چكيده به لاتين
The studies conducted in the field of size effects in cracked parts made of brittle and semi-brittle materials confirm that the fracture toughness obtained from laboratory samples is considered as one of the inherent characteristics of different materials, especially brittle and semi-brittle materials. Contrary to expectations, it is dependent on the dimensions and geometry of the tested parts and changes with the change of geometric dimensions.
Therefore, in most of the researches conducted in this field, they have tried to find a valid relationship between the fracture toughness of cracked parts and the dimensions of the sample, in order to apply this problem in practice. In the current study, the effect of the geometric dimensions of the graphite sample on the value of the fracture under pure loading modes I and III has been investigated. This type of graphite has ultimate tensile strength of 13.6 MPa, Young's modulus of 3.3 GPa and Poisson's ratio of 0.18. For this purpose, at first, the samples were subjected to three-point and antisymmetric four-point bending tests and their fracture load was obtained. Then, by theoretical relations and numerical simulation, the corresponding fracture toughness values have been calculated many times. In the following, the changes in the radius of the damage area with the characteristic dimension of the material (crack growth direction) have been investigated. By substituting the radius of the damage zone as a function of the dimensions of the part in the existing relationships, the changes in the fracture toughness of the material will be obtained with the dimensions of the part. Finally, using the criterias of MTS and GMTS, the fracture load corresponding to the graphite sample is estimated.
According to the conducted research, the fracture load for graphite samples in loading mode III was recorded to be approximately 6.5 times larger than the fracture load in loading mode I. The values of fracture toughness have increased by increasing the sample size in each loading mode, so that in pure loading mode III, the average value of fracture toughness has increased by 16% with the increase of the dimensions of the samples. The value of the fracture toughness in the mode I loading was more than that of the pure mode III. The MTS criterion for this type of geometry of the sample has a good estimate of the fracture load due to the negligible value of the second term of stress expansion around the crack tip. So that in the fourth sample, the value of the estimated fracture load of the MTS criterion value has a difference of 1.67% with the fracture load obtained from the laboratory tests.
