5. Moiré Patterns Formed by Remote Gratings

In this chapter, we present two moiré methods that use remote gratings for the determination of the gradients of the out-of-plane displacements or the sum of the two in-plane principal stresses for plane stress conditions. The moiré patterns are created by projecting the rulings of one grating onto the rulings of a second grating after interacting with the specimen. The first method developed by Theocaris (Moiré fringes in strain analysis. Pergamon Press, pp 178–218, 1969 [1]), Theocaris and Koutsambessis (J Sci Instr 42:607–610, 1965 [2]), Theocaris and Koutsambessis (Exp Mech 8:82–87, 1968 [3]), Theocaris and Koutsambessis (Strain 4:10–15, 1968 [4]) is based on geometric moiré and uses while light. The second method termed “Coherent Gradient Censor (CGS)” was developed by Tippur et al. (Int J Fract 48:193–204, 1991 [5]), Tippur et al. (Int J Fract 52:91–117, 1991 [6]), Tippur and Rosakis (J Exp Mech 31:243–251, 1991 [7]), Krishnaswamy et al. (J Mech Phys Sol 40:339–372, 1992 [8]), Tippur (Appl Opt 31:4428–4439, 1992 [9]), Bruck and Rosakis (Opt Lasers Eng 17:83–101, 1992 [10]), Mason et al. (J Mech Phys Solids 40:641–661, 1992 [11]), Rosakis (VCH Publishers, pp 327–425, 1993 [12]), Bruck and Rosakis (Opt Lasers Eng 18:25–51, 1993 [13]), Rosakis (Special issue of optics and lasers in engineering devoted to photomechanics applied to dynamic response of materials, pp 19, 3–41, 1993 [14]), Lee et al. (Opt Lasers Eng 25:25–53, 1996 [15]), Rosakis et al. (Thin Solid Films 325:42–54, 1998 [16]), Mello et al. (Exp Mech 49:277–289, 2009 [17]). It is based on the diffraction of light by two gratings and uses coherent light. Both methods involve simple optical setups.