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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Development of test methods for textile composites

NASA's Advanced Composite Technology (ACT) Program was initiated in 1990 with the purpose of developing less costly composite aircraft structures. A number of innovative materials and processes were evaluated as a part of this effort. Chief among them are composite materials reinforced with textile preforms. These new forms of composite materials bring with them potential testing problems. Methods currently in practice were developed over the years for composite materials made from prepreg tape or simple 2-D woven fabrics. A wide variety of 2-D and 3-D braided, woven, stitched, and knit preforms were suggested for application in the ACT program. The applicability of existing test methods to the wide range of emerging materials bears investigation. The overriding concern is that the values measured are accurate representations of the true material response. The ultimate objective of this work is to establish a set of test methods to evaluate the textile composites developed for the ACT Program.

Masters, John E.↗
NASA NTRS1993

Preparing polymeric matrix composites using an aqueous slurry technique

An aqueous process was developed to prepare a consolidated composite laminate from an aqueous slurry. An aqueous poly(amic acid) surfactant solution was prepared by dissolving a poly(amic acid) powder in an aqueous ammonia solution. A polymeric powder was added to this solution to form a slurry. The slurry was deposited on carbon fiber to form a prepreg which was dried and stacked to form a composite laminate. The composite laminate was consolidated using pressure and was heated to form the polymeric matrix. The resulting composite laminate exhibited high fracture toughness and excellent consolidation.

Johnston, Norman J.↗
NASA NTRS1994

Prediction of thermal cycling induced cracking in polmer matrix composites

The work done in the period August 1993 through February 1994 on the 'Prediction of Thermal Cycling Induced Cracking In Polymer Matrix Composites' program is summarized. Most of the work performed in this period, as well as the previous one, is described in detail in the attached Master's thesis, 'Analysis of Thermally Induced Damage in Composite Space Structures,' by Cecelia Hyun Seon Park. Work on a small thermal cycling and aging chamber was concluded in this period. The chamber was extensively tested and calibrated. Temperatures can be controlled very precisely, and are very uniform in the test chamber. Based on results obtained in the previous period of this program, further experimental progressive cracking studies were carried out. The laminates tested were selected to clarify the differences between the behaviors of thick and thin ply layers, and to explore other variables such as stacking sequence and scaling effects. Most specimens tested were made available from existing stock at Langley Research Center. One laminate type had to be constructed from available prepreg material at Langley Research Center. Specimens from this laminate were cut and prepared at MIT. Thermal conditioning was carried out at Langley Research Center, and at the newly constructed MIT facility. Specimens were examined by edge inspection and by crack configuration studies, in which specimens were sanded down in order to examine the distribution of cracks within the specimens. A method for predicting matrix cracking due to decreasing temperatures and/or thermal cycling in all plies of an arbitrary laminate was implemented as a computer code. The code also predicts changes in properties due to the cracking. Extensive correlations between test results and code predictions were carried out. The computer code was documented and is ready for distribution.

Mcmanus, Hugh L.↗
NASA NTRS1994

Fatigue resistance of unnotched and post impact(+/- 30 deg/0 deg) 3-D braided composites

The fatigue resistance of a multiaxial braided (3-D) graphite/expoxy composite in both unnotched and post impacted conditions has been evaluated. The material tested is a (+/- 30/0 deg) multiaxial braid constructed from AS4/12K tow graphite fibers and British Petroleum E905L epoxy resin. These materials were braided as dry preforms and the epoxy was added using a resin transfer molding process (RTM). The unnotched and post-impact specimens were tested in compression-compression fatigue at 10 Hz with a stress ratio of R=10. The unnotched tension-tension fatigue specimens were tested at S Hz with a stress ration of R=0.1. Damage initiation and growth was documented through the application of radiography and ultrasonic through transmission (C-scans). Visible inspection of surface and edge damage was also noted to describe the initiation and progression of damage in these materials. The mechanisms leading to damage initiation were established and failure modes were determined. Stiffness and strength degradation were measured as a function of applied cycles. These 3-D braided composite results were compared to strain levels currently used to design primary structure in commercial aircraft composite components made from prepreg tape and autoclave cured.

Portanova, Marc A.↗
NASA NTRS1994

Polymer infiltration studies

During the past three months, significant progress has been made on the preparation of carbon fiber composites using advanced polymer resins. The results are set forth in recent reports and publications, and will be presented at forthcoming national and international meetings. Current and ongoing research activities reported herein include: textile composites from powder-coated towpreg - role of surface coating in braiding; prepregger hot sled operation in making tape from powder coated tow; ribbonizing powder-impregnated towpreg; textile composites from powder-coated towpreg - role of bulk factor in consolidation; powder curtain prepreg process improvements in doctoring of powder; and hot/cold shoe for ATP open-section part warpage control.

Marchello, Joseph M.↗
NASA NTRS1994

Through-the-thickness tensile strength of textile composites

A series of tests was run to characterize the through-the-thickness tensile strength for a variety of composites that included 2D and 3D braids, 2D and 3D weaves, and prepreg tapes. A new test method based on a curved beam was evaluated. The through-the-thickness deformations were characterized using moire interferometry. Failures were significantly different between the 2D and 3D materials. The 2D materials delaminated between layers due to out-of-plane tensile stresses. The strength of the 2D textile composites did not increase relative to the tapes. The 3D materials failed due to the formation of radial cracks caused by high circumferential stresses along the inner radius. A circumferential crack similar to the 2D materials produced the final failure. Final failure in the 3D materials occurred at a lower bending moment than in other materials. The early failures were caused by radial crack formation rather than low through-the-thickness strength.

Jackson, Wade C.↗
NASA NTRS1994

Effects of ply thickness on thermal cycle induced damage and thermal strain

An experimental study was conducted to determine the effects of ply thickness in composite laminates on thermally induced cracking and changes in the coefficient of thermal expansion, CTE. A graphite-epoxy composite material, P75/ERL 1962, in thin (1 mil) and thick (5 mils) prepregs was used to make cross-ply laminates, ((0/90)(sub n))s, with equal total thickness (n=2, n=10) and cross-ply laminates with the same total number of plies (n=2). Specimens of each laminate configuration were cycled up to 1500 times between -250 and 250 F. Thermally induced microdamage was assessed as a function of the number of cycles as was the change in CTE. The results showed that laminates fabricated with thin-plies microcracked at significantly different rates and reached significantly different equilibrium crack densities than the laminate fabricated with thick-ply and n=2. The CTE of thin-ply laminates was less affected by thermal cycling and damage than the CTE of thick-ply laminates. These differences are attributed primarily to differences in interply constraints. Observed effects of ply thickness on crack density was qualitatively predicted by a combined shear-lag stress/energy method.

Tompkins, Stephen S.↗
NASA NTRS1992

Dynamic testing and analysis of extension-twist-coupled composite tubular spars

The results from a study aimed at improving the dynamic and aerodynamic characteristics of composite rotor blades through the use of extension-twist elastic coupling are presented. A set of extension-twist-coupled composite tubular spars, representative of the primary load carrying structure within a helicopter rotor blade, was manufactured using four plies of woven graphite/epoxy cloth 'prepreg.' These spars were non-circular in cross section design and were therefore subject to warping deformations. Three cross-sectional geometries were developed: square, D-shape, and flattened ellipse. Results from free-free vibration tests of the spars were compared with results from normal modes and frequency analyses of companion shell-finite-element models developed in MSC/NASTRAN. Five global or 'non-shell' modes were identified within the 0-2000 Hz range for each spar. The frequencies and associated mode shapes for the D-shape spar were correlated with analytical results, showing agreement within 13.8 percent. Frequencies corresponding to the five global mode shapes for the square spar agreed within 9.5 percent of the analytical results. Five global modes were similarly identified for the elliptical spar and agreed within 4.9 percent of the respective analytical results.

Lake, Renee C.↗
NASA NTRS1992

Design of fabric preforms for double diaphragm forming

Resin Transfer Molding (RTM) has the potential of becoming one of the most cost effective ways of producing composite structures since the raw materials used, resin and dry fabric, are less costly than prepregs. Unfortunately these low material costs are offset by the high labor costs incurred to layup the dry fabric into 3D shapes. To reduce the layup costs, double diaphragm forming is being investigated as a potential technique for creating a complex 3D preform from a simple flat layup. As part of our effort to develop double diaphragm forming into a production capable process, we have undertaken a series of experiments to investigate the interactions between process parameters, mold geometry, fabric weave, tow size, and the quality of the formed part. The results of these tests will be used to determine the forming geometry limitations of double diaphragm forming and to characterize the formability of fabric configurations. An important part of this work was the development of methods to measure and analyze fiber orientations, deformation angles, tow spreading, and shape conformation of the formed parts. This paper will describe the methods used to mark plies, the double diaphragm forming process, the techniques used to measure the formed parts, and the calculation of the parameters of interest. The results can be displayed as 3D contour plots. These experimental results have also been used to verify and improve a computer model which simulates the draping of fabrics over 3D mold shapes.

Luby, Steven↗
NASA NTRS1992

Resin transfer molding of textile preforms for aircraft structural applications

The NASA LaRC is conducting and supporting research to develop cost-effective fabrication methods that are applicable to primary composite aircraft structures. One of the most promising fabrication methods that has evolved is resin transfer molding (RTM) of dry textile material forms. RTM has been used for many years for secondary structures, but has received increased emphasis because it is an excellent method for applying resin to damage-tolerant textile preforms at low cost. Textile preforms based on processes such as weaving, braiding, knitting, stitching, and combinations of these have been shown to offer significant improvements in damage tolerance compared to laminated tape composites. The use of low-cost resins combined with textile preforms could provide a major breakthrough in achieving cost-effective composite aircraft structures. RTM uses resin in its lowest cost form, and storage and spoilage costs are minimal. Near net shape textile preforms are expected to be cost-effective because automated machines can be used to produce the preforms, post-cure operations such as machining and fastening are minimized, and material scrap rate may be reduced in comparison with traditional prepreg molding. The purpose of this paper is to discuss experimental and analytical techniques that are under development at NASA Langley to aid the engineer in developing RTM processes for airframe structural elements. Included are experimental techniques to characterize preform and resin behavior and analytical methods that were developed to predict resin flow and cure kinetics.

Hasko, Gregory H.↗
NASA NTRS1993

In situ processing methods for composite fuselage sandwich structures

Conventional sandwich structure fabrication methods are labor intensive and high in cost. A low cost method is needed to produce lightweight sandwich structures. Sundstrand has developed a series of in situ composite fabrication methods in which the raw materials (skin and core materials) are placed in a closed mold, and the component is produced in one heating cycle. Internal pressure is generated by chemical agents during the thermal cycles, which consolidates the skins and produces the foam core. The finished part is a net-shape composite sandwich structure with skins and a foamed core. The in situ process reduces cost by eliminating several secondary operations that are used in conventional fabrication methods. Further, a strong molecular bond is produced between the core and skin, which eliminates adhesive bonding and prevents a weak bond section in the sandwich structure. In this investigation, we evaluated the feasibility of the in situ process using thermoset materials currently under consideration for commercial airplane fuselage applications, such as keel sections. The materials used were Hercules 855340 toughened epoxy resin in both liquid and powder forms, and 3M Scotchply PR500 resin, manufactured by 3M Corporation, in powder form. We successfully foamed these resins and produced experimental panels with AS-4/855340 Hercules prepreg skins. Chopped fibers were added to the core to increase performance of the foam. Mechanical property testing on these panels showed properties competitive with other foams. Additional experiments are required to optimize the in situ foam core sandwiches for specific properties and applications.

Saatchi, Hossein↗
NASA NTRS1993

Novel cost controlled materials and processing for primary structures

Textile laminates, developed a number of years ago, have recently been shown to be applicable to primary aircraft structures for both small and large components. Such structures have the potential to reduce acquisition costs but require advanced automated processing to keep costs controlled while verifying product reliability and assuring structural integrity, durability and affordable life-cycle costs. Recently, resin systems and graphite-reinforced woven shapes have been developed that have the potential for improved RTM processes for aircraft structures. Ciba-Geigy, Brochier Division has registered an RTM prepreg reinforcement called 'Injectex' that has shown effectivity for aircraft components. Other novel approaches discussed are thermotropic resins producing components by injection molding and ceramic polymers for long-duration hot structures. The potential of such materials and processing will be reviewed along with initial information/data available to date.

Dastin, S. J.↗
NASA NTRS1995

Composite-Material Propfan Blades For A Wind-Tunnel Model

Report discusses design and sequence of operations performed in fabricating set of graphite-fiber/epoxy-matrix composite propfan blades. Describes major phases of fabrication of blades, including machining of metal master blades, fabrication of molds from master blades, cutting of graphite-fiber/epoxy prepreg sheets to form plies, stacking of plies to obtain preforms, assembly of pressure-and suction-side preforms into unitary blade preforms, curing preforms in molds, final light finishing, and inspection.

Fite, E. Brian↗
NASA NTRS1995

Out-of-plane properties

This paper summarizes three areas of research which were performed to characterize out-of-plane properties of composite materials. In the first investigation, a series of tests was run to characterize the through-the-thickness tensile strength for a variety of composites that included 2D braids, 2D and 3D weaves, and prepreg tapes. A new test method based on a curved beam was evaluated. Failures were significantly different between the 2D materials and the 3D weaves. The 2D materials delaminated between layers due to out-of-plane tensile stresses while the 3D weaves failed due to the formation of radial cracks between the surface plies caused by high circumferential stresses along the inner radius. The strength of the 2D textile composites did not increase relative to the tapes. Final failure in the 3D weaves was caused by a circumferential crack similar to the 2D materials and occurred at a lower bending moment than in other materials. The early failures in the 3D weaves were caused by radial crack formation rather than a low through-the-thickness strength. The second investigation focused on the development of a standard impact test method to measure impact damage resistance. The only impact tests that currently exist are compression after impact (CAI) tests which incorporate elements of both damage resistance and damage tolerance. A new impact test method is under development which uses a quasi-static indentation (QSI) test to directly measure damage resistance. Damage resistance is quantified in terms of the contact force to produce a unit of damage where a metric for damage may be area in C-scan, depth of residual dent , penetration, damage growth, etc. A final draft of an impact standard that uses a QSI test method will be presented to the ASTM Impact Task Group on impact. In the third investigation, the impact damage resistance behavior of a variety of textile materials was studied using the QSI test method. In this study, the force where large damage initiates was measured and the delamination size as a function of force was determined. The force to initiate large damage was significantly lower in braids and weaves. The delamination diameter - impact forace relationship was quanitfied using a damage resistance parameter, Q(*), which related delamination diameter to imapct force over a range of delamination sizes. Using this Q(*) parameter to rate the materials, the stitched uniweaves, toughened epoxy tapes, and through-the-thickness orthogonal interlock weave were the most damage resistant.

Jackson, Wade C.↗
NASA NTRS1995

Effects of preform architecture on modulus and strength of 2-D triaxially braided textile composites

Laminates formed using braided fibrous preforms have been extensively investigated during the course of the past few years as alternatives to unidirectional prepreg tape systems. This paper focused on one aspect of that work. It defined the role of the fibrous preform architecture in controlling a laminate's mechanical properties. The presentation was divided into four sections as the outline listed above illustrates. The presentation began with a brief introduction which defined the objectives of the study and detailed the materials studied. This was followed by a review of empirical test results. The materials' moduli and strengths were measured in both tension and compression. Their shear moduli were also experimentally determined. The review of the empirical data comprised the bulk of the presentation. A comparison of the experimental data to results predicted analytically was then presented. The presentation concluded with a few summary remarks. The specimens studied in this investigation featured 2-D triaxially braided AS4 graphite fiber preforms impregnated with Shell 1895 epoxy resin.

Masters, John E.↗
NASA NTRS1995

Processing and properties of fiber reinforced polymeric matrix composites: I. IM7/LARC(TM)-PETI-7 polyimide composites

A phenylethynyl terminated imide oligomer formed from the reaction of benzophenone tetracarboxylic acid dianhydride, an 75:25 molar ratio of 4,4'-oxydianiline and meta-phenylenediamine and 4-phenylethynylphthalic anhydride as the endcapper at a theoretical number average molecular weight (Mn) of approximately 3,700 g/mol was evaluated as a composite resin matrix. A glass transition temperature (Tg) of 315 deg C was reached after 250 deg C/1 hr annealing of the matrix resin. Unidirectional prepreg was made by coating an N-methylpyrrolidinone solution of the amide acid oligomer onto unsized IM7 graphite fibers. The thermal and rheological properties and the solvent/volatile depletion rates of the amide acid/NMP system were determined. This information was used to successfully design a molding cycle for composite fabrication. Composites molded under 800 Psi at 371 C consistently yielded good consolidation as measured by C-scan and optical photomicrography. The composite's short beam shear strength (SBS), longitudinal and transverse flexural strengths and moduli were measured at various temperatures. These composites exhibited excellent room temperature (RT) longitudinal flexural strength and modulus and RT SBS strength retention at 177 C.

Hou, Tan-Hung↗
NASA NTRS1996

Compression Testing of Textile Composite Materials

The applicability of existing test methods, which were developed primarily for laminates made of unidirectional prepreg tape, to textile composites is an area of concern. The issue is whether the values measured for the 2-D and 3-D braided, woven, stitched, and knit materials are accurate representations of the true material response. This report provides a review of efforts to establish a compression test method for textile reinforced composite materials. Experimental data have been gathered from several sources and evaluated to assess the effectiveness of a variety of test methods. The effectiveness of the individual test methods to measure the material's modulus and strength is determined. Data are presented for 2-D triaxial braided, 3-D woven, and stitched graphite/epoxy material. However, the determination of a recommended test method and specimen dimensions is based, primarily, on experimental results obtained by the Boeing Defense and Space Group for 2-D triaxially braided materials. They evaluated seven test methods: NASA Short Block, Modified IITRI, Boeing Open Hole Compression, Zabora Compression, Boeing Compression after Impact, NASA ST-4, and a Sandwich Column Test.

Masters, John E.↗
NASA NTRS1996

Database of Mechanical Properties of Textile Composites

This report describes the approach followed to develop a database for mechanical properties of textile composites. The data in this database is assembled from NASA Advanced Composites Technology (ACT) programs and from data in the public domain. This database meets the data documentation requirements of MIL-HDBK-17, Section 8.1.2, which describes in detail the type and amount of information needed to completely document composite material properties. The database focuses on mechanical properties of textile composite. Properties are available for a range of parameters such as direction, fiber architecture, materials, environmental condition, and failure mode. The composite materials in the database contain innovative textile architectures such as the braided, woven, and knitted materials evaluated under the NASA ACT programs. In summary, the database contains results for approximately 3500 coupon level tests, for ten different fiber/resin combinations, and seven different textile architectures. It also includes a limited amount of prepreg tape composites data from ACT programs where side-by-side comparisons were made.

Delbrey, Jerry↗