Modeling and Simulation Strategies for Thickness Reinforced Composite

Fiber reinforced composites have been extensively used in aerospace applications due to their higher strength, stiffness and lightweight nature. Despite superior in-plane properties, out-of-plane loads such as impacts can cause the fiber layers to separate from each other causing delamination. Low impact energies can cause delamination to form, which causes a significant reduction in in-plane strength.;Through thickness reinforcement (TTR) aims to provide mechanical means to prevent individual layers from separating. Reinforcement in the form of binding fiber (in case of 3D woven composites) or pins (in case of Z-pin reinforced composites) can improve the delamination resistance of composites. Damage and failure modes of TTR composites are significantly different than unreinforced laminated composites. Additionally, the damage and failures are influenced heavily by nature and type of reinforcement, manufacturing defects, loading conditions and other factors. Due to lack of reliable computational methods, evaluation and validation of reinforced and improved composite structures relies heavily on experimental techniques which are expensive and time consuming.;The present work is dedicated to the implementation of the failure mechanisms of 3D woven composites (3DWC) and Z-pinned composites in computational models to accurately predict the response of specimens subjected to different loading conditions. Since the reinforcement mechanisms are different in these two composites they will be investigated separately. The first half of this work is dedicated to 3DWCs and the second half deals with Z-pin reinforced composites.;3DWC specimen show inherent geometric imperfections due to manufacturing processes. Such localized defects act as failure initiation sites when the specimen is loaded in compression leading to micro buckling and kink band formation. A Digital elements (DE) approach of modeling is implemented in this study to adequately replicate the imperfections throughout the specimen. This model is utilized to predict the buckling behavior of a specimen using finite element analysis. Results indicate that this model can accurately predict formation of kink bands as observed in the experiments.;Z-pinned reinforced composites were investigated next. Experimental tests were carried out at the Air Force Research Laboratory (AFRL) where impact and compression after impact (CAI) tests were investigated. A detailed finite element model was constructed to understand and simulate mixed mode delamination behavior of single Z-pin reinforcement. Results of this model were utilized to create an equivalent material model to simulate the effect of Z-pin reinforcement without modeling the details of pin geometry. This equivalent material model was implemented in the finite element simulations for impact and CAI testing of Z-pin reinforced laminates. The results show that this simplified approach can predict the impact response and delamination damage on laminates with good accuracy.;Aerospace structures need to undergo stringent certification procedures to ensure safety. High fidelity virtual testing results can potentially be used to replace a large number validation tests required for certification. Accurate simulation techniques are essential part of modern manufacturing to understand the material response. Reliable virtual testing serves as a bridge between the manufacturing and testing steps by providing accurate prediction of response...