Photoelastic Method Research Articles

Piezooptical properties of ice

In connection with the use of ice as a building material, it is interesting to study its strength characteristics. To increase the strength of ice, the ice matrix was reinforced by adding various materials of natural origin. The greatest significance in this study is the assessment of the stress-strain state of ice composites. The possibility of the photoelasticity method applimentation to obtain stress fields in ice composites was considered. Due to the lack of data about the piezo-optical properties of ice, we performed numerous studies in this direction. As a result of research, it was found that it is not possible to quantify the piezo-optical properties of ice, because ice has a low optical sensitivity, high brittleness, and the ability to melt at room temperature. It is possible to qualitatively characterize stress fields, identify stress concentrators, and describe the effect of reinforcing materials on the stress state of the ice matrix using the photoelasticity method. The use of ice-composite materials in the Arctic is a promising direction.

Features of the stress-strain state in the corner zones of structures

Corner regions of structures are characterized by a complex stress-strain state (SSS), caused by both the shape of the boundary or the “geometric factor”, and the finite rupture of the specified forced deformations, mechanical properties that go to the contact surface of the elements of composite structures. The relevance of the work consists in the analysis of local SSS in the corner zone of the region boundary with various study approaches: theoretical, experimental. For the theoretical analysis of the SSS of the region, we consider the resolving system of equations of the problem of the theory of elasticity in the zone of the cutout of the boundary and elements of the similarity theory; the photoelasticity method is used for experimental studies. An experimental solution was obtained on polymer composite models with an angle cut of the boundary under the action of forced deformations in one of the regions. The analysis of possibilities of theoretical, experimental approaches of SSS studies in zones with irregular point of border allows to reveal zones of applicability of solutions of linear elasticity problem.

Search for plastics with piezo optic properties

The work is devoted to the search for plastics used in 3D printing, which have pronounced piezo optic properties. A review of the works devoted to the study of various characteristics of plastics is performed. It is established that there is currently no data on the piezoelectric properties of consumables for 3D printing. The various brands of plastics widely represented on the market are analyzed. Between them, those that transmit light well were selected. Experimental studies of widely available transparent plastics were performed. The photoelasticity method is used in the work. Several grades of plastics with birefringence have been identified. In the future, it is planned to use these materials for printing models of nodes of spatial structures on a 3D printer in order to obtain stress fields in them.

Fringe modelling and Photoelastic stress Measurement method in tetragonal PWO observed in the plane normal to a crystallographic a-axis

Scintillator crystals like the tetragonal PbWO4 (PWO) are largely used in calorimeters, for instance the CMS at CERN and in the PANDA calorimeter project. The performances of those calorimeters are directly linked to crystals homogeneity and quality, which can be assessed using indicators such as residual stress state condition. Since it is fundamental to develop and refine new mathematical models for the structural characterization and stress analysis of the crystalline materials, in this paper we present a novel approach aimed at the evaluation of stress or defect distribution and inhomogeneity, by analysing images obtained with photoelastic conoscopic system. A new model is proposed for the correct mathematical interpretation of the fringe pattern obtained by investigating the crystal in direction orthogonal to the uniaxial optical axis and parallel to the crystallographic a-axis. The model will be validated against experimentally acquired fringe patterns. A comparison with a simplified procedure, which operatively, reduces the computational effort, will be detailed. After a complete description of the fringe pattern, a linearized model is proposed in such a way that we obtain a simple stress intensity evaluation via experimentally easy measurable parameters, thus realizing a model based measurement of stress state. The results make the proposed procedure reliable, and useful also for other crystals symmetries.

Experimental and Numerical Study on Crack-Arrest Behaviours of Rock-Like SCDC Specimens

Studying rock crack-arrest toughness, which determines the arrest behaviours in rock materials with cracks, is important to prevent further damage to rock mass. To investigate the crack-arrest behaviours both qualitatively and quantitatively, dynamic impact experiments (using caustics and photoelastic methods) were performed on rock-like (epoxy resin) single cleavage drilled compression (SCDC) specimens to study the crack-arrest behaviours and the microscopic fracture characteristics. The crack-arrest toughness of the SCDC specimen was obtained and the extended finite element method (XFEM) in ABAQUS software was used to validate the experimental results. The results show that under the interaction between reflected compression and tensile waves, the running crack experiences two crack-arrest periods before the final arrest, and the crack-arrest toughness stabilizes at a value of 0.669 MPa m1/2. Moreover, the peak propagation velocity in each crack initiation process gradually decreases with the development of the crack path. Additionally, the observations by scanning electron microscopy (SEM) and photoelastic methods show that the roughness of the fracture surface is influenced by the crack-arrest behaviours, causing the spalling fragments as well as wing micro-cracks in the fracture surface.

A Comparison of Photoelastic and Finite Elements Analysis in Internal Connection and Bone Level Dental Implants

This study is a contribution to our understanding of the mechanical behaviour of dental implants through the use of the finite element and the photoelastic methods. Two internal connection and bone level dental implants with different design have been analysed (M-12 by Oxtein S.L., Zaragoza, Spain, and ASTRA, from Dentsply Sirona, Charlotte, NC, USA), evaluating the stress distribution produced by axial stresses and a comparison has been established between them, as well as between the two methods used, in order to validate the adopted hypotheses and correlate the numerical modelling performed with experimental tests. To load the implant in laboratory testing, a column was placed, such that the loading point was about 9.3 mm from the upper free surface of the resin plate. This column connects the implant with the weights used to define the test load. In turn, support for both plates was achieved by two 6 mm bolts 130 mm apart and located on a parallel line with the resin (flush with the maximum level of the implant), at a depth of 90 mm. The results obtained with both methods used were similar enough. The comparison of results is fundamentally visual, but ensures that, at least in the range of forces used, both methods are similar. Therefore, the photoelastic method can be used to confirm in a real way the virtual conditions of the finite element models, with the implications in the investigation of dental implants that this entails.

Mesoscopic structure characterization of plugging zone for lost circulation control in fractured reservoirs based on photoelastic experiment

Drilling fluid loss into formation fractures is one of the most common and costly problems encountered during the exploration and development of oil and gas resources. The stability of fracture plugging zone formed by lost circulation materials has a great impact on the lost circulation control effect. Mesoscale structure stability of force chain, which is the way to transferring contact force between particles, determines the strength of the macroscale plugging zone. In the current paper, photoelastic method is introduced and photoelastic experimental system is developed to characterize the mesoscopic structure evolution of plugging zone under shear. The key parameters for the mesoscopic structure characterization of fracture plugging zone are proposed, including the proportion, distribution, elastic energy, and angle of the strong force chain. Experimental results show that the mesoscopic structure of plugging zone experiences continuous formation and destruction during the shear process. Initially, the distribution of the mesoscopic force chain under a vertical load presents a vertical tree branch structure. Then, the proportion of force chains in the horizontal direction increases and form a staggered network structure after a horizontal shear force is continuously applied. During the shear process of plugging zone, the mesoscopic force chain network experiences rearrangement and structural failure. The distribution density of elastic energy in the particles transferring strong force is reduced. Based on the developed photoelastic experimental system, the mesoscopic structure evolution of the plugging zone is accurately described. It can be used to guide the selection of the lost circulation materials and optimization of the loss control formula.

One example of image digital processing of experimental data obtained by the method of photoelasticity

The article is devoted to the digital processing interferograms (isochromatic fringe patterns) obtained by the photoelasticity method. The application to interpretate the isochromatic fringe patterns allows us to automate image processing, gets to avoid the routine and time-consuming work. The features of the developed software package are described in detail by means the problem of stretching plate with two symmetrical edge notches. The application’s algorithm includes following main steps: image pre-processing, localization interference fringe, fringe tracing. The application creates a text file containing all the necessary data to further determine the stress-strain state (isochromatic fringe’s number and locus of fringes).

A new integral equation method for calculating interacting stress intensity factor of multiple crack-hole problem

In this paper, a new integral equation method is proposed based on elementary solutions and superposition principle to study multiple crack-hole problem under both remote and surface stresses. Several typical crack-hole problems are solved in order to compare the calculation results of interacting stress intensity factors (SIFs) obtained by the newly proposed method with those obtained by Green’s function method, dislocation density method, finite element method and digital photo-elasticity method, and further to investigate the effect of loading conditions and crack-hole geometric parameters on SIFs. Research results show that there exists a neutral inclination angle α0 of the crack (with x-axis) for eliminating the crack-hole interaction when σx∞ ≥ σy∞. Increase of σx∞ is helpful for cracking arrest and the compressive stresses on crack and hole surfaces are helpful for crack propagation. In addition, the normal stress (p) on the crack surface only affects KI and α0 of KI, while the normal stress (n) on the hole surface affects both KI and α0 of KI, and KII and α0 of KII. The hole radius (R) and the crack-hole spacing distance (t) have little effect on α0 of KI and KII. The new integral equation method not only has almost the same calculation results of SIFs as the Green’s function, dislocation density and finite element methods, but also has advantage of simplicity and wide applicability over Green’s function and dislocation density methods in calculating the interacting SIFs of the cracks under both remote and surface stresses.

Optical method to quantify the evolution of whole-field stress in fractured coal subjected to uniaxial compressive loads

Understanding and quantitatively characterizing the hidden distribution and evolution of the whole-field stress in fractured coal is crucial for accurately predicting crack propagation and connection that affect the transport capability of coalbed methane in reservoirs during fracture stimulation. However, it is challenging to directly reveal and quantify the hidden stress field using traditional methods because of the difficulties associated with extracting and identifying the stress distribution around randomly distributed, discontinuous, irregular-geometry cracks as well as with fabricating a model with embedded cracks. This study presents an optical method to reveal and quantify the continuous evolutions of the whole-field principal stress difference and shear stress in fractured coal by combining three-dimensional (3D) printing technology and digital photoelasticity. 3D printing technology is used to replicate the transparent model of fractured coal. The ten-step phase-shifting method and quality-guided phase unwrapping algorithm in photoelasticity are used to estimate the whole field photoelastic parameters (isoclinic and isochromatic) in the fractured coal model. The comparison between experimental and numerical results is provided to further prove the efficacy of this method in handling the natural fractured coal structure. The effects of crack irregularity and discontinuity on the stress evolution in fractured coal are evaluated as well.

Experimental evaluation of coefficients of multi-parameter asymptotic expansion of the crack-tip stress field using digital photoelasticity

In this study coefficients of the multi-parameter Williams series expansion for the stress field in the vicinity of the central crack in the rectangular plate are obtained by the use of the digital photoelasticity method and finite element analysis. The higher-order terms in the Williams asymptotic expansion are retained. It allows us to give more accurate estimation of stress, strain and displacement fields and to extend the domain of validity for the Williams series expansion. The program is specially developed for the interpretation and processing of experimental data from the phototelasticity experiments. The developed tool allows us to find points that belong to isochromatic fringes with minimal light intensity. The digital image processing with the aid of the developed tool is performed. The points determined with the adopted tool are used further for the calculations of stress intensity factor, T-stresses and coefficients of higher-order terms in the Williams series expansion. Numerical simulations of the same cracked specimen as in the experimental photoelasticity method are performed. The higher-order coefficients are extracted from finite element method calculations implemented in Simulia Abaqus software package and the outcomes are compared to experimental values. The plate with a small central crack has been considered either. This kind of the cracked specimen has been utilized for comparison of coefficients of the Williams series expansion obtained from finite element analysis with the coefficients known from the theoretical solution based on the complex variable theory in plane elasticity. It is shown that the coefficients of the Williams series expansion match with good accuracy.

Visualization of the photoelastic method data in the angular zone of a region

The experimental photoelastic method, which is a continuum method, and the method of forced deformations defrosting, allow us to obtain a stress-strain state in the stress concentration zone on the contact surface of composite structures with a surge of forced deformations on models made of optically-responsive material. The experimental solution is obtained on the model with an angular cut of the boundary, the vertex of which is occupied by the rupture of forced (temperature) deformations. The fringe contour obtained on the model by photoelastic method and the method of defrosting in the stress concentration area, in the zone of angular cut of the boundary and forced deformations surge, is characterized by significant gradients of experimental data: the order of fringes (isochromes), isoclines parameters, stresses, deformations. Modern methods of visualization of experimental data based on digital shooting expand the capabilities of the photoelastic method. The study purpose is to estimate the capabilities of using photoelastic method and the method of deformations defrosting to obtain the stress-state deformation in the angular zone of a region, which is as close as possible to the stress concentration area during digital shooting and data processing, by the standard methods of stress separation.

Experimental Verification of Cell Shape-Collapse Relationships in Metallic Foams by Photoelasticity Method

In the present study, the effect of cell morphology on mechanical properties in metallic foams with Al based closed cells was examined. AlSi8Mg0.8 alloyed metallic foam materials were produced by the Powder Metallurgy (PM) method. The elastic stresses and their distributions which were created by the compression load in the metallic foams, producing in similar density and different cell sizes were investigated by the photoelasticity method. The fracture and collapse mechanisms of the foaming materials, having the same density but different cell size and shape factor, exhibited discrepancy as well. The elastic stress concentrations and their distributions that were created by compression at the cell walls could be determined by the photoelasticity method. It was detected that the fringe orders in the photoelasticity images provided important and accurate information about the stress concentration areas at the foam walls. It was specified that the collapse at the end of the compression tests started mostly at these areas.

Determination of stresses in truss rods: numerical and physical experiment

The paper presents the results of an experimental numerical study of the stress state of the rods of a flat truss. The values of internal forces and stresses in the truss model were obtained using the SCAD software package, as well as experimentally using the polarization-optical method. The finite element method (FEM) is used to calculate two plane truss models. In the first case, the truss was modeled by rod elements, in the second – by plates. The fields of normal, tangential and principal stresses are obtained. Based on them the longitudinal internal forces in the rods of the farm are determined. The captins of interference bands in the model of a flat truss are obtained by the method of photoelasticity. Stresses are calculated with their help. The quantitative and qualitative analysis of the stress state, described in the numerical and physical experiment, is carried out.

Photoelastic Analysis of the Stressed State of a Flat Element with Geometrical Stress Concentrators (Cutout and Cuts)

The investigation of the stressed state of flat element with the geometrical stress concentrators was performed. The sample was made of piezo optic material. The oval-shape cutout and linear cuts were issued as the stress concentrators. On the symmetrical axles was cut out an oval. Linear cuts were made one by one under the different angles. Each time after appearing of the new cut, we fixed the stress fields in the model. Issue was performed using the axial compression at the elastic linear stage of loading. We used the method of photoelasticity. We estimated the quality and quantity of the influence of the size and the angle of cuts on the element stressed state, and received the stress fields during the different loadings. Also we estimated the stress coefficients near the tops of the cuts.

Investigation of the Hydrophility of a Polyurethane Film by the Photoelasticity and Nuclear Magnetic Resonance Relaxometry Methods

Investigation of the Hydrophility of a Polyurethane Film by the Photoelasticity and Nuclear Magnetic Resonance Relaxometry Methods

Visualization of Stress Distribution by Photoelastic Method Under Ultrasonic Grinding Condition

This study investigates phenomena in ultrasonic vibration-assisted grinding. The appropriateness of a stress visualization method is proven through comparison of a Hertzian contact stress analysis using finite element methods. The stress distribution on soda-lime glass caused by a 3-mm-diameter diamond electro-deposited wheel is visualized using a photo-elasticity method. The study compares the local stress concentrations caused by grains with and without ultrasonic wheel vibration. The global reaction force is measured by a dynamometer. The ultrasonic vibration leads to a reduced fluctuation of force, as well as a reduced time-averaged force. It is thought that the ultrasonic vibration causes a smaller local stress beneath the grains, which generates chips. In contrast, typical photo-elasticity methods are applicable for plane stress conditions. However, the stress distribution in a workpiece under a face grinding condition is distributed three-dimensionally, and the stress distribution cannot be recognized directly from the phase difference. Assuming that the stress distribution is sufficiently stable in a wheel rotation, continuously-captured images can be reconstructed to produce a 3D stress distribution, using computed tomography. The experimental tomographic images show a spatially-dispersed phase difference image caused by the electro-deposited wheel, with several discontinuous diamond grains on the end face of the wheel.

Research on Stress-Strain State in the Angular Domain Cut-Out Zone Using Photoelastic Method

Current challenges in civil engineering practice call for creating and substantiating structural theory research approaches that correlate with the objects, processes and phenomena under study (physical models) to the fullest possible extent, and that provide reliable results for tackling engineering challenges. The topicality of this paper is associated with the choice of research method, i.e. the experimental photoelastic method. This method allows to obtain the local stress-strain state with the highest reliability when solving problems with specified rupturing forced deformations in the domain of significant structural non-homogeneity, i.e. geometrical stress concentrators. Boundary shape and finite discontinuity of specified forced deformations translate into emerging singularity of stress-strain state. The research aim consists in theoretical and experimental analysis of the stress-strain state in neighborhood of irregular point on the boundary of plane domain which the finite discontinuity (jump) of forced deformations emerges into. The singularities of strain-stress state of structures and constructions that are characterized by “structural non-homogeneity” and rupturing forced deformations are determined using polymer models of the photoelastic and deformation freezing method as stress concentrators whose domains are the focus of this article. Research results demonstrate that a zone of stress-strain state similarity exists in the domain of stress concentration under the action of rupturing forced deformations. This is confirmed by the similarity of experimental data. A formula for experimental data extrapolation is proposed that helps restore band orders in the domain of elasticity problem singular solution where isochromatics on the photoelastic method polymer model are unreadable or “poorly” readable.

Modelling of the Effect of Piece-Wise Potential Volume Forces in the Photo-Elasticity Method

The authors note a special feature complicating the direct modelling process in the photo-elasticity method (using the models made from widely-used epoxy materials) with volume forces even of the simplest kind (mass forces). This problem is more complicated than the problems with constant parameters ν, E, γ, α in the whole body. The authors give some additional relationships between the similarity factors, the use of which may create some restrictions and difficulties in the process of modelling non-uniform problems through traditional methods. The authors use the theoretical principles of mechanics of deformable rigid bodies and show that the solution of the problem in case, when the volume forces in different domains of the body have different potential values, may be reduced to the solutions of the problems with potential volume forces and to the solution of the problem with given stress values. The authors have developed both methodology and engineering questions of modelling piece-wise potential volume forces using the equivalence of effects. They note that the methods of solution of the problems with the loadings substituting the volume forces have been developed in full and give efficient results.

Computational and experimental study of mixed mode loading of the cracked semi-circular disc under bending

Numerical and experimental works are performed on the mixed-mode I/II brittle fracture by two types of edge cracked semicircular bend specimens. The first type is edge cracked semi-circular bend (SBC) specimens with vertical and inclined notches. The second type of specimens is semi-circular samples in which the contribution of mode I and mode II components varies by changing the bottom loading support. In the test numerical program fracture tests were conducted at crack inclination angles of 0, 10, 45, 49, 50, 80 degrees. The stress intensity factors for Mode I and Mode II loading and T-stresses are obtained numerically in FEM package SIMULIA Abaqus/CAE. The results of FEM modeling are compared with the experimental results obtained by the photoelasticity method. The experimental approach based on the photoelasticity method allows us to observe the von Mises equivalent stress distribution in the whole specimen and confirm the FEM studies performed. The comparison shows that in the specimens the pure mode II loading can’t be realized. Analysis of numerical studies and the isochromatic fringe patterns allows us to conclude that in the semicircular disc the mixed mode loadings are realized for all angles from 0 to 85 degrees. The pure mode II or sliding fracture can’t be created by so- called mode II loading in disc-type specimens SCB, because there are opening displacements observed for all the specimens investigated numerically and experimentally.