A Study On Token Methods And Theories Of Fracture Toughness About 3D Braided Carbon Fiber Reinforced Silicon Carbide Matrix Composites

Lately, there are some disputes on the characterization methods for fracture toughness of 3D braided CMC, and many researchers usually insisted in their own viewpoints on this question. After continuous researches on macroscopic fracture theories and microscopic fracture theories, characterization parameters of the material fracture toughness have been defined, and the microscopic fracture theories applied in 3D braided CMC also have been proposed. Enough numerical simulations have been conducted, and also comparison with the theories has been done. At last, the characterization method of fracture toughness and theories of 3D braided CMC came to an end ideally. The specified works finished are listed:1. With experimental load-displacement curves and acoustic emission technology, it was found that the material shows pseudo-plastic behavior under tensile loads and quasi-elastic behavior under the flexural loads.2. Because the fracture mode of 3D braided CMC is the interfacial fracture of mode I and mode II, energy release rate GC and complex stress intensity factor KC was designated as characterization parameters of fracture toughness in the linear fracture theories.3. Bi-material interfacial fracture theories are researched with a phenomenon that cracks extend along interface. A gradient invading model of interface crack at the crack tip can gain stress fields along interface. So a formula of energy release rate for 3D braided CMC is deduced and the corrected formula of the energy release rate is examined with test results. The single specimen formula can show the material fracture characters in details other than the flexibility method, and save a great number of specimen.4. Interface failure model and fiber bundle fracture model are formed with the damage mechanics of 3D braided CMC, and a scope of the models application is preliminarily divided. Then, the two models can predict the material toughness, and the predicted results are accordant to test results.5. The fracture theory of interfacial layer is developed for engineering application of 3D braided C/SiC composites. A curve with fracture mode mixity is depicted to predict the material toughness.6. Influence of holes in the 3D braided CMC on fracture toughness is discussed preliminarily at last. And studying results indicate that the holes toughening effect isn't ideal at a cost of material strength.Above studies have solved the characterization problems of fracture toughness of 3D braided CMC, and given some preliminary microscopic fracture theories. The researched results are also original innovation to fracture toughness of 3D braided CMC in this thesis.