NUMERICAL SPECTRAL-ANALYSIS OF A GRID - APPLICATION TO STRAIN-MEASUREMENTS

We present a nondestructive and noncontact extensometry technique, whi ch has been developed in order to study the behavior of different mate rials such as polycarbonate, araldite, urethane, etc. A spectral, anal ysis of a crossed grating marked on the surface of a specimen and reco rded by a charge coupled device camera is presented. The procedure is based on the use of a bidimensional fast Fourier transform algorithm c oupled with an interpolation process. It leads to the accurate determi nation of frequencies. Numerical simulations have been made to evaluat e the strain sensitivity; it depends on both the initial phase and the number of cycles characterizing the grid. Theoretically, the strain s ensitivity can reach 10(5)in best cases, but practically it is not bet ter than 10(-4) several applications on different materials are presen ted: polyurethane, paper and cloth.