Maximum Fringe Order Research Articles

APPLYING PHOTOELASTICITY WITH OPTICAL MAGNIFICATION METHOD TO INCREASE PRECISION OF DETERMINING FRINGE ORDER UNDER MONOCHROMATIC LIGHT

Photoelasticity method was used to measure strain and stress fields to address the precision problem of determining fringe order of loading and cracking and notch areas. In this study, microscope was used as an additional tool in a photoelastic device for magnification of isochromatic fringe size. An experimental method was used by utilizing optical system and reflective polariscope. A simple compression tool was designed to provide a compression load on the solid disk-shaped specimens. Isochromatic fringe field was observed in the loading zone for different loading and captured image based on the following conditions: 9x, 15x and 20x without magnification and with magnification using SLR type Nikon camera Serie D3000. The image was processed using OpenCV software while C + + was used to produce clearer isochromatic fringe pattern formed on the surface of the specimen in the determination of the maximum fringe order observed. The average of additional fringe could be obtained as the optical magnification result. Data was analyzed using Finite Element Analysis Software. The result showed an increasing trend in fringe order with increasing levels of optical magnification. Its maximum value was obtained on20x optical zoom, which was 2-order fringe.

A study on the development of photoelastic experimental hybrid method for colour isochromatics (I)

Using the photoelastic experimental hybrid method, we can obtain stress intensity factors and separate the stress components near the crack tip of any material under various loading conditions from only isochromatics. However, monochromatic light has, until now been used in the photoelastic experimental hybrid method to produce black and white isochromatics. The existing photoelastic experimental hybrid method using black and white isochromatics requires high order fringes to obtain sufficient experimental data for photoelastic hybrid techniques. If a photoelastic experimental hybrid method for colour isochromatics is developed, isochromatics with a maximum fringe order of 2 will be adequate for gathering the necessary experimental data for stress analyses using photoelastic hybrid techniques. Therefore, in this work, photoelastic experimental hybrid method for colour isochromatics of a crack problem was developed, and its superiority demonstrated by comparing its results with the results from black and white isochromatics. The use of few fringe orders in photoelastic experimental hybrid method for colour isochromatics is an important feature of this method. Because of this feature, this method, specifically the reflection type photoelastic method where low fringe orders are produced compared to the transmission type photoelastic method, can be widely used in stress analysis of real components.

RGB Photoelasticity: Review and Improvements

Abstract: This paper considers the main developments of RGB photoelasticity with reference to the maximum measurable retardation. In this paper, a new procedure based on the standard error function evaluated on a subset of the calibration array is also proposed and experimentally tested. The experiments show that the filament lamp makes it possible to find retardations until approximately 4 fringe orders while the fluorescent lamp makes it possible to determine higher fringe orders (12 fringe orders in this paper) owing to the discrete spectrum of the source. The paper shows that, by using the incandescent lamp, the primary limiting factor is the lack of modulation of the R, G and B signals whereas, by using the fluorescent lamp, the limitation of the maximum fringe order derives mainly from the gradient of the fringes and the procedure of search of the retardation.

Photoelastic Investigation of Bimaterial Interfacial Stresses Induced by Thermal Loading

Abstract: In this paper, the photoelastic technique and finite element method were employed to investigate the interfacial stresses induced by thermal loading in bimaterial structures. By observing the photoelastic fringe patterns, in comparison with those theories developed by other investigators, severe temperature and stress gradients were found across the specimens’ heights. The maximum fringe order near the interface was six times that near the top surface of the photoelastic material when the temperature was raised to 65 °C. Furthermore, for the bimaterial structures, differences were found between measured stress fields as a result of increasing and decreasing temperature differences.

The Effect of Bending on the Normalized Stress at Roots of Threaded Connectors

Abstract Three-dimensional photoelasticity was used to analyze the effect of bending on the normalized stress at the roots of threaded connectors. Loading was effected by steel cages and a combination of eccentric weights (to provide the bending load) and concentric weights (to provide the axial load). The ratio of the bending stress to the axial stress was determined and various levels of this stress ratio, Rσ, were tested. The connections were analyzed by taking thin slices in the plane of bending and perpendicular to it. The position of the maximum fringe order at the roots was determined using Mesnager’s theorem and the maximum fringe order found by Tardy compensation. The fringe orders were normalized using the nominal axial stress and the total nominal stress (bending plus axial stress), which were calculated from the loads applied. The results, when normalized using the nominal axial stress and compared to those in connections without bending, exhibit a lower and broader peak of normalized stress values plotted against the helix length. The normalized stress values are also periodic in relation to the bending plane due to the variation in stress around the longitudinal axis of the bolt. It was found that bending in connectors will affect the normalized stress and that it is possible to determine this effect in a similar way to the method used for axially loaded connections.

The use of carrier fringes in shearography for locating and sizing debonds in GRP plates

Debonds in laminated plates when subjected to vacuum stressing are readily revealed using conventional shearography. In estimating the depth of these debonds from the perturbed fringes, difficulty often arises in assigning accurately the maximum fringe order which corre-sponds to the maximum deflection gradient of the surface of the debonded region. By superposing a set of linear reference carrier fringe pattern onto a conventional shearogram, it is shown in this paper that debonds embedded in a glassfibre-reinforced plastic (GRP) plate may be detected easily from the resulting perturbed fringe pattern. The size and depth of debonds may also be estimated with good accuracy, whether the perturbed carrier fringes are overdriven or non-overdriven. Analysis also indicates that it is generally desirable to use a dense set of reference carrier fringe patterns, which results in a non-overdriven fringe pattern, because it enables a more accurate estimation of the maximum fringe order; the accuracy in the determination of the debond depth is thereby improved.

Stress intensity factors for near edge cracks by digital image analysis

Experimental stress intensity factors (SIFs) are evaluated for single, straight near edge cracks in a plate of finite width. The cracks are positioned at various angles and distances with respect to the edge of the plate. The far field stress is uniform tension parallel to the edge. Online digital image analysis procedures are used to extract photoelastic data from the whole field isochromatic fringe patterns. Two different techniques are used to obtain information on the near crack tip stress field: (1) Half fringe photoelasticity (HFP), which requires adjusting the load to keep the maximum fringe order less than 0.5 in the data extraction zone, and (2) Trace, a digital fringe sharpening procedure, which accurately produces traces of all half- and full-order fringes. The load is adjusted to yield multiple fringes in the field of data extraction. Experimental SIFs from HFP and Trace are compared to each other and to the numerical results that are determined by the boundary integral equation method.

Angulation loading effects on cast-gold endodontic posts: A photoelastic stress analysis

Two-dimensional photoelastic stress analysis was used to investigate the effects of post diameter on stress distribution under angulated (26-degree) loads. Ten models were used; five for each diameter were tested. The models were placed in the straining frame of the polaroscope, and compressive forces of 30 and 60 pounds were applied to each of the models. Instantaneous photographs were made immediately after each load was applied. The fringe patterns were quantitated by determining the maximum fringe order in the test model. The results of this investigation suggest that (1) post diameter should be restricted with heavy anterior masticatory forces and (2) restricting post diameter conserves tooth structure and substantially reduces internal stresses within teeth that have cast-gold posts.

Photoelastic stress analysis of cast-gold endodontic posts

Two-dimensional photoelasticity was used to compare stresses generated upon loading of cast gold endodontic posts of two different diameters. Ten models were used, five for each diameter size tested. The models were constructed to simulate maxillary central incisors. Drill bits were used to prepare the root canal, and after root canal preparation was completed the post preparations were constructed using the Parapost system. The gold posts, made of type III gold, were cast using the Beauty Cast technique. A composite core was then placed on each of the posts to be tested. These models were positioned in a circular polariscope and vertical loads of 30 and 60 pounds were applied to each of the models, with the fringe patterns photographed immediately after each load was applied. The stresses were evaluated by determining the maximum fringe order in the simulated root and periodontium of the model. The results of this study suggest that (1) the diameter of a post with a core affects the magnitude of stress, (2) stress generally increases as post diameter increases, and (3) stress increases as the vertical load increases. It seems advisable to restrict the diameter of the post used in providing support for restorations, particularly when large occlusal for forces are exerted by the patient. The use of smaller diameter posts reduces the stresses exerted upon the dentin of the tooth and the tooth supporting structures, limits the amount of tooth material removed during preparation and, possibly, aids operative procedures by limiting the amount of tooth preparation.

On some differences between dynamic- and static-stress distributions

Dynamic-stress concentrations due to elastic waves are analyzed by high-speed photoelasticity, and some differences between dynamic- and static-stress distributions are clarified. Specimens such as long struts with shoulders or notches are loaded dynamically by an elastic wave behind the front of a rectangular pulse with a plateau of constant stress ρcv (ρ=density,c=elasticwave velocity,v=tensile velocity) and of long duration. Dynamic isochromatic patterns caused in polyurethanerubber specimens are recorded with a “Himac 16H” high-speed camera (framing speed: 10,000 pictures/ sec). It is found that dynamic-stress-concentration factors, that is, maximum fringe order in passage of the wave front and the initial part of the plateau, divided by the fringe order corresponding to the height of the plateau, are about 0.5–0.6 times the static-stress-concentration factors, for the specimen dimensions considered. An approximate theory of dynamic-stress concentration factor is derived by considering notches as the discontinuities in the cross-sectional area of strut. This theoretical consideration correlates somewhat with the values of dynamic-stress-concentration factors obtained experimentally.