Photoelastic Model Research Articles

Measurement of stress optical coefficients for GFRP based on terahertz time-domain spectroscopy

Glass Fiber-Reinforced Polymer (GFRP) finds extensive applications in the high-end equipment manufacturing industry owing to its advantages of light weight, high strength, and corrosion resistance. Since the residual stress in GFRP builds up during the curing process and affect its mechanical properties and service life, the characterization of the residual stress in GFRP is crucial. In this study, we establish a theoretical model based on the anisotropic stress-optics law for the orthorhombic crystalline system to describe the terahertz-elasticity of GFRP and calibrate the stress optical coefficients of GFRP. First, the residual stress in GFRP at different curing temperatures are measured by fiber Bragg grating sensors. Then, the refractive index of GFRP with different residual stress are obtained based on transmission-type THz-TDS. Finally, based on the proposed photoelastic model of GFRP, the stress optical coefficients of GFRP are measured by combining the measurement results of residual stress and refractive index. The experimental results show that the refractive index of GFRP decreases with the increase of residual stress; the stress optical coefficients of GFRP are determined as q11 = −5.612 × 10−9 Pa−1, q12 = −2.548 × 10−9 Pa−1, q21 = −1.305 × 10−8 Pa−1, q22 = −1.408 × 10−9 Pa−1. The modeling of terahertz photoelasticity in GFRP and the determination of stress optical coefficients provide a basis for characterizing residual stress in GFRP by THz-TDS.

A new thermo-optical system with a fractional Caputo operator for a rotating spherical semiconductor medium immersed in a magnetic field

PurposeUnderstanding the mechanical and thermal behavior of materials is the goal of the branch of study known as fractional thermoelasticity, which blends fractional calculus with thermoelasticity. It accounts for the fact that heat transfer and deformation are non-local processes that depend on long-term memory. The sphere is free of external stresses and rotates around one of its radial axes at a constant rate. The coupled system equations are solved using the Laplace transform. The outcomes showed that the viscoelastic deformation and thermal stresses increased with the value of the fractional order coefficients.Design/methodology/approachThe results obtained are considered good because they indicate that the approach or model under examination shows robust performance and produces accurate or reliable results that are consistent with the corresponding literature.FindingsThis study introduces a proposed viscoelastic photoelastic heat transfer model based on the Moore-Gibson-Thompson framework, accompanied by the incorporation of a new fractional derivative operator. In deriving this model, the recently proposed Caputo proportional fractional derivative was considered. This work also sheds light on how thermoelastic materials transfer light energy and how plasmas interact with viscoelasticity. The derived model was used to consider the behavior of a solid semiconductor sphere immersed in a magnetic field and subjected to a sudden change in temperature.Originality/valueThis study introduces a proposed viscoelastic photoelastic heat transfer model based on the Moore-Gibson-Thompson framework, accompanied by the incorporation of a new fractional derivative operator. In deriving this model, the recently proposed Caputo proportional fractional derivative was considered. This work also sheds light on how thermoelastic materials transfer light energy and how plasmas interact with viscoelasticity. The derived model was used to consider the behavior of a solid semiconductor sphere immersed in a magnetic field and subjected to a sudden change in temperature.

Evaluation of the biomechanics of Aramany class I obturators of different designs using numerical and experimental methods. Part II: Stress distribution

Evidence regarding stress evaluations of removable obturators with Aramany class I defects is lacking. Whether the stress distribution on Aramany class I prostheses can be improved by modifying the currently used designs is also unclear. The purpose of part II of this study was to evaluate the stress distribution in different designs of Aramany class I obturators using finite element analysis (FEA) and photoelastic stress analysis. Four finite element and 8 photoelastic models, including 2 acrylic resin base obturators retained with 2 Adams clasps, 2 linear, 2 tripodal, and 2 fully tripodal design obturators, were used in this study. The frameworks were fabricated on the casts obtained from a modified printed model. Vertical and oblique loads were applied on 2 points (anterior and posterior) of the models. The quantitative measurement was done by measuring the fringe orders and von Mises values to compare the influences of occlusal forces on the obturator components and their supporting structures. The qualitative evaluation was done by visual color mapping to identify the stress concentration. In the photoelastic analysis, the anterior abutments of the tripodal showed the highest stress, followed by the fully tripodal obturators, while, in FEA, the anterior abutments of the linear design received the most in both vertical and oblique load. The central incisor received the most stress in photoelastic (3 or more fringe orders) and FEA (687.3 and 150.1MPa for vertical and oblique loads, respectively), followed by the lateral incisors. Upon posterior loading, the base of the defect of the linear design demonstrated the most stress in photoelastic (3 or more fringes) and FEA (94.3 and 130.5MPa for vertical and oblique loads, respectively). The acrylic resin base obturator retained with Adams clasps demonstrated the lowest stress distribution in abutments and their supporting bone upon anterior and posterior loads. Upon vertical and oblique load application, the fully tripodal design was comparable with the tripodal in terms of stress distribution. Both designs were better than the linear in response to the same loading. The stress was concentrated at the anterior palatal part of the obturator, the base of the defect, and the junction of the metal and acrylic resin part of the prostheses upon anterior and posterior loading, respectively.

Photoelastic Stress Induced by Maxillary Denture on the Mandibular Denture and Overdentures Lined with Silicone-based Material

Objectives: This study evaluated the stress induced by maxillary denture over mandibular denture and overdentures lined with silicone-based material. Materials and Methods: Three maxillary dentures, a mandibular denture, an overdenture supported by a single implant, and other supported by two implants were made for the following groups: MCD—Mandibular complete denture lined with silicone-based material, OSI—Overdenture lined with silicone-based material supported by single implant, and OTI— Overdenture lined with silicone-based material supported by two implants. The mandibular photoelastic model was axially loaded by the maxillary denture with intensities of 10, 20, or 30 kgf. Photoelastic analysis took place with the maxillary denture in maximum intercuspation with the mandibular prostheses. Results: Photoelastic analysis showed that the 10 kgf load promoted in the mandibular denture greater stress along the photoelastic model mainly in the anterior region. Stress occurred around the single implant with higher intensity at the apex. Stress occurred between the two implants and no stress was shown along the implants and apices. For 20 kgf, stress occurred in the anterior region of the mandible denture; Higher stress is shown around the single implant and apex and smaller at the mandible left side. Stress occurred between the two implants, and few stress at the apex of the left implant. For 30 kgf, stress occurred in the anterior region of the mandible with more intensity on the left side. Stress is shown around and at the implant apex and both mandible frontal sides. Stress is shown between the two implants and little stress at the apex of the left implant. Conclusions: The lined overdentures supported by single or two implants exhibited stress concentration in the implant region compared to lined mandibular denture showing stress along the mandible body. The stress in the overdentures occurred mainly around the single implant or between the two implants. The increase in load intensity had little influence on the increase in stress for the prostheses with lined bases. Clinical Relevance: Intercuspation in the overdenture must have the same importance given to the vertical relationship to relieve the chewing effort in overdenture lined with silicone-based material.

Evolution law and transformation mechanism of force chains in the flow of photoelastic particles under a flexible barrier

The essence of large deformation flow in loose geological bodies lies in the breaking and rebuilding of force chains at the microscopic scale. Therefore, it is crucial to investigate force chains during particle flow processes. This study focuses on particle flow during the ore drawing process, develops a photoelastic experiment model, investigates the evolution law of force chains, reveals the transformation mechanism of strong and weak force chains, and provides insights for removing particle blockage in mining engineering. The results show that strong force chains are primarily located on both sides of the model, with fewer numbers, and extend toward the side boundaries in a direction close to 70°. Weak force chains are predominantly distributed in the middle section of the model, with numerous and uniformly developed directions. In addition, under the effects of self-organization and hetero-organization, the particles' sliding and rolling cause the buckling of force chains and the rearrangement of particles, resulting in the transformation between strong and weak force chains and accelerating the particles' flow. The research results deepen the understanding of the granular material large deformation flow-slip problem and provide a reference for similar engineering analyses.

Photoelastic Stress in Mandibular Overdenture Retained by Two Implants

Objectives: The aim of this study was to verify the effect of occlusal and axial single loads in the photoelastic stress in overdenture retained by two implants placed in the anterior region of the mandible. Materials and Methods: Occlusal load (10, 20, and 30 kgf) was exerted by the conventional maxillary complete denture in occlusion with the overdenture adapted in the mandibular photoelastic model. Axial single load was also exerted on the first left or right molar of the overdenture. Qualitative analysis was made in polariscope images and quantitative analysis by the FRINGES program. Results: Qualitative analysis: The occlusal load showed similar stress in the region between the two implants. Axial single load induced in the first molars promoted stress in the implant apexes, and between the implant and mandible posterior region for the three loads. Quantitative analysis for occlusal load: 10 kgf (T = 265.03; N = 0.57); 20 kgf (T = 989.81; N = 2.11); 30 kgf (T = 1055.40; N = 2.26). Axial single load in the left molar: 10 kgf (T = 267.38; N = 0.57); 20 kgf (T = 270.07; N = 0.58) and 30 kgf (T = 281.84; N = 0.60). Axial single load in the right molar: 10 kgf (T = 952.14; N = 2.03); 20 kgf (T = 1038.58; N = 2.22); and 30 kgf (T = 926.92; N = 1.98). Conclusions: Different photoelastic stress levels occurred in the overdenture retained by two implants when submitted to occlusal or axial single load in the first left or right molar. Clinical Relevance: Artificial teeth intercuspation in the overdenture supported by two implants must have the same importance given to the vertical relationship to relieve the chewing effort.

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.

Optical anisotropy of transparent polymer materials fabricated via 3D printing and their application in photoelasticity

Additive manufacturing has been used to fabricate photoelastic models for several years. In the 3D-printing techniques that use photosensitive resins, models are built layer by layer, and the influence of these layers on light propagation and photoelastic fringes is still ambiguous. In this study, a plane polarization arrangement is used to examine the influence of thin layers on light propagation using three disks printed along different directions. The influence of the layers on the light intensity is negligible for the disk where the layer planes are perpendicular to the direction of light propagation. Identical light polarization is observed for the disks where the layer planes are parallel to the direction of light propagation. This is consistent with a polarizer. The evaluation of the thin layers using scanning electron microscopy indicated that the influence of the microstructures on light propagation is the potential cause of these phenomena. The influence of light polarization on photoelastic fringes was presented by comparing the fringes of different disks under circularly polarized light. The results can improve the application of the 3D-printed models in photoelasticity and the understanding of the optical anisotropy of 3D-printed polymer.

Stress Distribution by Improved Designs of Obturator Frameworks Using Photo-elastic Resin Models. In Vitro Study

Obturator is a prosthetic appliance that is used to restore irreparable maxillary defects. The frames were supposed to form the infrastructure of the obturator that intends to close the maxillary arch defects. Five main types of maxillary defects according to Aramany classification were selected for this study. 15 different design frameworks were fabricated using Co-Cr dental alloy. The defects were simulated using premade maxillary arch models that were reproduced in photo-elastic resin. The fabricated metal frames were seated on the photo-elastic resin models and then were exposed to standardized stress at 3 different locations on the frames using a global testing machine. A custom made plane polariscope was arranged to provide polarized illumination for the light emission and analysis through the models. The photo-elastic resin models were photographed during loading under the testing machine to display the supporting areas deformation in frontal plane. The results showed that the proposed metallic designs variably distributed the stresses around the abutments and the remaining hard tissues on the photo-elastic models.

Comparative photoelastic stress analysis between all-on-four implant-supported cobalt-chromium framework and carbon fiber reinforced framework with varying cantilever lengths - An in vitro study

CONTEXT: In implant prosthodontics, metal frameworks are used to rigidly spilt the implants together to provide rigidity and stiffness to the prosthesis. However, due to the limitations associated with the metal framework fabrication, the recent advances have made it possible to fabricate metal-free implant-supported prostheses using fiber-reinforced composite frameworks.AIM: This study aimed to evaluate and compare photoelastic stresses between all-on-four implant-supported cobalt-chromium (Co-Cr) framework and the carbon fiber-reinforced composite (CFRC) framework at varying cantilever lengths.MATERIALS AND METHODS: Two photoelastic models of an edentulous mandible were fabricated according to the all-on-four concept. Following this, frameworks with a bilateral cantilever extension of 20 mm, one with CFRC and the other with Co-Cr, were fabricated and were subjected to a progressive load of 120–180 N with an interval of 20 N. The principal stresses (σ) were calculated using the mean maximum fringe order.STATISTICAL ANALYSIS: The quantitative analysis was performed using the Kruskal–Wallis test and Mann–Whitney U-test, P ≤ 0.05.RESULTS: The CFRC framework showed mean principal stress values significantly lower than the Co-Cr framework under all loading conditions; however, deformation of the framework was observed at the cantilever lengths of 15 mm and 20 mm.CONCLUSION: The study and its findings have indicated that the CFRC framework appears suitable for the fabrication of a framework for an “All-on-four” prosthesis. Within the confines of this study, the use of 10 mm as an arbitrary cantilever length recommendation for the CFRC framework and 15 mm for the Co-Cr framework seems reasonable.

FEA and Experimental Investigation of Prosthetic Knee Joint

The aim of this study is to look at how stresses are distributed in a polyethylene-based prosthetic knee joint. Ansys 18.1 software is used for finite element analysis. In experimental studies, the effects of sagittal radius, flexion angle, and external load on knee joint stresses were investigated. The prosthetic knee joint is created as a 2D model using parameters like sagittal radius and flexion angles in design modeling software. The experimental work has been carried out in a polariscope machine setup, The photo elastic model is tested in different loading conditions and finds stress in that model. Finally, results are validated experimental results with FEA results.

Biomechanical analysis of stress around the tilted implants with different cantilever lengths in all-on-4 concept

BackgroundMany clinical studies have reported the high success rate of the All-on-4 concept. In the present study, we aimed to compare the stress distribution with different tilted distal implants and cantilever lengths in an All-on-4 system using the two-dimensional photoelastic method and to establish the All-on-4 implant photoelastic model by computer-aided design (CAD) and rapid prototyping (RP). MethodsThe data of the human edentulous mandible were acquired by computed tomography (CT). Three human edentulous mandible All-on-4 implant models with different distally inclined implant holes were fabricated using Mimic, Geomagic Studio software, and a light solidifying fast shaping machine. Then the final photoelastic models were established through the traditional method. Each of the three models had four NobelSpeedy Replace implants between the interforaminal regions. The two posterior implants were placed 0, 15, and 45 degrees distally before the mental foramen. The four implants were splinted by wrought cobalt-chromium alloy frameworks. Each of the three photoelastic models was submitted to a 150 N vertical load at five points on the framework: the central fossa of the mandibular first molar, and 0 mm, 5 mm, 10 mm, and 15 mm of the cantilever length. The stress produced in the models was photographed with a digital camera, and the highest value of the stressed fringe pattern was recorded.ResultsThe All-on-4 implant photoelastic model established by CAD and RP was highly controllable and easy to modify. The position and inclination of implants were accurate, and the frameworks could be passively emplaced. The stress values were higher around a single tilted implant compared with the distal implant in All-on-4 with the same inclination. The 0-degree distal implant and 45-degree distal implant demonstrated the highest and lowest stress when loading at the central fossa of the mandibular first molar, respectively. With the same inclination of distal implant, the peri-implant bone stress increased as the length of cantilever increased.ConclusionThe method of establishing the All-on-4 implant photoelastic model by CAD and RP was highly controllable, convenient, fast, and accurate. The tilted implants splinted in the fully fixed prosthesis with reduced cantilever lengths did not increase the stress level compared with the vertical distal implants.And this illustrated that the influence of cantilever on stress distribution was greater than the influence of implant inlination.

Photoelasticity for Stress Concentration Analysis in Dentistry and Medicine.

Complex stresses are created or applied as part of medical and dental treatments, which are linked to the achievement of treatment goals and favorable prognosis. Photoelasticity is an optical technique that can help observe and understand biomechanics, which is essential for planning, evaluation and treatment in health professions. The objective of this project was to review the existing information on the use of photoelasticity in medicine and dentistry and determine their purpose, the areas or treatments for which it was used, models used as well as to identify areas of opportunity for the application of the technique and the generation of new models. A literature review was carried out to identify publications in dentistry and medicine in which photoelasticity was used as an experimental method. The databases used were: Sciencedirect, PubMed, Scopus, Ovid, Springer, EBSCO, Wiley, Lilacs, Medigraphic Artemisa and SciELO. Duplicate and incomplete articles were eliminated, obtaining 84 articles published between 2000 and 2019 for analysis. In dentistry, ten subdisciplines were found in which photoelasticity was used; those related to implants for fixed prostheses were the most abundant. In medicine, orthopedic research predominates; and its application is not limited to hard tissues. No reports were found on the use of photoelastic models as a teaching aid in either medicine or dentistry. Photoelasticity has been widely used in the context of research where it has limitations due to the characteristics of the results provided by the technique, there is no evidence of use in the health area to exploit its application in learning biomechanics; on the other hand there is little development in models that faithfully represent the anatomy and characteristics of the different tissues of the human body, which opens the opportunity to take up the qualitative results offered by the technique to transpolate it to an application and clinical learning.

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.

Experimental fracture investigations concerning the hyoid bone fracture

IntroductionExtensive literature exists concerning the etiology and morphology of fractures of the hyoid bone (h.b.) in cases of fatal strangulation. There is an agreement to the effect that fractures are mostly located within the distal third of the cornua majora (c.m.). Although several predictors have been described very accurately, the fracture analysis has neither been based on the osseous construction nor on the stress distribution of the c.m. under strangulation resulting from the configuration and its details. This especially applies to the apex of the distal ends of the big horns, the bulbi. The objective of the experimental break tests that were performed was to contribute to elucidating the biomechanics of the horizontal and vertical fractures and to fractures of the bulbi. Materials and methodsBreak tests in the a.p. direction line were carried out on 28 unfixed h.b. of adults on a specially constructed test bench by continuously increasing the tension until a fracture/dislocation occurred. The test arrangement followed the constellation of typical-symmetrical hanging. The selection criteria were the symmetry of the h.b. and the gender. Before and after the experiments, a radiological depiction (DIMA system 20 kV, 10 sec; Institute for Diagnostic Radiology, University-Hospital Goettingen) was carried out, followed by a preparative depiction under magnifying glass control. The h.b. configuration was classified according to the following types: hyperbole-, parabola-, and horseshoe-type. Following this classification, the results were related to the findings achieved by the photo-elastic model experiments. By this, the results of the experimental fracture tests could be specifically compared to the tension distribution within the model. ResultsA total of 70 % of the experimentally-produced fractures were located within the distal third of the c.m. This matches with the frequency distribution in real typical symmetrical hanging. Following the radiological and preparative investigations that were carried out, the c.m. have to be viewed as tubular bones. The transition regions of different osseous strength/elasticity are thus to be considered as areas of increased vulnerability. For the distal third of the c.m., it is the level at which the dense spongiosa/compacta of the shaft part turn into wide-meshed spongiosa and tender compacta of the bulbi. Additionally, the bulbi themselves represent a locus of reduced strength in which the fractures were located basally and/or apically in the transition. It was not only in the whole c.m. that the direction in which the fragment was snapped off or fractured was not random, as all fractures were located on the broad side of the horn, following the applied force. ConclusionsThe experimental fracture tests explained the known accumulation of fractures in the distal third of the c.m. in cases of hanging with the knot of the rope located against the neck. It could be demonstrated radiologically and preparatively that, anatomically, the big horn of the h.b. is a tubular bone. From this, a new approach to the forensic reconstruction of trauma can be derived. The transitional area from the shaft into the bulbus represents a locus minoris resistentiae. In case of pressing the h.b. towards the cervical spine under ventral application of force, one could expect a point load of the bulbi. Two different types of bulbus fractures showed that this load is diagnostically relevant.The fracture direction is also of diagnostic value. It depends on the angle that is formed by the c.m. with their broad side towards the horizontal. This angle may even change for about 90° for the c.m. in the course from proximal to distal movement.By adjustment of the alignment of the broad side with the stress distribution within the different types of the h.b., a mechanically justified answer can be given to the question of why a horizontal fracture appears in the one case and a vertical fracture in the other.

Digital Photoelastic Analysis of TAD-Supported Maxillary Arch Distalization

The objective of this study was to determine whether the distribution of compressional and tensional stress around tooth roots is influenced by the position of a temporary anchorage device and the length of the retraction hook during the distalization of the maxillary dentition. A photoelastic orthodontic model was made of photoelastic epoxy resin. Six combinations of three retraction hook lengths and two posterior Temporary skeletal anchorage devices (TAD) positions were established. Stress was applied through an elastic chain for each of the combinations. Digital photoelastic stress analysis measured the compression, tensional stress, and direction around the tooth root. Using this novel photoelastic model, we found that the distribution of compressional and tensional stress during the retraction of the maxillary dentition was significantly influenced by the position of the TAD and the length of the retraction hook.

Photoelastic models applied to dental restorations

To improve the understanding of the changes generated by restorative procedures through photoelastic models. Dental reconstruction should be based on principles of occlusion in order not to cause premature contact points during preparation and restoration procedures that can lead to disorders such as occlusal trauma, temporo-mandibullary joint dysfunction, or fractures. This paper presents a didactic method to objectively visualize occlusal loads and their distribution. Photoelastic models of teeth and maxillary segments were made to visualize the effects of dental preparations and restorations, as well as the wear of premature contact points. The direction, intensity and distribution of stresses in the crown and bone support were analyzed by photoelasticity. The technique allows visualization of how the structures are affected from the cavity preparation, the different stresses with diamond or carbide burs, in addition to observing how a premature contact point not only affects the organ that contains it but also the surrounding structures.

Stress distribution around dental implants, generated by six different ceramic materials for unitary restoration: An experimental photoelastic study.

Various ceramic materials have been used for esthetic rehabilitation with implants, but the issues regarding the dissipation of masticatory loads are not well understood. This in vitro quasi-static study aimed to evaluate with the photoelasticity test the dissipation of stress around dental implants with regard to different rehabilitation materials. A photoelastic model was elaborated in resin, where a conical Morse-tapered implant was inserted. On the abutments (1 per crown), 6 single crowns were prepared using different materials to form 6 groups: feldspathic ceramic (G1); chrome-cobalt alloy covered with ceramic (G2); hybrid ceramic (G3); zirconia covered with ceramic (G4); zirconia (G5); and lithium disilicate (G6). Axial loads of 100 N (load 1) and 300 N (load 2) were applied in the center of the crowns, and photoelastic images were captured and analyzed. The total area of stress dissipation was measured for each group. Then, a computational program was developed to measure the number of pixels of the colors generated in each group. Two image sizes were analyzed - total image and crestal image. Counting the numbers of pixels of the colors in the total images showed that G6 > G4 > G5 > G1 > G2 > G3 when load 1 was applied. When load 2 was applied, the sequence was G6 > G4 > G1 > G3 > G2 > G5. In the evaluation of the crestal area, the obtained results were G4 > G5 > G1 > G3 > G2 > G6 with load 1 and G5 > G1 > G2 > G6 > G4 > G3 with load 2. Within the limitations of this in vitro quasi-static study, the findings indicate that the zirconia crown (G5) presented higher stress in the crestal images, while the lithium disilicate crown (G6) presented higher stress in the total images.

Impact of Endodontic Kinematics on Stress Distribution During Root Canal Treatment: Analysis of Photoelastic Stress

IntroductionStructural defects created by endodontic treatment are the most common cause of major dental failures. This study analyzed levels of stress produced by endodontic instruments during the root canal treatment by photoelastic analysis of stress. MethodsTwenty-four human premolars were randomly divided into 4 groups (n = 6) according to instrumentation protocol: ProTaper Next (GPT), One Shape (GOS), Wave One Gold (GWO), and TF Adaptive (GTF). The evaluation of the photoelastic model was performed at 4 dental zones: dental-crown region, cervical third of root, middle third of root, and apical third of root. Silicone molds were prepared (2 × 15 mm), and pinjets were used inside the root canals to fixate teeth. Photoelastic resin (2:1 ratio) was poured into the silicone molds to form photoelastic models. A transmission polariscope was used to analyze the positions of interest and recorded with a digital camera. Tardy’s method was used to quantify the fringe order (n) and calculate the maximum stress value (τ) at each selected point. Data were analyzed with two-way analysis of variance, Tukey test (P < .05), fringe descriptive analysis. ResultsAll groups showed a significant increase in the level of stress created during biomechanical preparation of the root canals. In the quantitative analysis, there was no statistically significant difference among the groups (P > .05). In the qualitative analysis, GPT and GTF instruments achieved greater levels of stresses compared with GWO and GOS instruments. At the beginning of instrumentation, stresses were concentrated at the coronary level and the end of instrumentation at the middle and apical root level. ConclusionsAll endodontic systems resulted in accumulation of stress along the dental structure. Stress was found in different concentrations along the tooth and at different levels.

Study on the Geo-Stress Loading and Excavation Unloading Devices of the Large-Scale Photoelastic Model Test for Deep-Buried Tunnels

At present, theoretical analysis, numerical simulation, and other methods cannot be used to properly solve the problems associated with the stability and bearing capacity of the surrounding rock and its supporting system, the interaction between the supporting structure and surrounding rock, and the sharing role of each supporting structure, all of which commonly occur in deep tunnels. The model test method represented by the photoelastic test is still an important approach to study this kind of problem. In view of the deficiency of the current loading system of the photoelastic model test, we developed a geo-stress loading system for the photoelastic model test, which can simulate the in situ geo-stress environment of unidirectional loading, bidirectional equal pressure, bidirectional unequal pressure, and tridirectional unequal pressure. The universal retaining force loading rod can realize the stability and effective compensation of loading, which is an original design. According to the principle of umbrella-shaped expansion and contraction mechanism, an excavation unloading device for the photoelastic model test is developed, which can realize the simulation of various degrees of displacement release in the excavation process of deep tunnels and other underground projects. The loading simulation test and excavation unloading simulation test show that the geo-stress loading system and excavation unloading device developed in this paper are flexible, exhibit good performance, and can fully achieve their respective test functions. The combination of two devices can compensate for the insufficiency of the current photoelastic model test and will promote the application of photoelastic model tests in underground engineering applications such as deep tunnel projects.