Papers by Keyword: Brazilian Disc

Abstract: A sensitive determination of mechanical properties of brittle building materials of existing buildings, requires gathering of samples by core drilling. Due to the fact, that the material samples may change their properties during transportion, it is preferred to test samples on site. Development of portable machine for Split test was therefore the first choice. The specimen is compressed with diametrically opposite symmetric line loads. The main goal of this paper is to document fine-tuning of experimental setup of the device to provide consistent and reproducible measurement. Therefore the paper concentrates on the biggest problem which is the boundary conditions of the test. For this reason, different types of contacts were studied. For boundary condition analysis was used photoelasticity. Set of experiments was performed in order to fulfill the condition of crack initiating in the middle of the specimen. Location of crack initiation was determined by the Digital Image Correlation - DIC. Various materials for contact were evaluated and it was found that the lead bar and grips with radius are best.

Abstract: According to classical definition of crack deformation modes, the constant stress term (Tstress) exists only in presence of mode I. However, some studies show that this term can exist in mode II problems as well; and significantly affect the elastic stress field around the crack tip. Based on the previous analytical results, T-stress changes the photoelastic fringe patterns from symmetric closed shapes to asymmetric and discontinuous loops. In this research, the effects of T-stress on the fringe patterns in mode II cracks is investigated experimentally. Test specimens are Brazilian disks made of polycarbonate, and thermal treatment is performed to remove the residual stresses after generation of the cracks. Observed isochromatic fringes are in good agreement with theoretical predictions. Also, experimental results indicate that this specimen contains a negative T-stress in pure mode II condition.

Abstract: A series of fracture toughness tests have been conducted on a type of coarse grain marble. The tests were carried out either in pure mode I or in pure mode II conditions by using the cracked chevron notched Brazilian disc (CCNBD) specimen. A total number of 44 CCNBD specimens were tested, half in pure mode I and the rest in pure mode II, to obtain reliable values for mode I and mode II fracture toughness (KIc and KIIc) of the tested marble. The average value of KIc for this marble was approximately about 1.12 MPa m showing a good agreement with those reported for similar coarse grain marbles. The mode II fracture toughness was found to be 2.25 MPa m in average which is approximately twice the mode I fracture toughness. However, the conventional fracture criteria suggest that the mode I fracture toughness should be higher than the mode II fracture toughness, (KIc>KIIc). According to these criteria, the ratio of KIIc / KIc is a figure typically between 0.63 and 0.96. It is shown in this paper that enhanced mode II fracture toughness of CCNBD specimen could be due to the effects of highly negative T-stress when the specimen is subjected to mode II. It is also shown that an improved prediction for the ratio KIIc / KIc can be achieved when the effect of T-stress is taken into account.