Sandi G. Miller

Increased Tensile Strength of Carbon Nanotube Yarns and Sheets through Chemical Modification and Electron Beam Irradiation

The inherent strength of individual carbon nanotubes (CNTs) offers considerable opportunity for the development of advanced, lightweight composite structures. Recent work in the fabrication and application of CNT forms such as yarns and sheets has addressed early nanocomposite limitations with respect to nanotube dispersion and loading and has pushed the technology toward structural composite applications. However, the high tensile strength of an individual CNT has not directly translated into that of sheets and yarns, where the bulk material strength is limited by intertube electrostatic attractions and slippage. The focus of this work was to assess postprocessing of CNT sheets and yarns to improve the macro-scale strength of these material forms. Both small-molecule functionalization and electron-beam irradiation were evaluated as means to enhance the tensile strength and Youngs modulus of the bulk CNT materials. Mechanical testing revealed a 57% increase in tensile strength of CNT sheets upon functionalization compared with unfunctionalized sheets, while an additional 48% increase in tensile strength was observed when functionalized sheets were irradiated. Similarly, small-molecule functionalization increased tensile strength of yarn by up to 25%, whereas irradiation of the functionalized yarns pushed the tensile strength to 88% beyond that of the baseline yarn.

Effects of hygrothermal cycling on the chemical, thermal, and mechanical properties of 862/W epoxy resin

The hygrothermal aging characteristics of an epoxy resin were characterized over a one-year period, which included 908 temperature and humidity cycles. The epoxy resin quickly displayed evidence of aging through color change and increased brittleness. The influence of aging on the material’s glass transition temperature (T g) was evaluated by Differential Scanning Calorimetry and Dynamic Mechanical Analysis. The T g remained relatively constant throughout the year-long cyclic aging profile. Chemical composition was monitored by Fourier Transform Infrared spectroscopy, where evidence of chemical aging and advancement of cure was noted. The tensile strength of the resin was tested as it aged and this property was severely affected by the aging process in the form of reduced ductility and embrittlement. Detailed chemical evaluation suggests many aging mechanisms are taking place during exposure to hygrothermal conditions.

Trade-off between the Mechanical Strength and Microwave Electrical Properties of Functionalized and Irradiated Carbon Nanotube Sheets

Carbon nanotube (CNT) sheets represent a novel implementation of CNTs that enable the tailoring of electrical and mechanical properties for applications in the automotive and aerospace industries. Small molecule functionalization and postprocessing techniques, such as irradiation with high-energy particles, are methods that can enhance the mechanical properties of CNTs. However, the effect that these modifications have on the electrical conduction mechanisms has not been extensively explored. By characterizing the mechanical and electrical properties of multiwalled carbon nanotube (MWCNT) sheets with different functional groups and irradiation doses, we can expand our insights into the extent of the trade-off that exists between mechanical strength and electrical conductivity for commercially available CNT sheets. Such insights allow for the optimization of design pathways for engineering applications that require a balance of material property enhancements.

Out-of-autoclave processing and properties of bismaleimide composites

*This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. The emergence of bismaleimide composites has fulfilled some of the increasing demand for higher temperature performance aeronautics and space exploration vehicles. This study examines and evaluates three bismaleimide matrix resins and two bismaleimide adhesives and reports on the processing properties of these resins and composites by out-of-autoclave–vacuum-bag-only oven processing. Measurements were conducted under various cure cycles to characterize and correlate thermal and viscoelastic properties of the materials. Specimens of all three aged matrix resins exhibited an out-time life up to 30 days when stored at room temperature. Solid and sandwich panels were fabricated with the out-of-autoclave–vacuum-bag-only process. Because of tooling limitations in industry practices, composite fabrication of these bismaleimides was restricted to a maximum 177℃ curing, followed by a free-standing postcuring at elevated temperatures in an oven. The adhesive foaming characteristics, composite resin/void content, flat wise tensile strength, and fracture surface features were evaluated. Due to the unique temperature limitations of this work, the resulting panel properties were not necessarily representative of manufacturer specifications or recommendations.

Assessment of mode-II fracture tests for unidirectional fiber reinforced composite laminates

Three basic mode-II test methods (ENF, JIS, and ASTM D7905M-14) are assessed using the material system AS4/8552 carbon/epoxy unidirectional composite laminate to understand similarities and differences. The modified JIS method uses a PTFE film coated stainless steel rod instead of the PTFE strip that was proposed in JIS. The ASTM D7905M-14 test method determines FEP film crack front (NPC) and shear precrack front (PC) fracture toughnesses. Alternately, wedge precracked specimens were also tested to assess the shear versus opening mode precracking on mode-II fracture toughness. The analysis and test results revealed that the JIS method is a mixed-mode I-II test and result in lower value of mode-II fracture toughness. The GI loading is about 51 J/m2 for the material tested and GIIc measured by JIS is always less than pure mode-II fracture toughness. The GIIc measured from the ASTM D7905M-14 NPC and the ENF tests are almost identical, but the ASTM test offers a compliance equation that may be beneficial in fatigue crack growth studies. As suggested in ASTM standard, shear precracked specimen is appropriate to measure mode-II fracture toughness.

Polymer-Layered Silicate Nanocomposites for Cryotank Applications

Previous composite cryotank designs have relied on the use of conventional composite materials to reduce microcracking and permeability. However, revolutionary advances in nanotechnology derived materials may enable the production of ultra-lightweight cryotanks with significantly enhanced durability and damage tolerance, as well as reduced propellant permeability. Layered silicate nanocomposites are especially attractive in cryogenic storage tanks based on results that have been reported for epoxy nanocomposite systems. These materials often exhibit an order of magnitude reduction in gas permeability when compared to the base resin. In addition, polymer-silicate nanocomposites have been shown to yield improved dimensional stability, strength, and toughness. The enhancement in material performance of these systems occurs without property trade-offs which are often observed in conventionally filled polymer composites. Research efforts at NASA Glenn Research Center have led to the development of epoxy-clay nanocomposites with 70% lower hydrogen permeability than the base epoxy resin. Filament wound carbon fiber reinforced tanks made with this nanocomposite had a five-fold lower helium leak rate than the corresponding tanks made without clay. The pronounced reduction observed with the tank may be due to flow induced alignment of the clay layers during processing. Additionally, the nanocomposites showed CTE reductions of up to 30%, as well as a 100% increase in toughness.

Thermal Characterization of IM7/8552-1 Carbon-Epoxy Composites

Thermal properties of composite materials such as, thermal conductivity, diffusivity, and specific heat are very important in engineering design process and analysis of aerospace vehicles as well as space systems. These properties are also important in power generation, transportation, and energy storage devices including fuel cells. Thermal conductivity is the property that determines the working temperature levels of the material; it plays a critical role in the performance of materials in high temperature applications, and it is an important parameter in problems involving heat transfer and thermal structures.The objective of this paper is to develop a thermal properties data base for the carbon fiber-epoxy (IM7/8552-1) composite. The IM7 carbon fiber is a continuous, high performance, intermediate modulus, PAN based fiber. This fiber has been surface treated and can be sized to improve its interlaminar shear properties, handling characteristics, and structural properties. The 8552 is a high performance tough epoxy matrix for use in primary aerospace structures. It exhibits good impact resistance and damage tolerance for a wide range of applications. The IM7/8552-1 is an amine cured unidirectional prepreg. The manufacturer recommended cure cycle for this material was followed, which includes consolidation under vacuum and autoclave pressure. The composite has a service temperature up to 121°C (250°F).The thermal properties of IM7/8552-1 carbon-epoxy have been investigated using experimental methods. The flash method was used to measure the thermal diffusivity of the composite. This method is based on the American Society for Testing and Materials standard, ASTM E1461. In addition, the Differential Scanning Calorimeter was used in accordance with the ASTM E1269 standard to measure the specific heat. The measured thermal diffusivity, specific heat, and density data were used to compute the thermal conductivity of the IM7/8552-1 carbon-epoxy composite.Copyright