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NASA NTRS1973

One-dimensional transient waves in anisotropic plates.

The stresses and displacements induced in anisotropic plates by short-duration impact line forces are calculated in this paper. The anisotropy is related to the lay-up angles of the fibers of a laminated fiber composite plate. A modification of Mindlin's theory of crystal plates is used. The plate responses are calculated for various fiber lay-up angles and inclination of the line load to the axes of symmetry. Use is made of the computational method of the fast Fourier transform.

Moon, F. C.↗

NASA NTRS1975

Effect of longitudinal surface finish on elastohydrodynamic lubrication

The effect of longitudinal-lay surface finish on the elastohydrodynamic film thickness and the percentage of film between rolling disks in contact were evaluated using a rolling disk apparatus. Film thickness was measured by transmitted X-rays, and percentage of film was monitored by an alternating-current continuity circuit. Disk finish was varied on both the crowned upper disk and the cylindrical lower disk. A type-2 ester and a synthetic paraffinic oil were used as lubricants. It was shown that the roughness with longitudinal lay has a deleterious effect on both film thickness and percentage of film. Measured film thicknesses for the two lubricants were comparable at equivalent test conditions. The percentage of film where a change in surface topography was observed was approximately 20 percent for the synthetic paraffinic oil and 10 percent for the type-2 ester.

Dow, T. A.↗

NASA NTRS1978

Toughening of graphite-epoxy composites by interlaminar perforated Mylar films

Fracture and notch strength tests of graphite-epoxy composites showed that unidirectional lay-ups generally exhibit longitudinal cracking before failure, whereas multidirectional lay-ups fail transversely with little longitudinal cracking. A simple qualitative analysis suggested that the higher matrix shear stresses in unidirectional materials cause the longitudinal cracking, and that this cracking was responsible for the high toughness of unidirectional composites. In a series of comparative tests, the interlaminar strength of multi-directional composites was reduced by placing perforated Mylar films between laminae; tests on notched and slotted specimens showed that the interlaminar films promoted delamination and longitudinal cracking near the notches and that, as a result, toughness, notch strength, and impact strength were substantially increased.

Elber, W.↗

NASA NTRS1978

Mars - Topographic control of clouds, 1907-1973

Mariner 9 high-resolution photos and topographic information were used to make a topographic analysis of 'blue' and 'red' cloud positions reported over a 66-year period. A sample of 77 'blue' cloud sites lay preferentially at the highest Martian elevations; 60% centered precisely on the seven major volcanic mountain peaks (unknown when the clouds were observed); another 16% lay on substantial slopes or contacts between cratered terrain and lower plains. The median altitude of blue cloud sites was 2.1 km above the global topographic median. These results agree with other evidence that most earth-detected blue clouds are orographic uplift clouds, composed of condensates. Over half of 131 sporadic yelowish or red clouds were associated with blue clouds or volcanoes, and thus probably did not represent dust storm phenomena. Of 88 'possible dust clouds', about two-thirds occur at borders between light and dark areas, in the light regions. These sites may have thin veneers of dust, and current depositional or denudational activity. Median altitude of 'possible dust cloud' sites was 0.5 km below the global topographic median.

Hartmann, W. K.↗

NASA NTRS1982

Advanced Strain-Isolation-Pad Material with Bonded Fibrous Construction

The feasibility of utilizing air lay and liquid lay felt deposition techniques to fabricate strain isolation pad (SIP) materials for the Space Shuttle Orbiter was demonstrated. These materials were developed as candidate replacements for the present needled felt SIP used between the ceramic tiles and the aluminum skin on the undersurface of the Orbiter. The SIP materials that were developed consisted of high temperature aramid fibers deposited by controlled fluid (air or liquid) carriers to form low density unbonded felts. The deposited felts were then bonded at the fiber intersections with a small amount of high temperature polyimide resin. This type of bonded felt construction can potentially eliminate two of the problems associated with the present SIP, viz., transmittal of localized stresses into the tiles and load history dependent mechanical response. However, further work is needed to achieve adequate through thickness tensile strength in the bonded felts.

Seibold, R. W.↗

NASA NTRS1985

Altimetry data and the elastic stress tensor of subduction zones

The stress field in the lithosphere caused by the distribution of density anomalies associated to the geoidal undulations observed by the GEOS-3 and SEASAT Earth satellites in the Tonga region was studied. Different models of the lithosphere were generated with different assumptions on the density distribution and geometry, all generating a geoid profile almost identical to the observed one. The first model is the Airy isostatic hypothesis which consists of a crust of density 2.85 laying on a lithosphere of density 3.35. The models obtained with different compensation depths give residual shortwavelength anomalies of the order of several tens of mgal and several tens of meters geoidal undulations. It indicates that there is no isostasy of the Airy type in the Tonga region because the observed geoid has very smooth undulation of about 25 m over a distance of 2000 km. The Pratt isostatic hypothesis is used in a model consisting of a crust of variable density laying on a lithosphere of higher density. This model gives smaller residual anomalies but still shows that there is no isostasy of the Pratt type in the Tonga region because the observed geoidal undulation are much smaller and smoother than the residual undulations associated to the Pratt model of isostasy.

Caputo, M.↗

NASA NTRS1985

Differences in interfacial bond strengths of graphite fiber-epoxy resin composites

The effect of epoxy-size and degree of cure on the interfacial bonding of an epoxy-amine-graphite fiber composite system is examined. The role of the fiber-resin interface in determining the overall mechanical properties of composites is poorly understood. A good interfacial adhesive bond is required to achieve maximum stress transfer to the fibers in composites, but at the same time some form of energy absorbing interfacial interaction is needed to achieve high fracture toughening. The incompatibility of these two processes makes it important to understand the nature and basic factors involved at the fiber-resin interface as stress is applied. The mechanical properties including interlaminar shear values for graphite fiber-resin composites are low compared to glass and boron-resin composites. These differences have been attributed to poor fiber-matrix adhesion. Graphite fibers are commonly subjected to post-treatments including application of organic sizing in order to improve their compatibility with the resin matrix and to protect the fiber tow from damage during processing and lay-up. In such processes, sized graphite fiber tow is impregnated with epoxy resin and then layed-up i nto the appropriate configuration. Following an extended ambient temperature cure, the graphite-resin composite structure is cured at elevated temperature using a programmed temperature sequence to cure and then cool the product.

Needles, H. L.↗

NASA NTRS1985

Chronic acceleration and egg production in domestic fowl

A study of the influence of chronic acceleration on egg production of commercially raised hens placed on a large animal centrifuge at 90 days of age, was performed. S8 generation hens were stepped up from 1.25 G to 2 G, which was maintained for 30 days. Fifty percent ceased to be in the laying condition at 1.5 G, and 10.8 percent suffered oviduct prolapse above 1.8 G. S21 generation hens had no incidents of oviduct prolapse despite 170-day retention at 2 G, and, assuming a 30 percent population not in the laying condition, approximated the commercial production rate. Chronic acceleration did not appear to affect the relative sizes of albumen or yolk, but it did appear to reduce the relative shell size, consistent with a decrease in plasma calcium. Dry matter content was not affected.

Smith, A. H.↗

NASA NTRS1987

Use of lightweight composites for GAS payload structures

A key element in the design of a small self-contained payload is the supporting structure. This structure must support the experiments and other components while using as little space and weight as possible. Hence, the structure material must have characteristics of being both strong and light. Aluminum was used for the structure on the first Purdue University payload, but consumed a relatively large percentage of the total payload weight. The current payload has a larger power supply requirement than did the previous payload. To allow additional weight for the batteries, a composite material has been chosen for the structure which has the required strength while being considerably lighter than aluminum. A radial fin design has been chosen for ease of composite material lay-up and its overall strength of design. A composite plate will connect the free ends of the fins and add strength and reduce vibration. The physical characteristics of the composite material and the method of open lay-up construction is described. Also discussed are the testing, modifications, and problems encountered during assembly of the experiments to the structure.

Spencer, Mark B.↗

NASA NTRS1986

The pultrusion process for structures on advanced aerospace transportation systems

The pultrusion process, which has the potential for use in the manufacture of structures for aerospace hardware, is described. In this process, reinforcing fibers are pulled continuously through a resin system for wetting and subsequently through a heated die for polymerization. By using this process, fabrication of very long lengths of high strength, lightweight structures with consistently high quality for aerospace applications is possible. The more conventional processes involve hand lay-up, vacuum bagging, autoclaving or oven curing techniques such that lengths of structural elements produced are limited by the lengths of autoclaves or curing ovens. Several types of developmental structural elements are described in which fiberglass, aramid, graphite, and hybrid fiber systems have been used as reinforcements in an epoxy matrix and their flexural properties compared. Reinforcement fibers having tailor-made orientations which achieve tailor-made strength in the pultrusions are described. The potential aerospace applications for the pultruded products are described with advantages cited over conventional hand lay-up methods.

Wilson, Maywood L.↗

NASA NTRS1989

Effects of tacky mat contamination on bond degradation for Chemlok/liner and NBR/liner bonds

Tacky mats are placed by the rubber lay-up areas for the solid rocket motor segments. These mats dust off the shoes prior to entering the platform where the lay-up work is performed. The possibility exists that a tacky mat could be touched with gloved hands prior to handling the uncured nitride butadiene rubber (NBR). Tests were run to determine if NBR were accidentally touched would there be any degradation of the liner/NBR bond. The tacky mats were judged solely on the basis of bond degradation caused by either direct or indirect contamination. Test results all indicate that there was no notable NBR/Chemlok or liner/NBR bond degradation on samples that came into contact with the tacky mat material. Testing procedures are described. The tacky mat adhesive composition does not contain fluorocarbons or release agents that would affect bonding.

Padilla, A. M.↗

NASA NTRS1989

X-ray diffraction measurements of residual stresses in SiC/Ti composites

Residual stresses are developed in metal matrix composites during consolidation because of the difference in the coefficient of thermal expansion of the fiber and matrix constituents. X-ray diffraction techniques were used to measure tensile residual stresses in the matrix component of two different titanium alloy composites reinforced with continuous silicon carbide fibers. The titanium alloy matrices were a beta-stabilized alloy Ti-15V-3Al-3Cr-3Sn, and an ordered multiphase (alpha + beta) alloy, Ti-14Al-21Nb. Residual tensile stresses as high as 62 percent of the yield of Ti-15V-3Al-3Cr-3Sn and 70 percent of the yield of Ti-14Al-21Nb were measured in the longitudinal direction on unidirectionally reinforced composites. By measuring residual stresses in four different composite lay-ups, the composite lay-up was determined to have a significant effect on the residual stress state near the surface.

Brown, K. M.↗

NASA NTRS1987

Method for stabilizing thick honeycomb core composite articles

A method of stabilizing thick honeycomb core composites is disclosed. The method comprises bonding a layer of resin impregnated woven graphite fibers to one surface of the honeycomb core, then curing the resin impregnates forming a stabilized honeycomb core. This stabilized core is then cut to net shape and the composite structure is formed by laying one or more resin impregnated fiber layers onto the stabilized core surf aces and curing the lay-up under heat and pressure forming the composite article. Also disclosed is a stabilized honeycomb subassembly formed by bonding a graphite fiber, resin impregnated layer onto one of the honeycomb core surfaces and curing the resins, then shaping the stabilized honeycomb core to the desired form.

Gary J Jacaruso↗

NASA NTRS1990

New options for IRI electron density in the middle ionosphere

This paper reviews the present International Reference Ionosphere (IRI) model of electron density in the middle ionosphere and explores two new options for future editions of IRI. The first of these options is a better description of the bottomside thickness parameters, and the second is an analytical representation from E- to F2-peak using LAY-functions. For this analytical representation a table of standard parameters and constraints for the four LAY-functions recommended for IRI have been established.

Bilitza, Dieter↗

NASA NTRS1992

Scaling effects in angle-ply laminates

The effect of specimen size upon the response and strength of +/- 45 degree angle-ply laminates was investigated for two graphite fiber reinforced plastic systems and several stacking sequences. The first material system was a brittle epoxy based system, AS4 fibers in 3502 epoxy, and the second was a tough thermoplastic based system, AS4 fibers in PEEK matrix. For the epoxy based system, two generic +/- 45 degree lay-ups were studied: (+45 degrees sub n/-45 degrees sub n) sub 2S (blocked plies), and (+45 degrees/-45 degrees) sub 2nS, for n=1 and 2. The in-plane dimensions of the specimens were varied such that the width/length relationship was 12.7 x n/127 x n mm, for m=1, 2, 3, or 4. It is shown that the stress/strain response and the ultimate strength of these angle-ply laminates depends on the laminate thickness and the type of generic lay-up used. Furthermore, it is shown that first ply failure occurs in the surface plies as a result of normal rather than shear stresses. The implications of the experimental findings upon the validity of the +/- 45 degree tensile test which is used to determine the in-plane shear response of unidirectional composites are discussed.

Kellas, Sotiris↗

NASA NTRS1992

Process comparison study. MSFC Center Director's Discretionary Fund (CDDF)

A process comparison study was conducted using four different advanced manufacturing techniques to fabricate a composite solid rocket booster systems tunnel cover. Costs and labor hours were tracked to provide the comparison between the processes. A relative structural comparison of the components is also included. The processes utilized included filament winding, pultrusion, automated tape laying, and thermoplastic thermoforming. The hand layup technique is also compared. Of the four advanced processes evaluated, the thermoformed thermoplastic component resulted in the least total cost. The automated tape laying and filament winding techniques closely followed the thermoplastic component in terms of total cost; and, these techniques show the most promise for high quality components and lower production costs. The pultruded component, with its expensive tooling and material requirements, was by far the most expensive process evaluated, although the results obtained would not be representative of large production runs.

Golden, T.↗

NASA NTRS1993

Automated fiber placement composite manufacturing: The mission at MSFC's Productivity Enhancement Complex

Automated fiber placement is a manufacturing process used for producing complex composite structures. It is a notable leap to the state-of-the-art in technology for automated composite manufacturing. The fiber placement capability was established at the Marshall Space Flight Center's (MSFC) Productivity Enhancement Complex in 1992 in collaboration with Thiokol Corporation to provide materials and processes research and development, and to fabricate components for many of the Center's Programs. The Fiber Placement System (FPX) was developed as a distinct solution to problems inherent to other automated composite manufacturing systems. This equipment provides unique capabilities to build composite parts in complex 3-D shapes with concave and other asymmetrical configurations. Components with complex geometries and localized reinforcements usually require labor intensive efforts resulting in expensive, less reproducible components; the fiber placement system has the features necessary to overcome these conditions. The mechanical systems of the equipment have the motion characteristics of a filament winder and the fiber lay-up attributes of a tape laying machine, with the additional capabilities of differential tow payout speeds, compaction and cut-restart to selectively place the correct number of fibers where the design dictates. This capability will produce a repeatable process resulting in lower cost and improved quality and reliability.

Vickers, John H.↗

NASA NTRS1992

Customized ATP towpreg

Automated tow placement (ATP) utilizes robotic technology to lay down adjacent polymer-matrix-impregnated carbon fiber tows on a tool surface. Consolidation and cure during ATP requires that void elimination and polymer matrix adhesion be accomplished in the short period of heating and pressure rolling that follows towpreg ribbon placement from the robot head to the tool. This study examined the key towpreg ribbon properties and dimensions which play a significant role in ATP. Analysis of the heat transfer process window indicates that adequate heating can be achieved at lay down rates as high as 1 m/sec. While heat transfer did not appear to be the limiting factor, resin flow and fiber movement into tow lap gaps could be. Accordingly, consideration was given to towpreg ribbon having uniform yet non-rectangular cross sections. Dimensional integrity of the towpreg ribbon combined with customized ribbon architecture offer great promise for processing advances in ATP of high performance composites.

Sandusky, Donald A.↗