Compression And Compression After Impact Performance Of3D Woven Composites

Three dimensional (3D) woven composites possess higher through-the-thickness mechanicalproperties, damage resistance, delamination resistance and impact resistance in comparison withtraditional laminated composites. Apart from the good mechanical properties,3D woven compositesare generally weak in compression and consequently put a limit on their strength. From the literaturereview, it was found that this area is less understood field in composites, especially for woven fabriccomposites.3D woven composites, due to good impact resistance properties, are gaining popularityin armor industry but there is very limited literature available on the characterization of their damagetolerance performance. Since the compression after impact (CAI) is the measure of damage tolerance,therefore, it is important to determine the compression and compression after impact performance of3D woven composites.The objective of this research was to investigate the compression and compression after impactperformance of a new material named “3D woven orthogonal-interlock hybrid composite(3DWOHC)”. This objective was achieved by conducting experimental study and through analyticaland FE based predictive modeling. The predictive parameters were compared with the experimentaldata. The scope of the thesis includes literature review, geometric modeling, stiffness analysis,compression performance analysis, low velocity impact analysis and compression after impactanalysis, summary and recommendations. Some innovative research results are pointed out. And afew topics for further study are recommended.A novel geometrical model called “Generic Geometric model (GG-model) has been proposed todescribe internal geometry of the composite. The GG-model is capable to deal with a variety of towcross-sectional shapes through a novel “Generic Shape Function”, hybrid reinforcement system andvarious methods to represent geometry of3D woven interlock preforms/composites. The geometryestablished by GG-model was used to develop of a novel analytical stiffness model called “GenericStiffness model (GS-model). Based on volume averaging method and iso-strain boundary conditions,the model can determine the engineering elastic constants of3D woven interlock composites.A finite element (FE) based model is also developed at repeating unit cell level to understand theload carrying mechanism of various constituents of3DWOHC and to determine the mechanicalproperties of3DWOHC. Through digital image analysis, it was observed that constituent tows of3DWOHC are not perfectly straight. Therefore, a mathematical model has been proposed to modelthe initial geometrical imperfections of the tows. The geometrical imperfection was implemented through a user defined subroutine “ORIENT”. Another subroutine UMAT was used to implement thematerial properties and failure criteria of constituents of3DWOHC. The analysis was executed andpredicted compression strength was compared well to the experimental data. Sensitivity studies werealso been conducted to investigate the influence of void fraction, geometrical imperfection of tows,and material hybridization on the compression strength of3DWOHC.FE based method has been used to simulate the damage resistance of3DWOHC subjected to lowvelocity impact. The FE model of3DWOHC plate was developed at microstructure level in order tocapture the damage at microstructure level. The impact simulation was conducted for5J,10J,13.33J,15J,16.67J and20J initial impact energies. The results of impact induced damage area werecompared well to the experimental data. To simulate the compression after impact performance, FEbased plate-model composed of virgin monolithic material, virgin hybrid material and damagedmaterial was developed. The damaged material was provided with the discounted properties. Themodel was subjected to displacement controlled compressive loading. The compressive residualstrength was determined and compared with the experimental data.The predicted compression strength has been found in good agreement with the experimental data;the relative difference is within1%for4.25%geometric imperfection and5.8%voids in the purematrix. Both, voids and geometric imperfection of tows have been found to reduce the compressionstrength non-linearly. Addition of carbon fibers has been found to enhance stiffness only. Hence,high stiffness and high strength fibers are required to enhance both stiffness and strength of3DWOHC.The impact simulation and experimental data shows that impact induced damage area increasesnon-linearly with the initial impact energy. The compression after impact simulation andexperimental data shows that compressive residual strength reduces after10J impact energy. Hence10J impact energy can be regarded as minimum threshold energy to affect the compressive residualstrength of3DWOHC.