GRATING INTERROGATIONS - FROM SMALL TO LARGE-STRAIN MEASUREMENT

Our purpose is the direct strain measurement from the interrogation of a crossed grating marked on the surface of a specimen. The observatio n of the object through a master grid (as in moire method) is replaced by a direct characterization of the pattern using a Fourier transform . This gives direct access to the modification of the pitches which le ads to quantification of the strain without the intermediary of a frin ge pattern. The comparison between the undeformed and deformed states allows the determination of the magnitude and orientation of principal strains and of the local rigid-body rotation. We describe three analy sis techniques, one using the diffraction phenomenon, another utilizin g a numerical spectral evaluation and the third combining diffraction and phase-shifting procedure. These grid interrogations have different domains of application and a suitable choice of these analysis techni ques allows a very large measurement range (10(-5) to high strain acco rding to the grating resistance).