Showing papers on "Photoelasticity published in 2016"

Measurements of flow-induced birefringence in microfluidics

Abstract: In this study, we demonstrate the use of a microscopic circular polariscope to measure the flow-induced birefringence in a microfluidic device that represents the kinematics of fluid motion optically. Unlike the commercial birefringence microscope employed in the previous studies, our approach is able to provide direct measurement of retardance, which quantifies the difference in refractive index of the fluid experienced by the ordinary and extraordinary rays, from one single image frame. This capability facilitates unsteady full-field quantitation of flow-induced birefringence in microfluidics that has never been achieved before. At low flow rates, we find that the value of the retardance is independent of the microfluidic design and proportional to the nominal strain rates. This linearity bridges the measurement of birefringence and the deformation rate in the microflow environment, which yields the stress information of the fluid flow. In addition, the μPIV results confirm that both extensional and shear strain rates contribute to the flow-induced birefringence so that the retardance distribution can be used to represent the field of the principal strain rate in a microfluidic device. The outcome of this study proves that our approach provides a non-invasive method that enables an intuitive full-field representation of stress in the instantaneous flow field in a microfluidic device.

Gradient scattered light method for non-destructive stress profile determination in chemically strengthened glass

Abstract: A new non-destructive gradient scattered light method is presented for micron-scale stress profile measurement in chemically strengthened (chemically tempered, ion exchanged) glass. Direct non-destructive stress measurement in the surface layer (<100 µm) of chemically strengthened glass is reported for the first time. This is accomplished by passing a narrow laser beam through the surface layer of the glass at a considerably large incidence angle of 81.9°. The theory of gradient scattered light method is based on the ray tracing of ordinary and extraordinary rays in chemically strengthened glass and calculating the optical retardation distribution along the curved ray path. The experimental approach relies on recording the scattered light intensity and calculating the optical retardation distribution from it. The stress profile is measured in a chemically strengthened (8 h at 480 °C in a salt mixture of 80 mol% KNO3 and 20 mol% NaNO3) lithium aluminosilicate glass plate to illustrate the capability of the method. Measured surface compressive stress was −1053 MPa and case depth 365 µm. Method is validated with transmission photoelasticity. The method could also be used for stress profile measurement in all transparent flat materials (such as very thin thermally tempered glass slabs or polymers). Additional new applications could be: (1) enhanced version of Bradshaw’s surface layer etching method for stress profile measurement in case of ultra-thin case depths <20 µm; (2) micron-scale non-destructive tomography of layered polymeric gradient-refractive-index materials. The experimental procedure is developed to the level of full automation and the measurement time is less than 10 s.

Methodology of contact stress analysis of gearwheel by means of experimental photoelasticity.

Abstract: The subject of this paper is the analysis of contact stresses that occur between the teeth of a gear. The analysis was carried out by means of reflection photoelasticity, which is an experimental method rarely used in this field. Contact stresses assessed in the experiment are compared with values assessed through an analytical calculation while using the Airy stress function or Hertzian relations.

Evaluation of interfacial strength between fiber and matrix based on cohesive zone modeling

Abstract: This paper presents a measurement technique of interfacial strength considering non-rigid bonding on a fiber/matrix interface modeled as a cohesive surface. By focusing on the stress concentration near a fiber crack obtained from a single-fiber fragmentation test, the stress contours in matrix observed by photoelasticity can be related to the interfacial strength by defining a characteristic length. An equation expressing the relationship between the characteristic length on the stress contour and the interfacial strength was derived, and validated using finite element analysis. The primary advantage of proposed measurement technique is that only a single fiber crack, which usually occurs within elastic deformation of matrix, is required for the evaluation of interfacial strength, whereas saturated fiber fragmentation is necessary in the conventional method. Herein, a sample application was demonstrated using a single carbon fiber and epoxy specimen, and an average interfacial strength of 23.8 MPa was successfully obtained.

Stress distribution in implant-supported prostheses using different connection systems and cantilever lengths: digital photoelasticity.

Abstract: Photoelastic analysis was used to evaluate the biomechanical behaviour of implant-supported, double-screwed crowns with different connection systems and cantilever lengths. Three models were made in PL-2 photoelastic resin and divided into six groups, on the basis of the implant connection system (external hexagon [EH] or Morse taper [MT]), type of abutment (Mini Pilar [Neodent, Curitiba, Parana, Brazil] or "UCLA") and number of crowns in the cantilever (one or two). The implant-prosthesis unit was placed in a circular polariscope. Occlusal surfaces of the crowns were subjected to 100-N loads in the axial and oblique (45°) directions in a universal testing machine (EMIC). Generated stresses were recorded and analysed qualitatively in a graphics program (Adobe Photoshop). Under axial loading, all of the groups had similar numbers of fringes, which were increased when the crowns were subjected to oblique loading. The highest number of fringes was found during oblique loading in the EH + Mini Pilar group. In conclusion, although the type of implant connection system did not have a direct influence on the stress distribution for axial loading, the cantilever length did have a direct influence on stress distribution. Models with two crowns in the cantilever showed more stress, with a greater concentration of force on the cervical part of the implant.

Numerical Modeling of Cooling Stage of Glass Molding Process Assisted by CFD and Measurement of Residual Birefringence

Abstract: Process parameters of Precision Glass Molding (PGM) are often sought by Finite Element (FE) simulation. Mechanical as well as thermal boundary conditions (BCs) are necessary for FE simulation in which mechanical BCs are usually known or easily determinable. However, most of the thermal BCs are generally assumed in the FE simulation as they cannot be measured directly. The focus of this article is to propose a novel method for evaluating the thermal BC of glass–N2 gas. FE simulations as well as thermal cycling experiments are carried out for a glass disk specimen for three different cooling rates. CFD analysis of N2 flow in the PGM machine is performed to understand the heat extraction mechanism. Based on this, adhoc values for equivalent heat transfer coefficient (heqv) are obtained by lumped system analysis. A novel methodology is then proposed for obtaining accurate heqv values by measurement of integrated residual birefringence in glass using digital photoelasticity. FE simulation is repeated for different values of heqv until the integrated birefringence based on simulation matches with that of the experiment. For the same cooling rates, two aspherical glass lenses are molded and their residual birefringence is measured and compared with the glass disk specimen.

A Direct Approach to In-Plane Stress Separation using Photoelastic Ptychography.

Abstract: The elastic properties of materials, either under external load or in a relaxed state, influence their mechanical behaviour. Conventional optical approaches based on techniques such as photoelasticity or thermoelasticity can be used for full-field analysis of the stress distribution within a specimen. The circular polariscope in combination with holographic photoelasticity allows the sum and difference of principal stress components to be determined by exploiting the temporary birefringent properties of materials under load. Phase stepping and interferometric techniques have been proposed as a method for separating the in-plane stress components in two-dimensional photoelasticity experiments. In this paper we describe and demonstrate an alternative approach based on photoelastic ptychography which is able to obtain quantitative stress information from far fewer measurements than is required for interferometric based approaches. The complex light intensity equations based on Jones calculus for this setup are derived. We then apply this approach to the problem of a disc under diametrical compression. The experimental results are validated against the analytical solution derived by Hertz for the theoretical displacement fields for an elastic disc subject to point loading.

A Revisit to the Frozen Stress Phenomena in Photoelasticity

Abstract: Stress freezing in photoelasticity is regularly employed for 3-D elastic stress analysis of geometrically complex models, wherein the models under load are soaked at the stress freezing temperature and slowly cooled to room temperature. During this process, a model may distort or fail undergoing large deformations at the stress freezing temperature owing to thermo-mechanical interactions. The contribution of thermal deformation to the model distortions is neglected in the available literature of stress freezing. This aspect of stress freezing is investigated in this paper, wherein Digital Image Correlation (DIC) is used to study the strain behavior during a complete stress freezing cycle for an epoxy made of CY230 resin cured with HY951 hardener. The results show that the thermal contributions to the model distortions at the critical temperature must be taken care of to estimate the failure margins. The distortions and failures would mainly depend upon the thermal and mechanical response of the model material and the complexity of the model.

Photoelastic sphenoscopic analysis of crystals

Abstract: Birefringent crystals are at the basis of various devices used in many fields, from high energy physics to biomedical imaging for cancer detection. Since crystals are the main elements of those devices, a great attention is paid on their quality and properties. Here, we present a methodology for the photoelastic analysis of birefringent crystals, based on a modified polariscope. Polariscopes using conoscopic observation are used to evaluate crystals residual stresses in a precise but time consuming way; in our methodology, the light beam shape, which impinges on the crystal surface, has been changed from a solid cone (conoscopy) to a wedge (sphenoscopy). Since the polarized and coherent light is focused on a line rather than on a spot, this allows a faster analysis which leads to the observation, at a glance, of a spatial distribution of stress along a line. Three samples of lead tungstate crystals have been observed using this technique, and the obtained results are compared with the conoscopic observation. The samples have been tested both in unloaded condition and in a loaded configuration induced by means of a four points bending device, which allows to induce a known stress distribution in the crystal. The obtained results confirm, in a reliable manner, the sensitivity of the methodology to the crystal structure and stress.

Numerical method to digital photoelasticity using plane polariscope.

Abstract: This research aims to find a new way to get the intensity equations for the phase-shifting model in digital photoelasticity. The procedure is based on the rotation of the analyzer itself. From the intensity equations, the isoclinic and isochromatic equations parameters are deduced by applying a new numerical technique. This approach can be done to calculate how many images allow the resolution of the polariscope. Each image indicates the stress forces in the object. In this study the plane polariscope was used. The amount of images will determinate the number of errors and uncertainties of the study, due to the observation that the veracity of the equations increases considerably with a large amout of images. Several analyses are performed with different amounts of photographic images. The results showed the possibility to measure stress forces with high precision using plane polariscopes.

Image Processing Algorithm for Fringe Analysis in Photoelasticity

Abstract: Photoelastic fringe patterns recognition are carried out using a circular polariscope on a four point beam loaded by pure bending and they are processed with image processing algorithm in a personal computer. The aim of the present paper stress analysis of the photoelastic fringe in a rectangular cross section beam using digital image processing technology. An algorithm is developed to visualize the stress pattern acting in photoelastic models using fringe sharpening in conjunction with MATLAB software. Photoelasicity is a non-destructive experimental tool can be used for visual representation of stress patterns allowing for stress analysis. The present paper presents an image processing algorithm executed in a personal computer and obtained stress pattern recognition using digital image processing with histogram.

Effects of liquid properties on the dynamics of under-liquid laser-induced shock process

Abstract: We compared the shock processes induced when focusing a single laser pulse (1064 nm, FWHM = 13 ns) onto the surface of epoxy resin blocks immersed in glycerol, water, liquid paraffin, and silicone oils. A custom-designed time-resolved photoelasticity imaging technique was applied to observe the strength of stress wave induced inside the solid target and the propagation of shock waves in the liquid with time resolution of nanoseconds. We demonstrated that the shock impedance of the liquid caused a noticeable effect on the strength of laser-induced stress wave: Ablation in the liquid with a higher shock impedance resulted in a stronger stress. By using glycerol instead of water as the confining medium, the pulse energy required to induce a certain level of stress was reduced by about 20 %. The dynamical behaviors of the main shock wave and the reflected wave in inverted V-shape in each liquid are also discussed in details.

Application of principal component analysis in phase-shifting photoelasticity.

Abstract: Principal component analysis phase shifting (PCA) is a useful tool for fringe pattern demodulation in phase shifting interferometry. The PCA has no restrictions on background intensity or fringe modulation, and it is a self-calibrating phase sampling algorithm (PSA). Moreover, the technique is well suited for analyzing arbitrary sets of phase-shifted interferograms due to its low computational cost. In this work, we have adapted the standard phase shifting algorithm based on the PCA to the particular case of photoelastic fringe patterns. Compared with conventional PSAs used in photoelasticity, the PCA method does not need calibrated phase steps and, given that it can deal with an arbitrary number of images, it presents good noise rejection properties, even for complicated cases such as low order isochromatic photoelastic patterns.

High stress concentration analysis using RGB intensity changes in dynamic photoelasticity videos

Abstract: Photoelasticity images and videos contain information about the stress distribution in birefringent materials. Yet, its processing has difficulties in regions with high-stress concentrations. Making the material fail under load application. This paper identifies such regions by analyzing the intensity changes contained in photoelasticity videos. The evaluation is by the summa of each Euclidean distance between a frame, and the previous. The summa produces a grayscale image. Which is segmented using a thresholding process based on the range of intensity changes. It allowed separating regions with higher summa of color changes. That in this case, it corresponds to the zones with high-stress concentrations reported for a disc under compression. This technique allowed identifying high-stress concentration avoiding the sub-processes used in traditional photoelasticity.

Photoelastic Investigation of Interaction Between Matrix Crack and Different Shapes of Inclusions

Abstract: The interaction between different shapes of inclusions and the mode I matrix crack was studied experimentally using the photoelasticity method. First, the stress intensity factor (SIF) at the matrix crack tip in the neighborhood of the inclusions was derived based on transformation toughening theory and the Eshelby equivalent inclusion method. Then, photoelastic experiments were conducted using specimens with different shapes of inclusions, and the typical isochromatic fringe patterns around the crack tip near the inclusions were obtained. Finally, a numerical simulation was conducted for the same problem using ABAQUS, and the stress intensity factors computed from the finite-element method were compared with the experimental results. The results show that the stress intensity factors extracted from isochromatic fringe patterns agree well with the numerical and theoretical results.

Application of the harmonic star method in photoelastic separation of principal stresses.

Abstract: This paper deals with automation of an experiment process by means of photoelasticity on the condition that only information which was previously collected by identification of isochromatic fringes is used. The analysis of this issue pointed to Poisson's and Laplace's equations. These equations were solved through the harmonic star method. The automation of an experiment was based on the development of a noncommercial program for processing of data which had been collected during photoelastic measurements. In addition, the program was used for verification of these measurements and comparison of measurement outcomes with results which were gained by the finite element method (FEM). Such a program enabled us to measure stresses in a certain point of the examined surface and, at the same time, separate these stresses, that is, determine the magnitude of individual stresses. A data transfer medium-a camera-was used to transfer the picture of isochromatic fringes directly into a computer. The chosen procedure of solving and evaluating the experiment provided us with reliable outcomes comparable to results gained by FEM.

Time-space analysis in photoelasticity images using recurrent neural networks to detect zones with stress concentration

Abstract: Digital photoelasticity is based on image analysis techniques to describe the stress distribution in birefringent materials subjected to mechanical loads. However, optical assemblies for capturing the images, the steps to extract the information, and the ambiguities of the results limit the analysis in zones with stress concentrations. These zones contain stress values that could produce a failure, making important their identification. This paper identifies zones with stress concentration in a sequence of photoelasticity images, which was captured from a circular disc under diametral compression. The capturing process was developed assembling a plane polariscope around the disc, and a digital camera stored the temporal fringe colors generated during the load application. Stress concentration zones were identified modeling the temporal intensities captured by every pixel contained into the sequence. In this case, an Elman artificial recurrent neural network was trained to model the temporal intensities. Pixel positions near to the stress concentration zones trained different network parameters in comparison with pixel positions belonging to zones of lower stress concentration.

Nonlinear propagation of stress waves during high speed cutting

Abstract: Stress waves induced by high speed cutting (HSC) were demonstrated visually, and the dependence of their nonlinear propagation characteristics on cutting speed was studied. The time-resolved photoelasticity imaging technique in the bright-field mode was used to observe stress waves in the workpiece, and the obtained photoelastic images were evaluated semi-quantitatively. The experimental results were quantitatively reproduced via the lattice model, which helped explain our observations by analyzing the superposition of stress waves. According to the further simulation, we find that as the cutting speed increases, the stress intensity of the workpiece near the cutting tool is not in a linear enhancement process, with strong distortion of stress field under the superposition of different stress wave components. These help us have a deep understanding about the HSC mechanism under stress waves' effects.

Full Field Evaluation of the Stress Tensor Components in 2D Photoelasticity Via Computer Software

Abstract: Abstract The aim of this work is to find out the components of stress tensor in plane specimens. For this purpose the photoelasticity methodology is used. In order to make this technique more comfortable for use, there was developed an algorithm in MATLAB program. The results are compared with numerical solution. The main advantages of the developed algorithm are the speed and the capabilities to extend to analyze the plastic deformation and strain conditions in the material during forming processes.