Study Of Three-dimensional Woven Ceramic Matrix Composite Mechanical Behavior

Angle-interlock woven ceramic composites are a promising way to improve the interlaminar fracture toughness and damage tolerance, and they have excellent characteristics as a structural material for use at high temperatures for high thrust-weight ratio aerial engine and rocket engine because of its heat stability, superior strength and modulus, oxidation resistant, ablation resistant at elevated temperatures and low density. Three-dimensional (3D) woven composites have powerful abilities of forming, which make these materials can be applied to wider fields.It is a long way to understand mechanical properties of three dimensional woven ceramic composite especially angle-interlock woven continuous carbon fiber reinforced silicon carbide composites (C/SiC). It is necessary to material design and application that an overall profound understand of mechanical properties of 3D woven C/SiC composites. Thus, the investigation on mechanical behaviors of 3D angle-interlock woven C/SiC composite was carried out.The following works are included in this thesis.1. Using volume averaging method, a micromechanical model for elastic behavior analysis of 3D angle-interlock woven ceramic composites is proposed in this paper. Two geometric models were introduced, and these two models take into account the actual fabric structure by considering the fiber's undulation and continuity in space, the cavities between adjacent yarns and the actual cross-section geometry of the yarn. The effects of manufacturing ceramic composite to the composite elastic properties have been investigated. Based on laminate theory, the nine elastic constants of 3D angle-interlock woven ceramic composites are deduced for two assumptions of strain condition. Good agreements between theoretical predictions and experimental results demonstrate that the feasibility of the proposed model in analyzing the elastic properties of 3D woven composites. This model provides valuable references for designing and optimizing composites. The straight bundle bar elastic properties prediction model has more accurate predictions than the crimp bundle bar model.2. The effects of fabric micro-structural parameters on elastic properties were analyzed using the elastic properties prediction model proposed by author. Parametric studies are performed to examine the effects of two important micro-structural parameters on fiber volume fraction, crimp fraction and void volume fraction, and variations of elastic properties with fiber volume fraction, crimp fraction and void volume fraction are presented. As fiber volume fraction increases, the longitudinal elastic modulus predicted decreases nonlinearly. The reason of these variable trends is that as fiber volume fraction increases the magnitude of local fiber angle is enlarged and the elastic moduli mainly depend on the angle of longitudinal composite bar, the larger angle of warp leads to small elastic longitudinal property. The effects of manufactureprocesses on ceramic composite on elastic properties are unimportant, and voids of intran-yarn have obvious effect on elastic properties. The analysis demonstrate fabric structure and the mixture rule of yarn composite are main factors to elastic properties, and different fabric structure parameters can cater to different applications.3. Experiments of 3D angle-interlock woven ceramic composites were preformed, and the characters of stress-strain curves and failure were analyzed. Longitudinal and transverse tensile had similar mechanical properties, and both represented nonlinear stress-strain behavior, and the tangent modulus appeared obvious increase when the applied tensile stress was higher. The reasons of this mechanical behavior probably are the damage of bundle and fabric structure together. The stress-strain curves of longitudinal and transverse compression expressed almost straight lines when the applied stresses were low to collapsing loads about 90 percent of strengths. When the applied stresses above collapsing loads the stress-strain curves represented obvious destabilizing mechanical behavior accompany by residual strain and great drop of tangent modulus. However, the strength and modulus along different compressive direction have discrepancies, and longitudinal strength is lower than transverse strength, and the modulus of longitude is higher than that of transverse. The fracture angle of longitudinal compression approach the maximum angle of warp, and transverse fracture angle is related to arrange of wefts.4. Based on the experimental results, the damage developments of tensile and in-plane shear were studied, and the predictive stress-strain relations of these two type loads were given. The study illustrate that the damage processes of longitudinal tensile is very similar to that of transverse tensile. As applied tensile stresses increase, damage accumulate with polygonal line, and damage increase rapidly firstly then slowly. The predictive stress-strain curves of longitudinal and transverse tensile are almost consistent. The damage mechanisms along different shear plane are discrepancy, and the variation trends of corresponding damage factor are different as applied shear strains increase. However, the predictive stress-strain curves of in-plane shear along two directions are similar.