ABSTRACT
Grid and moiré methods have been the cornerstone of deformation and strain measurements. The grid method has its foundations in the definition of normal strain as the change in length of a line segment and of shear strain as the change in the angle between two line segments originally
perpendicular to each other. If this concept is extended to all points on the specimen, a grid is formed, and by comparing the grid in the undeformed and deformed states the strain at each point can be deduced from first principles. Theoretically, strain is defined as the derivative of the displacement component whereas, practically, the line segment has to have a finite length (gauge length) for measurement. Thus, to overcome strain averaging over the gauge length, there is a need for shorter gauge lengths in certain applications. Note that in case of uniform or linearly varying strains, error due to finite gauge length is zero. A shorter gauge length increases sensitivity as well as resolution but leads to an increased number of grid elements to evaluate, which can be cumbersome. Thus, the moiré method evolved. In the moiré method, the two grids (or gratings) recorded before and after loading are superposed to reveal moiré fringes that give the difference between the two grating periods. This then is directly proportional to deformation and, hence, strain can be readily deduced. The sensitivity, as with the grid method, is proportional to the pitch of the undeformed (or reference) grating. In addition, moiré methods provide full-field visualization of the deformation [1-3]. Due to diffraction effects, the coarse moiré is limited to gratings with a frequency of 100 lines/mm or to those in which the smallest pitch is 10
m
m. The coarse moiré method has found a wide range of applications in both in-plane and out-of-plane deformation measurement for various engineering problems. In this chapter, we limit our discussion to in-plane deformation measurement.
