Digital Photoelasticity Research Articles

FringeNet: A cyclic U-Net model with continuity imposed hybrid cyclic loss for demodulation of isochromatics in digital photoelasticity

This study introduces FringeNet, an innovative deep learning-based cyclic model to enhance the fringe order demodulation from single isochromatic images. A Continuity-Imposed Hybrid Cyclic Loss (CHCL) function, which combines Mean Squared Error (MSE), Structural Similarity Index (SSIM), Peak Signal-to-Noise Ratio (PSNR), and a continuity loss is proposed for optimizing multiple objectives, while enforcing the fringe order continuity in the isochromatic data. The proposed FringeNet is trained using available open-source ischromatic image dataset and the effectiveness of the model is validated using realistic experimental isochromatics. The FringeNet demonstrates a 92.4% improvement in MSE over existing methods, indicating substantial gains in predictive accuracy and model robustness. Additionally, a 16.19% enhancement in PSNR is observed, highlighting superior fidelity compared to existing approaches. The cyclic model employed in FringeNet represents a significant advancement in enhancing the deep learning-based predictive modeling used in the field of digital photoelasticity.

New high sensitivity approach for a quantitative visualization of the continuous whole-field stress evolution in digital photoelasticity

Digital photoelasticity is a widely used technique for experimental stress analysis on mechanical models subjected to loading conditions, due to its ability to reveal the stress information through color fringe patterns and consequently demodulate it computationally to achieve a quantitative evaluation. In this process, some limitations persist, such as, expertise dependence, experimental repeatability, sensitivity to load increments, and weak performance under experimental conditions, especially in time-dependent scenarios. In response to these weaknesses, the method “Quantitative Visualization of the Continuous Whole-Field Stress Evolution” introduced the ability to perform a complete demodulation of the fringe patterns generated in time-dependent applications, achieving high performance compared to conventional photoelasticity methods. This paper proposes a new highly sensitive approach for a quantitative visualization of the continuous whole-field stress evolution in digital photoelasticity by incorporating an inverse signal process and a multi-stage strategy. The algorithm aims to ensure the performance under high fringe order conditions while exhibiting high sensitivity in cases where the maximum stress obtained is less than one fringe order. The results presented validate its effectiveness in demodulating full-field stress distributions, especially in scenarios where the loading condition induces fringe orders less than one, thus contributing to the broader applicability of digital photoelasticity.

Evaluation of light source and sensor combinations based on Fourier analysis of digital photoelasticity images

Evaluation of light source and sensor combinations based on Fourier analysis of digital photoelasticity images

Deep learning as a powerful tool in digital photoelasticity: Developments, challenges, and implementation

Stress field evaluation, through fringe order maps, has always been of great importance in various engineering domains, providing essential insights into the mechanical response of loaded structures and enabling the prevention of potential failures, thus ensuring the safety and reliability of critical components. In recent years, the advent of new algorithms has ushered in a transformative era, particularly through computational techniques such as machine learning and, in particular, deep learning. These advanced approaches have proven instrumental in optimizing fringe demodulation processes, providing more accurate predictions, and uncovering complex patterns within structural behavior. This paper reports a careful review of papers and articles discussing the application of deep learning to photoelasticity studies. It explores the impact and applicability of deep learning in the field of digital photoelasticity, highlighting its benefits and advancements. In addition, the challenges associated with the implementation of this technique are examined to provide a critical insight. The paper also discusses experimental results and possible suggestions for future work in this new paradigm.

Fabrication of a Three-Dimensional Microfluidic System from Poly(methyl methacrylate) (PMMA) Using an Intermiscibility Vacuum Bonding Technique.

In this study, the fabrication of microfluidic chips through the bonding of poly (methyl methacrylate) (PMMA) boards featuring designed patterns to create a three-dimensional sandwich structure with embedded microchannels was explored. A key focus was optimization of the interface quality of bonded PMMA pairs by adjusting the solvent, such as such as acetone, alcohol, and their mixture. Annealing was conducted below 50 °C to leverage the advantages of low-temperature bonding. Because of the differences in the chemical reactivity of PMMA toward acetone, alcohol, and their combinations, the resulting defect densities at the bonding interfaces differed significantly under low-temperature annealing conditions. To achieve the optimal sealing integrity, bonding pressures of 30 N, 40 N, and 50 N were evaluated. The interface was analyzed through microstructural examination via optical microscopy and stress measurements were determined using digital photoelasticity, while the bonding strength was assessed through tensile testing.

Collagen imaging reveals synergistic effects of sutures and host-donor misalignment on topographical irregularities in penetrating keratoplasty.

Mitigating unwanted refractive errors is crucial for surgeons to ensure quality vision after penetrating keratoplasty (PK). The primary objective of the present study is to highlight the importance of microstructural matching of the host and the donor cornea during PK on the distribution of the corneal tissue while suturing. For this purpose, the present study undertakes an in-vitro PK model to analyse the effect of suturing and host-donor misalignment on corneal birefringence. Five groups of experiments were performed using five corneoscleral buttons. In each group, N = 16 data points (corresponding to 16 simple interrupted sutures) were assessed before and after PK with five degrees of misalignments, 0°, 30°, 45°, 60° and 90° to detect the variations in corneal birefringence post-PK. The technique of digital photoelasticity is utilized to capture the corneal birefringence experimentally. The local and global features of corneal birefringence provided interesting insights into the nuances of corneal birefringence in PK. Statistical analysis was performed to study the effects of suturing on the birefringence around the suture bites. It was observed that the interaction of the suture tension and structural misalignment between the host and the donor cornea influences the corneal birefringence in PK. Conclusions The zero-degree structural misalignment of the host and the donor tissue is preferable to minimize the topographical irregularities and related astigmatism post-PK. The findings of the present study envisage an additional step of structurally aligning the donor tissue with the host before suturing to minimize topographical irregularities in PK.

Quantification of full-field stress in continuously loaded fractured rocks using 3D printing and digital photoelasticity

Accurately characterizing stress fields in fractured rocks is crucial for understanding fracture propagation and rock failure mechanisms. However, existing methods struggle to quantify stress fields in continuously loaded fractured rocks with complex fracture networks. In this study, we propose a novel method that utilizes 3D printing and digital photoelasticity to quantify full-field stress in complex fractured rocks under continuous loading. Our method incorporates three key components: a modified global fringe thinning technique for automatic extraction of fringe skeletons, a proposed algorithm for automatic identification of near-zero-order fringe regions, and an optimized phase calculation process for efficient phase unwrapping. The effectiveness and accuracy of our method are validated through theoretical analysis and comparison with results obtained from the ten-step phase shifting method. Furthermore, our proposed method can also be applied to quantify stress fields in fractured rocks under static loading.

Intraoperative collagen imaging of sutured cornea: A way towards managing post-penetrating keratoplasty astigmatism

The birefringent nature of the human cornea plays an important role in comprehending its structural behavior in both diseased and surgical conditions. During corneal transplantation, irregular astigmatism is a common post-surgical complication that depends on the characteristics of suturing. Four human cadaver corneas are subjected to an in-vitro model of a typical full-thickness penetrating keratoplasty (PK) procedure using 16 simple interrupted 10–0 vicyrl sutures. The birefringence of these four corneas is analyzed using digital photoelasticity and compared with the control cornea (without PK). It is found that the sutures and their mutual interaction influence the morphology of the peripheral birefringence of the cornea. The findings of the present investigation are pertinent to intraoperative suture management during PK. Results suggest conserving the typical diamond-shaped morphology of peripheral birefringence would ensure uniform distribution of sutures. Therefore, birefringence imaging could be useful in suture management to ensure proper apposition of the graft-host junction, thus minimizing the risk of irregular astigmatism.

Advancing instantaneous photoelastic method with color polarization camera

Digital photoelasticity is an essential experimental method. For dynamic objects and transient stress fields, the utilization of the instantaneous photoelastic method becomes necessary. Existing instantaneous photoelastic methods, represented by the conventional RGB method and instantaneous phase shifting techniques, encounter a significant challenge in simultaneously obtaining both the isochromatic and isoclinic parameters while ensuring a considerable measurement range for the isochromatic fringe order (retardation). This severely limits the application scope of instantaneous photoelasticity. In this paper, a novel approach is proposed by introducing a color polarization camera into the field of photoelasticity. The proposed method enables simultaneous determination of the full-field continuous isochromatic fringe order and the isoclinic parameter, ensuring a considerable measurement range for the isochromatic fringe order. Additionally, the obtained isoclinic parameter is consistent, eliminating the need for tedious and time-consuming phase unwrapping procedures. To validate the accuracy and effectiveness of the method, it was applied to standard problems involving a disc and a ring under diametral compression and an instantaneous problem concerning a rotating blade. The proposed method expands the application potential of instantaneous photoelasticity, enabling the resolution of a broader range of problems.

Stress Separation in Digital Photoelasticity Part A - Photoelastic Data Unwrapping and Smoothing

The advent of digital photoelasticity saw the development of various techniques to get photoelastic data of isoclinics and isochromatics at every pixel in the domain. Unlike in other interferometric techniques, in photoelasticity one gets phase data of two quantities and their mutual dependence and interaction affect their evaluation. For stress separation studies, one requires both isochromatics and isoclinics accurately free of any kinks in the domain. With this in view, a robust approach is evolved to get these parameters. Initially wrapped isoclinics are obtained using a plane polariscope based phase shifting technique. These are unwrapped by a quality guided approach. However, the result has several kinks due to isochromatic-isoclinic interaction. An outlier smoothing algorithm is proposed for getting a smooth variation of the isoclinics. Six-step phase shifting method using a circular polariscope is used for determining the isochromatic data. The smoothed isoclinic values obtained are used in isochromatic calculation to get isochromatic phasemap free of ambiguities. This phasemap is also unwrapped by a quality guided approach and smoothed by the outlier algorithm. These methodologies are validated for two benchmark problems of a disk under diametral compression and a ring subjected to internal pressure. The models are subjected to moderate loads showing a high level of isochromatic-isoclinic interaction.

Analysis of the Stress Field in Photoelasticity Used to Evaluate the Residual Stresses of a Plastic Injection-Molded Part.

The degree of quality of thermoplastic injection-molded parts can be established based on their weight, appearance, and defects. However, the conditions of the injection process may induce effects on the mechanical performance of the injected parts, and the residual stresses can cause cracks or early failures when an external load or force is applied. To evaluate these mechanical behaviors, different experimental techniques have been reported in the literature, where digital photoelasticity has stood out both for being a non-contact technique and for achieving quantitative results through sophisticated computational algorithms. Against this background, our proposal consists of analyzing the overall residual stress distribution of parts injected under different molding conditions by using digital photoelasticity. In this case, the specimens are subjected to bending strength tests to identify possible effects of the injection process conditions. The findings show that, at mold temperatures of 80 °C, flow-induced residual stresses increase with packing pressure. However, these internal stress levels do not affect the external load applied by the mechanical bending test, while the mass injected at higher levels of packing pressure helps to increase the bending strength of the injected part. At lower mold temperatures (50 °C), the mechanical strength of the injected part is slightly reduced, possibly due to a lower effect of the packing pressure.

Genesis of Various Optical Arrangements of Circular Polariscope in Digital Photoelasticity

In the early developments for automation of photoelasticity several researchers have proposed phase shifting techniques (PST) with different optical arrangements. Several ways of calculating the isoclinic and isochromatic parameters were also reported. In many cases though the optical arrangements were different, the intensity equations remained the same. Although theoretically, each of the techniques provide the evaluation of isoclinics and isochromatics, when applied to experimental images only some of the techniques remain robust. This indicates that specific optical arrangements do play a very important role on the success of the phase shifting technique experimentally. One of the main sources of error is due to the mismatch of quarter wave plate. In this paper, the role of quarter wave plates in influencing the selection of appropriate optical arrangement for PST is studied systematically. Different possibilities of obtaining the photoelastic parameters with least error are explored.

A Generic Error Simulation in Digital Photoelasticity By Jones Calculus

Photoelasticity is a whole field experimental technique to obtain stress fields in both 2-D and 3-D elasticity problems. With advancements in Digital Image Processing (DIP), a separate branch of photoelasticity namely digital photoelasticity came into existence wherein the intensity of the images is used for photoelastic parameter estimation. The accuracy of parameter evaluation by digital photoelastic techniques depends on the choice of the optical arrangements for parameter estimation, the inherent error in optical elements and any misalignments introduced during experimentation. A generic approach to simulation of error in digital photoelasticity by Jones calculus is presented in this paper. The simulation accounts for inherent error in quarter-wave plates and also accommodates the error due to the optical misalignment of the various optical elements. The applicability of the new methodology is shown for the most popular six-step phase shifting technique. The methodology proposed is generic and is applicable to study the influence of error sources on parameter estimation for various techniques proposed in digital photoelasticity.

Identification of subject-specific fibrillar disposition in healthy rabbit cornea through birefringence analysis

The cornea is an essential component in ocular biomechanics. The modifications in the collagen microstructure of the cornea affect its biomechanical behavior in various ocular conditions. Birefringence is an indicator of the microstructural changes in the cornea. The characterization of birefringence can benefit the patient-specific assessment of disease and surgically driven effects. According to the existing literature, optical birefringence of the cornea is best described as of biaxial nature. However, the biaxial model does not concur with the varying degree of birefringence observed amongst individuals. Using digital photoelasticity, the present study explores the applicability of birefringence of the healthy rabbit cornea as a benchmark model for human cornea. Eight freshly excised healthy rabbit corneas are imaged using the polariscope in the transmission mode. A fringe analysis is performed for each configuration of the polariscope to discern the collagen fiber distribution in the cornea. The birefringence of the rabbit corneas showed inter-corneal variability with varying degrees of biaxiality like the human corneas. Based on the fringe analysis, the fiber families in the cornea were identified in the form of geometric patterns distributed over the corneal surface and incorporated in the finite element model of the cornea. The present study envisages the utilization of birefringence-derived structural information to develop a reliable treatment design based on microstructural features of the cornea.

Remove ambiguous zones using floodfill algorithm in digital photoelasticity

Digital photoelasticity is a widely used technique for the full-field measurement of stress/strain distributions. However, ambiguous zones are present in the isochromatic phase map obtained with the commonly used ten-step phase shifting technique. Obtaining accurate isochromatic phase maps without wrapping can be challenging due to the presence of ambiguous zones. Existing methods to address this issue are complex and not fully automated. In this study, a novel approach is proposed to automatically and accurately remove the ambiguous zones using the Floodfill algorithm, an advanced digital image processing algorithm. The validity and robustness of the proposed method is demonstrated through benchmark tests on a disc and a ring subjected to diametral compression and a four-point bent beam, as well as a model with a complex geometry. The proposed method enhances the automation and simplicity of full-field stress/strain measurement in terms of the removal of ambiguous zones.

Quantification of the stress field in extremely complex pores by digital photoelasticity

Quantification of stress fields in complex pores is essential for understanding the failure mechanisms of engineering geomaterials. Photoelasticity has been widely used to analyze stress fields in engineering. However, difficulties in model fabrication and stress field extraction limit its application for models with extremely complex structures, such as pores and fractures in rocks. In this study, the regional phase unwrapping algorithm was improved to efficiently determine the isoclinics by dividing the regions of the isoclinics in the interval (0, π/2). Furthermore, a photoelastic model with extremely complex pore structures in conformity with natural rocks was fabricated using a 3D printing technique. The isoclinics and isochromatics were calculated using the improved method. The good accuracy was verified by comparing the photoelastic fringe orders with the darkest and brightest fringes. The fabrication of complex photoelastic models and the efficient extraction of full-field stress will expand the application of photoelasticity in broader fields.

Full Field Solution for Remotely Loaded One Side Completely Debonded Short Rigid Line Inclusion Embedded in Soft Matrix: Two-Dimensional Analytical and Experimental Insights

Abstract The problem of a completely debonded short fiber (rigid line inclusion/anticrack) embedded in a 2D isotropic elastic soft matrix subjected to the remote loading condition is of fundamental interest. The current work investigates completely debonded anticrack embedded in a soft (isotropic) matrix using Kolosov Muskhelisvili's complex potential framework. Here two configurations are studied: debonded inclusion oriented (i) parallel and (ii) perpendicular to the loading direction. In particular, the potentials take the form of a non-homogeneous Riemann—Hilbert equation for the given problem. Upon solving analytical forms of potentials, the stress fields were obtained. The stress field for the fully debonded anticrack exhibited oscillatory singular behavior between r−3/4 and r−1/4 with the dependence on the oscillatory index ε and material constants. The correctness of the analytical solution was validated using numerical simulation and experiments based on the digital photoelasticity technique. The analytical results were in good agreement with the experimental and numerically obtained stress fields confirming the accuracy of it. The magnitude of singularity is quantified by defining a complex stress intensity factor at the tip of the discontinuity and compared with the experimentally estimated value. So far in the literature, no full-field analytical solution exists for the completely debonded rigid inclusion embedded in an isotropic soft matrix. The solution obtained in the present work is of fundamental importance in developing the constitutive properties of short fiber reinforced thermoplastic (SFRT) composites.

Fringe pattern analysis to evaluate light sources and sensors in digital photoelasticity.

When experimental photoelasticity images are acquired, the spectral interaction between the light source and the sensor used affect the visual information of the fringe patterns in the produced images. Such interaction can lead to fringe patterns with an overall high quality, but also can lead to images with indistinguishable fringes, and bad stress field reconstruction. We introduce a strategy to evaluate such interaction that relies on measuring the value of four handcrafted descriptors: contrast, an image descriptor that accounts simultaneously for blur and noise, a Fourier-based descriptor to measure image quality, and image entropy. The utility of the proposed strategy was validated by measuring the selected descriptors on computational photoelasticity images, and the fringe orders achieved when evaluating the stress field, from 240 spectral configurations: 24 light sources and 10 sensors. We found that high values of the selected descriptors can be related to spectral configurations that lead to better stress field reconstruction. Overall, the results show that the selected descriptors can be useful to identify bad and good spectral interactions, which could help to design better protocols for acquiring photoelasticity images.

Characterization of the dynamic stretching stages of polymeric fibers using dispersion measurements.

In this paper, we characterize the dynamics of the molecular chains at different stages of stretching polymeric monofilament fibers. The main stages characterized in this work are shear-bands, necking, crazes, cracks, and fracture regions. Digital photoelasticity and white-light two-beam interferometry are utilized to study each phenomenon by finding the dispersion curves and the 3D birefringence profiles using a single-shot pattern, which is performed for the first time, to our best knowledge. We also suggest an equation to find the full-field distribution of the oscillation energy. This study presents a clear vision on the behavior of the polymeric fibers at a molecular level during dynamic stretching until reaching its fracture point. Patterns of these deformation stages are given for illustration.

Three-wavelength digital photoelasticity.

A three-wavelength photoelasticity method is proposed to simplify the optical setup and speed up data acquisition. By recording six intensity images with circularly polarized illuminations of three close wavelengths, the phase retardation and corresponding inner stress can be computed accurately with a correspondingly developed computational algorithm. Since the mechanical rotations of wave plates and polarizers required by classic photoelasticity techniques are avoided, the data acquisition of this proposed method is very speedy, and measurement of a dynamic sample can be achieved with a very simple and compact optical setup. Besides theoretical analyses, numerical and experimental evidences are also used to confirm the feasibility of this suggested three-wavelength digital photoelasticity method.