Measurement of in plane strain with shearography and electronic speckle pattern interferometry

In experimental mechanics, the relative displacements induced by mechanical or thermal load can be measured by optical techniques such as Electronic Speckle Pattern interferometry (ESP) and then used to determine strains, stresses, or bending moments by additional calculations. These quantities are of interest because they affect the strength, safety, and lifetime of mechanical structures or components. The stress analysts are usually interested in displacement gradients, rather than in displacement measurements. Therefore, optical methods that can directly render displacement derivatives are of special interest. This is achieved by using Electronic Speckle Pattern Shearing Interferometry (ESPSI). ESPSI is an optical differentiation technique that can be considered an appropriate method for the measurement of the spatial derivatives in non-destructive inspections. The method is based on the digital correlation of two speckled wavefronts representing two states (loaded and unloaded) of the object under test. In this work, an optical setup that gives the possibility of using either ESPI or ESPSI has been implemented to assess in-plane strains induced on a composite sample. First, in-plane ESPI was used to measure displacement fields, which allowed us latter to evaluate the corresponding strain fields. Next, we applied ESPSI to measure the derivative of in-plane surface displacements (the strains). The experimental results obtained by applying both techniques (ESPI and ESPSI) were compared. We found that the difference between the strain fields obtained by ESPSI and ESPI was roughly a constant. This result was expected since, although ESPI in turn allows computing absolute strain values, the strains measured by ESPSI are relative to a reference that must he measured by an additional method.