Enabling High Resolution Strain Measurement at Elevated Strain Levels through the Use of Confocal Image Microscopy

Background: Digital Image Correlation (DIC) is widely utilized in experimental mechanics for full-field strain and displacement measurements due to its applicability across various length scales and its relatively low equipment cost. In materials science, high-resolution DIC enables detection of localized plasticity and deformation mechanisms, particularly at the grain or sub-grain scale. However, at elevated deformation levels, especially in coarse-grained materials, significant out-of-plane displacements introduce surface distortions that compromise image quality and correlation accuracy, challenges that become more severe under high magnification. Objective: This study aims to evaluate the effectiveness of confocal microscopy in improving DIC measurement accuracy under high strain (> 5%) and high magnification conditions, compared to conventional optical microscopy. Methods: Quasi-static deformation experiments were conducted on coarse-grained metallic samples exhibiting pronounced slip localization. Full-field strain measurements were obtained using DIC, with imaging performed via both optical and confocal microscopy. The correlation quality, error metrics, and image focus stability were assessed across increasing strain levels to quantify the impact of out-of-plane displacements on DIC accuracy. Conclusions: Out-of-plane motions significantly degrade the accuracy of DIC when using conventional optical microscopy, particularly at higher strains and magnifications. In contrast, confocal microscopy effectively mitigates correlation errors by maintaining surface focus, enabling reliable strain measurements beyond 5% strain. These findings demonstrate the utility of confocal imaging in extending the practical limits of high-resolution DIC for local deformation analysis in materials with heterogeneous strain fields.