Three Optical Procedures for Local Large-Strain Measurement

Optical methods have been developed to measure large strains. Two alternatives are presented: the first one uses a cross grating, obtained by an innovative manufacturing process and the second one uses a mark-tracking technique. The production of the grating is based on the deposit of a thin film through a master grid under vacuum. This creates small studs independent of each other, on the specimen surface. For the grid interrogation two techniques are discussed, one using the analysis of the optical diffraction figure and the other one the Fast Fourier Transform algorithm. Markers are simply deposited on the studied surface with a pen and strain determination is then obtained by mark-tracking. The experimental validation consists of several simple tensile tests. Experimental results for polymers and latex materials confirm the quality of measurement. Strains are measured for values up to 2,200 % without loss of information, with a strain accuracy close to 5x10(-4) at minimum gauge lengths of 0.225 mm.