Showing papers on "Photoelasticity published in 1997"

Full-field automated photoelasticity by use of a three-wavelength approach to phase stepping

Abstract: An overdeterministic least-squares phase-stepping method for automated photoelasticity is described. Problems associated with isochromatic–isoclinic interaction are solved by use of a three-wavelength method to calculate the value of the isochromatic parameter and the isoclinic angle. The ramped isoclinic phase map can now be unwrapped to give the orientation of the principal stresses with respect to a reference axis of the polariscope unambiguously. A three-wavelength approach to determination of the absolute value of the isochromatic parameter is shown to give reliable results also.

Investigation of the mechanics of intersonic crack propagation along a bimaterial interface using coherent gradient sensing and photoelasticity

Abstract: This paper describes the first experimental observations of various phenomena characteristic of dynamic intersonic decohesion of bimaterial interfaces. Two separate but complementary optical methods are used in conjunction with high speed photography to explore the nature of the large scale contact and mach wave formation at the vicinity of running cracks in two different bimaterial systems. Theoretical predictions of crack‐tip speed regimes where large scale contact is implied are confirmed. Also, the theoretically predicted mach wave emanating from the intersonically propagating crack‐tip is observed. Direct visual evidence is also obtained for another travelling mach wave emanating from the end of the intersonically moving contact zone.

On determining stress intensity factors for mixed mode cracks from thermoelastic data

Abstract: — An alternative methodology is presented for determining stress intensity factors for cracks subject to mixed-mode displacements. The methodology involves thermoelastic data generated from a SPATE (Stress Pattern Analysis by Thermal Emission) system and has been adapted from one used successfully in photoelasticity. The thermoelastic data is collected throughout the elastic stress field dominated by the crack tip singularity. The stress field is described using a Fourier series within Muskhelishvili's approach. This method allows different applied stress fields to be described which may include transient or non-uniform stress fields. The results obtained using the new methodology are at least as good as those obtained previously for pure mode I cases, and generally better for mixed mode displacement conditions.

Full field photoelastic stress analysis

Abstract: A structural specimen coated with or constructed of photoelastic material, when illuminated with circularly polarized light will, when stressed; reflect or transmit elliptically polarized light, the direction of the axes of the ellipse and variation of the elliptically light from illuminating circular light will correspond to and indicate the direction and magnitude of the shear stresses for each illuminated point on the specimen. The principles of this invention allow for several embodiments of stress analyzing apparatus, ranging from those involving multiple rotating optical elements, to those which require no moving parts at all. A simple polariscope may be constructed having two polarizing filters with a single one-quarter waveplate placed between the polarizing filters. Light is projected through the first polarizing filter and the one-quarter waveplate and is reflected from a sub-fringe birefringent coating on a structure under load. Reflected light from the structure is analyzed with a polarizing filter. The two polarizing filters and the one-quarter waveplate may be rotated together or the analyzer alone may be rotated. Computer analysis of the variation in light intensity yields shear stress magnitude and direction.

Photoelastic analysis of a three-dimensional specimen by optical slicing and digital image processing

Abstract: The authors show a nondestructive method for obtaining the isochromatic and isoclinic fringes in a three-dimensional photoelastic specimen. The basic idea is to delimit a slice between two plane laser beams. The properties of polarization of the scattered light (Rayleigh's law) and the interference possibilities of the diffused beams are used. By introducing speckle pattern properties, the correlation factor of the two scattered beams is similar to the illumination given in a plane polariscope for the investigation of a slice (in a classical frozen-stress technique). The authors use a monochromatic laser beam, a CCD camera and a personal computer. Because they cannot obtain the correlation factor directly, they do a statistical analysis of the speckle patterns. The variance (function of the correlation factor) is computed from the light intensities of three images corresponding to the speckle pattern for plane 1 alone, plane 2 alone, and both planes together.

A photoelastic study of contact between a cylinder and a half-space

Abstract: Three-dimensional photoelasticity was employed to study a cylinder in contact with a half-space. Both bodies were modeled in epoxy resin. Three loading cases were examined, namely, the cylinder lying on its side subject to a load normal to the plane, the cylinder on its side subject to both normal and tangential loads and the cylinder standing on its end and subject to a normal compressive load, i.e., as a circular punch. The cylinders and the half-space, which was represented by a large block, were stress frozen with a known coefficient of friction and using relatively small loads so that the strain levels were low. After slicing the cylinders, which resulted in lower fringe orders than could be readily analyzed manually, an automated system based on phase stepping was used to record and process the data. Distributions of maximum shear stress and Cartesian shear stress were obtained for a large area of the slice. Stress separation was performed, using the shear difference method, to obtain the Cartesian stress components in the plane of symmetry of the half-space. These results provide confirmation, by experiment, of the theoretical and numerical models of this type of contact obtained by other investigators.

Photoelastic stress analysis using an object step-loading method

Abstract: Sensitive, computer-aided retardation phase measurement methods suffer from the problem of discontinuity, which, in turn, hinders faultless phase unwrapping for stress determination in photoelasticity. An object step-loading method is introduced in this paper to overcome such a problem. One advantage of the technique is that the entire loading-to-stress history is computed. In this paper, the theoretical aspects related to implementation of the object step-loading method in computer-aided photoelasticity are outlined, and experimental verification is performed on a birefringent model subjected to compressive loading by comparing stress values with those computed using the theory of elasticity.

Digital determination of photoelastic birefringence using two wavelengths

Abstract: A new approach for digital determination of photoelastic birefringence is proposed. The relationships between the intensity values and the fringe orders of two wavelengths are derived. This scheme allows for automatic determination of fringe orders of a full-field photoelastic fringe pattern without using zero-order fringes in the fringe pattern. The usefulness of this method was demonstrated on two experimental fringe patterns with different wavelengths.

Initiation, propagation and arrest of an interface crack subjected to controlled stress wave loading

Abstract: An experimental study has been conducted to investigate the initiation, propagation, and arrest of bimaterial interface cracks subjected to controlled stress wave loading in the form of a tensile dilatational stress wave pulse. The tensile pulse is generated by detonating lead azide explosive in a specially designed specimen. Dynamic loading of the bimaterial interface results in crack initiation, propagation, and arrest, all in the same experiment. This failure event is observed using photoelasticity in conjunction with high speed photography. Full field data from the experimentally obtained isochromatic fringe patterns is analyzed to determine time histories of various fracture parameters such as the crack tip speed, the dynamic complex stress intensity factor, the energy release rate, and the mixity. The experimental data is also used to quantify the values of the dynamic initiation and arrest toughness and to evaluate a recently proposed dynamic interface fracture criterion.

Derivation of retardation phase in computer-aided photoelasticity by using carrier fringe phase shifting.

Abstract: Previous phase-shifting schemes in computer-aided photoelasticity required the processing of six fringe patterns to derive the phase difference due to retardation. A technique in which a carrier fringe is used is demonstrated to reduce to four the number of fringe patterns required. The use of fewer fringe patterns lowers the computation time and the number of phase-step errors. The basis of the technique is outlined in detail, and experimental results are presented as well.

Photoelastic tomography for three-dimensional flow birefringence studies

Abstract: The possibility of applying photoelastic tomography (integrated photoelasticity) for investigating a three-dimensional birefringent flow is studied. Tomographic equations of the strain rates are reduced to a vector analogue of the Radon equation by dividing the flow-velocity field into an axial component, a transversal rotational, and a transversal potential component. It is shown how the axial and potential parts can be determined by tomographic photoelastic measurements.

Development of Scanning Stress Measurement Method Using Laser Photoelasticity

Abstract: We have developed an optical equipment that possesses high detection sensitivity for measuring the small optical retardation induced by small stress by means of laser photoelasticity. A He-Ne laser is used as a light source to measure small stress in transparent materials. We explain the theory and process of the measurement of optical retardation in the materials. The magnitudes of principal stress difference and the directions of the principal stress are obtained simultaneously and quantitatively using our equipment. To evaluate the validity of the measurement results of the equipment, the stress distribution of a pulled rectangular glass plate with notches at both sides is measured using the equipment. The experimental results of stress distribution agree well with the analytical results of FEM. The stress distribution can be determined quickly by using the equipment and scanning stress distribution measurement has been realized.

Automation of data acquisition in reflection photoelasticity by phaseshifting methodology

Abstract: An optical arrangement for application of the phaseshifting technique to reflection photoelasticity is presented. The intensity equations for application of phaseshifting methodology are obtained by Jones' calculus. A new portable reflection polariscope is designed to effect the optical arrangement. The use of a commercially available Handycam is shown to meet the requirement of image capturing for digital photoelastic analysis. Fringe order variations for three arbitrary lines in a circular disc under diametral compression are obtained by the new methodology and compared with theoretical predictions. The comparison is found to be good.

Photoelasticity for the investigation of internal stress in BGO scintillating crystals

Abstract: Quality control of scintillating crystals production requires the on-line monitoring of the state of internal stress levels. The determination of the photoelastic constant in crystals is fundamental in this measurement, rendered necessary by the crystal fragility, a limiting factor in the construction of calorimeters (radiation detectors consisting in large arrays of crystals). In this work high-density BGO (Bismuth Germanate Bi3Ge4O12) crystals for calorimeters have been studied by means of photoelasticity. A test bench, based on a circular polariscope, an image acquisition system, a data processing unit and a suitable mechanical loading unit have been developed and applied to crystal sample testing. The results provide new data on photoelasticity of BGO and outline the basic features for the construction of an apparatus for on-line inspection of crystal quality.

Photoelasticity and quadratic permittivity of wide-gap semiconductors

Abstract: Expressions for the photoelastic constants are obtained on the basis of an analogy between the photoelastic and elastic properties of semiconductor crystals. Calculations were performed for wide-gap semiconductors (silicon carbide and boron, aluminum, and gallium nitrides) with sphalerite and wurtzite structures. The quadratic permittivity is also calculated for hexagonal structures.

Photoelastic determination ofK I andK II : A numerical study on experimental data

Abstract: The most efficient photoelastic methods to obtain stress intensity factors are those based on stress functions series expansions. The coefficients of these expansions are fitted to the experimental isochromatic pattern using an overdeterministic Newton-Raphson least squares method. In this paper, a study has been carried out to analyze the influence on the results of several numerical and experimental factors. It is shown that accurate values of the stress intensity factorsK I andK II can be obtained by following some recommendations given in the text and summarized in the conclusions at the end of the paper.

Phase unwrapping in photoelasticity

Abstract: A novel phase unwrapping algorithm for photoelasticity, based on the modulation ordering, is proposed and experimentally verified. The path for phase unwrapping is determined by the intensity modulation in the pixels, on the boundary of the phase unwrapped areas. Therefore, the algorithm is very robust, and in the worst case the error, if any, is limited to local minimum areas.

A Measurement Method of Scattered Light Photoelasticity using Unpolarized Light

Abstract: This paper describes an automatic measurement method for the stress analysis of a three-dirnensional photoelastic model having the rotation of the principal stress by scattered-light photoelasticity using unpolarized light. The relative phase retardation and the principal stress directions of a linear retarder for a distance in the solid model are expressed in terms of measurable Stokes parameters. The method was used for measurements on a frozen stress sphere under diametral compression.

Application of the photo-elastic method to measurement of dynamic stress distribution for NS-GT cut quartz crystal resonators

Abstract: NS-GT cut quartz crystal resonators are widely used as a frequency standard element in consumer products and communication equipment. The vibration mode of the resonators was analyzed by the finite element method (FEM) because they have a complicated shape. As a result, an asymmetrical vibration mode at the main resonant frequency has been obtained by the FEM simulation. But, it is necessary to confirm the asymmetrical vibration mode experimentally because it is just a simulation. In this paper, stress distributions of the NS-GT cut quartz crystal resonators are measured experimentally by using a dynamic photo-elastic method when the resonators are vibrating in the resonant frequency; thereafter, vibration modes of the NS-GT cut resonators are estimated with the experimental data of the stress distributions. This experiment for the NS-GT cut quartz crystal resonators exposes the existence of a twisted asymmetrical vibration mode at the main resonant frequency, with the magnitude of the twisted vibration in proportion to thickness of the resonators.

Recent advances in photoelastic applications

Abstract: Photoelasticity is one of the oldest methods for experimental stress analysis, but has been overshadowed by the Finite Element Method for engineering applications over the past two/three decades. However, certain new and novel developments and applications have revived the use of photoelasticity. Among them are image processing for fringe analysis, polarimetric fiber optic sensors, infra-red photoelasticity, low cost dynamic photoelasticity and photoelastic applications in stereolithography. This paper will, after a brief introduction to photoelasticity, highlight some of these advances and developments.

Transformation of embedded shape memory alloy ribbons

Abstract: Shape memory alloy (SMA) wires can be embedded in a host material to alter the stiffness or modalresponse and provide vibration control. The interaction between the embedded SMA and the host materialis critical to applications requiring transfer of loads or strain from the wire to the host. Although there hasbeen a significant amount of research dedicated to characterizing and modeling the response of SMA alone,little research has focused on the transformation behavior of embedded SMAs. Photoelastic experimentswith SMA wires in polymer matrices had previously provided a qualitative understanding of stress transferin SMA composites. In the current work, 2-D photoelasticity is utilized to quantify the internal stressesinduced by the actuation of a thin SMA ribbon in a pure polymer matrix. Through the use of a CCD cameraand a frame grabber, photoelastic images are digitally recorded at discrete time increments. Shear stressesinduced during the actuation are calculated as a function of time. Computational predictions of shear stressare made using finite element analysis and compared with experimental observations.Keywords: Shape memory alloys, phase transformation, composites, photoelasticity