Showing papers on "Photoelasticity published in 2007"

Towards a new model of crack tip stress fields

Abstract: This work introduces a novel mathematical model of the stresses around the tip of a fatigue crack, which considers the effects of plasticity through an analysis of their shielding effects on the applied elastic field. The ability of the model to characterize plasticity-induced effects of cyclic loading on the elastic stress fields is assessed and demonstrated using full-field photoelasticity. The focus is on determining the form of the shielding stress components (induced by compatibility requirements at the elastic–plastic interface along the crack flank and via the crack tip plastic zone) and how they influence the crack tip elastic stress fields during a load cycle. The model is successfully applied to the analysis of a fatigue crack growing in a polycarbonate CT specimen.

Stress Distribution after Installation of Fixed Frameworks with Marginal Gaps over Angled and Parallel Implants: A Photoelastic Analysis

Abstract: Purpose: The objective of this work was to compare by photoelastic analysis the stress distribution along a fixed framework placed over angled or parallel implants with different gap values between the framework and one of the implants. Materials and Methods: Two photoelastic models were created: (i) with parallel implants; (ii) with a 30° angled central implant. In both cases, three implants were used, and CP titanium frameworks were constructed with commercial components. A plane polariscope was used to observe the photoelastic fringes generated after initial framework assembly, and also when an axial load of 100 N was applied over the central implant. For both models, stress analysis was conducted on well-fitting frameworks and on another with a 150 μm vertical gap between the framework and the central implant. Results: The photoelastic analysis indicated that in the model with parallel implants, stress distribution followed the implant axis, and in the model with an angled implant, a higher and nonhomogeneous stress concentration was observed around the apical region of the lateral implants. The placement of an ill-fitting framework resulted in increased preload stress patterns. Conclusion: Stresses were generated after screw tightening of the frameworks, increasing when a load was applied and when a vertical gap was present. Angled implants resulted in oblique stress patterns, which were not transferred with homogeneity to the polymeric model.

Modern Photoelasticity for Residual Stress Measurement in Glass

Abstract: Residual stress is one of the important indicators of the quality of any glass product. The paper gives a review of the methods, which are nowadays used in glass industry for measuring residual stress both in architectural and automotive glass panels and in hollow glassware.

Colour Adaptation in Three Fringe Photoelasticity

Abstract: Three fringe photoelasticity (TFP) can give the total fringe order from a single colour isochromatic fringe field by suitably comparing the colour with a calibration specimen. The fringe order evaluation can be erroneous when the materials for the calibration specimen and the application specimen are different. This is because of the colour variation between the two materials. This is conventionally handled by preparing individual calibration tables for each application. A new methodology to tune the calibration table obtained for a single material to accommodate the tint variation in TFP is proposed for the use of different specimen materials. Discontinuities in fringe order variation are smoothed using the refined TFP (RTFP) procedure. The elegance of the new methodology for solving a multi-material system is bought out by solving the problem of a bi-material Brazilian disc. The results obtained are compared with the phase shifting technique.

A Review of Dynamic Fracture Studies in Functionally Graded Materials

Abstract: This article presents a review of dynamic fracture studies on functionally graded materials. A brief literature review on the fracture mechanics of graded materials is presented first. This is followed by a discussion on the higher-order asymptotic analysis of the transient elastic field surrounding the tip of a dynamically growing crack in a functionally graded material. A comprehensive experimental study of dynamic crack growth in model functionally graded material using the optical method of reflection photoelasticity and high-speed photography is then presented. The results are analysed to establish a generalised relationship between the crack velocity and the dynamic mode-I stress intensity factor (SIF). This relationship is found to be unique and is distinctly different from that previously established for the matrix material (polyester). Finally, an innovative experimental procedure is used to demonstrate the necessity of employing a fully transient stress-field representation in the analysis of optical data for an accurate prediction of the dynamic SIF history.

PWO photo-elastic parameter calibration by laser-based polariscope

Abstract: We systematically measured the photo-elastic parameter f σ of PbWO 4 (PWO) for crystals of different thicknesses. We observed a linear behaviour of f σ as a function of the thickness for low level applied stresses. This result was corroborated by numerical simulations. f σ allows the evaluation of the residual internal stress, useful for crystal quality control. In this work, we also propose a new polariscope based on laser light, which allows a fast measurement and mapping of the internal stress of crystals with high resolution owing to the reduced laser-beam dimension.

T-Stress Effects on Isochromatic Fringe Patterns in Mode II

Abstract: The theory of photoelasticity is used to study analytically the effects of T-stress on the fringe patterns around the crack tip in mode II crack specimens. The locus of an isochromatic fringe determined by taking into account the T-stress is compared with the locus of a fringe with no T-stress. It is shown for mode II cracks that in the presence of T-stress, the fringe loops are neither symmetric nor continuous. Asymmetric and discontinuous fringe patterns predicted in this paper are consistent with the experimental results observed previously in photoelasticity tests.

Stress and strain analysis method and its equipment

Abstract: Provided are a stress analysis method and stress analysis equipment that enable a detailed stress measurement, by using both a photoelasticity measurement method and a stress measurement (mechanoluminescence measurement) which utilizes a mechanoluminescent substance to measure a stress state of an object. Physical quantities that are measurable include individual principal stress component and a principal stress direction. The photoelasticity measurement method alone cannot measure individual principal stress component values.

Analysis of the interface between a thermoplastic fiber and a thermosetting matrix using photoelasticity

Abstract: The photoelastic images of the isochromatic fringes for a pull-out specimen were used to measure and to analyze the maximum interfacial shear strength for a thermoplastic–thermoset interface (PET/Epoxy). The photoelastic technique allows the loading process to be followed in-situ, as well as the determination of the shear stress distribution on a tested sample. A description of the stress transfer process and the measurement of the shear stress level along the embedded fiber were done. The maximum shear strength was located along the fiber surface at a distance of 2.5 times the diameter from the specimen border. The maximum shear stress values obtained from the photoelastic analysis were corrected to include the tri-dimensional axisymmetric distribution of the shear stress around the fiber. The corrected shear stress measured τcMax ranged from 0.27 to 1.17 MPa.

Tensor field tomography of residual stresses

Abstract: An approximate method is proposed for determining residual stresses in elonagted transparent articles featuring weak variation of the stress field along the axis. The proposed method is a generalization of the well-known method of determining internal stresses in optic fibers based on the integrated photoelasticity measurements. Complete determination of the stress tensor components is performed within the framework of the concept of temperature-dependent residual stresses.

Determination of Stresses from the Stress Trajectory Pattern in a Plane Elastic Domain

Abstract: Identification of stresses acting in a plane (homogeneous and isotropic) elastic domain is performed based on the analysis of principal stress trajectories. This problem, originated in photoelasticity, is now of great importance in geodynamics. Given stress trajectories, in photoelasticity stresses are found by solving a certain boundary value problem. We propose the solution of the problem without appealing to boundary conditions, which is advantageous to geodynamics where boundary stresses are poorly constrained. The analysis of the given stress trajectory pattern is equivalently reduced to the investigation of the argument, , of the complex-valued bi-holomorphic function, D, which represents the stress deviator of the 2D stress tensor. Necessary and sufficient conditions for the stress trajectory pattern to be admissible in elasticity are established. A procedure to obtain a particular solution, D1, is presented. The general solution for D derived from D1 depends on four (if is a harmonic function) or one (otherwise) arbitrary real constants. The procedure is illustrated by model examples for West European and Australian platforms.

Some applications of combined thermoelastic-photoelastic stress analysis

Abstract: Combined thermoelasticity-photoelasticity integrates the use of reflection photoelasticity to determine the difference in the principal stresses with thermoelastic stress analysis to evaluate the sum of the principal stresses, the advantage gained from this combination being the independent separation of the principal stresses at all points in the field of view. The principle has been established in prior research and suitable instrumentation designed and employed on classical test specimens. This work is extended here with the analysis of two more complex components, namely a composite panel subject to a biaxial strain field and a compressor blade excited at approximately 250 Hz. The methodology for applying combined thermoelasticity-photoelasticity under these conditions is described and the difficulties encountered highlighted and discussed. The results demonstrate that the technique and instrumentation are relatively robust and can be successfully used in these demanding applications.

A Non-linear Algorithm of Photoelastic Tomography for the Axisymmetric Problem

Abstract: A non-linear algorithm of photoelastic tomography for the measurement of axisymmetric stress fields has been elaborated. It is free of any assumptions concerning the value of the birefringence or rotation of the principal stress axes along the light rays. The algorithm is based on the measurement of characteristic directions and phase retardation in two parallel sections of the test object. Stress components are presented in the form of power series along the radial coordinate. A differential evolution algorithm has been used for finding the stress field parameters, which fit the measurement data best. Application of the method is illustrated by residual stress measurement in a drinking glass.

Automated Stress Separation Along Stress Trajectories

Abstract: A procedure for the separation of principal stresses in automated photoelasticity is presented. It is based on the integration of indefinite equations of equilibrium along stress trajectories, also known as Lame-Maxwell equations. A new algorithm for precise and reliable stress trajectory calculation, which is an essential feature of the procedure, has also been developed. Automated stress separation is carried out along stress trajectories starting from free boundaries. Experimental tests were performed on a disc in diametral compression and on a ring with internally applied pressure. Full-field principal stress values were obtained and results were compared with those from the theory of elasticity and with those obtained from the classical shear difference method. It was shown that the proposed method is more accurate and less affected by the presence of residual stresses or experimental errors at the boundaries than the shear difference method. In addition, the method requires little human interaction and is therefore well-suited for automated photoelasticity.

Thermal loading of short fibre composites and the induction of residual shear stresses

Abstract: A study has been undertaken to identify the effect of thermal residual stresses on the stress transfer between a short fibre and resin using the photoelastic method. As expected, it was observed that fibre fracture occurred at a higher applied load for the fibre embedded in the thermally (80 °C) cured epoxy matrix than in the room-temperature-cured epoxy. Under plane polarised light bright birefringent patterns were observed in the hot-cured epoxy matrix around the fibre-ends prior to loading. These were not present in the room-temperature-cured epoxy, indicating that thermal residual stresses had been induced during thermal-curing. On loading, the birefringent patterns in the hot-cured matrix at the fibre-ends were almost extinguished but at a particular stress reappeared as a bright region, and increased in intensity on further loading. Using a phase-stepping polariscope, four images of the fibre-ends were captured simultaneously so that detailed contour maps of fringe order could be created. To examine the micromechanical response in the matrices at the interfaces the profile of interfacial shear stress at the fibre-ends was calculated. Under a given external load the shear stress at the interface in the hot-cured matrix was significantly lower than that in the cold-cured epoxy matrix. The thermal load which is applied to a resin on cooling from manufacture requires a shear stress at the interface to put the fibre into compression. At the fibre-ends a residual shear stress of opposite sign (to that induced mechanically) leads to extension of birefringent patterns on loading.

New Initiatives on Comparison of Whole-field Experimental and Numerical Results

Abstract: A simple approach to plot photoelastic fringes in grey scale and also in colour from finite element (FE) results is presented for better recognition and comparison with experiments. This requires proper identification of the plotting variable from FE results. For comparison with transmission photoelasticity, post-processing of principal stress difference is needed and for reflection photoelasticity the principal strain difference is to be used. The importance of the use of appropriate correction factors for comparison with reflection photoelastic results is emphasised. A newer approach to evaluate Rf for complicated geometries is indicated. Plotting of experimental fringes from finite elements is useful not only for validating the numerical model based on experiments but also for validating the experiments. To illustrate this, the problem of an interfacial crack in a bi-material Brazilian disc is discussed.

Expansion stress analysis of ferroconcrete corrosion by digital reflection photoelasticity

Abstract: This study used digital photoelastic birefringent coatings, to measure the expansion stress of a steel bar boundary with concrete caused by corrosion. The distribution of expansion stresses were measured in the initial cracking and partial cracking stages. A numerical model was used to estimate and compare the expansion stress generated by the steel bar during different corrosion and crack stages. The feasibility of applying digital image processing photoelasticity to long-term monitoring and measurement of steel bar corrosion and expansion stress was verified by the consistent results obtained.

Time-resolved photoelasticity imaging of transient stress fields in solids induced by intense laser pulses

Abstract: In this study, the spatial distribution of laser induced transient stress wave have been observed successfully by both shadowgraph and photoelasticity images using transparent materials. It has been found that photoelasticity images of polymer materials, such as epoxy resin, observed in laser irradiation under water provide clear images which would allow quantitative estimation of the magnitude of laser induced stress. Obtained photoelastic images show clear black-and-white patterns from which laser-induced stress distribution and its dynamical change can be deduced. When a metal film was coated on the surface of an epoxy block, obtained images from the sample indicate the interaction of laser with the metal surface. A semi-quantitative estimation of intensity of the laser-surface interaction has been carried out by comparing images to those obtained for designated pulse energies.

Load Extraction from Photoelastic Images Using Neural Networks

Abstract: Photoelastic materials develop colored fringes under white light when subjected to mechanical stresses, which can be viewed through a polariscope. This technique has traditionally been used for stress analysis of loaded components, however, this can also be potentially used in sensing applications where the requirement may be measurement of the stimulating forces causing the generation of fringes. This leads to inverse photoelastic problem where the developed image can be analyzed for the input forces. However, there could be infinite number of possible solutions which cannot be determined by conventional techniques. This paper presents neural networks based approach to solve this problem. Experiments conducted to prove the principle have been verified with theoretical results and finite element analysis of loaded specimens. The developed technique, if generalized, can be implemented for whole-field analysis of the stress patterns involving complex fringes under different loading conditions. This can also provide direct visualization of the stress field, which may find application in a variety of specialized areas including biomedical engineering and robotics.

Photoelastic Stress Analysis on Patient-Specific Anatomical Model of Cerebral Artery

Abstract: In this paper, we propose a novel method to develop biomimetic models using polyurethane elastomer, which has high photoelastic coefficient, and a method to analyze stress states on the model by photoelastic effect. By using this method, stress condition on vascular wall is clearly visualized on vascular fringe as rainbow-colored photoelastic pattern, and stress can be quantitatively measured from that pattern. Ideally, retardation and path length of transmitted light through the polyurethane model should be observed for photoelastic analysis. Our method has capability to observe these two parameters simultaneously. Retardation and path length were determined from RGB value of rainbow-colored photoelastic stress pattern and permeability of light, respectively. The accuracy of stress analysis was evaluated by tensile test using cylindrical polyurethane models. 5.73% error was found in the result of photoelastic stress analysis. Lastly using the model produced by new method, we could analyze stress states quantitatively on the model of cerebral artery. Consequently, our method should be valuable for not only surgical simulations but also hemodynamic studies and pathological studies.