Showing papers on "Photoelasticity published in 2015"

Vertical-cavity surface-emitting lasers with birefringence splitting above 250 GHz

Abstract: Using the elasto-optic effect, increase of the frequency difference between the two orthogonally polarised modes, the so-called birefringence splitting, in standard single-mode oxide-confined AlGaAs-based vertical-cavity surface-emitting lasers is achieved to values beyond 250 GHz. A large birefringence is required for the generation of ultra-fast polarisation oscillations for potential future high-speed communication applications.

A photoelastic measurement system for residual stress analysis in scintillating crystals by conoscopic imaging

Abstract: The assessment of the stress state of scintillating crystals is an important issue for producers as well as users of such materials, because residual stress may arise during growth process. In this paper, a measurement system, based on the use of a photoelastic, conoscopic optical setup, is proposed for the assessment of stress state in scintillating crystals. Local stress values can be measured on the crystal in order to observe their spatial distribution. With the proposed system, it is possible to vary the dimensions of the inspected measurement volume. It has been validated with reference to a known stress state induced in a birefringent crystal sample and it has been tested for the case of loaded and unloaded samples, showing sub-millimetric spatial resolution and stress uncertainty ≤0.25 MPa. The proposed measurement system is a valid method for the inspection of scintillating crystals required by producers and users of such materials.

Review of photoelastic image analysis applied to structural birefringent materials: glass and polymers

Abstract: Photoelasticity is particularly suitable for the analysis of the stress state in structural materials that are transparent and birefringent. Some techniques of digital photoelasticity (phase shifting and RGB) are applied to the analysis of stress field in two classes of structural materials. The first one consists of tempered glasses, such as those used in the automotive and architectural fields. The second one consists of thermoset polymers, typically used as matrices in fiber reinforced plastic structural composites. The birefringence of such resins is, in particular, exploited to investigate the development of swelling stresses and changes in fracture toughness as induced by water uptake aging.

Inspection of birefringent media by photoelasticity: from diffuse light polariscope to laser conoscopic technique

Abstract: A reliable and noninvasive measurement method for the inspection of naturally birefringent transparent media is presented in this paper. It allows us to achieve a spatially resolved analysis of the stress state of birefringent materials. The developed system is based on photoelasticity and exploits a controlled laser conoscopy technique and a scanning system to perform local measurements in the volume dimensions of the media, which can be displaced over a grid of points. The configuration of the proposed laser conoscopic system is presented, and a dedicated algorithm, designed to perform digital analysis of the fringe patterns, is also described. The design and the realization of the system are discussed, as well as the advantages of the proposed system over the classic diffuse light polariscope technique. The method has been experimentally validated through laboratory tests on birefringent samples with known induced stress. The system has demonstrated its sensitivity to stress and its capability to achieve a spatial resolution on the order of 0.1 mm to resolve stress gradients (uncertainty on the stress amplitude of ± 0.1 MPa ).

New initiatives in single-colour image-based fringe order estimation in digital photoelasticity:

Abstract: The use of single-colour image for determining the fringe orders in digital photoelasticity has gained importance in recent years, and in this, the fringe orders are obtained by comparing the colou

Photoelastic analysis of edge residual stresses in glass by the automated tint plate method

Abstract: The analysis of residual stress in glass is usually carried out by means of photoelastic methods. This article considers the automation of the white light photoelastic method based on the use of a full-wave plate placed behind the glass plate. In particular, the method in based on the use of RGB photoelasticity in white light in conjunction with a full wave plate. The proposed method have been applied to the analysis of membrane residual stresses in tempered glass, showing that it can effectively replace manual methods of photoelastic analysis of residual stresses in glass when a low photoelastic retardation is present.

A CAE approach for the stress analysis of gear models by 3D digital photoelasticity

Abstract: The use of numerical and experimental methods to determine the stress field of mechanical components is well known. In particular, 3D photoelasticity can be considered the only experimental technique for the complete stress state evaluation of 3D components. The advent of rapid prototyping techniques has allowed the manufacturing of complex models in a matter of hours by using birifrangent materials. The present paper is focused on the description of a Computer Aided Engineering (CAE) approach which combines Finite Element (FE) simulations and automatic photoelastic investigations for the stress analysis of face gear drives, made by stereolithography. Computer Aided Design (CAD) geometries, used to manufacture the stereolithographic models, are directly used to perform FE analyses, thus allowing the stress analysis process to become simpler and easier. The substantial agreement observed between experimental and numerical results proved the potentialities of the adopted approach and the usefulness of FE simulations to optimize photoelastic analyses through cost- and time-effective experiments even for complex 3D shapes.

Influence of crown-to-implant ratio on stress around single short-wide implants: a photoelastic stress analysis.

Abstract: Purpose The aim of this study was to evaluate the photoelastic fringe patterns around two short-wide implants supporting single crowns with different crown-to-implant (C/I) ratios. Materials and Methods External hexagon (EH) cylindrical implants (5 × 7 mm) or Morse Taper (MT) conical implants (5 × 6 mm) were embedded individually into photoelastic resin blocks. Each implant received a single metal-ceramic crown, with a C/I ratio of 1:1 or 2:1 (n = 10). Each set was positioned in a polariscope and submitted to a 0.5 kgf compressive load, applied axially or obliquely (30°). The polariscope images were digitally recorded, and based on isoclinal and isochromatic fringes, the shear stress was calculated at 5 predetermined points around each implant. Data were analyzed by two-way ANOVA (α = 0.05). Results Under axial loading, the stress was concentrated at the crestal region, and there were no differences between C/I ratio or implant types. In contrast, under oblique loading, EH implants showed lower stress values than the MT group and the 2:1 C/I ratio showed higher stress concentration for both implant types (p < 0.05). Moreover, MT implants showed stress distribution through a higher area than the EH implant did, with a tendency to direct the stress toward the implant's apex under oblique loading. Conclusion MT conical short-wide implants showed higher stress values that were distributed through a higher area directed to the implant apex. The C/I ratio influences the stress distribution only under oblique loading.

Digital image analysis around isotropic points for photoelastic pattern recognition

Abstract: Fringe tracking and fringe order assignment have become the central topics of current research in digital photoelasticity. Isotropic points (IPs) appearing in low fringe order zones are often either overlooked or entirely missed in conventional as well as digital photoelasticity. We aim to highlight image processing for characterizing IPs in an isochromatic fringe field. By resorting to a global analytical solution of a circular disk, sensitivity of IPs to small changes in far-field loading on the disk is highlighted. A local theory supplements the global closed-form solutions of three-, four-, and six-point loading configurations of circular disk. The local theoretical concepts developed in this paper are demonstrated through digital image analysis of isochromatics in circular disks subjected to three-and four-point loads. (C) 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)

An experimentally validated contour method/eigenstrains hybrid model to incorporate residual stresses in glass structural designs:

Abstract: Contour method–based finite element models together with knowledge of the surface deformation resulted from the stress relaxation along a newly cut-plane were used to construct the residual stresses in commercially available float glass. The results show that the residual stress depth profile of float glass is parabolic. The constructed residual stress profiles, validated to some extent against results of scattered light polariscope experiments, were then used to establish the misfit strains (i.e. eigenstrains) existed in the original glass specimens. It is shown that despite the modelling uncertainty of the contour method and the limitations associated with the scattered light polariscope measurements, the eigenstrain depth profile in a given float glass specimen can be determined to an acceptable accuracy. The article shows that once the underlying eigenstrain distribution in a given thickness of glass has been determined, the complete residual stress distribution can simply be determined by incorporati...

Robust multidirectional smoothing of isoclinic parameter in digital photoelasticity

Abstract: In digital photoelasticity, one gets full-field information about the difference in principal stresses and their orientations by postprocessing the intensity data. Isoclinic data obtained using phase shifting techniques have inherent noise. The higher the external load, the higher the noise. Further, noise removal becomes complex if the isoclinic data has π-jumps, isotropic points, etc. Initially, the origin of the noise in isoclinic data obtained using phase shifting techniques is discussed. An explanation for the presence of excessive noise in a circular polariscope-based algorithm is provided. Three methods for noise removal of isoclinic data calculated using a plane polariscope are proposed using an outlier smoothing algorithm. The first two use quality and standard deviation measures to identify the noise-free pixel from each individual scan. The last method involves progressive multidirectional smoothing. The effectiveness of the proposed smoothing schemes is demonstrated using the benchmark problem of a circular disc under diametral compression. Multidirectional progressive smoothing is found to be effective in removing the noise at lower as well as at higher loads. Finally, this scheme is used to smooth isoclinic data in two other problems, one that has a π-jump and the other that has both an isotropic point and a π-jump.

Residual Stress Analysis of Commercial Float Glass using Digital Photoelasticity

Abstract: Residual stresses are generated in float glass at the time of manufacturing due to thermal gradients created during the cooling process. The quantification of these residual stresses is important in glass industries as they affect their cutting quality. Photoelasticity can be used for residual stress analysis of glasses, as glass exhibits stress-induced birefringence. In this study, a methodology involving carrier fringes in conjunction with digital photoelasticity is used to quantify the residual stress in float glass. The results are verified by six-step phase-shifting technique (a subset of ten-step phase-shifting method) using an automatic polariscope. Finally, to demonstrate the utility of the proposed method, the residual stress is measured in float glasses of different thicknesses. A method for approximate estimation of residual stress which does not require sophisticated digital image acquisition and processing systems is also reported.

A New Approach to Edge Stress Measurement in Tempered Glass Panels

Abstract: An important characteristic of glass panels is residual stress, especially stress at the surface. To measure this stress different devices have been developed, which use either the tunneling effect in light propagation near the surface or the scattered light method. Both devices are rather expensive. At the same time, many glass manufacturers use measurement of the stress at the edge of the panel. This measurement can be carried out with traditional methods of 2D photoelasticity. However, until now the data of edge stress measurement has not been used to the full extent. The edge stress itself is an estimation of the degree of temper of the panel. Besides, the region, where average through thickness stress is tensile, can be found. However, real stress distribution in the panel can not be determined. In this paper we show that since the edge stress and surface stress are in correlation, the edge stress permits to estimate also the value of the surface stress. Experimentally and with mathematical modelling of the tempering process it has been shown that practically edge stress equals the surface stress. Knowing the surface stress, it is possible to determine distribution of the parabolic tempering stresses through the panel thickness. That permits also to determine real stress field in the so-called tensile stress area. Thus complete analysis of residual stresses near the edge of the panel can be carried out.

Analysis of stress distribution in dry masonry walls using three fringe photoelasticity

Abstract: Masonry being a composite system is very complex to study with analytical methods alone. Quantitative digital photoelastic analysis using Three Fringe Photoelasticity (TFP) is carried out on a model of a dry masonry wall with bricks made up of epoxy, loaded with a concentrated load. The result obtained provides insight into the micromechanics of force transmission and interactions between bricks in masonry. The result shows that force transmission through the wall occurs at discrete points. A tree like hierarchical pattern of stress flow is observed. It is also observed that the stress percolation results in the existence of stress-free zones in the model domain.

Birefringence characterization of injection molded microplates

Abstract: Birefringence affects the quality of image analysis in injection molded micro-plates. Depending upon their manufacturing / production processes and the type of material, different plates exhibit varying amounts of birefringence. This birefringence is attributed to residual stress generated during the molding process. Polarimeter is the standard tool for birefringence distribution visualization and quantification. Broad chemical resistance and high mechanical stability of the plates are the desirable properties that can be characterized by birefringence measurement. Birefringence, expressed in nm/cm is light retardance (nm) after passing through a sample with certain thickness (cm). Low or uniform birefringence plates provide high-resolution demonstrating higher performance, hence suitable for bio-chemical analysis.

Original computer method for the experimental data processing in photoelasticity

Abstract: Optical methods in experimental mechanics are important because their results are accurate and they may be used for both full field interpretation and analysis of the local rapid variation of the stresses produced by the stress concentrators. Researchers conceived several graphical, analytical and numerical methods for the experimental data reduction. The paper presents an original computer method employed to compute the analytic functions of the isostatics, using the pattern of isoclinics of a photoelastic model or coating. The resulting software instrument may be included in hybrid models consisting of analytical, numerical and experimental studies. The computer-based integration of the results of these studies offers a higher level of understanding of the phenomena. A thorough examination of the sources of inaccuracy of this computer based numerical method was done and the conclusions were tested using the original computer code which implements the algorithm.

Experimental evaluation of plasticity-induced crack shielding from isochromatic data

Abstract: The plasticity-induced crack shielding effect is evaluated during fatigue crack growth using transmission photoelasticity. The proposed methodology is based on the evaluation of the stress intensity factors calculated from the analysis of the isochromatic fringe patterns observed at the vicinity of a crack tip. Four different mathematical models describing the crack tip stress fields (namely models based on Westergaard’s, Williams’s, and Muskhelishvili’s equations and a new model called Christopher–James–Patterson) have been employed. Thus, a comparative study to evaluate which of the models is more suitable for fatigue crack shielding evaluation has been performed. A set of fatigue experiments on polycarbonate middle-tension specimens at different R-ratios have been conducted. Experimental results reveal the presence of plasticity-induced crack shielding on growing fatigue cracks for specimens tested at a low R-ratio. In addition, a retardation effect on the fatigue growth rate has been observed due to the shielding effect induced by the plasticity generated both at the crack tip and along the crack flanks. All these results highlight the enormous potential of transmission photoelasticity for the evaluation of plasticity-induced crack shielding on growing fatigue cracks.

Experimental study on the matrix crack-fiber interaction using photoelasticity method

Abstract: The stress field of the matrix crack perpendicular to fiber bundles was studied by the method of photoelasticity. First, the stress intensity factor at the matrix crack tip in the neighbor of the fiber bundles are derived based on transformation toughening theory and Eshelby equivalent inclusion method. Then, a photoelastic experiment was conducted using a specimen with a fiber bundle in the center as an inclusion, and the typical isochromatic fringe pattern around the crack tip near the fiber bundle was obtained. Finally a numerical simulation was carried out based on the plane stress assumption, and the stress intensity factor measured using photoelasticity was compared with the numerical and theoretical results. These results will play an important role for evaluating the mechanism of crack-fiber interaction in fiber reinforced composites.

Special Section Guest Editorial: Digital Photoelasticity: Advancements and Applications

Abstract: This PDF file contains the editorial “Special Section Guest Editorial: Digital Photoelasticity: Advancements and Applications” for OE Vol. 54 Issue 08

Stress measurement method, member for stress measurement, and set for stress measurement

Abstract: The present invention provides a stress measurement method, a member for stress measurement comprising a linearly polarizing film and a phase difference film, and a device for stress measurement comprising the member for stress measurement and a stress display member comprising a photoelastic layer. The stress measurement method comprises: a step of irradiating an article that has photoelasticity and includes an object to be measured with light transmitting in sequence through a linearly polarizing film and a phase difference film; and a step of detecting reflected light derived from the irradiating light and reflected from the article and in sequence through the phase difference film and the linearly polarizing film, wherein the in-plane retardation Re(550) of the phase difference film with respect to light with a wavelength of 550 nm satisfies 100 nm ≤ Re(550 nm) ≤ 700 nm, and the in-plane retardation Re(450) of the phase difference film with respect to light with a wavelength of 450 nm satisfies Re(450)/Re(550) ≥ 0.9.

Quantitative measurement of CO2 laser-induced residual stress in fused silica optics

Abstract: The residual stress field of fused silica induced by continuous wave CO2 laser irradiation is investigated with specific photoelastic methods. Both hoop stress and axial stress in the irradiated zone are measured quantitatively. For the hoop stress along the radial direction, the maximum phase retardance of 30 nm appears at the boundary of the laser distorted zone (680-μm distance to center), and the phase retardance decreases rapidly and linearly inward, and decreases slowly and exponentially outward. For the axial stress, tensile stress lies in a thin surface layer (<280 μm) and compressive stress lies just below the tensile region. Both tensile and compressive stresses increase first and then decrease along the depth direction. The maximum phase retardance induced by axial tensile stress is 150 nm, and the maximum phase retardance caused by axial compression stress is about 75 nm. In addition, the relationship between the maximum axial stress and the deformation height of the laser irradiated zone is also discussed.

Photoelasticity Raycasting

Abstract: We present a novel physically-based method to visualize stress tensor fields. By incorporating photoelasticity into traditional raycasting and extending it with reflection and refraction, taking into account polarization, we obtain the virtual counterpart to traditional experimental polariscopes. This allows us to provide photoelastic analysis of stress tensor fields in arbitrary domains. In our model, the optical material properties, such as stress-optic coefficient and refractive index, can either be chosen in compliance with the subject under investigation, or, in case of stress problems that do not model optical properties or that are not transparent, be chosen according to known or even new transparent materials. This enables direct application of established polariscope methodology together with respective interpretation. Using a GPU-based implementation, we compare our technique to experimental data, and demonstrate its utility with several simulated datasets.