چكيده به لاتين
Abstract:
In the present research, brittle failure in key-hole notches is investigated both experimentally and theoretically under mixed mode Ι/ΙΙ loading with negative mode Ι conditions. The Key-hole notched Brazilian disk (Key-BD) specimen that is specified by different geometric parameters, i.e. the notch length and the tip radius, and made of the general-purpose Polystyrene (GPPS) is utilized to conduct the fracture tests under mixed mode І/ІІ loading with negative mode І contributions. The finite element analyses revealed that four cases of loading conditions exist in the Key-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, and finally; (iv) mixed mode Ι/ΙΙ loading with 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. Totally, 90 fracture tests are performed at room temperature and the fracture loads and fracture initiation angles are experimentally reported for the notched BD specimens. In order to predict the experimental results theoretically, two stress-based brittle fracture models, namely the key-hole notch maximum tangential stress (Key-MTS) and key-hole notch mean stress (Key-MS) criteria are proposed. The critical distance for each stress-based criterion is obtained by two different approaches meaning that four types of prediction curves are obtained by these theories. Moreover, the new method based on the strain energy density criterion was presented in order to predict the values of experimental fracture load as the other novelty in this thesis. Finally it was shown that, stress-based fracture criteria with theoretical and experimental critical distances predicted the experimental results with 90% and 95% accuracies, respectively. Also, the energy-based criteria have 6% discrepancy with the experimental data. With a general comparison, it was found that, the Key-MS criterion based on the experimental critical distance is the most appropriate failure criterion to predict the fracture load of the tested specimens in this experimental research.
Keywords: Key-hole notch; GPPS; Brittle fracture criteria; Mixed mode І/ІІ loading; Negative mode Ι contribution; Brazilian disk specimen
