The moiré grid-analyzer method for strain analysis
Paper indicates that simultaneous observation of moiré fringes generated by two orthogonal sets of screens is practical and beneficial. Complete strain distributions for static or dynamic conditions are derived from one photograph
The complete state of strain throughout an extended field can be determined from a single photograph by the moiré grid-analyzer method. Grids are used for the active, or specimen screen, and for the rigid analyzer screen. Hence, two families of moiré fringes appear simultaneously, providing displacements inx andy directions throughout the field.
Interweaving of moiré fringes so as to disguise the identity of each family is prevented by use of an initial pattern. The initial pattern also eliminates uncertainties in assignment of moiré fringe orders throughout the field, and it provides numerous data points in any local region for reliable evaluation of fringe gradients. A rigorous derivation is presented for interpretation of such patterns in terms of strains.
Errors in computed shear strains caused by analyzer misalignment are automatically canceled. The effects of rigid-body rotation of elements within the specimen are routinely eliminated in strain computations. Since calculated strains are independent of analyzer orientation, no fine control of analyzer alignment is required, and small shifts of analyzer position during the experiment are permissible. Accordingly, the moiré grid-analyzer method not only makes possible analysis of problems that previously could not be approached, but also offers vast simplifications for all extended-field moiré analyses.