چكيده به لاتين
The present research, dealing with investigation of brittle fracture in the well-known Brazilian disk weakened by V-notches with end holes made of general-purpose Polystyrene (GPPS), called VO-BD, under mixed mode loading with negative mode I contributions. The finite element analyses revealed that five cases of loading conditions exist in the VO-BD specimen as follows: (i) pure positive mode I loading; (ii) mixed mode Ι/ΙΙ loading with positive mode I conditions; (iii) pure mode ΙΙ loading; (iv) mixed mode Ι/ΙΙ loading with negative mode I conditions, and finally; (v) when the loading angle gradually increases from βc to 90°, the specimen breaks from the edge of the central slit, while experiencing still negative mode I conditions. In fact, investigation of mixed mode loading with negative mode I condition on the new brittle material is the main purpose of this thesis. Two well-established stress-based failure criteria, namely, VO-MTS which was established based on the maximum tangential stress criterion (MTS) criterion, and VO-MS was extension of the mean stress (MS) failure criterion. Both the theoretical critical distances corresponding to sharp cracks and notches and the experimental critical distances regarding to brittle fracture test results of the VO-BD specimens under pure mode I, were utilized in order to anticipate fracture loads. To evaluate validation of the criteria, the predicted fracture loads of the stress-based criteria corresponding both the experimental and theoretical critical distances were compared with large number of experimental data collected from 81 new brittle fracture tests carried on the Brazilian disk containing VO-notches made of GPPS specimens with various notch angles and notch radii under pure tensile mode I and mixed mode loading with negative mode I contributions. The theoretical predicted results coincide to the experimental data with good accuracy. The predicted fracture loads of the VO-MS criterion regarding the experimental critical distances has most accuracy to the experimental results. Moreover, two energy-based brittle fracture criteria, namely the averaged strain energy density (ASED) and averaged strain energy density based on the equivalent factor concept (ASED-EFC) are proposed to predict the experimentally obtained fracture loads of the tested GPPS specimens. Additionally, the fracture initiation angles of the tested VO-BD specimens are predicted by using ASED criterion. The finite element (FE) analyses, as well as the experimental observations, show that although brittle fracture in the specimens under mixed mode I/II loading takes place from the applied-load side of the notch border by local tensile stresses, the notch bisector line and the other side of the notch border sustain compressive stresses. In fact, this phenomenon states the concept of mixed mode I/II loading with negative mode I contributions. Finally, it is shown that good agreement exists between the experimental results and the theoretical predictions of the two energy-based fracture criteria.
