THE GOVERNING EQUATIONS OF MOIRE INTERFEROMETRY .1. THE EFFECT OF OUT-OF-PLANE DISPLACEMENT GRADIENTS FOR APPLICATION TO CRACK-TUP FIELDS

The effects of tilting of the grating plane on moire interferometric f ringe patterns are studied using the method of Livnat and Post (Experi mental Mechanics, 25(4) (1985) 360-6). Tilting may result either from rigid-body rotations or from out-of-plane displacement gradients. Tilt ing may introduce a significant error if it is assumed that the fringe s represent contours of in-plane displacement, particularly if the in- plane deformation is small compared with the out-of-plane displacement gradient. Knowledge of the surface slope appears to be sufficient to correct the error. It is also shown that the fringe patterns do not ca pture the added displacement caused by rigid-body rotations exactly. T he implications of these effects are briefly discussed for an experime nt measuring the deformation at the tip of a crack in a ductile steel, in which the out-of-plane displacement gradients are relatively large .