Full-Field Measurements of Principal Strains and Orientations Using Moire Fringes

The principal strains at the microscale are the key parameters for evaluating the instability behaviors of materials. In this study, we developed a technique to acquire the full-field distributions of the principal strains and their orientations by integrating the scanning moire method and the theoretical analysis of strain status. The normal strains in two perpendicular directions were first measured using the SEM scanning moire fringes. The principal strains with orientations were then determined based on the analysis of strain status for plane stress problems. In this study, the maximum and the minimum principal strains and their orientations of carbon fiber reinforced plastics under a three-point bending test were measured. The maximum principal strain is greatest in the bottom-left region and the absolute value of the minimum principal strain is greatest in the upper-right region of this specimen. This technique is independent of the specimen grating direction and is useful to detect the potential failure characteristics of various composite materials.