Determination of Signs in Mire Method

In applying the moiré method for stress analysis, it is sometimes difficult to attribute proper signs to fringes. In the conventional way the knowledge of boundary conditions is an absolute necessity for proper interpretation of fringes. Presented herein are methods for the determination of signs of moiré fringes based on their intrinsic properties. These methods can be applied to any portion of a moiré pattern without a priori knowledge of the boundary conditions. As a result the moiré method can be effectively used as a means for stress separation in three-dimensional photoelasticity. Heretofore the moiré pattern obtained from a stress-frozen photo-elastic slice upon annealing has been most difficult to analyze, because the boundary conditions were either not well defined or sometimes even nonexistant due to the destructive nature of the stress-frozen technique. Also presented is a method for fringe ordering whereby the signs of both direct derivative and cross derivatives are obtained automatically once the fringes are properly ordered.