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

Abstract

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 secondorder 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.

The Influence of Specimen Thickness on the Lüders Effect of a 5456 Al-Based Alloy: Experimental Observations

“…The three-dimensional DIC (3D-DIC) method is applied in the present study for surface strain mapping; this non-contact method is used to measure the full-field space coordinate, displacement, and strain distributions in a material. This method is precise, accurate, does not require special quakeproof equipment, and is therefore widely used in material testing [28,29]. 3D-DIC is adept at measuring strain localization.…”

“…Specifically, the experiment value of the displacement and strain measurement error are~0.01 pixels and 150 µε, respectively [33]. In a previous study, we determined the effect of DIC parameters, such as the patch size, shape function, and the strain gradient, on the measurement error; based on the results, a moderate patch size was suggested for high-gradient inhomogeneous deformation [29].…”

The state of the art of two‐dimensional digital image correlation computational method

“…Remarkably, the selection of subset is closely related to the speckles' quality and the studies on speckles' quality assessment provide a reference. Moreover, Xu et al studied the effects of various shape functions on the selection of subset size. They pointed out that employing a first‐order shape function with a large subset size can obtain a better result in uniform deformation, and a second‐order shape function with a moderate subset size is more suitable for high gradient deformation.…”

A parametric study of DIC measurement uncertainties on cracked metals

“…[18] The influence of the step size was discussed in Haddadi and Belhabib [16] and its effects on the measurements were investigated in Xu et al and Wang et al, [37,38] whilst the impact of the applied strain level was studied in other studies. [15,37,39] The measurement window (MW), a user-defined area on a displacement/strain map, is usually utilised to obtain average displacements or strains for the region of interest. In this study, however, the effect of the size of the MW on the errors was examined by analysing the data within a MW.…”