چكيده به لاتين
In this thesis, the strain energy-based fracture models are extended to predict the mixed mode fracture toughness in brittle and quasi-brittle materials. The two developed energy-based methods are the strain energy density and the averaged strain energy density. The modified criteria contain three key crack parameters including: the mode I and mode II stress intensity factors and the T-stress. The main goal of the current thesis is to investigate the effect of T-stress on the critical mode I stress intensity factor, crack trajectory under mode I loading and the mixed mode fracture toughness of brittle materials using modified energy-based models. Also, the effect of geometry on crack growth rate and life in cracked components under fatigue loading has been investigated by the modified criteria.
The critical mode I stress intensity factor (SIF) obtained from fracture tests on laboratory specimens is often used as fracture toughness. However, the fracture resistance under mode I loading is considerably influenced by the geometry and loading conditions. For example, the critical stress intensity factor of PMMA varies from 0.7 to 1.4 MPa.m0.5. The proposed modified criteria could provide very good estimates of the critical SIF for this wide range of data. Also, the crack trajectory of PMMA samples was investigated experimentally and theoretically for five different geometries. Although all the specimens were tested under pure mode I loading conditions, but in some cases, the crack growth kinked from the original crack line in a non-coplanar way. The crack growth deviation was related to the geometry and loading conditions. The discrepancies observed between the crack trajectories and the fracture strengths of different tested specimens were found to be related to the magnitude and sign of the T-stress. The theoretical results obtained from the generalized strain energy density criterion were then compared with the experimental results reported in previous studies for PMMA and rock. It was shown that the results predicted using the modified criterion were in significantly better agreement with the experimental results compared with the conventional criterion. In some cases, the discrepancy between the results predicted using the conventional criterion and experimental results was higher than 100%. Also, a series of mixed-mode fracture experiments was conducted on Ghorveh white marble using proposed geometry. Very good agreement was shown to exist between the theoretical predictions and the experimental results.
Different samples with positive and negative T-stresses were used to conduct high cycle fatigue experiments on Al-7075. It was shown that the modified criteria can predict the crack growth rate and the fatigue life of the pre-cracked specimens very well.
