Buckling and Damage Resistance of Transversely-Loaded Composite Shells

Experimental and numerical work was conducted to better understand composite shell response to transverse loadings which simulate damage-causing impact events. The quasi-static, centered, transverse loading response of laminated graphite/epoxy shells in a [+/-45(sub n)/O(sub n)](sub s) layup having geometric characteristics of a commercial fuselage are studied. The singly-curved composite shell structures are hinged along the straight circumferential edges and are either free or simply supported along the curved axial edges. Key components of the shell response are response instabilities due to limit-point and/or bifurcation buckling. Experimentally, deflection-controlled shell response is characterized via load-deflection data, deformation-shape evolutions, and the resulting damage state. Finite element models are used to study the kinematically nonlinear shell response, including bifurcation, limit-points, and postbuckling. A novel technique is developed for evaluating bifurcation from nonlinear prebuckling states utilizing asymmetric spatial discretization to introduce numerical perturbations. Advantages of the asymmetric meshing technique (AMT) over traditional techniques include efficiency, robustness, ease of application, and solution of the actual (not modified) problems. The AMT is validated by comparison to traditional numerical analysis of a benchmark problem and verified by comparison to experimental data. Applying the technique, bifurcation in a benchmark shell-buckling problem is correctly identified. Excellent agreement between the numerical and experimental results are obtained for a number of composite shells although predictive capability decreases for stiffer (thicker) specimens which is attributed to compliance of the test fixture. Restraining the axial edge (simple support) has the effect of creating a more complex response which involves unstable bifurcation, limit-point buckling, and dynamic collapse. Such shells were noted to bifurcate into asymmetric deformation modes but were undamaged during testing. Shells in this study which were damaged were not observed to bifurcate. Thus, a direct link between bifurcation and atypical damage could not be established although the mechanism (bifurcation) was identified. Recommendations for further work in these related areas are provided and include extensions of the AMT to other shell geometries and structural problems

Morphology and processing of aligned carbon nanotube carbon matrix nanocomposites

Intrinsic and scale-dependent properties of carbon nanotubes (CNTs) have led aligned CNT architectures to emerge as promising candidates for next-generation multifunctional applications. Enhanced operating regimes motivate the study of CNT-based aligned nanofiber carbon matrix nanocomposites (CNT A-CMNCs). However, in order to tailor the material properties of CNT A-CMNCs, porosity control of the carbon matrix is required. Such control is usually achieved via multiple liquid precursor infusions and pyrolyzations. Here we report a model that allows the quantitative prediction of the CNT A-CMNC density and matrix porosity as a function of number of processing steps. The experimental results indicate that the matrix porosity of A-CMNCs comprised of ∼1% aligned CNTs decreased from ∼61% to ∼55% after a second polymer infusion and pyrolyzation. The model predicts that diminishing returns for porosity reduction will occur after 4 processing steps (matrix porosity of ∼51%), and that >10 processing steps are required for matrix porosity <50%. Using this model, prediction of the processing necessary for the fabrication of liquid precursor derived A-CMNC architectures, with possible application to other nanowire/nanofiber systems, is enabled for a variety of high value applications.National Science Foundation (U.S.) (Grant CMMI-1130437

Strength and Performance Enhancement of Multilayers by Spatial Tailoring of Adherend Compliance and Morphology via Multimaterial Jetting Additive Manufacturing

Material tailoring of bondlayer compliance is a known effective route to enhance performance of multilayers, and here spatial material-tailoring of compliance and morphology of the adherends is examined. Multimaterial jetting additive manufacturing (AM) allows us to realize for the first time compliance- and morphology-tailored adherends, and evaluate directly the mechanical performance, including failure, of the tensile-loaded multilayers. Adherend compliance-tailoring, unlike bondlayer tailoring, requires additional consideration due to adherend bending stiffness and moment influences on bondlayer stresses. We introduce anisotropic as well as layered/sandwich adherend tailoring to address this dependence. Numerical models show that for both sub-critical and critical bondlengths (at which shear-dominated load transfer occurs through the bondlayer), adherend tailoring reduces peak stresses significantly, particularly peel stress (reductions of 47–80%) that typically controls failure in such systems. At sub-critical bondlengths, the AM-enabled layered/sandwich adherend tailoring shows significantly increased experimental performance over the baseline multilayer: strength is increased by 20%, toughness by 48%, and strain-to-break by 18%, while retaining multilayer stiffness. The adherend tailoring demonstrated here adds to the techniques available to increase the performance of bonded multilayers, suggesting that adherend tailoring is particularly well-suited to additively manufactured multilayers, but can also have application in other areas such as layered electronics and advanced structural composite laminates.Abu Dhabi National Oil Company (Award EX2016–000010

Impact and quasi-static response of cylindrical composite shells

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1995.Includes bibliographical references (v. 2, leaves 368-381).by Brian L. Wardle.M.S

Buckling and damage resistance of transversely-loaded composite shells

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1998.Includes bibliographical references (leaves 254-264).by Brian L. Wardle.Ph.D

Process-morphology scaling relations quantify self-organization in capillary densified nanofiber arrays

Capillary-mediated densification is an inexpensive and versatile approach to tune the application-specific properties and packing morphology of bulk nanofiber (NF) arrays, such as aligned carbon nanotubes. While NF length governs elasto-capillary self-assembly, the geometry of cellular patterns formed by capillary densified NFs cannot be precisely predicted by existing theories. This originates from the recently quantified orders of magnitude lower than expected NF array effective axial elastic modulus (E), and here we show via parametric experimentation and modeling that E determines the width, area, and wall thickness of the resulting cellular pattern. Both experiments and models show that further tuning of the cellular pattern is possible by altering the NF-substrate adhesion strength, which could enable the broad use of this facile approach to predictably pattern NF arrays for high value applications.United States. National Aeronautics and Space Administration (Grant NNX17AJ32G

Load Transfer Analysis in Short Carbon Fibers with Radially-Aligned Carbon Nanotubes Embedded in a Polymer Matrix

A novel shortfiber composite in which the microscopic advanced fiber reinforcements are coated with radially aligned carbon nanotubes (CNTs) is analyzed in this study. A shear-lag model is developed to analyze the load transferred to such coated fibers from the aligned-CNT reinforced matrix in a hybrid composite application. It is found that if the carbon fibers are coated with radially aligned CNTs, then the axial load transferred to the fiber is reduced due to stiffening of the matrix by the CNTs. Importantly, it is shown that at low loading of CNTs in the polymer matrix, there is a significant reduction in the maximum interfacial shear stress, e.g., at 1% CNTs, there is an ~25 % reduction in this maximum stress. Further, the modification in the load sharing between the fiber and the matrix plateaus at ~2% CNT matrix loading, indicating a small but critical window for engineering the interface in this manner. Effects of the variation of the aspect ratio of the fiber, CNT volume fraction and the application of radial load on the load transferred to such CNT coated fibers are also investigated

Thermal and Electrical Transport in Hybrid Woven Composites Reinforced with Aligned Carbon Nanotubes

Carbon nanotubes (CNTs) are a potential new component to be incorporated into existing aerospace structural composites for multifunctional (mechanical, electrical, thermal, etc.) property enhancement. Although CNT properties are extraordinary when measured individually, they tend to degrade by a large factor when integrated in system (often in polymer matrices). Mechanisms and effectiveness of nano-scale CNT implementation into macro-scale structural composites are not well understood. Non-mechanical aspects of these composites are the focus of this work. As a CNT hybridized fiber polymer composite, fuzzy fiber reinforced plastic (FFRP) is developed using a scalable fabrication method that achieves uniform CNT distributions for thermal and electrical conductive networks without requiring intensive mixing which can damage CNTs. At small CNT volume fractions (~0.5- 8% Vf), characterization shows significant enhancement in electrical conduction (x106-108) but limited enhancement in thermal conduction (x1.9). In addition, aligned-CNT polymer nanocomposites (A-CNT-PNCs) are being characterized as a representative volume element (RVE) of the FFRP. Experimentally obtained data on consistent A-CNT-PNC samples sets provide engineering knowledge and to achieve effective utilization of CNTs' multifunctional properties. Theoretical studies, both analytical and numerical, have been recently developed, suggesting interface effects may be a key to explaining the above limitations, including electron tunneling/hopping or phonon scattering at CNT-CNT and CNT-polymer interfaces. Multiple test techniques and property extraction methods for A-CNT-PNCs are developed and/or employed for cross-comparison. Applications of nano-engineered composites enhanced with CNTs can include lightning protection layers, electromagnetic interference shields, thermal management layers, and thermoelectrical sensor layers for airplane structures.Airbus IndustrieMassachusetts Institute of Technology (Richard and Linda Hardy Graduate Fellowship)Boeing CompanyEmpresa Brasileira de AeronáuticaLockheed MartinSpirit AeroSystems (Firm)Textron, inc.Composite Systems Technology (Firm)Toho Tenax Co., Ltd.Massachusetts Institute of Technology (Nano-Engineered Composite aerospace STructures (NECST) Consortium

Exohedral Physisorption of Ambient Moisture Scales Non-monotonically with Fiber Proximity in Aligned Carbon Nanotube Arrays

Here we present a study on the presence of physisorbed water on the surface of aligned carbon nanotubes (CNTs) in ambient conditions, where the wet CNT array mass can be more than 200% larger than that of dry CNTs, and modeling indicates that a water layer >5 nm thick can be present on the outer CNT surface. The experimentally observed nonlinear and non-monotonic dependence of the mass of adsorbed water on the CNT packing (volume fraction) originates from two competing modes. Physisorbed water cannot be neglected in the design and fabrication of materials and devices using nanowires/nanofibers, especially CNTs, and further experimental and ab initio studies on the influence of defects on the surface energies of CNTs, and nanowires/nanofibers in general, are necessary to understand the underlying physics and chemistry that govern this system.National Science Foundation (U.S.) (NSF Grant No. CMMI-1130437)National Science Foundation (U.S.) (NSF Award Number ECS-0335765)United States. Army Research Office (contract W911NF-07-D-0004