Study On The Mechanical Behavior Of Three Dimensional Braided Composites

Three dimensional braided composite is one kind of textile composite. Structures with complex shape can be manufactured easily by three dimensional braiding technologies. Three dimensional braided composite has higher impact resistance and higher damage tolerance than other textile composite have. That attracts much more attentions. Three dimensional braided ceramic matrix composites (CMC) also have characters such as low density, high temperature resistance and wash out resistance. It is a potential high temperature structure material used in airplane engine and space shuttles.It is a long way to understand mechanical properties of three dimensional braided composite. In order to instruct material design and material application, mechanical properties of three dimensional braided composite should be investigated thoroughly. Based on previous reason, study on mechanical behaviors of three dimensional braided composite was carried out.The following works are included in this thesis.1. Experiments of three dimensional braided T300/QY9512 composite and three dimensional braided C/SiC composite was conducted. And characters of stress-strain curves, damage development, and failure were analyzed. The following properties were found. Tensile and compress failure mode of two composites are the same. Tensile failure was caused by fiber fracture and compress failure was caused by interface failure and fiber bundle's shear, compress failure. For two materials shear modulus of four point bending are greater than those of tensile test. It is because of the different properties between interior and surface of specimens.2. For three dimensional braided C/SiC composite, stress-strain curve shows a nonlinear character. It induced by initial voids and microcracks in materials. Acoustic Emission data reveal that damage develops slowly before the load reaches its failure value. Damage develops fast after that period. And a nonlinear formula with parameter of unloading modulus and residual strain was brought out to describe damage development in tensile test. It is also found that mechanical properties of C fiber are much less than those of T300 fiber bundles.3. An unit cell model taking account of the difference among unit cells was established to predict elastic properties of three dimensional braided T300/QY9512 composites and three dimensional braided C/SiC composite. Predicted results agreed well with test results. Influence of fiber volume content and braiding angle on specimens' elastic properties was analyzed. Furthermore the influence of shape and dimension were analyzed. Elastic properties of three dimensional braided SiC/SiC composite were predicted. The conclusion that anisotropy of fiber bundles and modulus difference between matrix and fiber bundles influence specimens' Poission's ratio was also concluded.4. Experimental and Finite Element Analysis of three point bending (TPB) test were conducted. It is found material's anisotropy of anisotropic materials and poission's ratio of isotropic materials all had influence on contact deformation between specimen and fixture. So it is wise to get flexural properties using four point bending test.5. A failure criterion for three dimensional braided composite was introduced to predict composite's strength. Prediction values of two braided composites agree well with their experimental values. Influence of fiber volume content and braiding angle on specimens' strength was analyzed. The influence of shape and dimension were also analyzed...