Digital photoelasticity is a whole field experimental technique which can analyze both 2-D and 3-D models. In this paper, stress frozen 3-D models are sliced mechanically and the whole field principal stress differences (Isochromatics) and principal stress directions (Isoclinics) are evaluated by capturing only five images using the standard optical arrangements of a conventional polariscope. The wrapped isoclinic values obtained by processing the first four colour polarization stepped images in the range -π/4 to +π/4 are unwrapped using adaptive quality guided phase unwrapping algorithm to get isoclinics in the range of -π/2 to +π/2. The total fringe order is evaluated by three fringe photoelasticity or RGB photoelasticity with its latest developments like colour adaptation combined with refined three fringe photoelasticity. The methodology is validated for two slices cut from aero-structural components. © 2010 SPIE.

Krishnamurthi Ramesh

Department of Applied Mechanics

T. Kasimayan

3D models

ADAPTIVE QUALITY

DIGITAL PHOTOELASTICITY

Experimental techniques

FRINGE ORDERS

ISOCHROMATICS

ISOCLINICS

Latest development

OPTICAL ARRANGEMENT

PHOTO-ELASTIC ANALYSIS

Principal stress

REFINED THREE FRINGE PHOTOELASTICITY

THREE FRINGE PHOTOELASTICITY

Algorithms

ELASTICITY

mechanics

Photoelasticity

Physical optics

Refining

SIGNAL NOISE MEASUREMENT

Stress analysis

three dimensional

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Photoelastic analysis of stress frozen slices using five-step method

Proceedings of SPIE - The International Society for Optical Engineering

Digital photoelasticity has rapidly progressed in the last few years and has matured into an industry-friendly technique. This review thematically classifies all the developments in digital photoelasticity and highlights the relative merits and drawbacks of the various techniques. The overall objective is to provide enough information and guidance to allow an enduser to make an informed choice on the type of technique to be used in a particular situation.

Journal of Strain Analysis for Engineering Design

Digital photoelasticity - A comprehensive review

In digital photoelasticity, one gets whole field information of isoclinic and isochromatic parameters based on intensity processing. Isoclinic values are undefined at isochromatic fringe contours and this appears as noise in the isoclinic phasemap which affects stress separation adversely. A new adaptive smoothing algorithm to remove the noise due to isochromatic-isoclinic interaction that automatically takes care of the isotropic points and π jumps present in the isoclinic phasemap is proposed. It is validated for the benchmark problem of a ring under diametral compression and verified for an angled bracket. © 2009 Blackwell Publishing Ltd.

Strain

Adaptive smoothing for isoclinic parameter evaluation in digital photoelasticity

A simple five-step approach to obtain whole-field isoclinics and isochromatics using a conventional reflection polariscope is discussed. The only additional requirement was a suitable digital camera, while the benchmark problem of a ring under diametral compression was selected to show the effectiveness of the method. The methodology was extended to obtain the isoclinic and isochromatic fields for the problem of an angle bracket and for fields in a flange joint due to assembly stresses. The first four arrangements of the five-step method corresponded to a plane polariscope based polarization stepping algorithm and these optical arrangements were easily set in the commercially available reflection polariscope. The fifth optical arrangement was the conventional dark field circular polariscope arrangement that was easily set in the polariscope.

Experimental Techniques

Digital reflection photoelasticity using conventional reflection polariscope

With advancements in digital image processing and data acquisition, a separate branch of photoelasticity namely digital photoelasticity came into existence. Here, intensity information of the acquired image is used for the evaluation of whole-field photoelastic parameters. Digital photoelasticity provides only wrapped phasemaps of isoclinics and isochromatics and they have to be unwrapped in different ways for getting the continuous-phase values. In the case of the isochromatic phasemap, ambiguity removal prior to unwrapping is essential. In this paper, a 10-step phase-shifting methodology is proposed and a new strategy for obtaining the isochromatic phasemap free of ambiguity is demonstrated. Isoclinic unwrapping is performed by a new adaptive quality guided algorithm. Adaptive in the sense that isoclinic phase unwrapping is done autonomously even in the presence of isotropic points/π jumps occurring in the isoclinic phasemap. The isochromatic phasemap is also unwrapped using the quality guided path follower. The methodology is validated for the problem of a ring under diametral compression and later shown for three other models which have complex stress fields. Wherever possible, the parameters obtained by the new methodology are compared with analytical or numerical methods and the comparison is quite good. © 2008 Blackwell Publishing Ltd.

Adaptive quality guided phase unwrapping algorithm for whole-field digital photoelastic parameter estimation of complex models

Three-fringe photoelasticity (TFP) can give the total fringe order from a single-colour isochromatic fringe field. In TFP, the total fringe order is obtained by comparing the colour of the unknown photoelastic fringe pattern with a calibration specimen. Comparison is conventionally done by minimising the colour difference error. This leads to misidentification of fringe order in some regions. A new colour difference formula is proposed with an additional term, which ensures continuity of fringe order data over the domain. Fringe order data obtained by conventional TFP is used as base data and the new method is termed as refined TFP. To enable applicability of the technique to arbitrary geometries, a new boundary identification technique is developed. The new method is demonstrated with some example problems. © 2006 Elsevier Ltd. All rights reserved.

Optics and Lasers in Engineering

Noise removal in three-fringe photoelasticity by adaptive colour difference estimation

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. © Society for Experimental Mechanics 2007.

Experimental Mechanics

Colour adaptation in three fringe photoelasticity

Journal	Proceedings of SPIE - The International Society for Optical Engineering
ISSN	0277786X
Open Access	No