Envelope Element Local Digital Image Correlation Method for Crack Tip Deformation Field Measurement

The measurement of displacement and strain fields of smooth and accurate crack tips is one of the key issues for digital image correlation (DIC) method. The traditional subset-based DIC method is commonly used, but the results of subset cross crack calculation are invalid. The improved subset-splitting based DIC method gives low-accuracy results due to the reduction of the number of pixels in the subset. This study proposes a novel Hermite-element based local DIC (HELDIC) method. In HELDIC, the region of interest (ROI) is divided into independent meshes, the data with low precision of element boundary is eliminated by using large block regions of element envelopes, and then the Hermite element position is adjusted to minimize the amount of data removed. The improved pointwise least square (PLS) method is used to smooth displacement fields and obtain strain fields, and finally improves the smoothness and accuracy of strain fields at crack tips. The experimental results show that the deformation field computed by the proposed method is closer to the crack tip and crack face than that calculated by the traditional local DIC method, and the strain field mean error under the specific element and subset size conditions can be reduced by more than 30%, and the HELDIC method is an effective method for the crack tip deformation field calculation.