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At least 181 records · Page 10

A Biaxial-Bending Test to Observe the Growth of Interacting Delaminations in a Composite Laminate Plate

It is not easy to isolate the damage mechanisms associated with low-velocity impact in composites using traditional experiments. In this work, a new experiment is presented with the goal of generating data representative of progressive damage processes caused by low-velocity impact in composite materials. Carbon fiber reinforced polymer test specimens were indented quasi-statically such that a biaxial-bending state of deformation was achieved. As a result, a three-dimensional damage process, involving delamination and delamination-migration, was observed and documented using ultrasonic and x-ray computed tomography. Results from two different layups are presented in this paper. Delaminations occurred at up to three different interfaces and interacted with one another via transverse matrix cracks. Although this damage pattern is much less complex than that of low-velocity impact on a plate, it is more complex than that of a standard delamination coupon test and provides a way to generate delamination, matrix cracking, and delamination-migration in a controlled manner. By limiting the damage process in the experiment to three delaminations, the same damage mechanisms seen during impact could be observed but in a simplified manner. This type of data is useful in stages of model development and validation when the model is capable of simulating simple tests, but not yet capable of simulating more complex and realistic damage scenarios.

McElroy, Mark↗

Improved C/SiC Ceramic Composites Made Using PIP

Improved carbon-fiber-reinforced SiC ceramic-matrix composite (C/SiC CMC) materials, suitable for fabrication of thick-section structural components, are producible by use of a combination of raw materials and processing conditions different from such combinations used in the prior art. In comparison with prior C/SiC CMC materials, these materials have more nearly uniform density, less porosity, and greater strength. The majority of raw-material/processing-condition combinations used in the prior art involve the use of chemical vapor infiltration (CVI) for densifying the matrix. In contrast, in synthesizing a material of the present type, one uses a combination of infiltration with, and pyrolysis of, a preceramic polymer [polymer infiltration followed by pyrolysis (PIP)]. PIP processing is performed in repeated, tailored cycles of infiltration followed by pyrolysis. Densification by PIP processing takes less time and costs less than does densification by CVI. When one of these improved materials was tested by exposure to a high-temperature, inert-gas environment that caused prior C/SiC CMCs to lose strength, this material did not lose strength. (Information on the temperature and exposure time was not available at the time of writing this article.) A material of the present improved type consists, more specifically, of (1) carbon fibers coated with an engineered fiber/matrix interface material and (2) a ceramic matrix, containing SiC, derived from a pre-ceramic polymer with ceramic powder additions. The enhancements of properties of these materials relative to those of prior C/SiC CMC materials are attributable largely to engineering of the fiber/ matrix interfacial material and the densification process. The synthesis of a material of this type includes processing at an elevated temperature to a low level of open porosity. The approach followed in this processing allows one to fabricate not only simple plates but also more complexly shaped parts. The carbon fiber reinforcement in a material of this type can be in any of several alternative forms, including tow, fabric, or complex preforms containing fibers oriented in multiple directions.

Easler, Timothy↗

A sandwich plate with a part-through and a debonding crack.

The stress analysis of a metal base plate stiffened by a fiber-reinforced composite layer is considered. It is assumed that the two materials are bonded through an adhesive of constant thickness which is treated as a two-dimensional shear spring in the analysis. The metal plate is assumed to contain a through crack of finite length. The composite plate has no flaws. However, due to the high stress concentration around the crack and on the interface, a debonding crack is assumed to exist, encircling the crack in the base plate. The problem is reduced to a pair of integral equations of the second kind with Fredholm-type kernels. By solving these equations the stress intensity factor in the base plate, the boundary of the debonding crack, and the interface shear stresses acting on the adhesive are calculated.

Erdogan, F.↗

Preliminary studies on NiAl/Nb2Be17 reaction and effectiveness of BeO as an interfacial reaction barrier

The interfacial reaction between NiAl and Nb2Be17 (used as a reinforcement for the alloy) was studied by measuring diffusion bonding of NiAl and Nb2Be17 plates in a hot press under a vacuum of 10 exp -5 atm. It was found that, after 2 hrs of hot pressing at 1373 K, the reaction between NiAl and Nb2Be17 was extensive. A 40 to 50-micron-thick reaction zone consisted of three distinct layers at the NiAl/Nb2Be17 interface: layer A next to Nb2Be17, layer B in the middle, and layer C next to NiAl. Results of analysis of the reaction layers using energy dispersive spectroscopy (EDS) were inconclusive because of the inabiliaty of EDS to detect Be.

Misra, Ajay K.↗

Continuous fiber-reinforced titanium aluminide composites

An account is given of the fabrication techniques, microstructural characteristics, and mechanical behavior of a lightweight, high service temperature SiC-reinforced alpha-2 Ti-14Al-21Nb intermetallic-matrix composite. Fabrication techniques under investigation to improve the low-temperature ductility and environmental resistance of this material system, while reducing manufacturing costs to competitive levels, encompass powder-cloth processing, foil-fiber-foil processing, and thermal-spray processing. Attention is given to composite microstructure problems associated with fiber distribution and fiber-matrix interfaces, as well as with mismatches of thermal-expansion coefficient; major improvements are noted to be required in tensile properties, thermal cycling effects, mechanical damage, creep, and environmental effects.

Mackay, R. A.↗

Residual thermal stresses in composites for dimensionally stable spacecraft applications

An overview of NASA LaRC's research on thermal residual stresses and their effect on the dimensional stability of carbon fiber reinforced polymer-matrix composites is presented. The data show that thermal residual stresses can induce damage in polymer matrix composites and significantly affect the dimensional stability of these composites by causing permanent residual strains and changes in CTE. The magnitude of these stresses is primarily controlled by the laminate configuration and the applied temperature change. The damage caused by thermal residual stresses initiates at the fiber/matrix interface and micromechanics level analyses are needed to accurately predict it. An increased understanding of fiber/matrix interface interactions appears to be the best approach for improving a composite's resistance to thermally induced damage.

Bowles, David E.↗

Fiber reinforced solids possessing great fracture toughness: The role of interfacial strength

The high tensile strength characteristic of strong interfacial filament/matrix bonding can be combined with the high fracture toughness of weak interfacial bonding, when the filaments are arranged to have alternate sections of high and low shear stress (and low and high toughness). Such weak and strong areas can be achieved by appropriate intermittent coating of the fibers. An analysis is presented for toughness and strength which demonstrates, in broad terms, the effects of varying the coating parameters of concern. Results show that the toughness of interfaces is an important parameter, differences in which may not be shown up in terms of interfacial strength. Some observations are made upon methods of measuring the components of toughness in composites.

Atkins, A. G.↗

Raman Study of Uncoated and p-BN/SiC-Coated Hi-Nicalon Fiber-Reinforced Celsian Matrix Composites: Distribution and Nanostructure of Different Phases - Part 1

Hi-Nicalon fiber reinforced celsian matrix composites were characterized by Raman spectroscopy and imaging, using several laser wavelengths. Composite #1 is reinforced by as-received fibers while coatings of p-BN and SiC protect the fibers in composite #2. The matrix contains traces of the hexagonal phase of celsian, which is concentrated in the neighborhood of fibers in composite #1. Some free silicon was evident in the coating of composite #2 which might involve a {BN + SiC yields BNC + Si} "reaction" at the p-BN/SiC interface. Careful analysis of C-C peaks revealed no abnormal degradation of the fiber core in the composites.

Gouadec, Gwenael↗

Chemical compatibility issues related to use of copper as an interfacial layer for SiC fiber reinforced Ti3Ai+Nb composite

The reaction of Cu, a potential interfacial compliant layer for the Ti3Al plus Nb/SiC composite, with SiC, SCS-6 fiber, and the Ti3Al plus Nb matrix was examined at two temperatures: 1223 and 1273 K. Reaction of Cu with SiC resulted in the formation of a CuSi solution and free carbon, the reaction product being molten at 1273 K. Hot pressing the SCS-6 fiber in a Cu matrix at 1273 K resulted in cracking and delamination of the outer carbon-rich coating, thus allowing the Cu to penetrate to the SiC-carbon coating interface and react with SiC. In contrast, no such damage to the outer coating was observed at 1223 K. There was excessive reaction between Cu and the Ti3Al plus Nb matrix, the reaction product being molten both at 1223 and 1273 K. An interlayer of Nb between Cu and Ti3Al plus Nb matrix prevented the reaction between the two.

Misra, Ajay K.↗

Effect of HIPing on the effective thermal conductivity/diffusivity and the interfacial thermal conductance of uniaxial SiC fibre-reinforced RBSN

Hot isostatic pressing (HIPing) was found to increase the thermal diffusivity/conductivity of uniaxial silicon carbide fiber-reinforced reaction-bonded silicon nitride (RBSN) matrix composites, as the result of the densification of the matrix, the increase in the grain size of the silicon carbide and the improved thermal contact between the fibers and the matrix. Transverse to the fiber direction the thermal diffusivity/conductivity was found to be a function of the surrounding gaseous atmosphere due to the access of the gas phase to the fiber-matrix interface, which was facilitated by the existence of an interfacial gap due to the thermal expansion mismatch between the fibers and the matrix. The interfacial conductance was found to exhibit a strong positive temperature dependence as the result of the closure of the interfacial gap with increasing temperature.

Bhatt, H.↗

Transient thermal stresses in a reinforced hollow disk or cylinder containing a radial crack

The transient thermal stress problem in a hollow cylinder or a disk containing a radial crack is considered. It is assumed that the cylinder is reinforced on its inner boundary by a membrane which has thermoelastic constants different than those of the base material. The transient temperature, thermal stresses and the crack tip stress intensity factors are calculated in a cylinder which is subjected to a sudden change of temperature on the inside surface. The results are obtained for various dimensionless parameters and material constants. The special cases of the crack terminating at the cylinder-membrane interface and of the broken membrane are separately considered and some examples are given.

Tang, R.↗

Transient thermal stresses in a reinforced hollow disk or cylinder containing a radial crack

The transient thermal stress problem in a hollow cylinder or a disk containing a radial crack is considered. It is assumed that the cylinder is reinforced on its inner boundary by a membrane which has thermoelastic constants different than those of the base material. The transient temperature, thermal stresses and the crack tip stress intensity factors are calculated in a cylinder which is subjected to a sudden change of temperature on the inside surface. The results are obtained for various dimensionless parameters and material constants. The special cases of the crack terminating at the cylinder-membrane interface and of the broken membrane are separately considered and some examples are given.

Tang, R.↗

Acoustic characterization of composite cure

The relationship between the time evolution of ultrasonic longitudinal velocities, measured in the course of the cure of 3501-6 neat and fiber-reinforced resin, and the time evolution of the degree of cure was studied. The resin samples were placed in a mold, one side of which was a graphite plate, while the rest was made of stainless steel. After placing the mold in a press and applying a single-cycle 5-MHz pulse to the transducer bonded to the outer side of the graphite plate, the echo resulting from the resin-graphite interface and the subsequent echoes from within the resin layer were recorded with a 200-MHz, 8-bit digitizer. The degree of cure was calculated from the acoustic response, using the relationships derived by Winfree and Parker (1986). The calculated degree of cure had the expected time dependence, and the agreement between the degree of cure measured for the neat resin and the prepreg was reasonable.

Parker, F. R.↗

Investigation of a SiC/Ti-24Al-11Nb composite

A summary of ongoing research on the characterization of a continuous fiber reinforced SiC/Ti-24Al-11Nb (at percent) composite is presented. The powder metallurgy fabrication technique is described as are the nondestructive evaluation results of the as-fabricated composite plates. Tensile properties of the SiC fiber, the matrix material, and the 0-deg SiC/Ti-24Al-11Nb composite (fibers oriented unidirectionally, parallel to the loading axis) from room temperature to 1100 C are presented and discussed with regard to the resultant fractography. The as-fabricated fiber-matrix interface has been examined by scanning transmission electron microscopy and the compounds present in the reaction zone have been identified. Fiber-matrix interaction and stability of the matrix near the fiber is characterized at 815, 985, and 1200 C from 1 to 500 hr. Measurements of the fiber-matrix reaction, the loss of C-rich coating from the surface of the SiC fiber, and the growth of the Beta depleted zone in the matrix adjacent to the fiber are presented. These data and the difference in coefficient of thermal expansion between the fiber and the matrix are discussed in terms of their likely effects on mechanical properties.

Brindley, P. K.↗

Analysis of notched metal matrix composites under tension loading

Presented are techniques based on 3-D finite-element analysis for the examination of continuous fiber reinforced metal matrix composites. Examples are shown for specific metal matrix composites such as boron/aluminum and silicon-carbide/aluminum. Specimen stress-strain behavior and stress at first fiber failure were predicted for boron/aluminum laminates containing circular holes and crack-like slits. The predictions compared very well for (+ or - 45) sub 2s laminates. Mesh configuration was shown to have an effect on the calculation of stresses local to the notch. The presence of thin interface layers of matrix material had a significant influence on the slit-tip stress state, causing sharper stress gradients near the notch. Interface layers reduced the slit-tip fibers stresses in a (+ or - 45) sub s silicon-carbide/aluminum laminate but increased them in a (0/90) sub s laminate.

Bigelow, C. A.↗

Characterization of SiCSiC Composites in Support of Environmental Degradation Modeling

SiCSiC (silicon carbide fiber reinforced silicon carbide) composites are candidate materials for various turbine engine applications because of their high specific strength and good creep and oxidation resistance at elevated temperatures. This study was performed to characterize the microstructure of a melt infiltrated (MI) SiCSiC, and to examine environmental degradation mechanisms occurring in precracked MI SiCSiC CMC specimens under tensile stresses of 30 ksi or less at 815C in dry air or argon. In addition, the oxidation of the BN interface was characterized at815C, and crack opening displacement as a function of stress measurements were made. This material characterization is being performed to obtain data to support NASA GRC modeling of SiCSiC environmental degradation. The comparison of experimentally-observed phenomena with model predictions can lead to improved understanding of material degradation mechanisms.

SiC/SiC composites↗

Automated Reconfigurable Mission Adaptive Digital Assembly Systems (ARMADAS): Robotically Assembled Sustainable Lunar Infrastructure

Introduction: The Automated Reconfigurable Mission Adaptive Digital Assembly Systems (ARMADAS) project at NASA Ames Research Center is developing autonomous infrastructure, instrumentation, and spacecraft assembly and manufacturing capabilities for next generation exploration and science missions, with a goal to change the cost scaling of these missions relative to mission size and duration. Using a building-block approach with a 'kit of parts' composed of ultra-light, high-performance mechanical metamaterials, simplified robots leverage the period environment to achieve high levels of autonomy and reliability for in-space and surface assembly of large-scale apertures, solar-arrays, towers, habitats, and other infrastructure. Robots and structure break down into a compact form factor for launch. By leveraging economies of scale and achieving high-packing ratios, ARMADAS technology can revolutionize possible space missions by breaking the tyranny of the launch shroud, decreasing development times, decreasing mission costs for transformative science capability, and providing a scalable and versatile space infrastructure strategy. Capabilities: To date, the ARMADAS project has demonstrated autonomous assembly of large numbers of structural modules into a meters-scale structure in an earth gravity environment. The mechanical performance is on par with conventional space structures, at a fraction of the cost. By using highly repeatable manufacturing processes (injection molding carbon fiber reinforced space-rated polymers), the structures are both cost effective and highly precise. The ARAMADAS system carefully designs parts for high-precision assembly–the simple and cost-effective robots build structures much larger and more precise than themselves. While maintaining structural efficiency, the ARMADAS system encompasses many functional module types, including for solar power, comm/power routing, etc., that will enable entire infrastructure systems to be engineered and autonomously assembled with modular parts. With well-defined interfaces, custom instrumentation modules can easily be added to fit many mission objectives. Lunar Infrastructure Vision: At the LSIC spring meeting, ARMADAS will present a vision for a general-purpose lunar construction kit capable of meeting a wide variety of lunar surface infrastructure needs (Figure 1). Investing in an ecosystem of reconfigurable, discretely repairable parts and robots enables systems than can expand their capability, reconfigure to meet emergency or unforeseen needs, self-repair and reduce spare-part needs. With a small set of parts (module types), a wide variety of infrastructure needs can be met. Footing modules will allow infrastructure construction at locations with no surface preparation. Footer, primary structure, and rail modules can create reconfigurable rail systems. These rail systems can be used to reduce cost-of-transport between frequently accessed sites, provide dust mitigation, and convey power and communications. High-performance structure modules combined with power-routing and solar modules can create tall towers for power and communication. Concepts for habitats and garages encompass a structure to support regolith cover for a pre-integrated module to an entirely ARMADAS system-based structure. This system can be leveraged by many in-situ resource utilization (ISRU) technologies to simplify processes and augment capabilities.

C. E. Gregg↗

Application of Iosipescu specimen geometry to determination of shear strength in unidirectional composites

The Iosipescu (1967) test method, in conjunction with a finite element analysis of anisotropic, unidirectionally reinforced composite specimens, can be used to derive composite shear strength values despite the small size of the samples used. When such a specimen is loaded parallel to the fiber direction, fracture occurs by shear within a narrow region near the center of the specimen; this is consistent with the maximum shear area determined by finite element analysis. The resulting fracture surface, as observed, is characteristic of shear failure, occurring either within the matrix or at the fiber-matrix interface. Problems of compressive and mixed mode failure which accrue to short beam shear specimens are by the present method precluded.

Hurwitz, F. I.↗