Issue 41

S. Beretta et alii, Frattura ed Integrità Strutturale, 41 (2017) 269-276; DOI: 10.3221/IGF-ESIS.41.36 269 Focused on Crack Tip Fields Stress Intensity Factor calculation from displacement fields S. Beretta, L. Patriarca, S. Rabbolini Politecnico di Milano, Department of Mechanical Engineering, via La Masa 1, 20156, Milano, Italy stefano.beretta@polimi.it, luca.patriarca@polimi.it, silvio.rabbolini@polimi.it A BSTRACT . 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 Williams- type 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. K EYWORDS . Stress intensity factor; Digital image correlation; Crack-tip displacement fields. Citation: Beretta, S.., Patriarca, L., Rabbolini, S., Stress Intensity Factor calculation from displacement fields, Frattura ed Integrità Strutturale, 41 (2017) 269-276. Received: 28.02.2017 Accepted: 03.05.2017 Published: 01.07.2017 Copyright: © 2017 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION he 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). However, the adoption of displacement measurements to calculate the SIFs is critical, and special attention is required to properly select the DIC measurements. For example, the choice of the area of interest in front the crack tip (field-of-view) strictly depends on to the type of the analytical model selected for the fitting of the displacement field. It turns out that the accuracy of the SIFs measurements from full-field DIC displacement fields is a technical issue which T