Showing papers by "K. Ramesh published in 2007"

Colour Adaptation in Three Fringe Photoelasticity

Abstract: Three fringe photoelasticity (TFP) can give the total fringe order from a single colour isochromatic fringe field by suitably comparing the colour with a calibration specimen. The fringe order evaluation can be erroneous when the materials for the calibration specimen and the application specimen are different. This is because of the colour variation between the two materials. This is conventionally handled by preparing individual calibration tables for each application. A new methodology to tune the calibration table obtained for a single material to accommodate the tint variation in TFP is proposed for the use of different specimen materials. Discontinuities in fringe order variation are smoothed using the refined TFP (RTFP) procedure. The elegance of the new methodology for solving a multi-material system is bought out by solving the problem of a bi-material Brazilian disc. The results obtained are compared with the phase shifting technique.

New Initiatives on Comparison of Whole-field Experimental and Numerical Results

Abstract: A simple approach to plot photoelastic fringes in grey scale and also in colour from finite element (FE) results is presented for better recognition and comparison with experiments. This requires proper identification of the plotting variable from FE results. For comparison with transmission photoelasticity, post-processing of principal stress difference is needed and for reflection photoelasticity the principal strain difference is to be used. The importance of the use of appropriate correction factors for comparison with reflection photoelastic results is emphasised. A newer approach to evaluate Rf for complicated geometries is indicated. Plotting of experimental fringes from finite elements is useful not only for validating the numerical model based on experiments but also for validating the experiments. To illustrate this, the problem of an interfacial crack in a bi-material Brazilian disc is discussed.

Smoothing of Digital Photoelastic Data Using Robust Outlier Algorithm

Abstract: Photoelasticity is an optical technique for experimental stress analysis. It is widely used for 2-D and 3-D analysis of components for getting the information of principal stress difference (isochromatics) and principal stress direction (isoclinics) at every point in the domain. With the advent of digital computers, recording of images as intensity data became easier and a separate branch of photoelasticity namely digital photoelasticity came into existence [1]. In digital photoelasticity intensity information of the captured image is used for evaluating the isoclinic and isochromatic parameters. Thus, in principle one gets values of isoclinic (gq) and isochromatic (N) for the whole-field in the form of phase map [1]. Phase shifting / polarization stepping techniques are widely used in digital photoelasticity for getting the phase maps [1]. But these phase maps are wrapped and has to be unwrapped for getting continuous phase values. Both isoclinic and isochromatic phase maps have interdependence and this interaction affect their evaluation. For stress separation studies, one requires both isochromatics and isoclinics accurately free of any kinks in the domain.

Digital Photoelastic Evaluation of Isochromatic Fringe Order in a 3D Model of Complex Rocket Motor Strap- on Joint

Abstract: Digital Photoelasticity is a whole field technique, which provides the information on magnitude of difference in principal stresses (isochromatics) and direction (isoclinics) of principal stresses at every pixel in the image domain based on intensity processing [1]. Phase shifting techniques which are commonly used for quantitative estimation in digital photoelasticity give only a wrapped isochromatic phase map. This has to be unwrapped for obtaining the absolute phase values. One of the main issues in wrapped isochromatic phasemap is interpretation of the sign of the fractional retardation calculated. This is referred to as ambiguity in isochromatic phasemap. Depending on the problem one may have several zones where there may be ambiguity and these zones are labelled as ambiguous zones. The reason for ambiguous zones is due to the fact that isoclinic angle evaluated is not uniformly representing eitherσ1 or σ2 direction over the whole domain of the model [1]. For effective phase unwrapping, the isochromatic phasemap should be corrected for ambiguity. Identification and correction of ambiguity is not simple. There are different classes of ambiguous zones [2] and each of the available ambiguity removal methods can resolve ambiguity for only a certain class of problems. In cases involving complex geometries and loading one has to select appropriately, a combination of these algorithms for removing the ambiguity in the whole domain of the model.

Plotting of Isoclinic Phasemap from Finite Element Results and its Use in Digital Photoelasticity

Abstract: Digital photoelasticity is an experimental method for determining stresses in 2D and 3D models. In digital photoelasticity one gets a wrapped isoclinic phasemap. The main issue with wrapped isoclinic phasemaps is that the isoclinic values obtained do not uniformly represent the principal stress direction of one of the principal stresses consistently over the entire domain. These zones are labelled as inconsistent zones. Such zones need to be identified and corrected to get unwrapped values of continuous isoclinic phase values. In this paper, a method is developed to plot the simulated wrapped and unwrapped isoclinic phasemap from 2D Finite Element (FE) results so that one can use this as a convenient tool for identification and correction of inconsistent zones in isoclinic phasemaps obtained experimentally for complex problems. The method is explained by using the problem of a circular disc under diametral compression. The application of this method for handling complex problems is demonstrated by solving the cantilever bending of a binocular specimen.