Comparative measurement of in plane strain by shearography and electronic speckle pattern interferometry

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 later allowed us 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 allows computing absolute strain values, the strains measured by ESPSI are relative to a reference that must be measured using an additional method. Once calibrated the system ESPSI, the ESPI could no longer be used. The strain field obtained in ESPSI is corrected by the sum of constant value calculated.