Evaluation of automated full field analysis using phase stepping

A full field automated polariscope for photoelastic analysis has been developed in the author's laboratory and has been described in detail elsewhere. Briefly, the system uses phase-stepping to determine both the fractional isochromatic fringes and isoclinic parameter at all points in the field of view independently of their neighbouring points. A wrapping algorithm is then employed to produce continuous ischromatic and isoclinic data, which can be subsequently be used in stress separation procedures. The idea of using phase stepping in photoelasticity is a fairly recent innovation and can be described as changing incremently the absolute phase of the reference wave by rotating the output elements of the polariscope and measuring the local light intensity after each step. In the apparatus described here, the output elements are rotated to six different positions providing six images of the specimen. Maps of the periodic values of the isoclinic and isochromatic parameters are subsequently obtained by combining, mathematically, these six images. A number of full field techniques have been developed. Poloshin and Redner have developed half fringe photoelasticity, and two laboratories in Japan are working on the technique of phase stepping. It appears, however, that no detailed evaluation has been made of the accuracy and reliability of the results generated by the technique. The objective of the work described in this paper has been provided such an evaluation. Five different models were selected for analysis using the automated system and manually using the Tardy compensation method: (a) a disk in diametral compression: (b) a constrained beam subject to a point load: (c) a tensile plate with a central hole: (d) a turbine blade; and (e) a turbine disk slot. These models provided a range of different fringe patterns, orders and stress gradients to test the performance of the system.