13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- like graphite. The maximum tangential stress (MTS) criterion [12], the minimum strain energy density (SED) criterion [13] and the maximum energy release rate or G criterion [14] have been used more frequently by the researchers. Using a modified MTS criterion, Ayatollahi and Aliha [15] presented good estimates for the onset of mixed mode fracture in two grades of polycrystalline graphite containing sharp cracks. Cracks are generated in graphite mainly because of the manufacturing faults or due to the coalescence of the micro-structural pores or defects that are inherently embedded in graphite. Whereas cracks are viewed as unpleasant entities in most engineering materials, nevertheless, notches of U or V-shape are sometimes desired in design and manufacturing of products made from graphite. Graphite moulds, graphite heating elements and graphite chucks are only some examples for industrial components that contain U or V-shape notches. A review of literature shows that in spite of extensive studies on mode I and mixed fracture in cracked graphite specimens, very few papers have dealt with brittle fracture in V-notched graphite components. Ayatollahi and Torabi [16] recently conducted a series of fracture tests on three different V-notched test specimens made of a polycrystalline graphite material. They also proposed a mean stress criterion and estimated their experimental results with very good accuracy. However, the results presented by Ayatollahi and Torabi [16] are confined only to pure mode I loading conditions. There are various practical conditions where the notches in graphite components are subjected a combination of tensile and shear deformation (or mixed mode I/II loading). In a recent paper, the present authors investigated mixed mode brittle fracture in polycrystalline graphite both experimentally and theoretically [17]. First a series of fracture experiments were conducted on centrally notched Brazilian disk specimens made of graphite to determine the fracture loads under different combinations of mode I and mode II loading. Then a theory based on the SED criterion [18-22] was employed to estimate the experimentally obtained fracture loads. The main purpose of the present paper is to provide a new set of experimental results (70 new data) on fracture of graphite samples weakened by key-holes, with different values of loading mixity, inclination angles and notch radii, which may be helpful because enlarges the very scarce available data. By using the averaged value of the strain energy density over a well-defined volume, a fracture criterion for polycrystalline graphite under the above mentioned conditions is proposed to predict the static strength of the considered specimens. The third part of the paper deals with the analysis of fracture initiation direction and crack early propagation. 2. Fracture experiments The details of the graphite material, the test specimen and the fracture experiments are presented in this section. 2.1 Material The fracture tests were conducted on a commercial isostatic polycrystalline graphite. The mean grain size was measured by using the SEM technique and the density was determined from the buoyancy method. The basic material properties of the tested graphite are listed in Table 1: mean
RkJQdWJsaXNoZXIy MjM0NDE=