Showing papers on "Photoelasticity published in 1996"

Subsonic and intersonic crack growth along a bimaterial interface

Abstract: An experimental investigation has been conducted to study the dynamic failure of bimaterial interfaces. Interfacial crack growth is observed using dynamic photoelasticity and characterized in terms of crack-tip velocity, complex stress intensity factor, and energy release rate. On the basis of crack-tip velocity two growth regimes are established, viz. the subsonic and transonic regimes. In the latter regime crack-tip velocities up to 1.3 times the shear wave velocity of the more compliant material are observed. This results in the formation of a line of discontinuity in the stress field surrounding the crack tip and also the presence of a pseudo crack tip that travels with the Rayleigh wave velocity (of the more compliant material).

Full-field separation of principal stresses by combined thermo- and photoelasticity

Abstract: The combined use of thermoelastic stress analysis and full-field reflection photoelasticity based on the phase-stepping technique has been developed for twodimensional problems. The first method determines the sum of the principal stresses, the latter evaluates the difference of the principal stresses. Thus the principal stresses were separated at each point in the field of view without reference to neighboring points. An evaluation of this approach has been performed using a tensile plate with a central circular hole. The results show that the analysis carried out combining thermo- and photoelasticity incurred errors no larger than those of each system working independently.

The interaction of a single laser‐generated cavity in water with a solid surface

Abstract: This paper reexamines the collapse of a cavity in water and its role in possible damage processes. Laser‐generated cavities were formed near to a water–solid boundary and simultaneous observations made of the collapse processes and stresses on the solid surface. Using schlieren photography and a thin film transducer on the solid, the importance of fluid flow to induced material stress was determined. The stresses in the solid were also observed using dynamic photoelasticity. The cavity deformation in the collapse phase was observed with high spatial and temporal resolution. Two distinct surface stresses were observed associated with bubble collapse near the boundary. The results indicate that one was due to a shock wave. It is postulated that the second stress was associated with fluid flow through the bubble, set up by the so‐called ‘‘liquid jet’’ as opposed to jet impact on its own.

The effect of microstructural fabric on dynamic load transfer in two dimensional assemblies of elliptical particles

Abstract: Experimental and numerical studies have been conducted to investigate the effect of microstructural fabric, such as major axis orientation, contact normal and branch vector distributions, on dynamic load transfer behavior in two dimensional granular material. The granular medium was simulated by assemblies of elliptical particles. The experimental method utilizes the combination of high speed photography and photoelasticity to study the local load transfer behavior in granular assemblies subjected to explosive loading. Numerical studies employed a computational scheme based on the discrete element method. Results indicate that the microstructural fabric has significant effect on the load transfer phenomena, such as stress wave velocity, load pulse wavelength and contact load attenuation.

2D and 3D separation of stresses using automated photoelasticity

Abstract: A procedure for the separation of full-field photoelastic images for use with an automated polariscope is described. Regions of background in the image are identified thus producing the boundary of the model. The shear difference method is used to calculate the components of stress along all raster lines in the image using photoelastic parameters at the boundary points to calculate the initial values of stress. Algorithms were also used to evaluate the stress components along raster lines which did not contain boundary points. A plastic template was used to evaluate the efficiency of the boundary routine. It was found that it was able to identify edges to within approximately one pixel on screen. The complete procedure for stress separation was evaluated using a stress frozen disc in compression and a turbine slot. The values of stress found using the automated polariscope with the stress-separation procedure were found to agree well with theory and with results determined using the method of Tardy compensation and manual analysis. The automated polariscope was also used to analyze three-dimensional stress components along arbitrary lines of a 3D model. A two-model slicing regime was used to analyze a strut subjected to a vertical load. This work was compared to results obtained by Frocht and Guernsey on an identical model machined from Fosterite and subjected to a higher load. Good agreement was found between the results for points away from the region of loading. Significant differences were found near to the load point, however. A finite element analysis of the same problem suggested that this was due to the effects of plasticity.

Effects of processing conditions on birefringence development in injection molded parts. II. Experimental measurement

Abstract: The birefringence of injection molded parts was measured using a digital photoelasticity system, which combines a digital image analysis technique and the half-fringe photoelasticity (HFP) method The effects of processing conditions, including melt temperature, mold temperature, filling time and packing pressure, on the birefringence development in the molded parts were investigated. It was found that temperature and pressure are the two dominant factors that determine the birefringence development in the parts during the molding process. Frozen-in birefringence of the molded parts decreases with increasing melt temperature, mold temperature and injection speed. Birefringence of the parts also increases with increased packing pressure, especially around the gate area. Numerical simulations using the Leonov viscoelastic fluid model predict similar dependence of birefringence of parts on processing conditions. Simulated results are also consistent with measured values.

Analysis of phase transformation fronts in SMA composites

Abstract: Shape memory alloy (SMA) wires can be embedded in a host material to alter the stiffness or modal response and provide vibration control. The interaction between the embedded SMA and the host material is critical to applications requiring transfer of loads or strain from the wire to the host. Although there has been 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 SMA wires. In the current work, photoelasticity is utilized to quantify the development of internal stresses induced by the actuation of a SMA wire embedded in a pure polymer matrix. Through the use of a CCD camera and a frame grabber, photoelastic images are digitally recorded at discrete time increments. Shear stresses induced during the actuation are calculated as a function of time. Computational predictions of the transformation fronts are made using finite elements analysis and compared with experimental observations.

Birefringent properties of diametrically loaded gradient-index lenses

Abstract: Gradient-index (GRIN) lenses have been widely used as collimators in various fiber-optic sensors and as optical coupling devices in components designed for optical communication systems. However, relatively little attention has been paid to the birefringent properties of GRIN lenses and the potential for using them as photoelastic sensing elements in optical transducers. Analytical and experimental results are described that were obtained for the intensity distribution produced by studying a GRIN lens by using a polariscope. The residual birefringence inherent in an unloaded lens is initially studied. The lens is then assumed to be diametrically loaded and the superposition is studied by the method of ray tracing. When the results obtained from the simulation for a Selfoc, 0.25-pitch lens are compared with experimental data, an excellent agreement is obtained. Intensity increases monotonically with load, confirming that the lens would be a good choice for the sensing element of an optical transducer designed as part of a strain or acceleration measurement system. The numerical simulation is then used to study the influence of residual stress on sensitivity.

A compact polariscope/shearing interferometer for mapping stress fields in bimaterial systems

Abstract: A compact optical device, which is a combination of a polariscope and a shearing interferometer (PSI), is described for mapping stress fields in bimaterial systems. The PSI device uses commercially available wave plates, and a calcite crystal for beam duplicating. It can operate in a variety of modes, including as a circular polariscope and as a shearing interferometer. In its polariscope mode, it can be used on birefringent materials such as Homolite and epoxy; in its shearing mode, it can be used on optically isotropic materials such as polymethylmethacrylate (PMMA) and glass. As an example, the device is used to obtain full-field maps of stress fields in the vicinity of an interfacial crack in an epoxy-PMMA bimaterial plate.

Stress analysis of an orthotropic material under diametral compression

Abstract: The diametral compression test is commonly used to determine the tensile strength of brittle materials. For isotropic materials a simple relation based on specimen geometry and the applied load at failure is used to calculate the tensile strength. Previous to this work the effect of material orthotropy and material orientation on the specimen stress state had not been completely determined. In this study, both isotropic and orthotropic specimens were analyzed using a finite element analysis and experimentally verified by strain gage and photoelastic measurements. Further, this work investigated the effect of the applied load area on the specimen stress state. Results of this work show that there is a significant difference between the theoretical calculations based on the assumption of material isotropy when compared to an orthotropic material. This difference can be as much as 45 percent depending on the degree of orthotropy and the orientation. It was also determined that the applied load area and material orientation significantly influence the specimen stress state. An applied load area of 8 percent of the circumference was found to reduce the stresses in the applied load region.

Stress-induced vertical confinement of light in bulk GaAs and Si substrates

Abstract: We propose a new waveguide structure that can be formed on bulk semiconductor substrates without requiring any epitaxial or separate cladding layers for vertical confinement of light. In the proposed structure, vertical confinement of light is achieved via a photoelastic effect in semiconductor induced by thin-film stress, and lateral confinement is obtained by a semiconductor mesa. We have carried out numerical analyses on the stress distribution, dielectric constant changes, and mode profiles at 1.3 /spl mu/m or 1.55 /spl mu/m wavelength in GaAs or Si mesas. The results show that the proposed structure can support vertical modes with the amount of stress that can be obtained from typical thin-film/semiconductor interfaces.

A new method of determining the stress state in microelectronic materials

Abstract: This paper introduces a new technique of determining the stress in microelectronic materials by combining photoelasticity and Fourier analysis. The approach uses a continuously rotating analyser to determine the photoelastic parameters from a Fourier analysis of the measured emerging light intensity. The principle of operation is discussed and the apparatus is described. An example is demonstrated to illustrate the application of the technique.

Photoelastic Determination of Contact Stresses of Foundations

Abstract: The contact stress distribution underneath foundations under operating loads is important for a realistic estimation of settlements. Theoretical and numerical models are difficult to apply due to the nonlinear properties of the subsoil. Previously conducted modeland full-scale tests on real sand gave varying and contradictory stress distributions. A series of model tests using a photoelastic model of a rigid foundation resting on sand were conducted at Queen's University Belfast, Northern Ireland, to determine contact stresses underneath a rigid foundation. Models were tested for centrally and eccentrically applied loads at different load levels. It was found that the stress distribution for central loading was bell shaped, with zero corner stresses for small and nonzero contact stresses for higher loads. For eccentrically loaded foundations, the stress distribution was again bell shaped, whereby the effective width of the foundation was reduced. The photoelastic method was found a valuable tool for analy...

Stress distribution in and around a spherically ended fiber embedded in a polymer matrix

Abstract: The effect of fiber end geometry, especially one with an enlarged spherical end, on the stress distribution in a polycarbonate/short glass-fiber reinforced model composite was studied using photoelasticity and finite element analysis (FEA). The role played by the fiber/matrix interfacial strength was also investigated. The photoelastic method was applied to examine the stress distribution in the vicinity of a single fiber embedded in a thermoplastic polycarbonate matrix. Linear elastic, axisymmetric FEA was carried out to investigate the fibers effectiveness as a load bearing constituent, the effect of interfacial debonding, and stress concentration near the fiber ends. Both photoelastic analysis and FEA demonstrated the efficiency of the spherical enlarged fiber end in load transfer ability and in delayed debonding, which may be important in enhancing strength and modulus of thermoplastic matrices.

Using dynamic photoelasticity to evaluate the influence of parting planes on stress waves interacting with stopes

Abstract: Dynamic photoelasticity is used as a means for visualizing the complex interaction process between elastic waves and geometrical discontinuities. The photoelastic experiments are back-analysed by the dynamic finite difference program WAVE, and the code is assessed in terms of its accuracy and modelling capabilities. Three model geometries are investigated : (i) a stope situated within a homogeneous medium ; (ii) a stope surrounded by softened material, the interface between the softened and bulk material being bonded and (iii) a stope situated within softened material, with a non-cohesive material interface. Prominent waves resulting from the diffraction, refraction and reflection of incident waves, as well as normalized dynamic stress intensification factors at the stope face, stope back area and along the hanging-wall skin are analysed in this study. The parting planes are found to reflect a portion of the incident energy and thus shield the stope. However, a non-cohesive parting plane traps energy within the hanging-wall beam, and any shielding benefits are negated. WAVE has been proven to model accurately the diffraction, refraction and reflection of stress waves in a homogeneous medium and the interaction with cohesive and non-cohesive interfaces separating two material types.

Photomechanical properties of some birefringent polymers around their glass transition temperatures

Abstract: This paper describes an investigation and comparison of the optical and mechanical properties of the three polymers—PMMA (polymethyl methacrylate), CT200 and MY750 (both based on bisphenol-A epichlorohydrin). The work was undertaken with the specific purpose of assessing the suitability of these polymers for use in contact studies involving three-dimensional photoelasticity and the stress-freezing technique. The optical property investigated was the variation of photoelastic fringe constant with maximum stress-freezing temperature. The mechanical properties sought were the variation of Young's modulus with temperature and the stress-strain behavior at the stress-freezing temperature. The effects of the magnitude of stress and the soak time in the thermal cycle were also investigated for the MY750 resin. The results provide the optimum peak temperature for the thermal cycle to achieve repeatable values of Young's modulus and fringe constant during stress freezing. The nonlinear stress-strain behavior is quantified and should be a useful reference. The dependence of strain on load and soak time is also shown and is useful in specifying the optimum cycle time for stress freezing and the appropriate stress level. It was concluded that the resin MY750 was the most suitable for photoelastic applications which involved high localized stress, such as contact problems or fracture mechanics studies.

Residual stresses in epoxy systems by 3-D photoelastic method

Abstract: The new 3-D photoelastic method was applied to the studies of residual stresses around spherical inclusion in polymeric matrices. Full stress tensor for several model samples was measured. The extent of significant stresses is not greater than three radii of an inclusion. It was found that the stress follows the 1/R 3 rule at distances far from the inclusion, while in the narrow zone at the interface a plateau is observed. The level of stress ranges from few MPa up to the plastic yield of the polymeric matrix. The radial stress component is usually twice as large as the tangential stress component. Radial negative stress and tangential positive stresses are found in configuration with a hard inclusion, while radial positive stress and tangential negative stresses are in the systems with soft inclusion. The pressure in the matrix at points around inclusions calculated from the stress tensor is always near zero MPa, which indicates the action of purely deviatoric stresses in the matrix.

Application of photoelastic method to measurement of dynamic stress distribution for NS-GT cut quartz crystal resonator

Abstract: A dynamic photoelastic method is a very effective measuring technique for the stress distribution of the vibrating quartz resonators because the stress distribution is measured directly When this method was applied to observe stress distributions of BT cut quartz resonators, many interesting results were obtained In the present paper, stress distributions of the NS-GT cut quartz crystal resonators are measured experimentally by this method when the resonators are vibrating in the resonant frequency Vibration modes of the NS-GT cut resonators are estimated with experimental data of stress distributions This experiment for the NS-GT cut quartz crystal resonators has exposed existence of an asymmetrical mode of vibration at the resonant frequency and magnitude of asymmetrical modes are in proportion to a thickness of the resonator This asymmetrical mode of resonant vibration has been analyzed by the finite element simulation, but we could not give convincing proofs until this experiment The finite element simulation and the result of this experiment go to show that asymmetry modes at the resonant vibration are caused by elastic stiffness constant C/sub 16/ of the NS-GT cut quartz crystal resonator This constant c/sub 16/ is dependent upon thickness of the resonator

Photoelastic method of three‐dimensional stress determination around axisymmetric inclusions

Abstract: Using Aben's approach to three-dimensional photoelasticity, a method for the determination of the stress tensor in a transparent matrix around an isolated axisymmetric inclusion is detailed. The method is useful for studies on a microscopic scale, the inclusion preferably above 10 to 20 µm in diameter, especially when the total retardation is below the first fringe order of isochrom. The polarization and phase data for normal and oblique light beams are necessary for full determination of the stress tensor, including the information on pressure. The analysis is nondestructive of the composite material studied.

Birefringence induced by residual strain in optically isotropic III-V compound crystals

Abstract: By using a high-sensitivity high-spatial-resolution scanning infrared polariscope, we have quantitatively measured a small amount of birefringence induced by residual strain in commercial wafers of III-V compound crystals. From the measurement results in commercial (100) wafers of GaAs grown by the liquid encapsulated Czhocralski method, it is found that the 2D distribution maps of absolute difference (Delta) n between the principal refractive indices exhibit fourfold symmetry accompanying some fine structure such as stripes or line-segment patterns. The magnitude of (Delta) n is about 5 X 10-5 in maximum and the principal axes of birefringence are approximately radially or tangentially aligned. From the detailed analysis of photoelastic effect on the (100) wafers, the in-plane components of residual strains: Syy - Szz and 2Syz are deduced from the measured (Delta) n and the principal angle of birefringence (psi) .© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Nonaxisymmetric residual stress distribution in axisymmetric glass articles

Abstract: Residual stress measurements in many bottles, tumblers and in other axisymmetric glass articles by using integrated photoelasticity have shown that the residual stress distribution often deviates from the axially symmetric one. A method is developed which enables determination of nonaxisymmetric residual stress distribution. A computer-controlled polariscope has been developed for automatic measurements. Several examples of residual stress distributions, which deviate strongly from the axisymmetric one, are given. It is shown that deviations from axial symmetry lead to less favorable distribution of the residual stresses; e.g., they may lead to tensile residual stresses on the external surface of bottles.

Photoelastic characterization of undoped semi-insulating GaAs wafers with a high-spatial-resolution infrared polariscope

Abstract: In order to measure a small amount and spatial variation of birefringence induced by residual strains in III-V compound materials, we have developed a high-sensitivity high-spatial-resolution computer-controlled scanning infrared polariscope Several characterization results on commercial GaAs wafers are presented to demonstrate its performance It is found that local strain fields accompanied by crystal defects such as slip dislocations, lineages, inclusions, and voids can be observed

Development of Scanning Stress Measurement Method Using Laser Photoelasticity.

Abstract: We have developed an optical equipment which possesses high detection sensitivity for measuring small optical retardation induced by 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 magnitude of principal stress difference as well as the direction of the principal stress are obtained simultaneously and quantitatively using our equipment. In order to evaluate the performance of the optical retardation measurement equipment, the stress distribution of a pulled rectangular glass plate with notches at both sides was measured using the equipment. The results of stress distribution agreed with the analytical results. Stress at many points can be determined quickly using the equipment and scanning stress distribution measurement has been realized.

Experimental calculated determination of contact stresses in hinge joints

Abstract: The experimental-calculated method for determination of contact stresses is reported. The method is based on hybrid use of photoelasticity and finite element method (FEM). The results of contact pressure and contact angle determination in hinge joint as function of value and direction of load, clearance are presented. The contact stress distribution in three-dimensional hinge joint is shown which depend on elastic deformations of pin and lug.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Investigation of fracture problem mechanics by the nonlinear photoelastic method

Abstract: The paper deals with the photoelastic method of solving the problems of non-linear elasticity and plasticity under the large strains with the help of the birefringent materials, i.e. a polyurethane resin and a composition on the basis of epoxy. The transparent models and photoelastic coating method are used. The optical-mechanical dependencies of the non-linear photoelasticity under the strain in the range of 50 - 150% have been obtained. The interpretation of the optical patterns has been worked out when the object is under the large strains. There exist two ways of decoding. The first one uses Muny-Rilvin and Bartenev-Khazanovich's elastic potentials. The second one is much easier and describes the behavior of polyurethane in the whole range of the given strains, so it was used in the present investigation. It's worth mentioning that the processing of experimental data in non-linear problems is more complex than in classical photoelasticity, as it's necessary to take into account the change of the specimen's shape and geometry during the deformation. In this case the principle extent of lengthening lambda (a difference between the length of a deformed specimen and its initial length) was used as a strain value. The method of nonlinear photoelasticity gives the possibility to investigate the fundamental problems of the mechanics of solids. These problems have physical and geometrical non-linear properties, i.e. large elastic and plastic strains, mechanics of destruction. Siberian scientists have already investigated some problems of this kind. But they haven't reached the theoretical solution and solved it in the first approximation. So they need an experimental test. You'll find some of them below, i.e. the problem of thermal stresses in the multi-layered panels, determination of stresses in the reinforced beams with the naturally developing cracks, determination of forms and sizes of plastic zones near the sharp-notches tips in the metallic specimens, the problems of stress-strain condition in the necking of the plane steel specimens.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.