Showing papers on "Photoelasticity published in 2000"

Digital Photoelasticity: Advanced Techniques and Applications

Abstract: Transmission Photoelasticity.- Reflection Photoelasticity.- Digital Image Processing.- Fringe Multiplication.- Fringe Thinning and Fringe Clustering.- Phase Shifting, Polarization Stepping and Fourier Transform Methods.- Phase Unwrapping and Optically Enhanced Tiling in Digital Photoelasticity.- Colour Image Processing Techniques.- Evaluation of Contact Stress Parameters and Fracture Parameters.- Stress Separation Techniques.- Fusion of Digital Photoelasticity, Rapid Prototyping and Rapid Tooling Technologies.- Recent Developments and Future Trends.

An investigation into the effects of micro-compressive defects on interphase behaviour in hemp-epoxy composites using half-fringe photoelasticity

Abstract: Half-fringe photoelasticity was used to evaluate the two-dimensional state of stress in epoxy matrix micro-tensile specimens. The experimental technique is described in detail and demonstrated by the measurement of the stress distribution around a circular hole in a thin epoxy film subject to uniaxial tension. The results obtained using this photoelastic technique compare well with theoretical solutions. This system has enabled the mapping of stresses in the matrix of single (hemp) filament composites, strained parallel to the fibre axis, to be undertaken. Concentrations of stress were observed in the interphase region, adjacent to kink bands (micro-compressive defects) and stress concentration factors of ~ 1.4 were recorded. The role that these stress concentrations play in the failure of hemp fibre reinforced composites is discussed.

Strain measurements and stress analysis

Abstract: 1. Stress, Strain, and Stress-Strain Relationships. Introduction. Stress. Two-Dimensional Stress State at a Point. Principal Stresses. Mohr's Stress Circle. Differential Equations of Equilibrium. Strain and Displacement. Principal Strains and Mohr's Strain Cycle. Stress-Strain Relations. Problems. References. 2. Metal-Foil Resistance Strain Gages. Introduction. Principle of Operation. Gage Configurations and Fabrication. Strain Gage Bonding Agents and Procedures. Gage Factor and Transverse Sensitivity Correction. Environmental Effects on Metal-Foil Strain Gages. Problems. References. 3. Strain Gage Circuitry, Transducers, and Data Analysis. Introduction. Wheatstone Bridge. Correction for Long Lead Wires. Strain-Gage Transducers. Strain-Gage Rosette Data Analysis and Correction. Correction for the Wheatstone Bridge Nonlinearity. Gage Factor for Finite Deformation. Shunt Resistance Calibration. Potentiometer Circuit. Problems. References. 4. Photoelasticity. Introduction. Review of the Nature of Light and Wave Representation. Polarization of Light. Wave Plate and the Principle of Photoelasticity. Analysis of a Stressed Plate in a Plane Polariscope. Analysis of a Stressed Plate in a Circular Polariscope. Tardy Method. Calibration Methods. Determination of Whole-Field Isoclinic Fringe Patterns. The Separation of Principal Stresses. Relationship between Stresses in Models and Actual Structures. Materials for Photoelastic Models. Advanced Photoelasticity. Problems. References. 5. Photoelasticity-Coating Method. Introduction. Reflection Polariscope. Basic Principles of the Photoelastic-Coating Applications. Stress- or Strain-Optic Law for Coatings. Calibration of Photoelastic Coating. Principal Strain Separation Methods. Coating Sensitivity and Selections. Coating Applications. Problems. References. 6. Geometric Moire Techniques in Strain Analysis Introduction. Fundamental Property of Moire Fringes. Geometrical Method of Moire Fringe Analysis in Two Dimensions. Displacement Method of Moire Fringe Analysis in Two Dimensions. Instrumentation. Out-of-Plane Displacement Measurements. Out-of-Plane Slope Measurements. Grating and Their Applications. Problems. References. 7. Holographic Interferometry. Introduction. Interference and Diffraction. Wavefront Recording and Reconstruction by Holography. Displacement Measurement by Holographic Interferometry. Vibration Analysis by Time-Average Holography. Basic Equipment and Technique for Making a Hologram. Comparative Holographic Interferometry. Problems. References. 8. Computer Data Acquisition and Control System. Introduction. Basics of a Computer Data Acquisition and Control System. Components of a Computer Data Acquisition and Control System. Analog Signal Input and Output Channels. Interface for a Computer Data Acquisition and Control System. Summary Remarks. Problems. References. Index.

Recent Developments and Future Trends

Abstract: The application of digital techniques for data acquisition in 2-D transmission photoelasticity and reflection photoelasticity is well developed and these were discussed in the earlier chapters. The extension of digital techniques for data acquisition to integrated photoelasticity and scattered light photoelasticity have just made a beginning. In integrated photoelasticity, one has to determine three parameters experimentally and these are known as characteristic parameters. In scattered light photoelasticity, the illumination levels are quite low due to light scattering. This necessitates the use of high-resolution CCD cameras. The ultimate aim of integrated photoelasticity or scattered light photoelasticity is to evaluate the stress field interior to the model. The current developments on tensorial tomography are discussed in this chapter.

Stress distribution in the dento-alveolar system using digital photoelasticity.

Abstract: Past research has confirmed that the governing factors in cellular modelling and remodelling adhere to sound principles of engineering mechanics. Hence studies of stress distributions would provide better understanding of the functional adaptation of dental supporting structures. Photoelasticity is an established experimental tool to study whole-field stress distribution in structures subjected to forces. However, it has certain limitations that make its application in biological specimens tedious. In this investigation an advanced digital photoelastic system is used to visualize and study the nature of the stress distribution in dental supporting structures. These digital fringe patterns are analysed using a phase-shift technique. The present biomechanical study shows that dental supporting structures exhibit a characteristic stress distribution, promoting structural adaptation based on needs. Furthermore, the advantage of using a digital image processing system along with the circular polariscope is discussed.

Limitation of fourier transform photoelasticity: Influence of isoclinics

Abstract: The application of the Fourier transform to photoelasticity was used in the evaluation of the retardation using a carrier system of fringes. In photoelasticity, the light intensity from the analyzer in a circular polariscope depends on both the retardation (isochromatics) and the isoclinic parameter. The theoretical analysis shows that the angle between the principal stresses in the model and in the carrier system of fringes influences the evaluation of the retardation (isochromatics), as occurs when misaligned compensators (namely, Babinet) are used. As a consequence, this method may not be applied as a full-field technique, although the error is small if the angle between the principal stresses in the model and in the carrier is less than 25 deg. Numerical simulations and experimental tests were conducted to corroborate this prediction.

Isochromatic fringes in photoelasticity

Abstract: The structure of the isochromatic fringe pattern in a two-dimensional photoelastic model is investigated with the phase diagram method of the theory of dynamic systems. Isochromatics are interpreted as phase paths (or level curves) of a Hamiltonian system. Possible singularities of the fringe pattern are analyzed.

Hybrid Stress Analysis of Perforated Tensile Plates using Multiplied and Sharpened Photoelastic Data and Complex-Variable Techniques

Abstract: An experimental study is presented on the effects of varying the number of terms in a power-series representation of the stress function relative to determining the stresses around a circular hole or an elliptical hole in a finite-width, tensile loaded plate. Photoelastic data are hybridized with complex variable/mapping techniques to calculate the tangential stress on the boundary of the cutout. Accuracy/reliability is enhanced by twice-multiplying and sharpening the measured isochromatics using digital image processing. Actual and calculated fringes are compared qualitatively. Far quantitative comparison, percentage errors and standard deviations of the percentage errors are calculated for all measured input data by varying the number of terms in the stress function. The hybrid results agree within three percent with those predicted by theory and finite-element analyses.

A novel instrument for automated principal strain separation in reflection photoelasticity

Abstract: A novel instrument is presented capable of measuring the principal strains at a point on a photoelastic coating. The oblique incidence method of principal strain separation is employed, using three fringe order measurements, which are made by the automated method known as spectral contents analysis. The oblique incidence measurements are made after rotation of the polariscope about any two orthogonal axes. Three polariscopes are built into the instrument, as is a CCD camera, to facilitate accurate positioning of the instrument. The instrument was used to analyze the principal strain distributions in a disk under diametrical compression, a plate with a central hole under tension, and a stepped plate in tension. The strains in the disk and the plate with a hole were compared with theoretical results, and the strains in the stepped plate were compared with a finite element analysis of the problem. The correlation between the experimental and theoretical or numerical strains was better than 0.95 in most cases.

Complex fracture parameters for an interface crack between twohardening materials: a photoelastic study

Abstract: In this paper, complex stress intensity factors (SIFs) at an interface crack are determined for a range of applied loads, crack lengths and remote mode mixes using automated photoelasticity. The specimen geometries comprise epoxy resin and aluminium alloy halves bonded together, and are loaded in either compact tension in mixed-mode conditions or in three-point bend under mode I conditions. In the experiments, full-field isochromatic data were obtained from the epoxy half using an established phase-stepping technique. A reworked approach to the determination of the SIFs was developed by combining a least-squares over-deterministic method for fitting crack-tip stress equations to the data and a weighting factor that ensures that only data in the singularity zone are used. For comparison, some of the specimens were tested using a linear-elastic finite element (FE) analysis and/or by experiment using homogeneous test specimens. Excellent agreement between the experimental and numerical SIF moduli was achieved for remote mode I loadings. However, for good agreement to be made between the phase angle results requires an additional phase term to be added to the FE solution at each load to account for the development of a crack-tip plastic zone. Further, results for the SIFs from remote mixed-mode loadings of the compact tension specimen only have a meaningful interpretation in light of small-scale yielding conditions. It is shown, qualitatively, that the experiments verify some of the predictions made in the literature of asymptotic behaviour at interface crack tips from results of elasto-plastic FE analyses.

A new method of birefringence measurements using a Faraday modulator. Application to measurements of stress-optical coefficients

Abstract: The determination of the stresses in transparent thin plates (or films) is important in micro-electronic and optoelectronic technologies. Several polarimetric optical methods for measuring birefringence and hence stresses have recently been described in scientific papers. They use an image-analysis technique (whole-field analysis), a photoelastic modulator or a rotating polarizer (point-by-point analysis). We present a new method using a polarimetric device with Faraday modulation of the light beam. It allows one to determine with high precision (to within some 0.05?) at each point of the sample the birefringence (residual or induced) which is characterized by the direction and a phase shift introduced between optical waves polarized along orthogonal linear eigenpolarizations. This method can easily be automated. Two examples are given, namely measuring the birefringence of a ferrofluid plate subjected to a transverse magnetic field and determination of photoelastic coefficients of various glasses.

Photoelastic analysis of the singular stress field in a bimaterial wedge

Abstract: This paper presents an experimental investigation of the singular stress field near the vertex of a bimaterial wedge using a digital photoelastic technique. Special attention is given to the casting of bimaterial wedge specimens and analysis technique for extracting stress intensity factors from photoelastic samples. Different bimaterial wedge specimens are made of two different photoelastic materials bonded through a special casting procedure and loaded in simple tension. A new multiple-parameter method is developed to obtain the stress intensity factor reliably from the isochromatic fringe patterns and the series representation of the stress field at the vertex of the wedge. Experimental results are compared with finite element predictions, and good agreement is observed.

Hybrid mechanics for axisymmetric thermoelasticity problems

Abstract: The present article extends the integrated photoelasticity technique for the case where stresses are due to an unknown axisymmetric temperature field. Relationships that express radial and circumferential stresses through the axial and shear stresses are derived. Since residual stresses in glass can be described by the equations of thermoelasticity, the results of this article offer a means for complete determination of the residual stresses in axisymmetric glass articles using integrated photoelasticity. The results are verified by numerical and physical experiments.

Resonant vectorial wave coupling in cubic photorefractive crystals

Abstract: We present a theory of resonant photorefractive wave coupling in cubic photorefractive crystals that are in general optically active. This theory includes the resonance enhancement of the photorefractive response in a constant applied electric field, the influence of photoelasticity, and the spatial inhomogeneity of the light intensity. In a unified manner, it allows one to describe the polarization and energy properties of two-wave coupling for the optical configurations relevant to experiment. Applications of the theory are given to photorefractive crystals of the sillenite family.

The hole drilling method in photoelasticity ‐application of an optimisation approach

Abstract: This paper presents a least squares inverse problem solution for the case of photoelastic fringes around a hole in a biaxially loaded plate for two problems. Problem 1 relates to an infinite plate in which the circular hole is drilled first and then the loads are applied. Problem 2 is the residual stress problem in which the hole is drilled after the biaxial load is applied to the infinite plate. This hybrid experimental numerical approach permits utilisation of as many points of information as desired, away from the edge of the hole and the axes of symmetry. Also, no a priori knowledge is needed as to the orientation of the principal axes of symmetry. Its implementation, using simulated photoelastic fringes, yields accurate predictions of the magnitude and orientation of the far field stresses.

Experimental study on crack curving propagation in bending beams under impulsive load

Abstract: Dynamic fracture behaviour of crack curving in bent beams has been investigated. In order to understand the propagation mechanism of such cracks under impact, an experimental method is used that combines dynamic photoelasticity with dynamic caustics to study the interaction of the flexural waves and the crack. From the state change of the transient stresses in polymer specimen, the curving fracture in the impulsively loaded beams is analyzed. The dynamic responses of crack tips are evaluated by the stress intensity factors for the cracks running in varying curvature paths under bending stress wave.

Automated reflection photoelasticity : digital data acquisition and use.

Abstract: Automation of reflection photoelasticity has simplified the stress and strain analysis of real engineering components and reduced analysis time. However, images obtained from automated reflection photo elasticity contain noise and accuracy of the analysis will be affected by the degraded intensity images. In the present study, major sources of noise in automated reflection photoelasticity have been found to be the photoelastic coating and the electronic instrumentation. An automated reflection polariscope PSIOS developed by Patterson and Wang (1998) for the simultaneous observation and capture of four phase-stepped photo elastic images was used as an example. The majority of the noise is in the high spatial frequency domain. The zero-phase, low pass Butterworth filter was found to be the most effective and flexible smoothing method for reducing the effect of noise in the intensity images. Results from experiments performed for assessing the ability of the PSIOS indicated that it is capable of yielding accurate results for the stress analysis of real components in both static and dynamic conditions and that it is fast and easy to use. Full-field experimental methods are often used to validate the stress distribution generated from numerical analysis. A common practice is to plot data along a line across the maps and to include both experimental and numerical results on the same axes. This approach is used widely and usually a reasonable, quantitative conclusion can be made. However, it cannot obtain more information about the relationship between the stress maps. Another method is to compare hot spots on experimental maps to the numerical maps. If the hot spots on the two maps match well, the numerical method is considered valid. However, when designs are being optimised for weight or crack paths are being investigated, comparison of the positions of the hot spots alone will not be enough and the correlation elsewhere in the data field should be taken into account. It has been shown that fit between the stress map from an experimental method and the stress map from the numerical analysis can be represented by a statistical parameter, the scaled standard deviation. An evaluation of the method was performed using stress maps from transmission photoelasticity, thermoelasticity and the finite element method as examples. The results from experiments using a curved tiebar, a circular ring and a real engineering component in this case, a race car hub carrier indicated that the scaled standard deviation represents the fitness between the two stress maps. If the scaled standard deviation is smaller than 0.1, then the experimental map and the numerical map can be considered to be in good agreement.

Experimental and numerical simulations of the static and dynamic behaviour of sandwich plates

Abstract: For the purpose of mastering the behaviour of sandwich structures through the knowledge of the mechanical properties, static strain measurement was carried out using the classical strain gauge method and reflection photoelasticity, and dynamic modal measurement was made using a FFT analyser and an impact hammer. The photoelastic technique has the advantage of yielding full strain field information in the form of fringes which give the difference in principal strain and their orientation. Photoelasticity, strain gauges and FFT measurements are compared to the numerical results obtained through finite element analysis. For orthotropic material such as the honeycomb cores, nine independant elastic coefficients must be known in order to be introduced into the program. The finite element package used for the modelisation is Ansys with two-dimensional shell finite elements. Knowledge of edge effects in composite with honeycomb cores is also essential for the design of sandwich structures. This paper deals with the measurement of strain by the application of reflection photoelasticity. Because of the geometry of honeycomb cores a classical method by strain gauges is inaccurate and unsuitable. The measurements by photoelasticity is compared to analytical and numerical results. The final aim of this work is to master the behaviour of such structures by the exact knowledge of its mechanical properties deduced in part from edge effect results.

Automated Photoelasticity Applied With Thermoelasticity to Real Components

Abstract: A complete definition of stresses requires an evaluation of the magnitude and directions of the components of stress related to a co-ordinate system. The combination of thermoelasticity and photoelasticity offers the potential to achieve this objective since photoelasticity provides the direction of principal stresses and difference in magnitude, whilst thermoelasticity yields the sum of the magnitude. Recent experiments have shown that simultaneous measurements can be made. The demands of simultaneous measurement for a full-field of view imply that all the photoelastic information must be captured quickly. A hybrid technique based on spectral contents analysis and phase stepping has been developed. The methodology and instrumentation for combining the automated photoelasticity with thermoelasticity has been developed In conclusion, it is shown that the complete state of stress over an area of the surface of the component can be evaluated without any prior knowledge of the stress state.

An Approach to General 3-D Stress Analysis by Multidirectional Scattered Light Technique

Abstract: Scattered light photoelasticity provides a nondestructive analysis of stress components on a plane layer in the incident light path. For general three dimensional stress fields, however, the effect of birefringent axis rotation has to be taken into account, and hence the technique has been limited only in application to plane stress or symmetrical stress distributions.

Use of mechanical stress in design of a Faraday isolator for high power radiation

Abstract: Summary form only given. Many laser applications require propagation of high power radiation through transmissive optical elements such as Faraday isolators. Absorption of laser radiation in the optical element leads to not only thermal lensing but also self-induced depolarization, resulting in a limitation of the maximum isolation ratio. The spatial nonuniform distribution of temperature causes two effects contaminating laser polarization: temperature dependence of the Verdet constant and linear birefringence due to the photoelastic effect of thermal stress. The last phenomenon is more efficient for most magneto-optical materials and hence limits the isolation ratio. In order to suppress the absorption induced depolarization, we have developed a method in which we apply force to induce stress into a magneto-optical sample. The direction of stress is maintained (in contrast to thermal stress) over the entire cross-sectional area. Due to the photoelastic effect, the induced stress leads to linear birefringence with a uniform direction of eigenpolarizations (they are along x and y axes) and various phase delays between them.

Stress Analysis of a Curved Beam Plate by using Photoelastic Fringe Phase Shifing Technique

Abstract: The method of photoelasticity allows one to obtain principal stress differences and principal stress directions in a photoelastic model In the classical approach, the photoelastic parameters are measured manually point by point This is time consuming and requires skill in the identification and measurement of photoelastic data Fringe phase shifting method has been recently developed and widely used to measure and analyze fringe data in photo-mechanics This paper presents the test results of photoelastic fringe phase shifting method for the stress analysis of a curved beam plate The technique used here requires four phase stepped photoelastic images obtained from a circular polariscope by rotating the analyzer at 0˚, 45˚, 90˚ and 135˚ Experimental results are compared with those of ANSYS and calculated by the simple beam theory Good agreement among the results can be observed