Photoelastic Technique Research Articles

The use of photoelastic techniques in the measurement of die-wall stress in tabletting

Abstract A new technique is described for the observation of the stresses acting during a compaction operation. Pharmaceutical materials have been compressed in a Perspex die and viewed by polarised light. By using ancillary Perspex strain gauges both radial pressure and shear stress at the die wall may be determined from the interference pattern. Although Perspex is not a strong material, pressures up to about 12 tons/in2 could be contained, and a hard tablet extracted from the die. The entire pressing operation is recorded on ciné film for later analysis.

Photoelastic and Finite Element Stress Analysis of the Gap between the L4 and L5 Vertebrae

The purpose of this study was to analyze the stresses on the intervertebral disc between vertebrae L4 and L5 when a compressive load is applied on vertebra L4 using the photoelasticity transmission technique and the finite element method. Nine photoelastic models were used and were divided into three groups. Each group was formed by three models, according to the localization of the sagittal cut on vertebrae L4-L5. Simulation was carried out using a load of 23 N. The fringe orders were assessed by points close to the edge of the intervertebral disc using the Tardy compensation method. The analyses using the photoelasticity technique and the model of the finite elements showed that the stress generated by the vertebrae on the intervertebral disc was higher in the posterolateral region. Thus, this region is more susceptible to pathologies such as hernia and disc degeneration.

Experimental Study of Load Sharing in Roller-Bearing Contact by Caustics and Photoelasticity

Problem statement: In this study a comprehensive methodology for calculating load sharing in Roller-bearing contact was presented bas ed on the experimental stress-optical method of caustics and Photoelasticity. Approach: The theoretical equations describing the geometry o f transmitted caustic in relation to the length of th e contact zone were derived and a simple mathematical set of equations correlating the maxim um diameter of transmitted caustic with the magnitude of load was given.For this contact probl em, the basic theory of photoelasticity was given. Results: The techniques of caustics and photoelasticity were applied on a set of PMMA (Plexiglas) Roller-bearings and on a set of PCBA (L exan) Roller-bearings, respectively. Conclusion: The proposed method of the caustics is a reliable alternative for measuring load distribution in Roller-bearings compared to photoel asticity technique.

FOTOELASTICIMETRÍA: MODERNIZANDO UNA VIEJA TÉCNICA

Among the various strain measurement techniques, photoelasticity is one of the oldest and, in turn, more versatile and powerful. In recent years, important contributions are being carried out, which are transforming this technique in a modern measurement tool: software is being developed to implement it easily and without a highly skilled operator, and standards have been developed for the calibration of equipments, which will allow express measurements with their corresponding uncertainties, making them valid, comparable and reproducible. Moreover, significant progresses are being made in the use of stereolithography for the manufacture of photoelastic models, as well as their use for three-dimensional analysis by tomographic photoelasticity. This article describes all these contributions in order to provide potential users the understanding of the different photoelastic techniques, the extent to which these are developed and commercially available and their application possibilities.

Epoxy Specimen Fabricated by Rapid Prototyping Method for Photo-Elastic Technique

Stress concentration due to the circular hole within the mechanical part is concerned during design phase. The stress concentration of the circular hole within the mechanical mart can easily derived by the photo-elastic technique if the testing specimen is sufficiently developed. The testing specimen using the characteristic of the photo-elastic coefficient, with actual form like the mechanical part by rapid prototyping forming is thus used in this paper. The rapid mold for prototype is made by the RTV-2 silicone gel associated with the vacuum chamber to remove the air within it. Totally twelve specimens divided into two sets, the first set with the machined holes by milling at the centroid of the specimens with diameters of 5, 8, 10, and 12 mm. The second set with the formed holes by molding with the previous diameters. The testing scheme is by way of the simply supported beam where the load is imposed on the middle of the specimen. Results show that the smaller diameter of hole on specimen provides a larger stress concentration effect. When the diameter of the hole is small, the stress concentration effects caused by the machined hole and the forming hole have almost no difference. However, the stress concentration effects of the larger diameter of the machined hole and of the formed hole give significant difference, the stress concentration caused by the machined hole is much greater than one of the formed hole. In this paper, the actual shape of the specimen which is as same as the original mechanical components can be quickly obtained by the rapid prototyping technology for the photoelastic technique to reveal the experimental mechanics of real applications.

Experimental evaluation of stress field around the sharp notches using photoelasticity

A higher order representation of the stress field around the sharp notch has been utilized for calculating notch stress intensity factors (NSIFs) as well as the coefficients of higher order terms by the technique of photoelasticity. Adding the higher order terms to the singular term makes it possible to collect the data points from a larger zone, which helps to simplify the data collection from experiments. Moreover, the effects of higher order terms in the region near the notch tip are taken into account. To utilize the advantages of whole-field photoelasticity and minimize the experimental errors, a large number of data points have been employed and the overdeterministic least squares method combined with the Newton–Raphson method have been used to solve the resulting system of nonlinear equations. The NSIFs for a laboratory specimen called V-notched Brazilian disk (V-BD) were calculated for various notch angles under pure mode I, pure mode II and mixed mode I/II loading conditions. In addition to NSIFs, the coefficient of the first non-singular term of 30° notch was calculated experimentally and the effects of this term on the stress distribution in the vicinity of notch tip were investigated. A good correlation was observed between the experimental results and the numerical results obtained from finite element analysis.

Six image phase stepped photoelasticity for the quantification of the stress field around 25 μm reinforcing fibres

A six image phase stepped photoelastic technique has been applied for the measurement of the interfacial shear stress distribution in the matrix resin along a reinforcing E-glass fibre of ∼25 μm in diameter at its end and at a fibre-break. Contour maps of the isochromatic fringe order and its corresponding isoclinic angle, which are related to the principal stress difference and direction relative to the fibre axis, could be obtained continuously using this photoelastic technique. Thus the actual interfacial shear stress along the fibre can be calculated. Furthermore in the presence of a fibre-break, interfacial debonding and matrix yielding during stress transfer was investigated. The micro-mechanics are shown to differ at a fibre-break compared to a fibre-end.

A study on the development of photoelastic experimental hybrid method for colour isochromatics (I)

Using the photoelastic experimental hybrid method, we can obtain stress intensity factors and separate the stress components near the crack tip of any material under various loading conditions from only isochromatics. However, monochromatic light has, until now been used in the photoelastic experimental hybrid method to produce black and white isochromatics. The existing photoelastic experimental hybrid method using black and white isochromatics requires high order fringes to obtain sufficient experimental data for photoelastic hybrid techniques. If a photoelastic experimental hybrid method for colour isochromatics is developed, isochromatics with a maximum fringe order of 2 will be adequate for gathering the necessary experimental data for stress analyses using photoelastic hybrid techniques. Therefore, in this work, photoelastic experimental hybrid method for colour isochromatics of a crack problem was developed, and its superiority demonstrated by comparing its results with the results from black and white isochromatics. The use of few fringe orders in photoelastic experimental hybrid method for colour isochromatics is an important feature of this method. Because of this feature, this method, specifically the reflection type photoelastic method where low fringe orders are produced compared to the transmission type photoelastic method, can be widely used in stress analysis of real components.

Statistical properties of a 2D granular material subjected to cyclic shear

This work focuses on the evolution of structure and stress for an experimental system of 2D photoelastic particles that is subjected to multiple cycles of pure shear. Throughout this process, we determine the contact network and the contact forces using particle tracking and photoelastic techniques. These data yield the fabric and stress tensors and the distributions of contact forces in the normal and tangential directions. We then find that there is, to a reasonable approximation, a functional relation between the system pressure, P, and the mean contact number, Z. This relationship applies to the shear stress τ, except for the strains in the immediate vicinity of the contact network reversal. By contrast, quantities such as P, τ and Z are strongly hysteretic functions of the strain, e. We find that the distributions of normal and tangential forces, when expressed in terms of the appropriate means, are essentially independent of strain. We close by analyzing a subset of shear data in terms of strong and weak force networks.

Experimental Investigation of Static Interfacial Fracture in Orthotropic Polymer Composite Bimaterials Using Photoelasticity

The effects of static Mode I (opening mode) loading on orthotropic-orthotropic bimaterial interface cracks have been investigated using the experimental technique of transmission photoelasticity. For successful implementation of this experimental technique transparent and birefringent glass fiber reinforced polyester composite materials were developed, enabling the direct observation and recording of photoelastic fringes in the vicinity of the interface crack in the orthotropic bimaterial. It is our belief that this is the first of such experimental investigation. Opening and shearing mode stress-intensity factors and the strain energy release rates have been calculated for various combinations of the bimaterial halves. Results have been verified by mathematically regenerating the observed isochromatic fringe patterns. This study has made it feasible to investigate interfacial fracture in orthotropic-orthotropic bimaterials and orthotropic-isotropic bimaterials using photoelasticity.

Drawing parameters optimization for birefringence reduction in optical fibers

The impact of the drawing process on the stress-induced birefringence in optical fiber is experimentally analyzed. By means of the photoelastic tomographic technique, residual internal stresses are measured in fiber samples drawn under different conditions of temperature, drawing tension and speed. From measured residual stress distributions, the stress-induced birefringence and beatlength are mathematically derived and compared, proving that an effective birefringence reduction can be obtained by a proper choice of drawing parameters.

Experimental and Computational Method for Determining Parameters of Stress-Strain State from the Data Obtainable by Interference Optical Techniques

Experimental and computational method for determining parameters of stress-strain state is proposed which is based on estimation of compliance between the data sets obtained experimentally and the results of numerical calculations of the boundary problems in formulation of which all distinctive features of area geometry, character of the loads being considered and deformation characteristics of materials are taken into account. The procedure proposed was checked at a number of practically important problems. The methods for investigation into stress-strain state (SSS) of structures based on interference optical techniques for registration of strain or displacement fields (photoelastic techniques, holographic interferometry, speckle photography, electronic digital speckle pattern interferometry techniques) provide the acquisition of uninterrupted information at the surface of the object being studied (1,2). These methods being commonly used to solve the problems of mechanics of deformed solids, fracture mechanics, vibration-based diagnostics, structural dynamics etc., have distinct advantages over traditional measuring tools that allows only averaged (accumulated over known time base) values of strains or displacements in one direction and at the point given to be registered. The adequacy of the results of such measurements requires, as a rule, additional substantiation. In modern practice the photoelastic coatings technique and, in recent years, electronic digital speckle pattern interferometry (EDSI) became the most common among these methods; significant advantage of these techniques is the capability of creating compact and reliable measurement instrumentation which can be used effectively under full-scale conditions. Besides, the evolvement of modern computer technologies offers a considerable increase in accuracy of measurement results as well as development of conceptually new methodological approaches to solving mechanics problems. The determination of SSS parameters for the object being studied on the basis of processing of the experimental data obtainable by interference optical techniques includes three main stages: • registration of baseline information on the basis of one or another effect of interference optics (photoregistration of interference patterns, hologram recording, registration of images with speckle pattern etc.); • presentation of obtainable information on electronic media in digital format in terms of phase differences of light waves or (after primary processing) directly through components of three- dimensional vector of body surface point displacements;

Jamming for a 2D granular material

This paper focuses on the nature of jamming, as seen in two-dimensional frictional granular systems consisting of photoelastic particles. The photoelastic technique is unique at this time, in its capability to provide detailed particle-scale information on forces and kinematic quantities such as particle displacements and rotations. These experiments first explore isotropic stress states near point J through measurements of the mean contact number per particle, Z, and the pressure, P as functions of the packing fraction, ϕ. In this case, the experiments show some but not all aspects of jamming, as expected on the basis of simulations and models that typically assume conservative, hence frictionless, forces between particles. Specifically, there is a rapid growth in Z, at a reasonable ϕ which we identify with as ϕc. It is possible to fit Z and P, to power law expressions in ϕ – ϕc above ϕc, and to obtain exponents that are in agreement with simulations and models. However, the experiments differ from theory on several points, as typified by the rounding that is observed in Z and P near ϕc. The application of shear to these same 2D granular systems leads to phenomena that are qualitatively different from the standard picture of jamming. In particular, there is a range of packing fractions below ϕc, where the application of shear strain at constant ϕ leads to jammed stress-anisotropic states, i.e. they have a non-zero shear stress, τ. The application of shear strain to an initially isotropically compressed (hence jammed) state, does not lead to an unjammed state per se. Rather, shear strain at constant ϕ first leads to an increase of both τ and P. Additional strain leads to a succession of jammed states interspersed with relatively localized failures of the force network leading to other stress-anisotropic states that are jammed at typically somewhat lower stress. The locus of jammed states requires a state space that involves not only ϕ and τ, but also P. P, τ, and Z are all hysteretic functions of shear strain for fixed ϕ. However, we find that both P and τ are roughly linear functions of Z for strains large enough to jam the system. This implies that these shear-jammed states satisfy a Coulomb like-relation, |τ| = μP.

Light transmission in thermal stress-induced birefringent medium

In this paper the temperature and thermal stress distribution in a simplified-mode transparent slab is calculated and then variation of refractive index is acquired based on the photo-elastic theory. Using the vector differential algorithm the refractive behavior of rays in such media is simulated, and luminous intensity distribution and shape of fringes at the receiving plane is predicted. The output of the program is in coincidence with the check experiment based on photo-elasticity technique, which indicates the correctness of our analysis method and light transmission calculation method. This research provides a practical means for improving the imaging quality of optical instruments.

Analysis of a complex shape chain plate using Transmission Photoelasticity

Most chains are an assembly [1] of five parts namely, outer plate, inner plate, bush, pin and roller. Two inner plates are press fitted with two bushes to form an inner block assembly. The outer plates are press fitted with pins after keeping the pins through the assembled bushes of the inner block. Roller is a rotating member and placed over the bush during inner block assembly. Inner block assembly is the load transfer member from sprocket tooth. The outer block assembly helps in holding and also to pull the inner block over the sprocket teeth. If a chain length is in odd number of pitches, it requires an offset plate as shown in Figure 1 to connect two ends of the chain together to make chain endless. When the chain is assembled with an offset plate, the chain fatigue life was observed only 20 to 25% of the total life of a chain, assembled without an offset plate. The holes in the offset plate are of the same size as in the outer and inner plates respectively and it is a complex in shape chain plate. A inbuilt thinning zone at the centre of the chain plate as shown in Figure 1 is unavoidable. The stresses and its distribution in this complex shape chain plate geometry play a critical role in the fatigue life performance of a chain assembly. However, it is difficult identify the stress distribution and stress concentration zones precisely using only the conventional industrial friendly tools such as routine quality control test, breaking load test and numerical computations. In this context the transmission photoelastic technique has made it possible to identify the stress distribution, its concentration and also to quantify the stress and strain [2-3] at any point in the chain plate. This paper explains how transmission photoelastic technique is used to estimate the stress distribution and its concentration zones in a complex chain plate when it isloaded.An epoxy chain plate model was made through the casting method using a Perspex mould [2-3]. A loading fixture was used to load the model during stress freezing in the furnace. The integrated fringe pattern of the offset chain plate is shown in Figure 2. Slices are cut in the offset zone to understand the nature of the stress distribution. A Simple photoelastic analysis of this has revealed a wealth of information to take suitable decision on the design of such plates taking into account the constraints in manufacturing.

Photoelastic residual stress measurement in nonaxisymmetric glass containers

Residual stresses in axisymmetric glass containers are mostly measured with integrated photoelasticity. However, many containers, e.g. cosmetic bottles and bottles of expensive alcoholic drinks, often have a non-axisymmetric form. In this paper we consider residual stress measurement in such containers with a modified integrated photoelasticity technique, with the scattered light method and with a simple method of surface stress measurement.

Visualization of force networks in 2D dense granular materials

Dense granular matter is a conglomeration of discrete solid and closely packed particles. As subjected to external loadings, the stress is largely transmitted by heavily stressed chains of particles forming a sparse network of larger contact forces. To understand the structure and evolution of force chains, a photoelastic technique was improved for determining stresses and strains in the assemblies of photoelastic granular disks in this paper. A two-dimensional vertical slab was designed. It contains 7200 polydispersed photoelastic disks and is subjected to a localized probe penetrating at the top of the slab to mimic the cone penetration test. The interparticle contact force distribution was found a peak around the mean value, a roughly exponential tail for greater force and a dip toward zero for smaller force. The force chain network around the probe tip was depicted, and the contact angle distribution of particles in force chains was found to be well aligned in the directions of major principal stress.

Efficacy of photoelasticity in developing whole-field imaging sensors

The paper investigates the efficacy of developing a whole-field imaging sensor based on the principles of photoelasticity. A flatbed sensor has been designed that produces colored fringes under white light when load is applied. These fringes can be analyzed using conventional photoelastic techniques; however, for whole-field sensing applications the loading has to be on the plane rather than the conventional in-plane photoelasticity. This requires some new strategies to be developed to analyze the out of plane loading situations. Further, for such applications low modulus materials are to be used for appreciable in-plane deformation under vertical load and for pronounced photoelastic effect. The paper discusses efficacy of both RGB calibration and phase shifting techniques in sensing applications and proposes a neural network based approach in an expert system environment that can extract load information from photoelastic images in real time. The characteristics of fringe patterns obtained under vertical and shear forces have been studied and the results obtained under these conditions are discussed with their limitations. Finally, a case study has been conducted to analyze a foot image and conclusions drawn from this have been reported together with future research directions.

The analysis of residual stress in glass-to-metal seals for solar receiver tube

The solar receiver tube is a key component in the parabolic trough solar thermal power system. Breakage of glass-to-metal sealing is main cause for damages of receivers in existing power plants. The residual stresses that are generated during the cooling process of the seal can decrease the seal strength and induce the breakage of the glass-to-metal sealing. Residual stresses were simulated by finite element software ANSYS and measured experimentally by photoelastic techniques. In order to lower the seal failure probability, the effects of the component dimensions were analyzed using finite element method. The simulation results agreed with the measurement results. The dangerous tensile stresses occur not only at the glass–metal interface but also on the outer surface of glass tube near the sealing area. When the depth of metal embedded into glass increases, the magnitude of residual stress decreases and the sealing strength will increase. The results of this study have important implications on the optimization of seal configuration in the solar receiver tubes.

On Crack Propagation of I-II Mixed Mode Fracture in Concrete by Means of Photoelastic Coatings

Investigation on crack propagation is important to understand mechanical behaviors and fracture mechanism in concrete. Till now substantial work has been done on crack propagation under mode-I loading, and mixed-mode fracture mechanics problems, however, are more common for actual crack growth. This study aims to systematically investigate the crack initiation, stable propagation and unstable failure of concrete under I-II mixed mode condition by using photoelasticity technique. For this purpose, reflective photoelastic coatings were applied to the surface of the four-point shearing notched concrete specimens with the maximum size of 2500mm×600mm×200mm. The resulting interference fringe on photoelastic coatings was recorded to reflect a complete crack propagation process. Based on the colored strain sequence, the strain profile around the crack tip could be determined and thereafter the load-crack extension curve. Then finite element analysis was performed to simulate the process of crack propagation and corresponding stress intensity factors were solved for every step. Experimental results show obvious stable crack propagation in concrete before unstable failure under I-II loading condition. The conventional failure criterion with the absence of this stable propagation quantity is therefore conservative. Besides, the fracture mode is ultimately I-mode even the specimen is subjected to I-II mixed mode loading condition, that is, the crack will extend in a direction perpendicular to the peak principal stress.