Transmission Polariscope Research Articles

Development of a floating element photoelastic force balance

We present a floating element force balance design that uses an optical measurement of the force using photoelastic stress analysis. The force sensing element consists of pins embedded in photoelastic polyurethane pads, which generate an internal stress when the floating element is loaded that is observed via a transmission polariscope. A series of known loads and their corresponding fringe patterns allow a calibration matrix to be derived using a polynomial model solved by least squares regression. Finite element analysis (FEA) simulation is carried out to validate the proposed method. The balance then measured a lift curve of the NACA0015 wing at low speed. A comparison of the photoelastic balance and a commercial, 6-axis strain-gauge load cell showed typical differences of less than 6%. This optical approach enables accurate measurements with inexpensive and simple components inside the sensor. This work demonstrates that a photoelastic balance is a simple, inexpensive, and sensitive force transducer.

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.

Use of an automatic device for the determination of mechanical properties on photoelastic samples of small dimensions

Abstract Photoelastic materials and Photoelasticity have been very effective in the study of the effects of stress and strain in materials such as iron, steel, concrete, etc. However, mainly because of human interaction, systematic errors are introduced during the measurement process, which directly affects the final result. When using small photoelastic samples, these types of errors can escalate due to the increased difficulty of handling. Intending to minimize systematic errors in the process of data collection, this work presents the use of an automatic loading device, developed by us, to apply mechanical stresses in small photoelastic samples. The device created has shown great effectiveness in photoelastic applications that uses a transmission polariscope, but it can also be used in other photoelastic techniques, such as reflection photoelasticity, as well as in other areas of optics such as Holography and Speckle. An important benefit is the ease and speed of handling that can improve the organization and quality of the activities in labs of optics.

Photoelastic Analysis of Two Different Cementless Femoral Stems for Total Hip Arthroplasty in a Canine Model

Objectives The aim of this article was to analyse and compare internal stress generated at different points of a femoral photoelastic model after insertion and axial load application using two different cementless femoral stems for total hip arthroplasty in dogs: a collared stem combined with limited textured surface and absence of grooves and a collarless stem with fully textured surface and grooves. Methods Ten femoral photoelastic models, divided into two groups, were created using two different designs of cementless femoral stems. The models were submitted to axial loading on the femoral head in a universal test machine. The internal stress (kPa) around the femoral stems was evaluated at seven predetermined points using a transmission polariscope. Results The internal stress at the femoral calcar was larger in the models with collared stem combined with limited textured surface and absence of grooves (p &lt; 0.05). No differences were identified between the groups in the other points (p &gt; 0.05), corresponding to the tip of the stems and proximal lateral region of the femur. Conclusions The collar of femoral stem combined with the absence of grooves and more limited textured surface increase the axial load transmission to the femoral calcar, and in vivo, it may act to reduce complications, such as subsidence and stress shielding. However, other biomechanical tests and clinical evaluations must be performed to determine other important aspects for the implantation of these prostheses.

Comparative analysis of stress in a new proposal of dental implants

The purpose of this study was to compare, through photoelastic analysis, the stress distribution around conventional and modified external hexagon (EH) and morse taper (MT) dental implant connections. Four photoelastic models were prepared (n=1): Model 1 – conventional EH cylindrical implant (Ø 4.0mm×11mm - Neodent®), Model 2 – modified EH cylindrical implant, Model 3 – conventional MT Conical implant (Ø 4.3mm×10mm - Neodent®) and Model 4 – modified MT conical implant. 100 and 150N axial and oblique loads (30° tilt) were applied in the devices coupled to the implants. A plane transmission polariscope was used in the analysis of fringes and each position of interest was recorded by a digital camera. The Tardy method was used to quantify the fringe order (n), that calculates the maximum shear stress (τ) value in each selected point. The results showed lower stress concentration in the modified cylindrical implant (EH) compared to the conventional model, with application of 150N axial and 100N oblique loads. Lower stress was observed for the modified conical (MT) implant with the application of 100 and 150N oblique loads, which was not observed for the conventional implant model. The comparative analysis of the models showed that the new design proposal generates good stress distribution, especially in the cervical third, suggesting the preservation of bone tissue in the bone crest region.

Experimental study on elasto‐optic effect in bulk GaN crystal

AbstractElasto‐optic effect has been studied experimentally in bulk α‐GaN crystal. By using an imaging type transmission polariscope, two‐dimensional distribution of refractive index difference |Δn | was measured in the c‐plane of beam‐like samples (3×3×10 mm3) while applying the external stress compressive to its longitudinal directions. The two‐dimensional distribution of |Δn | revealed its local fluctuation reflecting residual strain distribution of the sample because there is no natural birefringence in c‐plane. At the initial stage of stress impression, the |Δn | was increased or decreased at each point according to whether the residual strain is compressive or tensile along the stress‐applied direction. The |Δn | was finally increased with a similar slope at the final stress‐impression stage, regardless of the residual strain level. The |p11‐p12| was obtained as 0.13 from the averaged slope of the final stress‐impression stage, which is useful for estimating the magnitude of residual strain in c‐plane GaN substrates. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Photoelastic stress analysis of endodontically treated teeth restored with different post systems: normal and alveolar bone resorption cases

The present study examined the influence of different post materials and their lengths on the mechanical stress of endodontically treated incisor roots in two alveolar bone conditions. Two-dimensional photoelastic models were fabricated to simulate the endodontically treated maxillary central incisors restored with three kinds of posts materials (low Young's modulus glass fiber post, high Young's modulus glass fiber post, and prefabricated stainless steel post) and two post lengths (8 and 4 mm). Completed models were placed in a transmission polariscope and loaded with a static force of 150 N at 45° to the tooth axis. Photoelastic photographs and the magnitudes of fringe order revealed stress distribution in the root, and suggest that the glass fiber post with a low Young's modulus and long length can reduce the stress concentration both in normal and alveolar bone resorption conditions.

Photoelastic stress analysis of different post and core restoration methods

The aim of this study was to compare three types of post and core systems by analyzing the stress magnitude within the root. To this end, two-dimensional photoelastic simulation models of endodontically treated maxillary central incisors were fabricated. Three different types of post and core systems were selected for this study: composite resin post and core, composite resin core in combination with a glass fiber post, and conventional cast metal post and core. The fabricated models were observed in a transmission polariscope with the same loading force (400 N) on 45 degrees palatal direction and the fringe orders registered were thereby analyzed. Results obtained in this study suggested that abutment build-up using composite resin core in combination with a glass fiber post model produced the lowest stress concentration and is hence effective in preventing stress concentration in the case of restored endodontically treated teeth.

Enhancement of Mechanics Education by Means of Photoelasticity and Finite Element Method

The goal of this project is to enhance mechanics education by incorporating experiments using photoelastic stress analysis and finite element analysis within the existing curricula. Specific instructional objectives for students are to: (a) increase conceptual understanding of stress distribution through photoelastic and finite element based visualization, (b) gain experience with photoelasticity and finite elements including their advantages/limitations, and (c) appreciate the synergism between experimental and numerical methods of stress analysis. At UOP, one transmission polariscope set, one reflection polariscope set, and accompanying accessories have been purchased and installed for students' use. This equipment has also been used for in-class demonstrations and motivational presentations to high school students. Additional apparatus have been made in-house and/or purchased by departments at UOP and USAFA. All mechanical engineering students at UOP have benefited from this project. Several experiments have been introduced in the curriculum, and several student projects have been completed. A similar combination of photoelastsic demonstrations plus finite element results has been used at USAFA, which has exposed both engineering and non-engineering majors to this educational enhancement.

On the feasibility of determining discrete photoelastic parameters using the multi-load method

An alternative method for the determination of three-dimensional stresses in photoelastic models is discussed. The model is sectioned into discrete, imaginary slices and a relationship is derived between the retardation and isoclinic angle of each slice and characteristic photoelastic parameters measured using a transmission polariscope. An illustration of the multi-load method is given and the practical application of the technique is discussed.

Análise fotoelástica da distribuição de tensões produzida na mandíbula pela simulação do esforço mastigatório em sobredentaduras muco-dento-suportadas e prótese total

O objetivo deste estudo foi verificar a distribuição dos esforços mastigatórios em mandíbulas sob próteses do tipo sobredentaduras e prótese total. Foram confeccionadas três mandíbulas em material fotoelástico, obtidas pela moldagem de uma mandíbula humana desdentada. Os moldes foram preenchidos com resina fotoelástica PLM-4 (Photolastic Inc., Raleigh, USA). Variou-se a disposição dos pilares radiculares nas réplicas, dividindo-as em três grupos: Grupo I, a mandíbula não apresentava raízes suportes (grupo controle); Grupo II, raízes dispostas bilateralmente na arcada dentária correspondendo às raízes dos dois caninos; e Grupo III, raiz de um canino e de um molar dispostos no mesmo hemiarco, de maneira unilateral. Sobre estas, foram confeccionadas prótese total convencional e sobredentaduras, respectivamente, as quais ocluíam em um modelo padrão de resina acrílica termopolimerizável montado num articulador semi-ajustável para a aplicação do carregamento (20 N), simulando a carga mastigatória. Após o congelamento das tensões, as réplicas foram cortadas em sua linha mediana e analisadas num polariscópio de transmissão (Série 060, Photolastic Inc., Raleigh, USA). Pelo padrão de distribuição de tensões observadas puderam concluir que: 1) os três modelos fotoelásticos testados apresentaram diferentes padrões de distribuição de tensão; 2) a mandíbula do Grupo I concentrou mais tensões na região de rebordo; 3) a mandíbula do Grupo II apresentou a melhor distribuição das tensões quando comparada com os outros grupos, pois concentrou a tensão no ápice dos dentes pilares minimizando sua concentração na região desdentada; 4) no Grupo III houve maior concentração dos esforços tanto nos ápices radiculares como na região intermediária desdentada.

The multi‐load method of determining discrete system parameters in a loaded three dimensional photoelastic model

AbstractThe determination of the secondary principal stress difference and their orientations for a discrete slice of thickness dz along a particular light path in a loaded three dimensional photoelastic model is discussed in this paper. It is well known that once the geometry of the loaded member and the geometry of loading on the member are fixed, the secondary principal stress directions at any particular point along any line of consideration remain constant. The secondary principal stress values will get multiplied by the factor through which the loading on the component is increased. This has been made use of in the present method and from the characteristic parameters obtained at the end of any light path for different loadings on the three dimensional model, the secondary principal stress difference and their orientations are obtained. The method proposed is a fully non‐destructive method of testing and involves measurements only on a transmission polariscope.

Observation of orthopedic force distribution produced by maxillary orthodontic appliances

A three-dimensional anatomic model was duplicated from a human skull, using different birefringent materials to simulate the various craniofacial structures. Individual bones of the midface were fabricated separately and then articulated in their correct sutural relation. One removable and four fixed maxillary appliances were used. The fixed appliances included the Haas, Minne-expander, Hyrax, and quad helix devices. The removable appliance incorporated an expansion screw in a full acrylic palate with appropriate retentive clasps. After the insertion of each appliance, intraoral forces were produced by incremental activation. The model was examined and photographed in the field of a transmission polariscope. Each appliance used produced a different range of load-activation characteristics. This was reflected by the differences in the stresses transmitted through the bones of the craniofacial complex and the effect on the various sutures. Stresses produced by the fixed appliances were concentrated in the anterior region of the palate, progressing posteriorly toward the palatine bone. The Haas, Minne-expander, and Hyrax appliances produced stresses that radiated superiorly along the perpendicular plates of the palatine bone to deeper anatomic structures, such as the lacrimal, nasal, and malar bones, as well as the pterygoid plates of the sphenoid. Similar stress characteristics were seen with the removable appliance. However, increased activation decreased retention of the appliance, thereby lessening the stress. The quad helix appliance proved to be the least effective orthopedic device. Although the effects of palate separation were seen with increased activation, this appliance primarily affected the posterior teeth.

A non‐destructive method to determine stresses in a three‐dimensional photoelastic model using a transmission polariscope

The determination of stresses within a loaded three dimensional photoelastic model without cutting it into slices has been the ultimate aim of many investigators. The standard methods using a transmission polariscope do not yield much information. Scattered light methods no doubt enable one to completely determine the state of stress at a point within the body. However, the methods proposed up to now necessitate the use of an arrangement where both the body under test and the sensing unit which picks up the scattered light or both will have to be rotated. This leads to a complicated experimental set‐up and increases the possibility of making errors, especially while rotating the model. In the proposed method, most of the information needed to determine the state of stress at a point within the body is obtained using the transmission polariscope set‐up. The model and the direction of observation of scattered light are fixed. This allows observations to be made in test models which have at least one flat face without using a liquid with matched refractive index and increases the accuracy of measurements. The method is an almost purely transmission polariscope type of method and where space permits tests can be done using ‘live’ loading without having to use the stress freezing technique.

Integrated Photoelasticity Of Axisymmetric Bodies

The paper reviews integrated photoelastic methods for stress determination in axisymmetric bodies. The specimens are put in an immersion bath and viewed in a common transmission polariscope. If birefringence of the specimen is high, integral fringe pattern as well as quasi-isoclinics may be photographed. By point-by-point technique the characteristic directions as well as the characteristic phase retardation on a number of light rays are measured. This experimental data, often combined with the application of the equilibrium equations and macrostatic equilibrium conditions, permit one to determine stress distribution in the specimen. Investigation of stresses in isotropic speci-mens as well as in cubic single crystals is considered. A new method of determining stress in cylinders, including measurement of the angular deflec-tion of light rays, is described.

Photoelastic investigation of a shrink-fit problem

Stress analysis of the components of a shrinkfit assembly has been carried out by the three-dimensional photoelastic technique. Experimental results obtained from the analysis of a typical shrink-fit assembly, wherein a cylindrical wheel is shrink fitted onto a hollow cylindrical hub in an asymmetrical position. Photoelastic models made from hot-setting Araldite B were loaded at the critical temperature in a stress-freezing oven and the meridional and the hoop slices were observed in a transmission polariscope. It is concluded from this investigation that the bending induced in the components due to the asymmetrical placement of the wheel and the contact shear at the interface influences the stress distribution quite considerably. Also, the dimensions of the components in the axial direction have an influence on the mode and contribute to the magnitude of the variation of stress components.

Photoelasticity with additional physical fields and optically active models

The aim of the paper is to enlarge the class of three-dimensional stress distributions which can be investigated in a transmission polariscope. The experimental data are shown to contain more information about the stress distribution on the wave normal if the rotation of the principal axes or that of the plane of polarization is induced. This can be obtained by additional physical fields or by using optically active models. Three examples are described. First, an exact theory for investigating bending stresses in plates by the aid of prefrozen models has been developed and applied to evaluate the experimental data. Second, the application of the magnetophotoelastic method to determine stress-concentration factors by cylindrical bending of plates is considered. Third, a nonlinear stress distribution along the wave normal has been determined experimentally in an optically active model.

Response of the integrated polariscope

A new photoelastic technique has been developed for simultaneous recording of the amount of birefringence in transmitted and in scattered light. This paper presents the basic theories of transmission and scattered-light polariscopes and hence deduces the theoretical foundation of the method and technique of the developed integrated polariscope. A laser is used as light source, or two identical lasers are used. In a one-laser system the linearly polarized ray is split into two beams, one of which is expanded and passes through the standard transmission polariscope, and the other, being about 1 mm in diameter, enters the photoelastic object parallel to its middle plane. An ‘integrating prism’ has been developed for simultaneous recording of the standard isochromatics and of three scattered light beams; a beam scattered parallel to the polariscope axis and two complementary beams scattered at angels of − π/4 and + π/4.

Applications of lasers to photoelasticity

This paper discusses briefly the principles of gaseous and ruby lasers and makes comparisons of pertinent properties of lasers and conventional light sources. Since the output from a laser is a highly collimated beam of monochromatic light that can be made extremely intense and plane polarized, the eminent suitability of lasers as light sources for photoelasticity is suggested. Lasers are shown to be superior to conventional light sources, especially for the scattered-light method and for high-speed dynamic work. Experiments are described and results are given for using lasers for (1) the scattered-light method, (2) a conventional transmission polariscope with static loads and (3) dynamic photoelasticity.

The Photoelastic Analysis of Transverse Bending of Plates in the Standard Transmission Polariscope

Abstract In this paper the ideas of three-dimensional photo-elasticity are applied to the analysis of plates under transverse bending, resulting in a simple photoelastic method, employing the standard two-dimensional transmission polariscope. The procedure used is to “freeze” a fairly high initial tension in bakelite or similar material, cut out the model at some angle to the direction of the tension, bend, and observe. Results of various tests are given; photographs of two cases, the circular hole and the semicircular notch, are included. Stress-concentration factors are also obtained and analyzed. The problem of the thick plate as contrasted with the thin plate, assumed in the usual theory, is discussed briefly.