Fatigue Crack Deformation Field Measurement Based on Zoom 2D-DIC System and Incomplete Second-Order Shape Functions

Fatigue damage represents a significant risk to the structural integrity of engineering components. However, current experiments on fatigue crack propagation struggle to fundamentally elucidate the mechanisms governing crack initiation and propagation. Based on the Digital Image Correlation (DIC) method, this article investigates techniques that effectively measure the displacement field at the fatigue crack tip. Methodologically, the traditional DIC displacement mode has been refined to introduce incomplete second-order displacement shape functions, findings indicate that a suitable incomplete second-order displacement function closely approximates the second-order shape function in terms of measurement accuracy, yielding an additional 12% improvement in measurement efficiency. Regarding instrumentation, the integration of an electrically tunable lens (ETL) into DIC is employed to establish a zoom Two-Dimensional Digital image correlation (2D-DIC) measurement system. This configuration effectively addresses the focusing challenge inherent in traditional small Field-of-View (FOV) 2D-DIC systems, facilitating efficient experimental measurements. Moreover, a speckle translation-based method is introduced for calibrating the distortion coefficients of zoom 2D-DIC, thereby alleviating the considerable calibration workload associated with ETL. Ultimately, the deformation field at the small-scale fatigue crack tip is measured to validate the practical utility of the developed measurement system and the effectiveness of the enhanced displacement shape functions.