Abstract
In the last two decades, visual image techniques such as Digital Image Correlation (DIC) enabled to experimentally determine the crack tip displacement and strain fields at small scales. The displacements are tracked during loading, and parameters as the Stress Intensity Factor (SIF), opening and closing loads, T-stress can be readily measured. In particular, the SIFs and the T-stress can be obtained by fitting the analytical equation of the Williamstype expansion with the experimentally-determined displacement fields. The results in terms of fracture mechanics parameters strictly depend on the dimension of the area considered around the crack tip in conjunction with the crack length, the maximum SIF (and thus the plastic tip radius), and the number of terms to be considered in the Williams-type expansion. This work focuses in understanding the accuracy of the SIF calculation based on these factors. The study is based on Finite Element Analysis simulations where purely elastic material behavior is considered. The accuracy of the estimation of the SIF is investigated and a guide-line is provided to properly set the DIC measurements. The analysis is then experimentally validated for crack closure measurements adopting the SENT specimen geometry.
Highlights
The digital image correlation (DIC) technique is nowadays largely adopted as a reliable, non-destructive, low-cost technique to measure real-time local displacements on a flat surface of the specimens [1, 2]
Real crack tip displacement fields are affected by several factors that cannot be controlled during Digital Image Correlation (DIC) displacement measurements
Starting from the displacement fields obtained from the finite element (FE) simulations, this work shows the comparison with the Stress Intensity Factor (SIF) predicted by the numerical analyses focusing on the parameters that are required to be properly set to obtain an accurate SIF measurement during DIC measurements
Summary
The digital image correlation (DIC) technique is nowadays largely adopted as a reliable, non-destructive, low-cost technique to measure real-time local displacements on a flat surface of the specimens [1, 2]. The full-field displacement and strain measurements found several engineering applications as, for example, in the study of the crack tip fields and in the characterization of the typical fracture mechanics parameters such as the stress intensity factor (SIF). The adoption of displacement measurements to calculate the SIFs is critical, and special attention is required to properly select the DIC measurements. In order to clarify and to provide a guideline on the SIF measurements, in this work we analyze the displacement field calculated according finite element (FE) simulations of two typical cracked body geometries: the center cracked tension (CCT) and single-edge notched tension (SENT) specimens. Starting from the displacement fields obtained from the FE simulations, this work shows the comparison with the SIF predicted by the numerical analyses focusing on the parameters that are required to be properly set to obtain an accurate SIF measurement during DIC measurements. The Williams expansion is written for the displacements v in the direction perpendicular to the crack surfaces in three different versions based on the number of terms involved: v
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