference between the light used during the first exposure and that used during the second exposure. It is shown that as the ratio of these two intensities is changed both the position and the intensity of the photoelastic-fringe pattern is altered. An interpretation of the photoelastic pattern as a simple combination of isochromatic and isopachic-fringe patterns is shown to be possible only under certain conditions.

Interfiber stresses in filamentary composites

Abstract: Theoretical and experimental results for the magnitude and distribution of interfiber stresses in a transversely loaded composite consisting of an elastic matrix reinforced with elastic fibers are presented. Interfiber stress distribution is also given for an in plane shear load condition. The mathematical model consists of uniformly spaced circular fibers in a square array. An approximate solution is obtained for the interfiber stresses. Theoretical results are compared with results of a photoelasticity investigation, Fil'shtinskii's rigorous elasticity solution for a plate containing rigid circular inclusions, and with results of a rigorous elasticity solution for a three-dimensional problem. The present approximate solution provides good agreement with the experimental results and with the theoretical results obtained by other authors. The method of solution presented can be used for predicting interfiber stresses in composites with various shape fibers arranged in arbitrary arrays, as well as for investigating interfiber thermal stresses, elasto-plastic deformations, and residual stresses.

Role of light ellipse in photoelasticity and new methods for isoclinic-parameter determination: Several optical setups used in practice are explained by means of the light-ellipse method

Abstract: In dealing with various types of polariscopes the transformations that the light ellipse undergoes as it traverses through several optical elements give a better physical insight than the conventional trigonometrical transformations that are generally applied to the light vectors. Further, an understanding of the transformations of the light ellipse suggests several interesting methods for the determination of the isoclinic parameter.

A photoelastic study of stress wave propagation in a quarter‐plane

Abstract: An experimental investigation was conducted on a quarter‐plane loaded with an explosive charge at one point on the boundary. Experimental methods of dynamic photoelasticity and interferometry were used to obtain full‐field data of the dynamic event. Results of the study were obtained for both the P and R‐wave interaction with the corner. Energy reflection coefficients were found for the P‐wave interaction. For the Rayleigh wave, where the response was more significant, transmission and reflection coefficients based on both stress and surface energy were established. Subsurface stress distributions for the transmitted and reflected Rayleigh waves were obtained in the region near the corner.

Photoelastic studies of fracture

Abstract: Fundamental optic laws for photoelasticity are derived on the basis of the wave theory of light. Two-dimensional and three-dimensional static and dynamic photoelasticity and birefringent coating techniques are explained. Applications of these photoelastic techniques to linear fracture mechanics are reviewed. Future areas of research involving the applications of photoelasticity, photoviscoelasticity, and photoplasticity to problems in fracture mechanics are discussed.

Transient creep of a block compressed on opposite sides by elastic punches of the same width as the block

Abstract: A method combining photoelasticity of an elastic body and photo-rheological stress analysis of a plastically flowing body may be applied to the experimental investigation of the contact problems between elastic tool and plastic workpiece which appear very often in plastic working.The transient creep of a block compressed on opposite sides by elastic punches of the same width as the block is analysed by the above-mentioned method.

A Scattered Light Polariscope for Three‐Dimensional Birefringent Flow Studies

Abstract: This paper reports in detail the design and construction of a new scattered light polariscope for three‐dimensional birefringent flow research. The unique features of this polariscope, which were employed to overcome the particular problems associated with birefringent flow analysis using scattered light, are pointed out. A photograph of typical birefringence patterns obtainable is shown. A comparison of theoretical and experimental results for a particular flow case is made indicating the accuracy that can be obtained using this polariscope.

Scattered light polariscope

Abstract: A photoelastic model is fixedly positioned in a body of fluid within an octagonal tank. The planar sides of the tank are provided with windows and the tank is supported for angular movement about a vertical axis relative to the model until desired fringe patterns are visible through one or more of the windows. Laser or polarized light beams scattered or transmitted by the model produce the fringe patterns for analysis of the model under different operational modes.

Establishing the boundary retardation with respect to the observed fringes in scattered-light photoelasticity

Abstract: This technical note presents a technique for estabalshing the boundary retardation with respect to the observed finges. Thus, the available data are increased and, thereby, improve the accuracy of determining fringe spacing near boundaries and, in particular, in thin models. The technique is applicable to scattered-light polariscopes utilizing a quarter-wave plate to circularize the entering polarized light.

An investigation on the crack arrest capability of a hole

Abstract: : Dynamic photoelasticity by a 16-spark gap Cranz-Schardin camera system was used to determine some of the conditions leading to fracture arrest by a circular hole ahead of a propagating crack. Photoelastic models of 3/8 in. x 10 in. x 10 in. Homolite-100 plates with a 1/2 inch edge crack were loaded in a fixed grip configuration and crack arrest was achieved by central holes of 1/2, 1/4, and 1/8 inch diameters. For a uniformly loaded plate, with a central hole of 1/8 inch diameter, the propagating crack continued through this hole. Changes in dynamic stress intensity factors, as the crack tip approaches the hole, as well as changes in the dynamic stress concentration factors at the far side of the hole were studied and these results were compared with the corresponding static results determined by finite element analysis. This comparison shows that the static analysis can be used to qualitatively asses the arrest capability of the hole using the maximum static stress concept or the proposed concept of strain energy released as the crack penetrates the hole. (Author)

Study of the micro-mechanical behaviour of concrete using reflective photoelasticity

Abstract: Tests are reported to study the micro-mechanical behaviour of concrete using a photoelastic reflection technique to determine the internal and bond strains in concrete. In this technique a layer of suitable birefringent material is bonded to the sliced surface of a specimen. When the specimen is loaded the resulting surface strains are followed by the coating. The isochromatic patterns observed through polarized light reflected from the surface can then be interpreted in terms of strain. The principle of the technique, the method of measurements, and of the separation of principal strains are described.

The photoelastic determination of the stresses in vibrating cantilevered beams

Abstract: A method of determining the dynamic stresses in vibrating cantilevered beams using photoelasticity is presented. The method uses the basic principles of photomechanics and the optic-stress laws. A high-intensity strobe light is timed with the frequency of vibration so that the beam image appears to be stationary. Data are recorded with a camera and analyzed to provide an experimental solution. The theoretical solution is derived from the Bernoulli-Euler equation of motion. Two basic types of beams were investigated, an aluminum beam coated with a birefringent material to simulate an actual structural member and a birefringent model that was dynamically similar to the aluminum beam. The feasibility of extending the technique to more complicated shapes is suggested by this investigation. The experimental results and feasibility of the concept are verified by close correlation with the analytical solution.

New stress-analysis procedures using the method of shear-stress differences

Abstract: The use of logarithmic scales, adaptation of the polariscope, and the principles of the moire method and the method of constant spacing between two lines are discussed.