Photoelastic Patterns Research Articles

Determination of the full-field stress and displacement using photoelasticity and sampling moiré method in a 3D-printed model

The quantitative characterization of the full-field stress and displacement is significant for analyzing the failure and instability of engineering materials. Various optical measurement techniques such as photoelasticity, moiré and digital image correlation methods have been developed to achieve this goal. However, these methods are difficult to incorporate to determine the stress and displacement fields simultaneously because the tested models must contain particles and grating for displacement measurement; however, these elements will disturb the light passing through the tested models using photoelasticity. In this study, by combining photoelasticity and the sampling moiré method, we developed a method to determine the stress and displacement fields simultaneously in a three-dimensional (3D)-printed photoelastic model with orthogonal grating. Then, the full-field stress was determined by analyzing 10 photoelastic patterns, and the displacement fields were calculated using the sampling moiré method. The results indicate that the developed method can simultaneously determine the stress and displacement fields.

Method for extracting the full-field dispersion data of birefringent objects based on the digital RGB photoelasticity

In this paper, we are presenting a new photoelastic pattern analysis technique to find the point-wise distribution of the dispersion data of birefringent materials. This technique can be applied to profile the dispersion data in three dimensions during static, online, real-time, and dynamic investigations using the well-known digital photoelasticity technique. This method is applied to characterize the dispersion parameters of monofilament isotactic polypropylene fibers. The photoelastic patterns are included for illustration.

An algorithm for direct birefringence measurements using intensities of digital photoelastic patterns of fibres

An algorithm for direct birefringence measurements using intensities of digital photoelastic patterns of fibres

Opto-Thermo-Mechanical Characterization of Crazing Phenomena Observed for Isotactic Polypropylene Fibers Using Digital Photoelasticity and the Computed Tomography Techniques

Digital photoelasticity was used to characterize the crazing phenomena of isotactic polypropylene (iPP) fibers as a precursor of fracture processes. In this study, the effect of stretching at different rates on the formation of crazes was investigated for the iPP fibers. Also, the influence of drawing the iPP fibers at different temperatures on the formation of the crazes was studied. In addition, the effect of heating the crazed iPP fibers on the formed crazes was characterized using digital photoelasticity. Computed tomography was used to obtain qualitative and quantitative measurements of the crazes densities along the cross-sections of the iPP fibers. Also, the birefringence (Δn) values of the crazes were measured for different conditions of stretching and temperature and compared to the Δn of the bulk material. The photoelastic patterns of the iPP fibers are given for illustrating the crazes behavior in the iPP fibers.

Machine learning approach to force reconstruction in photoelastic materials

Photoelastic techniques have a long tradition in both qualitative and quantitative analysis of the stresses in granular materials. Over the last two decades, computational methods for reconstructing forces between particles from their photoelastic response have been developed by many different experimental teams. Unfortunately, all of these methods are computationally expensive. This limits their use for processing extensive data sets that capture the time evolution of granular ensembles consisting of a large number of particles. In this paper, we present a novel approach to this problem that leverages the power of convolutional neural networks to recognize complex spatial patterns. The main drawback of using neural networks is that training them usually requires a large labeled data set which is hard to obtain experimentally. We show that this problem can be successfully circumvented by pretraining the networks on a large synthetic data set and then fine-tuning them on much smaller experimental data sets. Due to our current lack of experimental data, we demonstrate the potential of our method by changing the size of the considered particles which alters the exhibited photoelastic patterns more than typical experimental errors.

Determination of the stress and strain fields in porous structures by photoelasticity and digital image correlation techniques

Quantitative visualization of the full-field stress and strain in porous structures is of significance to reveal the mechanism of the damage and failure of engineering materials. To quantitatively characterize the full-field stresses and strains, optical measurement methods, such as photoelasticity for determining the stress fields, moiré and digital image correlation (DIC) methods for measuring the strain fields, have been developed in previous works. However, it is challenging to combine these methods to measure stress and strain fields simultaneously because the speckles and gratings for strain measurement will disturb the interference fringes in the photoelasticity. In this study, a method incorporating photoelasticity and DIC techniques was developed to determine the stress and strain fields simultaneously in different porous structures which were fabricated by three-dimensional (3D) printed technique. The tested models were printed with a highly transparent, colourless material and the speckles on the front surface of the tested models were printed with a translucent, magenta material. Under compressive loads, the stress fields in these models were obtained based on the photoelastic patterns and the strain fields were determined by DIC method. The results indicate the proposed method can simultaneously determine the full-field stresses and strains and the comparison of the distribution of the principal stress and strain difference and the distribution of the shear stress and strain verified its good accuracy. In addition, the stress and strain distribution in different pore structures were compared and found that the optimal design of the pore distribution can greatly change the concentration of the stress and strain fields.

Photoelastic characterization of shear-bands in mechanically stretched polymeric fibers.

In this work, the shear bands formed during the stretching of isotactic polypropylene fibers are studied. The digital photoelasticity technique is applied to provide full-field visualization of the stress of the shear bands regions. The mechanical testing device is modified to be suitable for mechanical measurements. The effect of different parameters, that is, the stretching rates and the ambient temperature on the formation of shear bands is investigated. Finally, the influence of heating pre-existed shear bands is studied. Photoelastic patterns of the shear bands at these conditions are included for illustration.

Fractographic characterization of isotactic polypropylene fibers.

In this article we present an opto-thermo-mechanical characterization of isotactic polypropylene monofilament during failure by fracture. The digital photoelasticity, the two-beam Pluta polarising interference microscope and the computed tomography are used to provide quantitative data and visual models of the fracture regions of isotactic polypropylene fibers suffering failure by fracture at different thermal and mechanical conditions. Birefringence values and the intrinsic stress-anisotropy are measured at the fracture surface. 3D models of the fracture regions are reconstructed. Photoelastic patterns and two-beam interferograms are included for illustration.

A quantitative study on using digital photoelasticity for characterising the effect of the stretching speed on the necking phenomenon

Abstract The digital photoelastic technique is used to characterise the necking behaviour of isotactic polypropylene (iPP) fibres. The effect of stretching rate on necking initiation is studied. The birth of necking is observed using photoelastic patterns of the stretched fibres to understand how the localised difference between the principle stresses grows to form a necking region. Finally, the formation of multiple necking regions is characterised photoelastically. These multiple necks are initiated using the same formation mechanism and conditions as if there is only a single necking region. It was evident that, fast stretching causes faster arrangement of molecular chains and hence decreases the time required for necking initiation. Recommendations are suggested for optimum mechanical processing conditions of iPP fibres to avoid failure by necking. Photoelastic patterns are given for illustration.

Quantification of photoelastic fringe orders using polarized light camera and continuous loading

Quantification of fringe orders is important in visualizing full-field stress in digital photoelasticity. Various methods have been developed to improve the calculated accuracy of fringe orders, such as the phase shifting, carrier, and RGB methods. However, in these methods, fringe orders are calculated using the photoelastic patterns under a certain load but not the patterns in the whole time series; this impedes quantification of the fringe orders in dynamic photoelasticity. In this study, a polarized light camera was employed to simultaneously capture the photoelastic patterns of the dark and bright fields under circular polariscope. Based on the light intensity difference of these patterns in the entire loading process, a method of quantifying fringe orders is proposed. Full-field fringe orders in a disk fabricated by 3D printer and subjected to three compressive loads were calculated using this method. Its good accuracy, improved by removing the background light and choosing a suitable capture frame rate, was verified by comparing the fringe orders calculated by the traditional and proposed methods. Furthermore, the determination of the full-field fringe orders in complex structures indicated that the complexity of tested models had no influence on the calculated accuracy.

Application of principal component analysis in phase-shifting photoelasticity.

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.

Digital image analysis around isotropic points for photoelastic pattern recognition

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)

Characterizing Frictional Contact Loading via Isochromatics

Isochromatic patterns in the vicinity of frictional contacts furnish vital clues for characterizing friction. Though friction effects are evident in a diametrally loaded circular disk, three-point loading provides better results towards highlighting friction. In this paper, a new method of characterizing friction at loading contacts using photoelastic isochromatics patterns is presented. Location of isotropic points (IPs) formed in three-point and four-point loadings of circular disk is used as a main tool to quantify the friction component using theoretical analysis. Bifurcation of isochromatic fringe loops near the distributed loads is explained by the presence of anti-symmetric Hertzian shear traction in addition to Hertzian normal traction. The classical solution by Flamant for point load at the edge of half plane is used to derive stresses in circular disk for all required loading configurations. A semicircualr ring under three-point loading is examined using photoelasticity to understand the isochromatics pattern theoretically by considering normal and shear traction components at loaded regions.

Distribution of Peri-implant Stresses with a Countertorque Device

To verify the effectiveness of a countertorque device in dental implants in redistributing stress to the bone-implant interface during tightening of the abutment screw. Two prismatic photoelastic samples containing implants were made, one with a 3.75-mm-diameter implant and the other with a 5.0-mm-diameter implant (both implants had an external-hexagon interface) and the respective abutments were attached (CeraOne). The samples were placed in a support and submitted to torques of 10, 20, 32, and 45 Ncm with an electronic torque meter. The torque application was repeated 10 times on each sample (n = 10) with and without a countertorque device. Photoelastic patterns were detected; thus, a photographic register of each test was selected. The fringe patterns were analyzed at discrete points near the implants' external arch. In both implants analyzed, a stress gradient reduction was observed through the implant with the countertorque device. The countertorque device used in this study proved to be effective in reducing the stresses generated in the peri-implant bone tissue during torque application.

Measurement of Stress and Displacement Fields in Particle Assembly subjected to Shallow Foundation Loading via Photoelasticity Technique

본 논문의 목적은 얕은 기초 하중을 받는 입상체 내의 응력의 분포와 변위를 측정하는 광탄성 기법의 적용 방안을 제시하는 데 있다. 광탄성 측정기법은 입자 집합체의 힘 전달을 시각화하기 위해 사용 되었다. 실내 모형시험은 스틸 프레임으로 경계면을 만들고 폴리카보네이트 탄성중합체로 만들어진 이차원 원형 입자들을 적층하여 구성하였다. 광탄성 시트를 원형 입자에 코팅함으로써 힘 전달의 패턴을 밝은 빛의 줄무늬 형태로 확인할 수 있었고, 광탄성 측정 기법을 통해 접촉되어 있는 입자들에서의 주 응력차의 크기, 방향 또한 측정 할 수 있었다. 변위장은 디지털 이미지 해석 기법을 사용하여 측정 하였다. 하중 재하 초기 상태와 파괴 근처의 상태에서 서로 다른 접촉력의 분포를 정량적으로 분석하였다. 파괴 이전 단계에서 관측된 광탄성 패턴과 변위장은 접촉력 사슬의 좌굴 발생 이후 즉시 사라짐을 확인하였다. The purpose of this paper is to present an photoelasticity technique for measuring the displacement and stress distribution in particle assembly subjected to shallow foundation loading. Photoelastic measurement technique was employed to visualize the force transmission of a particle assembly. A model assembly bounded by a steel frame was built by stacking bi-dimensional circular particles made of polycarbonate elastomer. Each particle was coated by a thin photoelastic sheet so that the force transmission represented by bright light stripes can be visualized. In a contacted particle, both magnitude and orientation of principal stress difference can also be measured via the photoelasticity technique. The different distributions of the contact stresses at the initial loading and near the failure were quantitatively compared. The photoelastic patterns and displacement fields observed in the pre-failure state disappears immediately after the buckling of confined force chains.

Full-Field Modelling of Crack Tip Shielding via the ‘Plastic Inclusion’ Concept

This paper presents an outline of the development, verification and application of a new model of crack tip stress fields in the presence of a plastic enclave around a growing fatigue crack. The approach taken rests on capturing the effects of this ‘plastic inclusion’, comprising the crack tip and crack wake plastic zones, via elastic stress distributions applied at the elastic-plastic boundary. The model is therefore independent of the mechanisms of plastic deformation and potentially applicable to a variety of materials. A Muskhelishvili complex potential extension to the Williams crack tip stress field is found for four stress parameters representing a K-stress, a T-stress, a crack retardation stress and a compatibility-induced shear stress at the elastic-plastic boundary. This model is validated via full field fitting to photoelastic stress fringe patterns, obtained from epoxy resin and polycarbonate specimens. It has also been extended to the strain fields measured in digital image correlation techniques, which allows its application to metallic alloys.

Photoelastic Study of the Support Structures of Distal‐Extension Removable Partial Dentures

The double system of support, in which the distal-extension removable partial denture adapts, causes inadequate stress around abutment teeth, increasing the possibility of unequal bone resorption. Several ways to reduce or more adequately distribute the stress between abutment teeth and residual ridges have been reported; however, there are no definitive answers to the problem. The purpose of this study was to analyze, by means of photoelasticity, the most favorable stress distribution using three retainers: T bar, rest, proximal plate, I bar (RPI), and circumferential with mesialized rest. Three photoelastic models were made simulating a Kennedy Class II inferior arch. Fifteen dentures with long saddles, five of each design, were adjusted to the photoelastic patterns and submitted first to uniformly distributed load, and then to a load localized on the last artificial tooth. The saddles were then shortened and the tests repeated. The quantitative and qualitative analyses of stress intensity were done manually and by photography, respectively. For intragroup analyses the Wilcoxon test for paired samples was used, while for intergroup analyses Friedman and Wilcoxon tests were used to better identify the differences (p < 0.05). The RPI retainer, followed by the T bar, demonstrated the best distribution of load between teeth and residual ridge. The circumferential retainer caused greater concentration of stress between dental apexes. Stress distribution was influenced by the type of retainer, the length of the saddle, and the manner of load application. The long saddles and the uniformly distributed loads demonstrated better distribution of stress on support structures.

Chaotic pattern of unsmoothed isochromatics around the regions of concentrated stresses

Abstract Photoelastic isochromatics are computationally reconstructed from the results of finite element analysis. Fast variation of the components of stresses cannot be approximated by a coarse non-adaptive finite element mesh. Visually chaotic and attractive looking pattern of numerically reconstructed photoelastic fringes prevents interpretation of the field of stresses around the regions of concentrated stresses. The phrase “chaotic pattern” is used in this paper in a loose sense to refer to the intricate and unpredictable patterns produced by this approach. One goal of this paper is to present these photoelastic patterns for their aesthetic appeal.

Full-field pulsed magnetophotoelasticity – a description of the instrument

This paper describes a novel instrument used for the analysis of full-field through-thickness stress distributions using the theory of magnetophotoelasticity (MPE) developed by Aben [1] and developed by Clarke et al. [2, 3]. MPE is an experimental stress analysis technique that involves the application of a magnetic field parallel to an electromagnetic wave propagating through a birefringent model within a polariscope. The effect viewed through the polariscope is then a combination of the model's birefringence and the Faraday rotation created in the model by the magnetic field. Aben developed this technique especially for use in the measurement of stress profiles where the integrated photoelastic pattern alone yields little information. Clarke et al. developed MPE in order to study toughened glass. To date, the technique of MPE has been a single-point measurement, which is of limited utility in the investigation of three-dimensional stress in toughened glasses. The pulsed magnetopolariscope (PMP), described here, enables the full-field application of MPE. This paper contains a description of the novel apparatus and demonstrations used to validate the performance of a proof-of-concept PMP instrument. The paper also highlights improvements in the application of MPE, which are now possible with this new equipment. These improvements include the extension of MPE to larger areas of analysis, three-dimensional stress analysis, and the possibility of analysing a general unknown stress distribution.

Determination of Energy Release Rate in Carbon Fiber-Thermoplastic Composites and Comparison of Crack Propagation in Amorphous and Crystalline Interface By Single Fiber Pullout Test

The fracture toughness in the fiber-matrix composite by means of the single fiber pull-out test can be achievable, if it is possible to measure the compliance of partial debonded fiber, which necessitates a stable crack propagation and the determination of the corresponding crack length. A combination of high stiff test equipment and a simultaneous monitoring of the photo-elastic pattern of the embedded fibers with the help of polarization microscope enables to measure crack length and determination of Gc [Hampe, A and Marotzke, C. (1997). The Energy Release Rate of the Fiber/Polymer Matrix Interface: Measurement and Theoretical Analysis, J. Reinf. Plast. and Compo., 16: 341-352.]. This method is well-known for glass fiber-amorphous polymer composite. In this investigation, we tried to implement the same technique to C-fiber with amorphous and semi-crystalline polymer composites. The energy release rate (ERR) of the fiber-matrix interface was evaluated for the carbon fiber-reinforced in polycarbonate (PC) by interfacial crack propagation arising in the single fiber pullout test. Further, the crack propagation at the fiber-matrix interface with different matrix morphology was discussed with the use of photo-elastic pattern observed with the aid of polarization microscope.