Advances in displacement and strain analysis by moiré interferometry

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Virginia Polytechnic Institute and State University


Moiré interferometry was developed and extended in two related areas: (1) high density displacement fringes of excellent quality were obtained with a sensitivity of 97.6% of the theoretical maximum, and (2) the complete state of strain on the surface of a tensile specimen with a central hole was obtained by two different shearing techniques.

In the high sensitivity experiment a highly reflective diffraction grating of 2000 lines/mm (50,800 ℓ/in) was applied to the surface. Load induced displacement fringes with a sensitivity of 0.24 µm/fringe (9.6 µin/fringe) were obtained by using a virtual reference grating of 4000 lines/mm (101,600 ℓ/in).

In a second experiment a silicone, cross-line specimen grating of 600 ℓ/mm (15,000 ℓ/in) was interrogated in the x, y, and 45° directions to obtain a full-field displacement rosette. Fringe patterns of normal strain components, ex, e₄₅ and ey were then produced by mechanically shearing the displacement patterns. A shearing distance of 0.6 mm (.025 in) was used. By using this strain rosette, the complete state of strain was obtained while avoiding errors in shear strain values caused by unintentional rigid body rotations.

In a third experiment the complete state of strain was found by shearing interferometry. Each of the warped wavefronts generated in a moiré interferometry system was separately sheared and recorded on film with a carrier pattern. An adjustable air wedge located near the common focal point of a telecentric lens system was used to shear the wavefronts. The two patterns were superimposed and optically filtered to yield normal strain contours of ex, e₄₅ and ey. The experimental result was found to be in good agreement with theory.