AUTOMATIC-ANALYSIS OF ISOCHROMATIC AND ISOCLINIC FRINGES IN PHOTOELASTICITY USING PHASE-MEASURING TECHNIQUES

Photoelasticity is one of the oldest and remains one of the most useful means of optically analysing the way in which a structure reacts under load. This is because it is still the only means by which internal surfaces and interfaces can be studied and it measures the most relevant parameter in fatigue life prediction, shear stress, directly. This paper presents two new methods that provide a complete solution for the automatic fringe analysis of photoelastic data. Previous solutions have not provided a complete full-field analysis of both fringe sets (isochromatic and isoclinic) over a number of fringes with an automatic measurement of absolute magnitude. This paper reviews previous approaches and discusses the aims toward which this work has been directed. Specifically, two new algorithms are presented. Firstly, the combination of phase maps at three wavelengths in order to remove the isochromatic ambiguity at every pi turning point that would occur in a single phase map is discussed. Additionally, this algorithm allows the automatic measurement of absolute isochromatic magnitude, provided that the neutral axis is within the field of view. Secondly, a new method is proposed that removes the areas of 2 pi ambiguity in an isoclinic phase map by cross correlating regions of low modulation at two different wavelengths. The algorithms are demonstrated by applying them to the analysis of a simple calibration disc. The disc was manufactured as a full-scale three-dimensional model in araldite (Ciba-Geigy CT200), from which a central slice was taken after the sample had been stress-frozen.