Stress-Strain Measurement of Rubber with Optical Moiré Fringes

We use the moiré fringes to measure true stress and true strain of a rubber specimen in tension, and analyze the stress-strain relationship. With the printed straight-line pattern on a specimen, the moiré patterns appear when the specimen is stretched. The geometrical relationship of bright and dark fringes is used to calculate the strain values both in the axial and transverse directions. Together with optical image processing, which can also be used to obtain the instantaneous cross sectional area, we can determine true stress and true strain. The results from the moiré method yield the true stress and true strain in good agreement with those calculated from the standard engineering measurement. Additional benefits of the method include simultaneous measurement of stress and strain, their inhomogeneity, and shear strain.