Photoelastic Material Research Articles

A study on the development of photoelastic model material with shape memory effect

The photoelastic model material with shape memory effect and the molding processes for the material are developed in this research. The matrix and fiber of the photoelastic model material developed in this research are epoxy resin (Araldite to hardner 10 to 3 (weight ratio)) and wire of shape memory alloy, respectively. It is called Ti50-Ni50 Shape Memory Alloy Fiber Epoxy Composite Ti50-Ni50 SMA-FEC is satisfied with the requirements of the photoelastic model material and can be used as a photoelastic model material. The maximum recovering strain of SMA-FEC is occurred at in any prestrain of shape memory alloy fiber and in any fiber volume ratio. Recovering strain(force) is increased with the increment of the prestrain and the fiber volume ratio. The best prestrain of SMA-FEC is 5% for the recovering force among 1%, 3%, 5%.

Parameter monitoring using neural-network-processed chromaticity

A PC-based neural-network processing system is described for the interpretation of chromatic sensor information monitored remotely using a CCD camera. Chromaticity based monitoring provides a means of simple data pre-processing to reduce system noise due to total intensity variation. However, sensing elements using chromaticity usually show complex variations in chromaticity with measurand changes. This contribution shows how these complex variations may be processed to yield high resolution values of a measurand. The paper presents a novel application of neural networks for monitoring temperatures using thermochromic materials and to perform 2-D pressure analysis using photoelastic materials. The inherent complex mapping and generalisation abilities of multi-layered perceptrons (MLP) make them ideal for processing the detected signals. Results are presented showing that the neural network can provide levels of resolution and performance for remotely addressing chromatic transducers, which are acceptable for detailed metrological applications.

A Photoelastic Determination of Stress Intensity Factors for Corner Cracks in a Bolted Joint

Three-dimensional photoelasticity was used to measure mode I stress intensity factors of corner cracks in a double-shear bolted joint. A model of the joint assembly was manufactured from a photoelastic material and three different corner cracks were introduced using a cutting wheel. After the stress freezing process, slices along the crack fronts were cut and analyzed using three different photoelastic procedures. There was good correlation between the methods of analysis. Stress intensity factors of these cracks were found to vary along the crack front with a maximum at the bore of the hole and a minimum on the surface of the plate. This implies that the cracks are likely to grow more rapidly along the bore.

Photoelastic study of the contact mechanics of fractal joints

The Earth's crust exhibits numerous fractures and weaknesses which significantly affect the mechanical behaviour of rock masses, such as the deformation, strength and conductivity, in which the surface roughness of rock fractures plays a dominant role. To take the influence of roughness into account, efforts have been made to characterize the surface roughness of rock joints. The brittle elastic properties of rock joints can be approximately simulated by using photoelastic material and an investigation has been performed on epoxy specimens cast against natural rock joints. In order to cntrol the roughness and to display directly the fractal effects on the mechanical behavior, a series of synthetic fractal joints with different fractal dimensions is tested in the present study by using the photoelastic method under uniaxial compression and direct shear.

PHOTOELASTIC STUDY ON THE MECHANICAL PROPERTIES OF FRACTAL ROCK JOINTS

In this paper, the fractal rock joints are manufactured on the photoelastic material plate. Based on the visualized photoelastic experiments, the mechanical properties of the fractal joints with different roughness (different fractal dimensions) are investigated under uniaxial and shear compression. The research results indicate that the joint roughness (the fractal dimension of the joints) significantly influences the peak shear strength, the position of the maximum shear stress, and the number of contact points.

Pilot study comparing the photoelastic stress distribution for four endodontic post systems

This pilot study compared stress distribution using photoelastic blocks for both vertical loading and oblique loading (26 degrees angle) for four groups of post systems (Flexi-Post, Flexi-Flange, ParaPost and AccessPost dowels). For each group, three test blocks of photoelastic material were prepared with simulated endodontic canals, and the specimens were then photographed. Minimal stresses were observed for all the posts after cementation in the unloaded state. Stress distribution for the Flexi-Post, Flexi-Flange and AccessPost were favourable under vertical loading at 134 N and oblique loadings of 89 N and 134 N. Stress patterns were symmetric over the entire length of these posts. Stress distribution for the ParaPost under vertical loading at 134 N generated stress patterns that were asymmetric with concentration of stresses apically compared to coronally. Stress distribution for the ParaPost during oblique loadings of 89 N and 134 N were asymmetric with stress concentrated apically as compared to coronally. Stress distribution for the Flexi-Post, Flexi-Flange and AccessPost were all favourable under loading conditions studied.

Sliding effect on branch crack

Brittle materials (ceramics, rocks and oce) may contain a distribution of small, grain-sized, cracks. When loaded in compression, these cracks propagate stably until they interact to give final structure. In order to understand the branching process from defects in geological conditions, a set of branch fractures, formed under uniaxial loading at the tip of a pre-existing oblique slot, was studied in PMMA (polymethyl methacrylate) plates. In actual experimental fracture of brittle plate, an elliptical slit is cut in a plate of photoelastic material. Here, the slits were made sharper than in the previous method in order to decrease the applied stress necessary to cause crack growth. The growth of branch cracks was slow and could be stopped or started at will. Most of the cracks could be stopped before they reached the loaded ends of the PMMA plates. It was found that : 1.- Two classical branch cracks, formed from the tip of a central preexisting oblique slot in PMMA plate submitted to an uniaxial loading, are symmetrical with the defect center and rejoined the direction of the applied compression ; 2.- As for an open slot (no friction), the wing cracks were initiated at a certain distance from the initial crack tip ; 3.- For a given friction coefficient, this distance and the branching crack angle (which is less than 90°) increased with the slot-loading axis angle. The study of the stress field linked to an elliptical slot under pure shear conditions is in good agreement with our experimental observations.

Experimental determination of stress intensity factors of cracks in sheet structures with bolted stiffeners

The optical method of photoelasticity was used to measure stress intensity factors of cracks in stringer stiffened structures. From a photoelastic material L-shaped stringers and plates with central holes were manufactured, and one stringer was attached vertically to one side of each plate by means of bolting. In each test a crack was initiated from the side nearest the stiffener and extended in a direction perpendicular to the tensile load in the plate. With every crack extension the associated stress intensity factor was determined from the crack tip isochromatic fringe pattern by the multiple point over-deterministic method. The results showed that the presence of a stringer produces a significant reduction in the stress intensity factor and this reduction is affected by bolting position, thickness of the stringers and the method of load transfer from parent plate to the stringer.

Precise optical force sensors made of polycarbonate or epoxyde disks

The interference effects in the centers of "disklike" whole cylinders of different photoelastic materials loaded by uniaxial forces acting along diameters are studied. The intensity of the light passing through the cylinder is analyzed and a few numerical models of the phenomenon are constructed and compared with the experimental results. The dispersive character of the "photoelastic constant" is shown and its consequence for the effect are emphasized. A computer-aided spectrometer was specially constructed for the research and is the heart of a semiautomatic measurement stand. The utilization of the effect to construct an optical force sensor is shown.

Polymer materials in practical uses of photoelasticity

A brief review of photoelastic materials comprising primarily high-sensitivity polymers and their mechanical and optical properties is presented. This review is followed by a discussion of their advantages, disadvantages, and the ranges of their applicability. The main selecting criteria of proper materials for photoelastic studies are listed and the relations between the chemical structure of polymers and their photoelastic and mechanical properties are discussed. The calibration methods are briefly described.

Biomechanical rationale for surgical-orthodontic expansion of the adult maxilla

Objective: To develop a biomechanical rationale for surgical-orthodontic correction of transverse maxillary deficiencies in adults by clarifying the internal stress responses to rapid palatal expansion in a photoelastic model. Materials and Methods: A three-dimensional photoelastic analog of an adult human skull was constructed by fabricating the individual facial bones from a photoelastic material and fixing them along their anatomic sutural areas. After determining the force-activation characteristics of a Hyrax expansion appliance in a straining frame, the appliance was applied to the analog and incrementally activated. The stresses developing at the different craniofacial areas were visualized and photographed in the field of a circular polariscope. Sequential cuts were performed to simulate midpalatal, zygomatic buttress, and pterygomaxillary osteotomies, and the alterations in the internal stress responses were recorded after each individual cut. Results: The force-activation characteristics of the Hyrax appliance are in the orthopedic range (500 g). The magnitude and distribution of the stresses induced by appliance activation differed notably between the simulated osteotomies. Analysis of fringe patterns showed that the midpalatal and pterygomaxillary articulations were the primary anatomic sites of resistance to expansion forces. The patterns of distribution and the increase in the magnitude of the stresses at superior sutural location were particularly pronounced after the pterygomaxillary cuts. The forces produced by the Hyrax appliance had deep anatomic effects, with internal stresses also manifesting at regions distant from the site of force application. Conclusions: Based on photoelastic observation, it may be concluded that complete midpalatal and pterygomaxillary osteotomies are essential for predictable skeletal expansion in adults. Exclusive use of bilateral zygomatic buttress osteotomies appears to be inadequate. The expansion forces exerted by the Hyrax appliance are orthopedic in nature and produce deep anatomic effects. Clinicians should be aware that the craniofacial stresses produced by appliance activation may be experienced by the patient as pain or discomfort.

Comparison of the Photoelastic Stress for a Split‐Shank Threaded Post Versus a Threaded Post

This study compared the stress distribution during insertion and after cementation for two prefabricated endodontic posts, (1) a split-shank threaded post, and (2) a threaded post with no split in the shank. Three test blocks of photoelastic material were prepared with simulated endodontic canals for each group studied. Specimens were then photographed after insertion and cementation. Minimal stresses were observed for the post with a split (group 1) throughout the length of the shaft, whereas asymmetric patterns of stresses for the post without a split (group 2) were present. Stress distribution for the post without a split were concentrated more apically rather than coronally. The presence of a split reduces insertion and cementation stresses for this post system. The split also acts as a vent during the cementation of the post into the canal and minimizes stress concentration during cementation.

47610 Determination of the elastic constants of photoelastic material at freezing temperature by the method of caustics (In Japanese: English Abstracts) : Wu, D.; Takahashi, S. Journal of JSNDI, vol. 40, no. 8, pp. 530–535 (Aug. 1991)

47610 Determination of the elastic constants of photoelastic material at freezing temperature by the method of caustics (In Japanese: English Abstracts) : Wu, D.; Takahashi, S. Journal of JSNDI, vol. 40, no. 8, pp. 530–535 (Aug. 1991)

Stress Analysis of Orthotropic Plates by using Isotropic Photoelastic Material

Stress Analysis of Orthotropic Plates by using Isotropic Photoelastic Material

Elastic impact loading of notched beams and bars

Four types of impact specimen, Charpy, Izod, Slender Cantilever and Double Notched Bar, are studied using the technique of dynamic photoelasticity. The growth of fringes with time in low modulus photoelastic material is analysed for each speciment and it is concluded that the stress wave behaviour of the Cantilever geometries and Double Notched Bar offer advantages over the traditional Charpy specimen.

Photoelastic material characteristics based on the stress-induced effect for sensor application

An investigation of the properties of photoelastic sensing materials via the stress-induced effect using an interferometric technique is presented. Sensing material properties such as stress relaxation, elastic limit, and its temperature dependency are investigated with a view to using such a material as a stress/ pressure sensing device.

Photoelastic evaluation of the standard bionator appliance.

A study was undertaken to demonstrate the forces in the craniofacial complex generated by the activation of the standard bionator appliance. A three-dimensional skull replica of an orthodontic patient with a Class II Division 1 malocclusion in the mixed dentition was fabricated using a urethane based photoelastic material to simulate bone. Four muscles of mastication were simulated on the photoelastic model. Various anatomical areas of the skull were viewed in a circular polariscope and recorded photographically before and after inserting the bionator. The downward and forward repositioning of the lower jaw by the bionator appliance caused various stresses on the craniofacial complex. The photoelastic stresses visualized in this investigation would most likely cause: an increase in the gonial angle; downward repositioning of the midface; and a posterior displacement and expansion of the maxilla.

An optical pressure sensor using a low-coherence light source with a photoelastic sensing element

Abstract This paper presents the results of an investigation of an optical pressure sensor, using photoelastic material as the sensing element. A change in the optical path of light propagating in the stressed sensing element has been observed by monitoring a corresponding change in the optical path in a ‘recovery’ optical interferometer. The pressure to be measured is applied by placing a calibrated mass on the sensing element. Two optical arrangements, each using a low-coherence light source, have been studied. Results are presented showing the potential of such a system for wider pressure-sensor applications.

The dispersion of birefringence in photoelastic materials

AbstractA spectral analyser was used to determine the variation with wavelength known as the dispersion of birefringence of the fringe constant f and the stress optic coefficient C for photoelastic materials. A disc was loaded diametrically and was subjected to the stress freezing thermal cycle. The intensity was measured along the diameter perpendicular to the load for wavelengths between 450 and 850 nm at 5 nm intervals. Using the theoretical solution to the stresses it was possible to calculate the stress optic coefficient for all wavelengths employed. A similar technique was used for an identical disc that was loaded at room temperature.It was found that the fringe constant was extremely linear with wavelength for both types of loading and for the materials CT200, CT1200 and MY750. The stress optic coefficient did not change much between sheets of the same material but was markedly different between materials. The dispersion of birefringence was found to be approximately constant but showed evidence of anomalous dispersion.

Novel Photoelastic Approach in Analysis of Elliptical Holes in Thick Plates

Nondestructive photoelastic stress analysis of elliptical holes in thick plates was carried out. Using Plexiglas as a model material, the scattered photoelastic technique was used. In a novel approach, the residing birefringence in unstressed commercially available polymer plates was introduced as an information carrier. Pairs of monochromatic coplanar rays of polarized light were sent, in two perpendicular directions. The experimental data captured from each pair, for points at the load‐free boundaries of the holes, were combined to achieve a complete determination of the stress state at the point of a pair's intersection. The nondestructive technique used allows optical scanning of stresses through the thickness. The photoelastic data obtained was utilized in determining stress‐concentration factors at the ends of the major and minor axes of the elliptical holes. The experimentally obtained distributions of the stress concentration through the thickness were compared with those cited in the literature. The introduced photoelastic approach along with the material array of equidistant parallel lines proved to work efficiently. Moreover, it has been shown that Plexiglas can be employed, in spite of its low stress‐optic coefficient, as a quasi‐sensitive photoelastic material.