Multi-scale characterization and failure modeling of carbon/epoxy triaxially braided composite

Carbon/Epoxy two-dimensional triaxially braided composites are known to have excellent damage tolerance, energy absorption and impact resistance. Recently, many aircraft manufactures have used such braided composite for engine fan cases. The mechanical interlocking and large unit cell of the fabric architecture enable the material to behave like a structure. On the other hand, this complicated architecture increases the difficulty of experimental characterization and numerical simulation, due to the existence of free edge effect induced premature local damage in standard coupon specimens. The goal of this research is an attempt to identify the damage mechanism of this braided composite, develop analytical and numerical tools to simulate the elastic and failure behavior, and obtain accurate material properties for design of structures and components.;Multi-scale modeling is a well established approach in simulating textile composites. In this work, a meso-scale finite element modeling framework is developed to simulate the local and global damage behavior and investigate the failure mechanisms. To achieve the accuracy of the meso-scale model, the micro-scale geometry features are examined in detail to provide support for building a realistic representative model; micromechanical finite element models are developed to predict the material constants of meso-scale fiber tows. On the macro-scale, both analytical models and numerical models are used to predict the global effective stiffness.;A comprehensive analysis of the experimentally measured and numerical predicted elastic and strength properties are presented to verify the accuracy of numerical models and evaluate the ability of different types of specimens. The material constants of an infinite large plate are predicted using the numerical models. This can help to make design of structures more efficient and build accurate numerical models representing behavior of braided composites.;Particular attention is paid on the analysis of free edge effect which was known to lead to premature local damage and failure. It is found in the numerical studies and confirmed by experiments that the free edge effect can also cause a reduction of effective stiffness. The edge effect that acts in the form of out-of-plane warping is found to be an inherent behavior of the triaxially braided architecture. It is also identified in experiments that some material properties may depend on the coupon width, especially in transversely loaded specimens. Through a numerical dimensional analysis, the relationship of specimen width and effective stiffness and strength is quantified.;The long-term performance of the material is also studied by conducting thermal cycling test. Acoustic Emission and X-ray CT scanning are used to detect the aging induced microcracking damage behavior and predict the saturation number of thermal cycles. Multi-scale finite element models are utilized to predict the degradation of mechanical properties and impact resistance of microcracked braided composite panels.