Showing papers on "Photoelasticity published in 2020"

A Comparative Study of Dynamic Fracture of Soda-Lime Glass Using Photoelasticity, Digital Image Correlation and Digital Gradient Sensing Techniques

Abstract: The dynamic fracture of high-stiffness and low-toughness materials such as soda-lime glass (SLG) typically involves crack initiation and growth prior to branching, underlying mechanics of which is not yet fully understood. Investigation of this issue using full-field optical techniques face numerous spatio-temporal challenges since crack speeds in these materials reach 1500 m/s or more and are accompanied by highly localized deformations. However, often it is tacitly assumed that most optical methods are equally capable of studying this challenging problem and it turns out not to be true. To this end, three prevalent optical techniques - transmission photoelasticity, 2D Digital Image Correlation (DIC) and transmission Digital Gradient Sensing (DGS) - are implemented concurrently to visualize crack-tip fields and quantify fracture characteristics associated with crack initiation, crack growth and macroscopic crack bifurcation in SLG plates subjected to nominally identical loading in three separate experiments. Each method is implemented in conjunction with ultrahigh-speed (1 Mfps) photography, flash/pulse illumination, and a modified-Hopkinson pressure bar for impact loading of specimens. The feasibility of measuring fracture parameters and imposed force histories along with the pros and cons of each approach for this material system are examined.

Relationship between Photoelasticity of Polyurethane and Dielectric Anisotropy of Diisocyanate, and Application of High-Photoelasticity Polyurethane to Tactile Sensor for Robot Hands.

Abstract: Eight types of polyurethane were synthesized using seven types of diisocyanate. It was found that the elasto-optical constant depends on the concentration of diisocyanate groups in a unit volume of a polymer and the magnitude of anisotropy of the dielectric constant of diisocyanate groups. It was also found that incident light scattered when bending stress was generated inside photoelastic polyurethanes. A high sensitive tactile sensor for robot hands was devised using one of the developed polyurethanes with high photoelasticity.

The algorithm for the determination of the Williams asymptotic expansion coefficients for notched semidiscs using the photoelasticity method and finite element method

Abstract: The paper proposes the algorithm for the determination of the coefficients of the Williams series expansion in notched semidisks with different angles of the notch. The algorithm is based on the experimental procedure of the photoelasticity method and the finite element analysis. The large series of experiments for semidiscs was performed. Digital photoelasticity method is used to analyze experimentally the complete Williams series expansion of the stress and displacement fields in the vicinity of the crack tip in isotropic linear elastic plates under Mixed Mode loading. The distribution of the isochromatic fridge patterns is employed for obtaining the stress field near the crack tip by the use of the complete Williams asymptotic expansion for various classes of the experimental specimens (plates with two collinear cracks under tensile loading and under mixed mode loading conditions). The higher order terms of the Williams series expansion are taken into account and the coefficients of the higher order terms are experimentally obtained. The stress field equation of Williams up to fifty terms in each in mode I and mode II has been considered. The comparison of the experimental results and the calculations performed with finite element analysis has shown the importance and significant advantages of photoelastic observations for the multi-parameter description of the stress field in the neighborhood of the crack tip.

An investigation of elastic waves producing stone fracture in burst wave lithotripsy.

Abstract: Burst wave lithotripsy is a method to noninvasively fragment urinary stones by short pulses of focused ultrasound. In this study, physical mechanisms of stone fracture during burst wave lithotripsy were investigated. Photoelasticity imaging was used to visualize elastic wave propagation in model stones and compare results to numerical calculations. Epoxy and glass stone models were made into rectangular, cylindrical, or irregular geometries and exposed in a degassed water bath to focused ultrasound bursts at different frequencies. A high-speed camera was used to record images of the stone during exposure through a circular polariscope backlit by a monochromatic flash source. Imaging showed the development of periodic stresses in the stone body with a pattern dependent on frequency. These patterns were identified as guided wave modes in cylinders and plates, which formed standing waves upon reflection from the distal surfaces of the stone model, producing specific locations of stress concentration in the models. Measured phase velocities compared favorably to numerically calculated modes dependent on frequency and material. Artificial stones exposed to bursts produced cracks at positions anticipated by this mechanism. These results support guided wave generation and reflection as a mechanism of stone fracture in burst wave lithotripsy.

Toward photoelastic sensors: a hybrid proposal for imaging the stress field through load stepping methods

Abstract: Extending photoelasticity studies to industrial applications, such as photoelastic sensors, implies to overcome limitations like need of experts, experimental over-calibration, or reduced number of experiments. This paper introduces a computational hybrid technique toward this goal.

A photoelastic and numeric study of the stress field in the vicinity of two interacting cracks: Stress intensity factors, T-stresses and higher order terms

Abstract: The multi-parameter description of the stress field in the neighborhood of the crack tips of two interacting cracks based on the photoelastic study and finite element analyses is given. Digital photoelasticity method is used for obtaining the isochromatic and isoclinic patterns in the plate with two collinear horizontal and two inclined cracks in anisotropic linear elastic material. Stress intensity factors, T-stresses and coefficients of the higher order terms in the multi-parameter Williams series expansion are experimentally determined. The finite element analysis for the cracked plates with the same configurations has been performed. Stress intensity factors, T-stress and coefficients of nine-term asymptotic expansion for the stress field are numerically obtained and are compared with the experimental results. The comparison shows good agreement of experimental and numerical estimations of these fracture mechanics parameters. Very good agreement is shown to exist between the digital photoelasticity method and finite element results confirming the effectiveness of the photoelasticity technique in obtaining the coefficients of higher order terms of the Williams series expansion from the experimental stress field around the crack tip.

Study of Crack Interaction Effects Under Thermal Loading by Digital Photoelasticity and Finite Elements

Abstract: The effect of an interacting internal crack on the edge crack in a transient thermal stress field is evaluated using digital photothermoelastic experiments and finite element (FE) analysis. Initially, a transient thermal stress field is simulated using FE analysis for which the thermal boundary conditions are set based on the experimental isochromatic fringes. In this simulated thermal stress field, single edge crack with and without an internal short crack interacting at different configurations are modeled and the non-dimensional Stress Intensity Factor (SIF) values are evaluated at different time intervals. It is observed that the asymmetric and collinear configurations tend to increase the SIF of the edge crack whereas a parallel crack tends to decrease the SIF of the edge crack. For experimental evaluation, specimens with a single edge crack and asymmetric configuration of interacting cracks are considered and the non-dimensional SIF values are evaluated under a thermal stress field generated by edge heating and cooling. The results are compared with those obtained using the FE analysis.

Experimental determination and finite element analysis of coefficients of the multi-parameter Williams series expansion in the vicinity of the crack tip in linear elastic materials. Part II

Abstract: This study aims at obtaining coefficients of the multi-parameter Williams series expansion for the stress field in the vicinity of the central crack in the rectangular plate and in the semi-circular notched disk under bending by the use of the digital photoelasticity method. The higher-order terms in the Williams asymptotic expansion are retained. It allows us to give a more accurate estimation of the near-crack-tip stress, strain and displacement fields and extend the domain of validity for the Williams power series expansion. The program is specially developed for the interpretation and processing of experimental data from the phototelasticity experiments. By means of the developed tool, the fringe patterns that contain the whole field stress information in terms of the difference in principal stresses (isochromatics) are captured as a digital image, which is processed for quantitative evaluations. The developed tool allows us to find points that belong to isochromatic fringes with the 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 the stress intensity factor, T-stresses and coefficients of higher-order terms in the Williams series expansion. The iterative procedure of the over-deterministic method is utilized to find the higher order terms of the Williams series expansion. The procedure is based on the consistent correction of the coefficients of the Williams series expansion. The first fifteen coefficients are obtained. The experimentally obtained coefficients are used for the reconstruction of the isochromatic fringe pattern in the vicinity of the crack tip. The comparison of the theoretically reconstructed and experimental isochromatic fringe patterns shows that the coefficients of the Williams series expansion have a good match.

Investigation of Stress Field and Fracture Development During Shale Maturation Using Analog Rock Systems

Abstract: The emergence of hydrocarbons within shale as a major recoverable resource has sparked interest in fluid transport through these tight mudstones. Recent studies suggest the importance to recovery of microfracture networks that connect localized zones with large organic content to the inorganic matrix. This paper presents a joint modeling and experimental study to examine the onset, formation, and evolution of microfracture networks as shale matures. Both the stress field and fractures are simulated and imaged. A novel laboratory-scale, phase-field fracture propagation model was developed to characterize the material failure mechanisms that play a significant role during the shale maturation process. The numerical model developed consists of coupled solid deformation, pore pressure, and fracture propagation mechanisms. Benchmark tests were conducted to validate model accuracy. Laboratory-grade gelatins with varying Young’s modulus were used as scaled-rock analogs in a two-dimensional Hele-Shaw cell apparatus. Yeast within the gelatin generates gas in a fashion analogous to hydrocarbon formation as shale matures. These setups allow study and visualization of host rock elastic-brittle fracture and fracture network propagation mechanisms. The experimental setup was fitted to utilize photoelasticity principles coupled with birefringence properties of gelatin to explore visually the stress field of the gelatin as the fracture network developed. Stress optics image analysis and linear elastic fracture mechanics (LEFM) principles for crack propagation were used to monitor fracture growth for each gelatin type. Observed and simulated responses suggest gas diffusion within and deformation of the gelatin matrix as predominant mechanisms for energy dissipation depending on gelatin strength. LEFM, an experimental estimation of principal stress development with fracture growth, at different stages was determined for each gelatin rheology. The interplay of gas diffusion and material deformation determines the resulting frequency and pattern of fractures. Results correlate with Young’s modulus. Experimental and computed stress fields reveal that fractures resulting from internal gas generation are similar to, but not identical to, type 1 opening mode.

Generalized adversarial networks for stress field recovering processes from photoelasticity images

Abstract: For overcoming conventional photoelasticity limitations when evaluating the stress field in loaded bodies, this paper proposes a Generative Adversarial Network (GAN) while maintaining performance, gaining experimental stability, and shorting time response. Due to the absence of public photoelasticity data, a synthetic dataset was generated by using analytic stress maps and crops from them. In this case, more than 100000 pair of images relating fringe colors to their respective stress surfaces were used for learning to unwrap the stress information contained into the fringes. Main results of the model indicate its capability of recovering the stress field achieving an averaged performance of 0.93±0.18 according to the structural similarity index (SSIM). These results represent a great opportunity for exploring GAN models in real time stress evaluations.

Stress Concentration and Shape Optimization for a Fillet Surface of a Step-Shaped Shaft

Abstract: An analytical solution to the direct boundary-value problem and the two-dimensional problem concerning the stressed condition in the median surface of a step-shaped shaft is presented In the boundary-value problem, a singular integral equation with the Cauchy kernel is used, the solution of which can be found in the form of an unlimited increase in stresses at the ends of the integration interval The two-dimensional problem is presented in trigonometric series, where constant coefficients can be determined from the boundary conditions that have been previously expanded into a Fourier series Comparison of the results obtained with the data taken from scientific sources and experimental studies on the stressed condition by means of a laser polariscope using flat transparent models of step-shaped parts with fillets having constant and variable curvature has confirmed the adequacy of the solution presented

Analysis of photoelastic properties of monocrystalline silicon

Abstract: . Photoelasticity is considered a useful measurement tool for the non-destructive and contactless determination of mechanical stresses or strains in the production of silicon wafers. It describes a change in the indices of refraction of a material when the material is mechanically loaded. As silicon has a diamond lattice structure, the stress-dependent change in the refractive indices varies with the loading direction. In this work, an anisotropic stress-optic law is derived, and the corresponding stress-optical parameters are measured using a Brazilian disc test. The parameters were determined to be ( π 11 - π 12 ) = 14.4 ⋅ 10 - 7 MPa −1 and π 44 = 9.4 ⋅ 10 - 7 MPa −1 . The results of this work are compared to previous works found in the literature, and the deviations are discussed.

Polarimeter for measuring the properties of birefringent media in reflective mode.

Abstract: This work presents a description of a polarimetric system for measuring the properties of birefringent media. In our reflection system the applied Stokes polarimeter acts both as a generator of the light’s selected polarization states as well as a light analyzer leaving the examined medium. The method is based on six intensity distribution measurements realized in six different configurations of polarizers/analyzers: four linear and two circular ones. Thus, we have achieved parallel polariscope for linear polarizers and the crossed polariscope for circular polarizers. Such a setup can be easily applied for linearly birefringent media properties measurements including dichroic ones. This measurement setup and the measurement method were successfully tested in a homogeneous medium and a medium with variable phase difference.

StressNet: A deep convolutional neural network for recovering the stress field from isochromatic images

Abstract: Extending photoelasticity studies to industrial applications is a complex process generally limited by the image acquisition assembly and the computational methods for demodulating the stress field wrapped into the color fringe patterns. In response to such drawbacks, this paper proposes an auto-encoder based on deep convolutional neural networks, called StressNet, to recover the stress map from one single isochromatic image. In this case, the public dataset of synthetic photoelasticity images `Isochromatic-art' was used for training and testing achieving an averaged performance of 0.95 +/- 0.04 according to the structural similarity index. With these results, the proposed network is capable of obtaining a continuous stress surface which represents a great opportunity toward developing real time stress evaluations.

Dual beam-shear differential interference microscopy for full-field surface deformation gradient characterization

Abstract: We propose a new experimental mechanics method: dual beam-shear differential interference microscopy (DInM) for full-field surface deformation measurement. The method integrates the principles of differential interference contrast, photoelasticity, digital image correlation and the theories of continuum mechanics for a 4D quantification of the surface topography. Our first DInM prototype provides the lateral resolution of 787nm for up to 12% measurable out-of-plane strains. The resolution can be improved by using the shorter spectrum light and higher magnification objective lens. We use our system to characterize the buckling profile of Si microribbon on an elastomer substrate, which was verified by atomic force microscopy. We also demonstrate the stress-induced phase transformation in NiTi alloy by our system. The evolution of the surface topographies and the full-field deformation gradients are successfully captured and quantified. The dynamic measurement provides the information to calculate the relative transformation strains between phases of different symmetries. The establishment of dual beam-shear DInM opens a new avenue in the field of experimental meso/micromechanics.

Thermal transient stepping: a powerful thermal-based approach for evaluating the stress field by using digital photoelasticity

Abstract: A thermal approach for measuring the stress field was developed by using digital photoelasticity. The approach relies on applying a thermal stimulation to the examined model, in conjunction with a computational hybrid algorithm of load stepping, to determine the isochromatic phase value from only three experimental images. The proposal was validated by using a PMMA disk under compressive load and exposed to thermal variations. This experiment was conducted in reflection photoelasticity where a face of the disk was used to observe the fringe patterns, and the back face to capture thermal variations. The results obtained in synthetic and experimental images, indicate that the approach is effective, easy to reproduce, and could enhance the capabilities of existing approaches to analyze stress fields.

Visualization of Delamination in Encapsulated Flexible Electronics Fabricated using Slot Die Coating

Abstract: A major challenge in the flexible electronics industry is the inability to quickly and accurately assess the mechanical properties of barrier materials used to encapsulate various devices. The feasibility of using a low-cost approach, digital photoelasticity (DP), to determine stress formation in barrier film is analyzed so that inherent weak areas cannot only be identified, but also reinforced. In this experimental study, ethylene vinyl acetate (EVA) is slot die coated onto untreated polyethylene terephthalate (PET) or onto PET treated with indium tin oxide (ITO) or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). An axial stress is imposed on each sample using a universal test machine, and DP was used to obtain the stress and grab profiles. It has been shown that highly concentrated and multilayer EVA coatings withstand higher stress. Also, it has been shown that the locations of delamination happen at the side edge of the encapsulation when EVA was coated on a homogenous surface, but for heterogeneous surfaces, the delamination occurred at the boundary between the PEDOT:PSS and ITO. All these data were depicted using DP in very quick fashion. Therefore, DP is a viable method for quickly and accurately determining stress in barrier films.

Experimental investigation of the stress field around the crack tip in a polymer material under dynamic loading

Abstract: A dynamic photoelasticity-caustic experimental system is established to quantitatively visualize and characterize the distribution of an entire stress field. The evolution of both the singular stress and far-field stress around the crack tip can be precisely determined using the proposed multiparameter solution. Moreover, the accuracy of the proposed stress inverse calculation method increased with an increasing number of parameters. The reconstructed isochromatic fringes based on 12-parameter solutions agreed well with the results obtained by the photoelasticity technique. Moreover, the influence of the parameters on the crack propagation behaviors was comprehensively investigated. In addition, the numerical results showed that the 12-parameter solution was sufficient for modeling the stress field around the crack tip, which enlarged the estimation region by at least 50 times compared with that of the traditional three-parameter (KId, KIId, and T-stress) solution.

Residual stress measurement methods of optics

Abstract: Residual stress is an important performance indicator of optics, which is of great significance to the fabrications and applications of optical components. Residual stress measurement methods of optics can be summed up into two categories: methods based on the strain measurement and on the stress induced birefringence measurement, respectively. The strain based methods, which are built upon crystal dynamics and elastic mechanics, including X-ray diffraction (XRD), Stoney curvature method, and micro-Raman spectroscopic method, are well developed and widely used. Methods based on the measurements of birefringence phase retardation induced by residual stress, including digital photoelasticity method, photoelasticitic modulator (PEM) method and polarization-dependent cavity ring-down method, show a higher precision. The principles, measurement precisions and application scenarios of these residual stress measurement methods are summarized in this overview. Comparisons between the performances of these methods are performed and correlations between them are analyzed in detail.

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

Abstract: 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.

Examination of internal stress by photoelasticity in laser cleaving of glass

Abstract: Laser cleaving is a glass-cutting technique in which thermal stress induced by laser heating and cooling produces cracks in the glass. Stress measurement during the laser cleaving process is critical in elucidating the crack-propagation mechanism and solving the problems of the laser cleaving method. In this study, we measured the birefringence retardation using a high-speed polarization camera and evaluated the relevance and accuracy of the measured values by comparing them with the results of a numerical calculation. The birefringence retardation at the crack tip was also observed in the experimental process. For the experiment, a soda lime glass was cleaved using CO2 laser irradiation. Then, the birefringence retardation and azimuthal angle obtained using the polarization camera were compared with the numerical calculation results. The birefringence retardation around the crack tip corresponded with that of the deformation caused by mode I. The crack propagation was arrested when the crack tip approached the edge of the glass. The birefringence retardation observed using the polarization camera confirmed that the mode I deformation decreased as the crack approached the edge.

Experimental study of wind impact on multilevel industrial scaffolding

Abstract: In the article, the local stress-strain state of structures and constructions is investigated, various variants for the design of the boundary are taken into account: special lines, points. The acting forced deformations don’t satisfy the compatibility conditions. They have a finite discontinuity along the contact line (surface) of the domains, including the irregular point of the boundary, causing stresses. The subject of article is stress concentrators the singularity of the stress-strain state of structures and constructions exhibiting “constructive heterogeneity” and discontinuous forced deformations determined on polymer models of photoelasticity and defrosting of deformations. A complex theoretical-numerical-experimental approach, for obtaining and analyzing the stress state in the neighborhood of the irregular point of the plane domain boundary, is proposed to extrapolate reliable experimental data to a domain where the fringe contour is not readable.

Measuring photoelastic dispersion coefficients in material samples with digital holography

Abstract: Polarized Digital Holography (PHD) is a fast and efficient tool for analyzing mechanical effects in materials. Especially when the task requires non-invasive techniques that do not damage the material in study, the use of PHD has great perspectives. The most common methods of digital reconstruction use the convolution theory to discretize the Huygens- Fresnel integral. When external stresses are applied to photoelastic materials, the relationship between these stresses and phase differences observed by polarization holography is an intrinsic characteristic of the material called the photoelastic dispersion coefficient. In photoelasticity, this coefficient depends on the wavelength. By using PHD the authors show in the present paper that the photoelastic dispersion coefficient also depends on the wavelength in Holography. A Mach- Zehnder interferometer, modified with the inclusion of linear polarizers, was built to verify this effect in a sample of photoelastic material. In this set-up, two coherent light sources with different wavelengths were used. For the analysis, a digital method was created that correlated the mean stresses differences on the photoelastic material sample and the mean phases differences at each distinct wavelength.