13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Recent developments in brittle and quasi-brittle failure assessment of graphite by means of SED approach Filippo Berto1,*, Paolo Lazzarin1, M.R. Ayatollahi2 1Department of Management and Engineering, University of Padua, Stradella San Nicola 3 , 36100, Vicenza, Italy 2Fatigue and Fracture Research Laboratory, Center of Excellence in Experimental Solid Mechanics and Dynamics, School of Mechanical Engineering, Iran University of Science and Technology, Narmak 16846, Tehran, Iran. * Corresponding author: berto@gest.unipd.it Abstract Brittle fracture of isostatic graphite is studied experimentally and theoretically using plates containing key-hole notches subjected to different degrees of loading mixity. The main purpose of this paper is twofold. First, to provide a new set of experimental results on fracture of notched graphite samples, with different values of loading mixity and notch radii, which may be helpful for researchers because enlarges the very scarce available data; and second, to provide a fracture criterion for polycrystalline graphite under the above mentioned conditions. The averaged value of the strain energy density over a well-defined volume is used to predict the static strength of the considered specimens. Good agreement is found between the experimental data for the critical loads to failure and the theoretical predictions based on the constancy of the mean strain energy density over the material volume. Some preliminary tests from compression tests on the same material are summarized in the paper as well. Keywords Brittle failure, Notches, Strain Energy Density (SED) 1. Introduction Several researchers have studied in the past fracture resistance of polycrystalline graphite either under mode I (crack opening) or under mixed mode I/II (crack opening-sliding) loading conditions. For example, Awaji and Sato [1] were among the earliest researchers who used the cracked Brazilian disk (CBD) specimen to study mixed mode I/II fracture toughness of two polycrystalline graphite materials experimentally. CBD is a test specimen of circular shape in which a centre crack is generated in order to measure fracture toughness of brittle materials. Yamauchi et al. [2, 3] also made use of disk type specimens (i.e. CBD specimen and semi circular bend (SCB) specimen subjected to three point bend loading) and investigated the mixed mode I/II fracture toughness of graphite. Other test specimens have also been used for exploring the fracture behavior of polycrystalline graphite materials. For instance, the single-edge notch bend specimen [4] and the three-point bend sandwiched specimen [5] are two other types of specimens used in the past for fracture testing on graphite. In another attempt, Etter et al. [6] investigated the mode I fracture toughness KIc of isotropic polycrystalline porous graphite in addition to graphite/aluminum composite by means of the single-edge notched beam specimen. The extensive applications of graphite fibers in the composite materials (such as graphite/epoxy composites) have also prompted some researchers to study the fracture behavior of these materials under pure mode I and mixed mode I/II loading conditions [7-10]. Lomakin et al [11] made use of an energy release rate criterion for analyzing the fracture initiation in cracked graphite specimens under pure mode I loading. There are also several fracture criteria in literature for predicting the onset of mixed mode I/II brittle fracture in various engineering materials
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