Effects of Various Shape Functions and Subset Size in Local Deformation Measurements Using DIC

The digital image correlation (DIC) method obtains comparable results with strain gauges and its reliability and accuracy are commonly accepted in the measurement of affine deformations. However, in engineering measurements, there are always substantial local deformations with high strain gradients, such as the Portevin-Le Chatelier (PLC) shear bands, deformations near gaps, and crack tips. In these situations, strain gauges are restricted because the results within the contact areas are smoothed. Although the DIC method can be employed to measure these local deformations, the calculation parameters (e.g., the order of the shape functions, and template size) seriously impact the results. By analyzing PLC shear bands with different gradients in tensile tests and simulated bands, the deep mechanism on how shape functions and templates impact on the accuracy of DIC results is established. This study also demonstrates that second-order shape functions are more suitable than first-order shape functions to describe local deformations. The theory that the results of second-order shape functions are reliable and accurate when the relative error between first- and second-order shape functions is less than 10 %, is proposed. In addition, improving the spatial resolution and the acquisition frequency is proposed, and proved to achieve reliable results.