Howard A. Katzman

Characterization of high thermal conductivity carbon fibers and a self-reinforced graphite panel

Abstract Extremely high thermal conductivity graphitic materials from mesophase pitch precursors (K-1100 fibers, four experimental high thermal conductivity fibers, and a ThermalGraph® panel) were examined utilizing X-ray diffraction (XRD) and high resolution field emission (FE) scanning electron microscopy (SEM). Of the four experimental fibers, two were produced from Amocos standard petroleum pitch, and two were produced from an Amoco experimental pitch precursor. The low d-spacings, narrow peaks, and presence of three-dimensional reflections in the XRD patterns of the five fibers and the ThermalGraph® panel indicate that they are all highly graphitic. The thermal conductivities of these materials correlate best with the graphite inter-basal-plane spacing (d002). All of the materials studied appear very graphitic in high resolution SEM micrographs of their transverse fracture surfaces. Well-developed graphene layer planes are clearly seen. High resolution SEM examination of the ThermalGraph® panel shows that the precursor fibers have coalesced into a continuous three-dimensional structure. The result of this fiber fusion is a “self-reinforced”, graphitic structure.

Fibre coatings for the fabrication of graphite-reinforced magnesium composites

A new fabrication technique for graphite-fibre-reinforced magnesium is presented. An air-stable silicon dioxide coating is deposited on the fibre surfaces from an organometallic precursor solution. The fibres are passed through this solution, followed by hydrolysis or pyrolysis of the organometallic compound to form silicon dioxide on the fibre surfaces. The silicon dioxide coating facilitates wetting and bonding when the fibres are immersed in molten magnesium. A modification of this coating technique was developed for coating fibres with amorphous carbon, which was found to improve the adhesion between the graphite fibres and the silicon dioxide coating. Composites containing either T300, P55, or P100 graphite fibres in pure magnesium, magnesium-1 wt% silicon, and magnesium alloy AZ91 have been fabricated. Preliminary mechanical property data are presented.

Characterization of low thermal conductivity pan-based carbon fibers

Abstract The microstructure and surface chemistry of eight polyacrylonitrile (PAN)-based carbon fibers heat treated at relatively low temperatures to retain low thermal conductivity were determined. Properties of these fibers (hereafter called LTC fibers) were compared with those of PAN-based carbon fibers subjected to higher heat treatment temperatures (HTTs). Wide-angle x-ray diffraction shows that LTC fibers have a turbostratic structure with large 002 d-spacings, small crystallite sizes, and only moderate preferred orientation. Small-angle x-ray scattering results indicate that LTC fibers do not have well-developed pores. Transmission electron microscopy shows that the texture of LTC fibers consists of multiple sets of parallel, wavy, bent layers that interweave with each other, forming a complex, three-dimensional network oriented randomly around the fiber axis. Crystallite size and extent of graphitization both increase with increased HTT. However, scanning electron microscopy indicates that HTT has little effect on the surface texture of PAN fibers. X-ray photoelectron spectroscopy analyses find that concentrations of surface oxygen and nitrogen decrease with increasing HTT, which is consistent with increased volatilization of nitrogen and greater extent of graphitization.

Carbide coatings for fabrication of carbon-fiber-reinforced metal matrix composites

A carbon fiber reinforced metal matrix composite is produced by carbide coating the surface of the fibers by passing the fibers through an organometallic solution followed by pyrolysis of the organometallic compounds. The carbide coated fibers, so produced are readily wettable without degradation when immersed in a molten bath of metal matrix material containing an active alloying element.

FIBER COATINGS FOR COMPOSITE FABRICATION

ABSTRACT Graphite fibers resist wetting when immersed in most molten metals. They must be coated with a material that allows wetting and that also protects the fibers against chemical degradation during composite fabrication. The titanium-boron chemical vapor deposition process, as well as the graphite-metalinterfacethat results from that process, is discussed. This is followed by a description of a solution coating process developed at The Aerospace Corporation to produce air stable coatings. In this process, the fibers are passed through appropriate organometallic solutions, followed by either hydrolysis or pyrolysis of the organometallic compounds to form the desired coating on the fiber surfaces. Fibers have been coated with selected oxides, nitrides, and carbides. A modification of this technique can also be used to coat fibers with amorphous carbon. These coatings can be applied to other fibers as well as graphite, and some are easily wet by metals. In addition, the coatings are expected to prove us...

Failure Modeling and Simulation of Composites Subjected to Bypass and Bearing Loads

An analytical model to predict the bypass and bearing strength of composites is developed. The model includes a contact algorithm, finite deformation theory, nonlinear material behavior, and the progressive damage of composites. The orientation and density of matrix-cracks and fiber damage are represented through a damage tensor, an internal state tensor that is thermodynamically consistent within the framework of continuum damage mechanics. The damage tensor evolves as damage associated with different failure modes accumulates. Matrix-cracks are predicted using a three-dimensional failure criteria, which is based on the stresses acting on a potential fracture plane. For use with the failure criteria, a new anisotropic shear constitutive law is postulated governing the transverse and the in-plane shear nonlinear mechanical behavior. For the prediction of initiation and progression of multiple delaminations through the thickness of the composite, a cohesive-decohesive constitutive law is adopted. The analytical model is implemented in a finite element commercial code via user subroutines. The problem of a notched composite loaded in tension is examined. The general characteristics of the predicted failure modes are in good agreement with the experimental observations obtained from the open literature. Numerical simulations are also conducted for the filled hole tension, double sided bypass and bearing load, and single sided bypass and bearing test configurations. The predicted strength and the strain behavior is in good correlation with in-house experimental data. The predicted failure modes are consistent with observations made in the open literature. A common challenge in the development of aircraft and spacecraft structures is maintaining structural integrity in the presence of mechanically fastened joints. This challenge is amplified when the structures include composite laminates, which have shortcomings from a microscopical and macroscopical standpoint. In the micro-scale, stress concentrations develop between stiff fibers and relatively compliant matrix material, and in the macro-scale stress concentrations develop near discontinuities such as a drilled hole. These stress concentrations lead to failure modes that can initiate at loads below the ultimate strength of the composite material. The focus of this paper is to quantify joint strength via an improved simulation of failure modes in composite structures. The design and analysis of bolted joints in composite structures is complex and uncertain because failure loads depend on a combination of factors such as material selection, stacking sequence, bolt clamping force, loading vector, geometric configuration, and manufacturing defects. The contact between the bolt and the bolt hole may induce large strains and high stress concentrations in the vicinity of the bolt hole boundary. Eventually an accumulation of localized failures ‐ such as fiber failure, delamination, and matrix-cracks ‐ that propagate from the edge of the bolt hole leads to the ultimate failure of the mechanically fastened joint. This complexity requires analytical procedures that can account for these variables in order to reasonably predict the response of new structures. Analytical work needs to be developed based on lessons learned from the extensive experimental work characterizing failure mechanisms that occur in mechanically fastened composite joints. 1‐6 Failure modeling of composite joints is challenging due to the multi-scale damage mechanisms that occur in composite laminates: micro-cracks in the micro-scale, and delaminations and through-the-thickness cracks in the macroscale. Significant work has been conducted in modeling and simulating the failure of composites using progressive

MgO DIFFUSION BARRIER FOR TITANIUM MATRIX COMPOSITES

ABSTRACT The feasibility of developing an effective diffusion barrier for titanium matrix composites was investigated. MgO was identified as a potential barrier and was applied to silicon carbide and B4C-coated boron filaments. The interface microstructure and fracture behavior of the resulting composites were characterized using ion microprobe mass analysis, optical microscopy, and scanning electron microscopy. The results indicate that MgO is a viable diffusion barrier for titanium matrix composites.