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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

The effects of uneven fiber spacing on thermal residual stresses in a unidirectional SCS-6/Ti-15-3 laminate

High residual stresses develop in SCS-6/Ti-15-3 composites during cooldown from the fabrication temperature; these residual stresses can effect the mechanical and physical properties of the composite. Discrete fiber-matrix finite element models were used to study the residual stresses due to the temperature change during the fabrication process, including the effects of uneven fiber spacing, the free surface, and increased fiber volume fractions. To accurately model the effects of the free surface, it is only necessary to model one fiber through the thickness. Below the first ply, the analysis predicts stress distributions that are identical to the infinite array predictions. For uneven fiber space less than 0.042 mm in an interior ply, the maximum hoop stress was predicted to occur between fibers within a ply and to increase as the fiber spacing decreased. The maximum hoop stress correlated well with the observed radial cracking between fibers. For the case of touching fibers, the analysis predicted tensile radial stresses at the fiber-matrix debonding during the fabrication cooldown. Identical trends were predicted for uneven fiber spacing in surface plies with slightly greater values of maximum stresses. The analysis predicted matrix yielding to occur upon cooldown when the edge-to-edge fiber spacing was less than or equal to 0.022 mm. The stress distributions preidcted for increasing fiber volume fractions were similar to those predicted for decreasing the fiber spacing for two adjacent fibers within a ply.

Bigelow, Catherine A.↗
DOE OSTI2025

Structure and Sulfur: Tuning the Viscoelastic and Surface Properties of Natural Keratin Fibers

Natural keratin fibers, such as wool, possess a complex hierarchical structure that governs their mechanical properties and surface energy. However, the extent to which these characteristics are influenced by combined contributions of structural variations (e.g., fiber diameter, intermediate filament (IF) packing) and chemical composition (e.g., disulfide bond density) remains poorly understood. In this study, we investigate wool fibers from five sheep breeds (Merino, Polwarth, Cheviot, Eider, and Devon) to elucidate how these factors influence viscoelasticity and surface interactions. Using a multimodal approach integrating interfacial and bulk characterization methods, including inverse gas chromatography (IGC), atomic force microscopy-infrared spectroscopy (AFM-IR), X-ray photoelectron spectroscopy (XPS), uniaxial tensile testing, and synchrotron small-angle X-ray scattering (SAXS), we show that the nanometer-thick 18-methyleicosanoic acid (18-MEA) layer is consistently present across all wool types and plays a key role in governing hydrophobicity and surface heterogeneity. A controlled isothermal treatment at 200 °C, designed to cleave disulfide bonds, results in a nearly 40% reduction in specific surface area across all fiber types, accompanied by a significant decrease in tensile strength and 80% reduction in elongation at break for Merino and Devon wool, but limited influence on the mechanical properties of Eider fibers. Furthermore, rate-dependent tensile testing within the elastic regime reveals distinct viscoelastic responses among the fiber types, suggesting that the sulfur-rich protein matrix surrounding IFs and its structure contribute actively to stress partitioning. Altogether, when combined with conclusions from SAXS measurements of IF spacing, our work offers compelling insights into the role of the keratin-associated protein (KAP) matrix in shaping wool fiber mechanics. Differences in mechanical behavior among wool types, despite similar IF spacing or sulfur content, highlight the importance of matrix composition and cross-linking density, suggesting that the molecular architecture of the KAP network may be a dominant factor in determining fiber performance.

X-ray scattering↗
NASA NTRS1976

Longitudinal residual stresses in boron fibers

A technique is proposed for measuring the longitudinal residual stress distribution in commercial CVD (Chemical Vapor Deposition) boron on tungsten fibers of 102, 142, and 203 microns in diameter. The experimental apparatus is so designed that continuous measurements are made of the length changes of a boron fiber specimen as the surface of the fiber is removed by electropolishing. The effects of surface removal on core residual stress and core-initiated fracture are discussed. The three sizes of boron fibers investigated show similar residual stress distributions, i.e., compressive at the surface, tensile near the core, and for the 102-micron fiber compressive again in the core. It is shown that an increase in UTS is due to the increase in the compressive stress at the core produced by fiber contraction during surface removal. An expression is derived for calculating the longitudinal residual stress at a given radius for an as-received fiber of a certain radius from measurements of the axial strain produced by removal of the surface by electropolishing.

Behrendt, D. R.↗
NASA NTRS1995

Study of surfaces using near infrared optical fiber spectrometry

The measurement and control of cleanliness for critical surfaces during manufacturing and in service provides a unique challenge for fulfillment of environmentally benign operations. Of particular interest has been work performed in maintaining quality in the production of bondline surfaces in propulsion systems and the identification of possible contaminants. This work requires an in-depth study of the possible sources of contamination, methodologies to identify contaminants, discrimination between contaminants and chemical species caused by environment, and the effect of particular contaminants on the bondline integrity of the critical surfaces. This presentation will provide an introduction to the use of optical fiber spectrometry in a nondestructive measurement system for process monitoring and how it can be used to help clarify issues concerning surface chemistry. Correlation of the Near Infrared (NIR) spectroscopic results with Optical Stimulated Electron Emission (OSEE) and ellipsometry will also be presented.

Workman, G. L.↗
NASA NTRS1997

Study of SRM Critical Surfaces Using Near Infrared Optical Fiber Spectrometry

The measurement and control of cleanliness for critical surfaces during manufacturing and in service operations provides a unique challenge in the current thrust for environmentally benign processes. Of particular interest has been work performed in maintaining quality in the production of bondline surfaces in propulsion systems and the identification of possible contaminants which are detrimental to the integrity of the bondline. This work requires an in-depth study of the possible sources of contamination, methodologies to identify contaminants, discrimination between contaminants and chemical species caused by environment, and the effect of particular contaminants on the bondline integrity of the critical surfaces. This paper will provide an introduction to the use of Near Infrared (NIR) optical fiber spectrometry in a nondestructive measurement system for process monitoring and how it can be used to help clarify issues concerning surface chemistry. In a previous conference, experimental results for quantitative measurement of silicone and Conoco HD2 greases, and tape residues on solid rocket motor surfaces were presented. This paper will present data for metal hydroxides and discuss the use of the integrating sphere to minimize the effects of physical properties of the surfaces (such as surface roughness) on the results obtained from the chemometric methods used for quantitative analysis.

Workman, G. L.↗
NASA NTRS1990

Numerical, micro-mechanical prediction of crack growth resistance in a fibre-reinforced/brittle matrix composite

Micromechanics fracture models are incorporated into three distinct fracture process zones which contribute to the crack growth resistance of fibrous composites. The frontal process zone includes microcracking, fiber debonding, and some fiber failure. The elastic process zone is related only to the linear elastic creation of new matrix and fiber fracture surfaces. The wake process zone includes fiber bridging, fiber pullout, and fiber breakage. The R-curve predictions of the model compare well with empirical results for a unidirectional, continuous fiber C/C composite. Separating the contributions of each process zone reveals the wake region to contain the dominant crack growth resistance mechanisms. Fractography showed the effects of the micromechanisms on the macroscopic fracture behavior.

Jenkins, Michael G.↗
NASA NTRS2015

Method for Coating a Tow with an Electrospun Nanofiber

Method and apparatus for enhancing the durability as well as the strength and stiffness of prepreg fiber tows of the sort used in composite materials are disclosed. The method involves adhering electrospun fibers onto the surface of such composite materials as filament-wound composite objects and the surface of prepreg fiber tows of the sort that are subsequently used in the production of composite materials of the filament-wound, woven, and braided sorts. The apparatus performs the methods described herein.

Kohlman, Lee W.↗
NASA NTRS1999

Fiber Effects on Minicomposite Mechanical Properties for Several Silicon Carbide Fiber: Chemically Vapor-Infiltrated Silicon Carbide Matrix Systems

Several different types of SiC fiber tows were coated with BN and composited using chemically vapor-infiltrated SiC to form single-tow minicomposites. The types of SiC fiber included Nicalon(sup TM), Hi-Nicalon(sup TM), and the new Sylramic(sup TM) polycrystalline SiC fiber. The interfacial shear stresses were determined from unload-reload tensile hysteresis-loop tests. The ultimate stress and strain properties also were determined for the minicomposites. The ultimate strengths of the newer Hi-Nicalon and Sylramic fibers were superior to that of Nicalon minicomposites with similar fiber volume fractions. The Sylramic minicomposites had the lowest strain to failure and highest interfacial shear strength, respectively, because of the high modulus of the fiber and the rough surface of this fiber type. The apparent interfacial shear strength increased as the stress increased for the Sylramic minicomposites, which also was attributed to the surface roughness of this fiber.

Morscher, Gregory N.↗
NASA NTRS2005

Polymer based nanocomposites with nanofibers and exfoliated clay

Polymer solutions, containing clay sheets, were electrospun into nanofibers and microfibers that contained clay sheets inside. Controllable removal of polymer by plasma etching from the surface of fibers revealed the arrangement of clay. The shape, flexibility, size distribution and arrangement of clay sheets were observed by transmission and scanning electron microscopy. The clay sheets were partially aligned in big fibers with normal direction of clay sheets perpendicular to fiber axis. Crumpling of clay sheets inside fibers was observed when the fiber diameter was comparable to the lateral size of clay sheets. Single sheets of clay were observed both by catching clay sheets dispersed in water with electrospun nanofiber mats and by the deliberate removal of most of the polymer in the fibers. Thin, flexible gas barrier films, that are reasonably strong, were assembled from clay sheets and polymer nanofibers. Structure of composite films was characterized with scanning electron microscopy. Continuous film of clay sheets were physically attached to the surface of fiber mats. Spincoating film of polymer and clay sheets was reinforced by electrospun fiber scaffold. Certain alignment of clay sheets was observed in the vicinity of fibers.

Meador, Michael A.↗
NASA NTRS1992

Thermal degradation study of silicon carbide threads developed for advanced flexible thermal protection systems

Silicon carbide (SiC) fiber is a material that may be used in advanced thermal protection systems (TPS) for future aerospace vehicles. SiC fiber's mechanical properties depend greatly on the presence or absence of sizing and its microstructure. In this research, silicon dioxide is found to be present on the surface of the fiber. Electron Spectroscopy for Chemical Analysis (ESCA) and Scanning Electron Microscopy (SEM) show that a thin oxide layer (SiO2) exists on the as-received fibers, and the oxide thickness increases when the fibers are exposed to high temperature. ESCA also reveals no evidence of Si-C bonding on the fiber surface on both as-received and heat treated fibers. The silicon oxide layer is thought to signal the decomposition of SiC bonds and may be partially responsible for the degradation in the breaking strength observed at temperatures above 400 C. The variation in electrical resistivity of the fibers with increasing temperature indicates a transition to a higher band gap material at 350 to 600 C. This is consistent with a decomposition of SiC involving silicon oxide formation.

Tran, Huy Kim↗
NASA NTRS1984

The Chemical Nature of the Fiber/resin Interface in Composite Materials

Carbon fiber/epoxy resin composites are considered. The nature of the fiber structure and the interaction that occurs at the interface between fiber and matrix are emphasized. Composite toughness can be improved by increased axial tensile and compressive strengths in the fibers. The structure of carbon fibers indicates that the fiber itself can fail transversely, and different transverse microstructures could provide better transverse strengths. The higher surface roughness of lower modulus and surface-treated carbon fibers provides better mechanical interlocking between the fiber and matrix. The chemical nature of the fiber surface was determined, and adsorption of species on this surface can be used to promote wetting and adhesion. Finally, the magnitude of the interfacial bond strength should be controlled such that a range of composites can be made with properties varying from relatively brittle and high interlaminar shear strength to tougher but lower interlaminar shear strength.

Diefendorf, R. J.↗
NASA NTRS1991

Development of an optical fiber interferometer for detection of surface flaws in aluminum

The main objective was to demonstrate the potential of using an optical fiber interferometer (OFI) to detect surface flaws in aluminum samples. Standard ultrasonic excitation was used to generate Rayleigh surface waves. After the waves interacted with a defect, the modified responses were detected using the OFI and the results were analyzed for time-of-flight and frequency content to predict the size and location of the flaws.

Gilbert, John A.↗
DOE OSTI2025

Influence of sizing concentration on strength, stiffness, and porosity in textile grade carbon fiber (TCF)-Epoxy composites: Revealing inverse trends

The effect of fiber sizing (i.e., surface treatment) concentration (0 %, 1.36 %, 1.52 %, 1.94 %, and 2.13 %) on the mechanical properties (tensile, flexural, interlaminar shear strength (ILSS), and low velocity impact) of textile grade carbon fiber (TCF)-epoxy composite is examined. An inverse relationship between the strength and stiffness of the composite is observed with increased sizing concentration. The root mean square (RMS) roughness of the fiber surface increased from 17.8 nm (unsized) to 22.7 nm with 2.13 % sizing concentration. It was found that the tensile strength increased by 131 % from 221.4 ± 18.5 MPa (unsized) to 510.8 ± 28.05 MPa (for 1.36 % sizing) and further by 155 %–563.7 ± 14.95 MPa at 2.13 % sizing. On the contrary, the stiffness is initially increased by 126 % from 33.52 ± 7.80 GPa (unsized) to 75.9 ± 3.21 GPa (for 1.36 % sizing) but reduced with further increase in the sizing concentration. A single fiber pull-out test is simulated using the finite element method to validate the reverse trend in strength and stiffness. The varying sizing concentration is simulated by introducing an interface of varying thickness between fibers and matrix. Simulation results confirm that a thicker interface, corresponding to a higher sizing concentration, decreases interfacial shear stress, enhancing material strength while decreasing stiffness. The reverse trend in strength and stiffness with the sizing concentration aligns with experimental observations. In conclusion, the present study emphasizes the importance of sizing concentration for mechanical properties and provide a design criterion for customized high-strength and high-stiffness applications.

Porosity↗
NASA NTRS1991

Effect of fiber reinforcements on thermo-oxidative stability and mechanical properties of polymer matrix composites

A number of studies have investigated the thermo-oxidative behavior of polymer matrix composites. Two significant observations have been made from these research efforts: (1) fiber reinforcement has a significant effect on composite thermal stability; and (2) geometric effects must be considered when evaluating thermal aging data. A compilation of some results from these studies is presented, and this information shows the influence of the reinforcement fibers on the oxidative degradation of various polymer matrix composites. The polyimide PMR-15 was the matrix material that was used in these studies. The control composite material was reinforced with Celion 6000 graphite fiber. T-40R graphite fibers, along with some very stable ceramic fibers were selected as reinforcing fibers because of their high thermal stability. The ceramic fibers were Nicalon (silicon carbide) and Nextel 312 (alumina-silica-boron oxide). The mechanical properties of the two graphite fiber composites were significantly different, probably owing to variations in interfacial bonding between the fibers and the polyimide matrix. The Celion 6000/PMR-15 bond is very tight but the T-40/PMR-15 bond is less tight. Three oxidation mechanisms were observed: (1) the preferential oxidation of the Celion 6000 fiber ends at cut surfaces, leaving a surface of matrix material with holes where the fiber ends were originally situated; (2) preferential oxidation of the composite matrix; and (3) interfacial degradation by oxidation. The latter two mechanisms were also observed on fiber end cut surfaces. The fiber and interface attacks appeared to initiate interfiber cracking along these surfaces.

Bowles, Kenneth J.↗
NASA NTRS1992

Effect of fiber reinforcement on thermo-oxidative stability and mechanical properties of polymer matrix composites

A number of studies have investigated the thermooxidative behavior of polymer matrix composites. Two significant observations have been made from these research efforts: (1) fiber reinforcement has a significant effect on composite thermal stability; and (2) geometric effects must be considered when evaluating thermal aging data. The polyimide PMR-15 was the matrix material used in these studies. The control composite material was reinforced with Celion 6000 graphite fiber. T-4OR graphite fibers, along with some very stable ceramic fibers were selected as reinforcing fibers because of their high thermal stability. The ceramic fibers were Nicalon (silicon carbide) and Nextel 312 (alumina-silica-boron oxide). The mechanical properties of the two graphite fiber composites were significantly different, probably owing to variations in interfacial bonding between the fibers and the polyimide matrix. Three oxidation mechanisms were observed: (1) the preferential oxidation of the Celion 6000 fiber ends at cut surfaces, leaving a surface of matrix material with holes where the fiber ends were originally situated; (2) preferential oxidation of the composite matrix; and (3) interfacial degradation by oxidation. The latter two mechanisms were also observed on fiber end cut surfaces. The fiber and interface attacks appeared to initiate interfiber cracking along these surfaces.

Bowles, K. J.↗
NASA NTRS1993

Thermal stability relationships between PMR-15 resin and its composites

A study was conducted to investigate the relationship between the thermo-oxidative stability of PMR-15 matrix resin and the stability of graphite-fiber-reinforced composites that contain this resin as the matrix material. Three areas were investigated. The first was the effect of fiber/matrix interfacial bond strength on the isothermal aging weight loss of composites. By using type-A graphite fibers produced by Hercules, it was possible to study composites reinforced with fibers that were processed to receive different surface treatments. One of the fibers was untreated, a second fiber was treated by oxidation to enhance fiber/matrix bonding, and the third type of fiber was coated with an epoxy sizing. These treatments produced three significantly different interfacial bond strengths. The epoxy sizing on the third fiber was quickly oxidized from the bare fiber surfaces at 288, 316, and 343 C. The weight loss due to the removal of the sizing was constant at 1.5 percent. This initial weight loss was not observed in thermo-oxidative stability studies of composites. The PMR-15 matrix satisfactorily protected the reinforcemnt at all three temperatures.

Bowles, Kenneth J.↗
NASA NTRS2009

Design and Manufacturing of Tow-Steered Composite Shells Using Fiber Placement

Advanced composite shells that may offer the potential to improve the structural performance of future aircraft fuselage structures were developed under this joint NASA-industry collaborative effort. Two cylindrical shells with tailored, tow-steered layups and continuously varying fiber angle orientations were designed and built at the National Center for Advanced Manufacturing - Louisiana Partnership. The shells were fabricated from unidirectional IM7/8552 graphite-epoxy pre-preg slit tape material fiber-placed on a constant-diameter mandrel. Each shell had the same nominal 8-ply [plus or minus 45/plus or minus Theta]s layup, where the nominal fiber angle in the tow-steered plies varied continuously from 10 degrees along the crown to 45 degrees on each side, then back to 10 degrees on the keel. One shell was fabricated with all 24 tows placed during each pass of the fiber placement machine, resulting in many tow overlaps on the shell surface. The fiber placement machine's individual tow cut/restart capability was also used to manufacture a second shell with tow drops and a more uniform laminate thickness. This paper presents an overview of the detailed design and manufacturing processes for these shells, and discusses issues encountered during their fabrication and post-cure evaluation. Future plans for structural testing and analyses of the shells are also discussed.

Wu, K. Chauncey↗