Assessment of Speckle Pattern for Use in Digital Image Correlation Analysis Using Simulated Displacement Field

Local stresses and strains near the crack tip control fatigue crack growth process. Calculation of elastic–plastic strains and stresses at the crack tip requires solving the nonlinear boundary value problem of a cracked body. Analytical solutions of such problems for varied structural geometry are rarely attainable. A popular method of structural integrity assessment involves stress field characterization ahead of the crack tip. Stress Intensity Factor (SIF) calculation using full-field displacement from digital image correlation (DIC) results is a popular experimental method. Reliable estimation of displacement field from successive images of a specimen under deformation relies largely on intensity profile and its gradient on the specimen, known as speckle patterns. Application of DIC in integrity assessment of structural components of infrastructure and power plant systems should begin with speckle pattern characterization. Speckle pattern characterization can be carried out using two images without any motion or images with rigid-body motions (rigid-body translation and rotation) or images obtained using constant strain tests. The goal of speckle pattern characterization is to quantitatively compare the imposed motion with the recovered motion from DIC and estimation of minimum displacement resolution. Although characterizing speckle pattern using the methods suggested above is relatively straightforward, reproducing representative speckles for multiple cases is a challenge. To overcome this challenge, a speckle pattern assessment procedure, which uses simulated displacement field, is proposed in the present study. A rigid transformation and constant strain condition are simulated and the effectiveness of speckle pattern in capturing simulated displacement from DIC analysis is observed. Error distribution in the estimation of displacement is plotted. For quantitative evaluation of error in length, image calibration is also carried out.