13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Evaluation Using Digital Image Correlation and Finite Element method of Stress Intensity Factors and T-stress M.L.Hattali*, H. Auradou, M. François, V. Lazarus Laboratoire FAST, Université Paris-Sud, Université Pierre et Marie Curie–Paris 6, CNRS, Bat. 502, Campus Univ., Orsay, F-91405, France * Corresponding author: hattali.lamine@gmail.com Abstract A controlled crack growth in PMMA was achieved in Wedge-Splitting Test (WST) by regulating the cross-head speed of loading by a computer-driving testing device. The fracture behavior of a crack propagating was investigated quantitatively by both numerical modeling and experiment using Finite Element Method (FEM) and Digital Image Correlation (DIC). A twoparameter fracture mechanics approach, describing the near-crack-tip stress field, was applied to determine the stress intensity factor (SIF) and the coefficient of higher order term (T-stress). In both methods, it was shown that the stress intensity factor versus crack length remains constant whereas the T-stress varies from negative to positive. This later variation is similar to the three point bend beam, but different from the compact tension specimen, for which the T-term is always positive. Using Digital Image Correlation (DIC), the stress intensity factor and T stress were estimated with 10% and 15% uncertainty in a complex loading set-up without having to do a numerical modeling of the experiment. Keywords Crack path propagation, Digital Image Correlation, Finite Element Method, Stress intensity factor, T-stress 1. Introduction Fracture propagation in brittle or quasi brittle material is dominated mainly by the near-tip stress field. The stress intensity factors “K” gives the magnitude of the stress field ahead of a crack tip and fracture propagates when a critical stress intensity factor is reached. There are experimental evidences that the stress contributions acting at largest distance from the crack tip may affect fracture mechanics properties [1]. The constant stress contribution (first “higher-order” term of the Williams stress expansion, denoted as the T-stress term) is the next important parameter. It is well known that for crack growth under mode I loading (i.e., KII = 0) the straight crack path is stable when T < 0 and unstable for T > 0 [2]. In the brittle or quasibrittle materials, the determination of a two-parameter fracture mechanics “K” and T-stress as function of crack length or crack velocity is often difficult. To measure the stress field during the fracture propagation, we need to perform stable fracture tests beyond the maximum load. This is not an easy task! Sophistical and expensive test stability control apparatus (i.e. closed loop control unit with e.g. crack tip opening displacement as a feedback signal) may be used to perform such tests. . However, these systems are often not available inlaboratories. The wedge-splitting test (WST), first used by Linsbauer and Tscheg [3] and later developed by Brühwiler and Wittmann [3], is a test that permits to propagate stable cracks. . The WST has been extensively used for experimental (e.g. [5-6]), numerical (e.g. [7-8]) and inverse analysis studies (e.g. [9]). In spite of its advantages, the fracture parameters for the WST are not widely reported. The purpose of this paper is to study, using both numerical and experimental approaches, the variation of stress intensity factor or T-stress as function of crack path position. Digital Image Correlation (DIC) is used to follow the crack propagation. Based on displacements evaluation, crack tip position, Stress intensity factor “K” and T-stress are determined. Secondly, we performed by means of a constraint-based two-parameter fracture mechanics approach, a numerical analysis of near-crack-tip stress field using the Finite Element Method (Abaqus 6.6 software) [10]. In the numerical approach, the wedge
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