Novel Scanning Scheme for White Light Photoelasticity
The use of white light based Twelve Fringe Photoelasticity/RGB Photoelasticity has gained popularity in the recent years. The main advantage of this technique is that it requires only a single image recorded under white light for isochromatic demodulation
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Novel Scanning Scheme for White Light Photoelasticity Vivek Ramakrishnan and Ramesh Krishnamurthi
Abstract The use of white light based Twelve Fringe Photoelasticity/RGB Photoelasticity has gained popularity in the recent years. The main advantage of this technique is that it requires only a single image recorded under white light for isochromatic demodulation. This makes it suitable for problems where multiple acquisitions are difficult. Accuracy of fringe order estimation using Twelve Fringe Photoelasticity is dependent on the spatial resolution of the isochromatic fringe pattern. The existing scanning schemes for refining the fringe order data do not take this into account and leads to the propagation of noise from the low resolution zones. In this work, a novel scanning scheme is proposed whose progression is guided by the spatial resolution of the isochromatic fringe pattern. Initially, a method based on the intensity gradients is developed to create a whole field map resembling the resolution of the fringe pattern. This map is used to guide the scanning scheme for refining the fringe order data. The proposed scanning scheme encapsulates the noise within the zones of low spatial resolution and eliminates their propagation. The working of the scanning scheme is demonstrated using the benchmark problem of a circular disc subjected to diameter compression. Keywords Digital photoelasticity • Image processing • RGB photoelasticity • Twelve fringe photoelasticity • Isochromatics
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Introduction
Recent developments in white light photoelasticity make it possible to determine whole field isochromatic data from a single colour image with high accuracy [1–3]. Twelve Fringe Photoelasticity (TFP) has been used for a variety of applications ranging from residual stress analysis in glass [4, 5] to analysis of civil structures [6]. Use of photoelasticity for interdisciplinary research demands that even personnel with a limited background in photoelasticity are able to conveniently perform post processing of the isochromatic fringe patterns. Further, the use of TFP/RGB photoelasticity to solve new problems requires it to be robust in order to handle models of complex geometry and accommodate fringe patterns of varied shapes and gradients. Mathematical modelling of intensity variation in the isochromatic image requires accurate consideration of various factors including spectral composition of the light source used, transmission response of the optical elements, quarter-wave plate error, dispersion of birefringence and spectral response of the camera. Hence, in practise an approach based on the use of a calibration table is generally followed. In RGB Photoelasticity (RGBP) or Twelve Fringe Photoelasticity (TFP), the isochromatic parameter is determined by comparing the colour components at a point on the model with that in the calibration table. Calibration table is generated using specimens of known fringe order variation such as beam under four point bending. Initially, the fringe orders are estimated by the colour dif
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