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.
Transient thermal stresses of a bimaterial specimen with interface edge cracks subjected to heating along an edge is analysed by refined three-fringe photoelasticity (RTFP). Whole-field, noise-free, fringe order estimation using a single colour image is made possible using RTFP combined with colour adaptation. The stress intensity factors (SIFs) of the interface crack are determined through a multiparameter overdeterministic system of equations by a least-squares approach using experimental data collected automatically. The transient SIFs are found to peak to a higher value than in steady state, and the opening mode is found to be dominant. An increase in thermal load causes the crack to propagate, and this is easily visualized on the basis of quantitative fringe order data available for the whole field. The SIFs of a propagating crack are found to be low. The study shows that the crack propagates easily when the opening mode is dominant.
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