Strain analysis using ESPI applied to fracture mechanics
Electronic speckle pattern interferometry (ESPI) has become an established technique for surface deformation studies. However, difficulties remain in the practical use of ESPI, primarily because results require skilled and time-consuming interpretation. The work reported here has enabled automated acquisition and processing of ESPI displacement data. Further processing has enabled the components of surface strain and stress to be, determined automatically. Such measurements are of great importance for experimental fracture mechanics studies, a quantitative approach to measuring the severity of defects in a loaded structure. Having established the importance of experimental optical methods in fracture mechanics, and the particular advantages of ESPI, a study has been undertaken to determine whether ESPI can be used for quantitative fracture mechanics measurements. Automated analysis for in-plane displacement measurements with ESPI was achieved by the phase-stepping technique. Numerical differentiation of the displacement data allowed surface strain to be evaluated. The accuracy of such measurements was investigated, particularly with regard to speckle noise inherent in the data. Speckle noise limits the accuracy of all measurements, and a practical threshold for displacement and strain of ±0.03~m and ±6~strain was found. From these considerations, two new phase-stepping algorithms have been proposed for ESPI. The first offers improved accuracy by the way it eliminates speckle noise; the second has allowed phase-stepping of ESPI addition fringes for the first time. · In the past ESPI has been restricted to uniaxial measurements. A new interferometer design enabled displacement to be measured along two axes simultaneously: extension to three-dimensional sensitivity is discussed. Automated displacement and strain measurements, recorded with the dual-sensitivity interferometer, are presented for a cantilever loaded at its free end.