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Tailoring the intergranular phases in silicon nitride for improved toughness

Description: Intergranular glass phases can have a significant influence on fracture resistance (R-curve behavior) of Si nitride ceramics and appears to be related to debonding of the {beta}-Si{sub 3}N{sub 4}/oxynitride-glass interfaces. Applying the results from {beta}- Si{sub 3}N{sub 4}-whisker/oxynitride-glass model systems, self- reinforced Si nitrides with different sintering additive ratios were investigated. Si nitrides sintered with a lower Al{sub 2}O{sub 3}: Y{sub 2}O{sub 3} additive ratio exhibited higher stead-state fracture toughness together with a steeply rising R-curve. Analytical electron microscopy suggested that the different fracture behavior is related to the Al content in the SiAlON growth band on the elongated grains, which could result in differences in interfacial bonding structures between the grains and the intergranular glass.
Date: December 31, 1996
Creator: Sun, E. Y.; Becher, P. F.; Plucknett, K. P.; Waters, S. B.; Hirao, K. & Brito, M. E.
Partner: UNT Libraries Government Documents Department

Interfacial composition analysis of silicon nitride whisker reinforced oxynitride glass systems

Description: Scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy and parallel-detection electron energy-loss spectroscopy was used to analyze the composition at interfaces in {beta}-Si{sub 3}N{sub 4} whisker reinforced oxynitride glass systems.
Date: June 1, 1995
Creator: Sun, E. Y.; Alexander, K. B.; Becher, P. F. & Hwang, S. L.
Partner: UNT Libraries Government Documents Department

Investigation of the Interphase Region in Polymer Matrix - Glass Fiber Reinforced Composites Using the Interfacial Force Microscope

Description: The proposed research was to provide a critical vehicle to enhance South Dakota researchers' abilities to participate in nationally important energy related research while building and strengthening partnerships between the South Dakota School of Mines and Technology and Sandia National Laboratory.
Date: June 30, 2004
Creator: Winter, R. M
Partner: UNT Libraries Government Documents Department

Transient and steady-state erosion of in-situ reinforced silicon nitride

Description: Relative to most other materials silicon nitride is very erosion resistant. However, the resulting surface flaws degrade strength - a serious concern for component designers. AlliedSignal Ceramic Components GS-44 in-situ reinforced silicon nitride was eroded in a slinger apparatus. Both transient (extremely low level) and steady-state erosion regimes were investigated. Alumina particles with effective average diameters of 140 Jim and 63 {mu}m were used at velocities of 50 m/s, 100 m/s, and 138 m/s. Biaxial tensile strength was measured. Strength decreased by about 15% after a very small erodent dosage and then remained virtually constant with further erosion. In-situ reinforcement produces R-curve behavior in which the fracture toughness increases with crack size. The effect of this is quite dramatic with strength loss being significantly less than expected for a normal silicon nitride with constant fracture toughness.
Date: October 1, 1994
Creator: Karasek, K.R.; Whalen, P.J.; Rateick, R.G. Jr.; Hamilton, A.C. & Routbort, J.L.
Partner: UNT Libraries Government Documents Department

On the statistical strength of nicalon fibers and its characterization

Description: Nicalon fibers are one of the most attractive ceramic fibers for reinforcing high temperature structural composites. Experiment show that the diameter variation (from 8 to 22 {mu}m) in a tow of commercial Nicalon fibers has an effect on statistical strength distribution of Nicalon fibers. Therefore, an appropriate characterization of the statistical distribution of fiber strength, capable of accounting for the effect of diameter variation, is required. The two-parameter single-modal Weibull distribution is found inadequate for characterizing the strength of Nicalon fibers. This work demonstrates that a 3-parameter modified Weibull distribution can successfully characterize the strength of Nicalon fibers; this distribution yields a higher {beta} value, which indicates less scatter in fiber strength than with the 2-parameter single modal Weibull distribution. It more accurately treats the strength variation caused by diameter variation. It is also much easier to use than the 4-parameter bimodal. Moreover, it is seen to more accurately treat the strength variation caused by diameter variation than the single modal 2-parameter analysis.
Date: February 1, 1997
Creator: Zhu, Y.T.; Taylor, S.T.; Stout, M.G.; Butt, D.P.; Blumenthal, W.R. & Lowe, T.C.
Partner: UNT Libraries Government Documents Department

The structure of carbon black and its composites with elastomers: A study using neutron scattering

Description: The authors have been exploring the use of small-angle neutron scattering and the method of contrast variation to give a new look at a very old problem--reinforcement of elastomers by carbon black in durable rubber products. The method has yielded some interesting information on the structure of an experimental carbon black, HSA, and on the associations of HSA in polyisoprene composites. Carbon black has a hierarchy of structures consisting of particles covalently bound into aggregates, which in turn associate by weak interactions into agglomerates. The authors found that in HSA the aggregates are rodlike, containing an average of 4--6 particles. The aggregates have an outer graphitic shell and an inner core of lower density carbon. The core is continuous throughout the carbon black aggregate. Contrast variation of swollen HSA-polyisoprene gels show that the HSA is completely embedded in polyisoprene and that the agglomerates are formed predominantly by end on associations of the rodlike aggregates. The surface structure of the carbon black appears smooth over length scales above about 10 {angstrom}. Further studies on production carbon blacks suggest that the shell-like aggregate structure is present in commercial carbon blacks.
Date: July 1, 1996
Creator: Hjelm, R.P.; Wampler, W. & Gerspacher, M.
Partner: UNT Libraries Government Documents Department

The in-situ generation of silica reinforcement in modified polydimethylsiloxane elastomers

Description: Structure and properties of a series of modified polydimethylsiloxane (PDMS) elastomers reinforced by {ital in situ} generated silic particles were investigated. The PDMS elastomer was modified by systematically varying the molecular weight between reactive groups incorporated into the backbone. Tetraethoxysilane (TEOS) and partial hydrolyzate of TEOS were used to generate silic particles. Chemistry and phase structure of the materials were investigated by {sup 29}Si magic angle spinning nuclear magnetic resonance spectroscopy and swelling experiments.
Date: June 1, 1996
Creator: Prabakar, S; Bates, S.E.; Black, E.P. & Ulibarri, T.A.
Partner: UNT Libraries Government Documents Department

Influence of fiber lay-up sequence on mechanical properties of SiC(f)/SiC composites

Description: Mechanical properties of Nicalon-fiber-reinforced silicon carbide matrix composites with two different fiber lay-up sequences (0{degree}/40{degree}/60{degree} and 0{degree}/45{degree}) were evaluated at various temperatures ranging from ambient to 1300{degree}C. Composites with 0{degree}/40{degree}/60{degree} fiber lay-up sequence showed a higher average first matrix cracking stress than that of 0{degree}/45{degree} composites. The measured room-temperature ultimate strength of the 0{degree}/40{degree}/60{degree} composites was 300 MPa compared to 180 MPa for the 0{degree}/45{degree} composites. These measured ultimate strengths were correlated to the predictions made with an analytical model and to in-situ fiber strength characteristics. The large difference in room-temperature ultimate strengths between the two sets of composites is attributed to the relative contributions of the off-axis fibers to the load-bearing capacity of each composite. Up to 1200{degree}C, ultimate strength and work-of-fracture in each set of composites increased, but then declined above 1300{degree}C. The decreases were correlated to in-situ Nicalon fiber strength and fiber/matrix interface degradation.
Date: March 1996
Creator: Singh, D.; Singh, J. P. & Sutaria, M.
Partner: UNT Libraries Government Documents Department

Mechanical properties, microscopy, and failure mechanisms of carbon fiber reinforced epoxy laminated composites

Description: The mechanical behavior of quasi-isotropic and unidirectional epoxy- matrix carbon-fiber laminated composites subjected to compressive loading at strain rates of 10{sup {minus}3} and 2000 s{sup {minus}1} are described. Failure in the studied composites was dominated by delamination which proceeded by brittle fracture of the epoxy matrix. The matrix-fiber bonding in these composites is very strong and prevented the occurrence of significant fiber-pullout. The mode I delamination strain energy release rate of the unidirectional composites was determined using the double cantilever beam and hole in plate compression methods. The DCB method indicated a significant R curve effect attributed to fiber bridging while the presently available hole in plate analytical methods show questionable validity for highly anisotropic materials.
Date: December 1995
Creator: Thissell, W. R.; Zurek, A. K. & Addessio, F.
Partner: UNT Libraries Government Documents Department

Engineering scale development of the vapor-liquid-solid (VLS) process for the production of silicon carbide fibrils. Phase 2

Description: As reinforcements for composites, VLS SiC fibrils have attractive mechanical properties including high-strength, high modulus, and excellent creep resistance. To make use of their excellent mechanical properties in a composite, a significant volume fraction (>10%) of aligned, long fibrils (>2 mm) needs to be consolidated in the ceramic matrix. The fibrils must be processed into an assembly that will allow for composite fabrication while maintaining fibril alignment and length. With Advanced Product Development (APD) as the yam fabrication subcontractor, Carborundum investigated several approaches to achieve this goaL including traditional yam-forming processes such as carding and air-vortex spinning and nontraditional processes such as tape forming and wet casting. Carborundum additionally performed an economic analysis for producing 500 and 10,000 pounds of SiC fibrils annually using both conservative and more aggressive processing parameters. With the aggressive approach, the projected costs for SiC fibril production for 500 and 10,000 pounds per year are $1,340/pound and $340/pound, respectively.
Date: April 1, 1995
Creator: Ohnsorg, R. W.; Hollar, W. E., Jr.; Lau, S. K.; Ko, F. K. & Schatz, K.
Partner: UNT Libraries Government Documents Department

Evaluation of interfacial properties of fiber-reinforced ceramic composites

Description: The mechanical behavior of fiber-reinforced ceramic composites is closely related to their interfacial properties. The application of a single-fiber push-out test to evaluate these interfacial properties is addressed, and the stress-displacement relation during the push-out process is analyzed. The interfacial bonding, Coulomb friction at the debonded interface, Poisson`s effect of the loaded fiber, residual stresses, and the interfacial roughness effect are included in the analysis. Closed-form analytical solutions are obtained. Based on these analytical solutions, a methodology is established to extract the interfacial properties from the experimental fiber push-out curve.
Date: October 1, 1996
Creator: Hsueh, Chun-Hway
Partner: UNT Libraries Government Documents Department

Effect of fiber architecture on mechanical behavior of SiC(f)/SiC composites

Description: We evaluated mechanical properties (first matrix cracking stress, strength, and work-of-fracture) of Nicalon-fiber-reinforced silicon carbide matrix composites with three different fiber lay-up sequences (0{degrees}/20{degrees}/60{degrees}, 0{degrees}/40{degrees}/60{degrees}, and 0{degrees}/45{degrees}) at various temperatures from room to 1300{degrees}C. Up to 1200{degrees}C, ultimate strength and work-of-fracture for the 0{degrees}/40{degrees}/60{degrees} and 0{degrees}/45{degrees} composites increased, but then declined at 1300{degrees}C. The decreases were correlated to in-situ Nicalon fiber strength and fiber/matrix interface degradation. However, for the 0{degrees}/20{degrees}/60{degrees} composites, ultimate strength and work-of-fracture reached their a minima at 1200{degrees}C. These measured ultimate strengths at room and 1300{degrees}C were correlated to the predictions made with an analytical model and to in-situ fiber strength characteristics. The large difference in room-temperature ultimate strengths between the three sets of composites is attributed to the relative contributions of the off-axis fibers to the load-bearing capacity of each composite.
Date: January 1, 1997
Creator: Singh, D.; Singh, J.P. & Sutaria, M.
Partner: UNT Libraries Government Documents Department

Influence of reinforcement morphology on the mechanical properties of short-fiber composites

Description: A major problem of short-fiber composites is that the interfaces between the fiber and matrix become a limiting factor in improving mechanical properties such as strength. For a short fiber, a strong interface is desired to effectively transfer load from matrix to fiber, thus reducing the ineffective fiber length. However, a strong interface will make it difficult to relieve fiber stress concentration in front of an approaching crack. Stress concentrations result in fiber breakage. The authors report in this paper an innovative approach to overcome this problem: reinforcement morphology design. Short-fibers with enlarged ends are processed and used to reinforce a polyester matrix. The initial results show that the bone-shaped short-fibers produce a composite with significantly higher strength than can be attained with conventional short, straight fibers.
Date: December 1, 1997
Creator: Zhu, Y.T.; Valdez, J.A.; Shi, N.; Lovato, M.L.; Stout, M.G.; Zhou, S. et al.
Partner: UNT Libraries Government Documents Department

The implications of diffraction in-situ loading macro-strain measurements in composites and finite element interpretations

Description: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The main objective of the project is to show that diffraction strain measurement, when complemented with numerical tools, can provide insights that traditional metallurgical methods cannot offer. In this project, we concentrate on issues concerning discontinuously reinforced metal and intermetallic matrix composites. We have studied load transfer between phases during loading, internal stress distribution during and as a result of inelastic stress relaxation (creep). We also studied the signatures of reinforcement fracture on the evolution of average elastic global strain, and used neutrons to study the micromechanics of slip in intermetallic composites. Of these subjects, we have found, most importantly, that the ratio of load partition in multiphase material does not stay as a constant; internal damage in composites leaves a distinct signature on the evolution of global average elastic strain; and, in reinforced low-symmetry materials, stress relaxation is dictated by the micromechanics of crystal slip, which cannot be accounted for by the continuum theory.
Date: December 31, 1998
Creator: Shi, Ning; Bourke, M.A. & Roberts, J.A.
Partner: UNT Libraries Government Documents Department

Evolution of microstructure and mechanical properties in laser induced reaction coating of Al{sub 2}O{sub 3} on SiC/Al{sub 2}O{sub 3} composite

Description: Protection of a SiC(p)/Al2O3 composite (SiC particulate-reinforced Al{sub 2}O{sub 3}-matrix) at high temperature from deleterious reactions occurring within and with the surrounding environment is required for high temperature applications. Development of a continuous Al2O3 coating on SiC(p)/Al2O3 ceramic composite for such protection is achieved using the laser assisted in-situ reaction technique. The as-deposited alumina coating was analyzed using optical microscopy and XRD. The coated samples were also evaluated for mechanical properties using 3-point bend tests.
Date: December 31, 1996
Creator: Dahotre, N.B.; Xiao, C.; Boss, W.; McCay, M.H. & McCay, T.D.
Partner: UNT Libraries Government Documents Department

The Mechanics of Creep Deformation in Polymer Derived Continuous Fiber-Reinforced Ceramic Matrix Composites

Description: The objective of this Cooperative Research and Development Agreement between Lockheed Martin Energy Research Corporation and Dow Corning Corporation was to study the effects of temperature, stress, fiber type and fiber architecture on the time-dependent deformation and stress-rupture behavior of polymer-derived ceramic matrix composites developed by the Dow Corning Corporation. Materials reinforced with CG-Nicalon{trademark}, Hi-Nicalon{trademark} and Sylramic{reg_sign} fibers were evaluated under fast fracture, stress-relaxation, and stress-rupture conditions at temperatures between 700 C and 1400 C in ambient air and for stresses between 50 and 200 MPa. Some of the stress-rupture tests conducted as part of this program are among the longest-duration experiments ever conducted with these materials. The possibility of using accelerated test techniques to evaluate the very-long term stress-rupture/creep behavior of these materials was investigated by means of stress-relaxation experiments. However it was found that because these materials exhibit non-linear stress-strain behavior at stresses larger than the matrix cracking stress and because of environmentally-induced changes in the micro and mesostructure of the material, particularly at elevated temperatures, this approach is impractical. However, the results of stress-relaxation experiments will be useful to predict the behavior of these materials in applications where stresses are thermally-induced and therefore driven by strains (e.g., when components are subjected to thermal gradients). The evolution of the microstructure of the fibers, matrix and fiber-matrix interface was studied as a function of stress and temperature, using analytical electron microscopy. The results from these analyses were essential to understand the relationships between environment, stress, temperature and processing on the microstructure and properties of these materials.
Date: January 30, 2001
Creator: Lara-Curzio, E.
Partner: UNT Libraries Government Documents Department

Relationship between High-Strain-Rate Superplasticity and Interface Microstructure in Aluminum Alloy Composites

Description: The Al alloy composites reinforced with Si<sub>3</sub>N<sub>4</sub> or SiC have been reported to exhibit superplasticity at high strain rate of faster than 1x 10<sup>-2</sup>s<sup>-1</sup>. It has been shown in many aluminum alloy composites that the optimum superplastic temperature coincides with an incipient melting temperature. The coincidence suggests a contribution of the liquid phase to the superplasticity mechanism. This paper shows a direct evidence of partial melting along matrix grain boundaries and matrix-reinforcement interfaces. Based on the obtained results, the role of the liquid phase in the high-strain-rate superplasticity is discussed. The sample was Al-Mg (5052) alloy reinforced with 20vol% Si<sub>3</sub>N<sub>4</sub> particles, fabricated by a powder metallurgy process. The sample showed an excellent superplasticity under the conditions given in Table 1. Partial melting was confirmed to occur at 821 K by differentail scanning calorimetry. The microstructural changes during heating were observed in situ by TEM using a heating stage. The structure of interfaces and grain boundaries was observed by HREM. Chemical analysis was performed with EDS attached to VG-STEM. A bright-field image of the composite is shown in Fig. 1. Notice that the edge of the Si3N4 particles are fragmented. Fig. 2 (a) shows a selected-area diffraction pattern taken at 821 K. A halo ring appears at this temperature, indicating partial melting. Fig. 2 (b) shows a dark- field image with an inverted contrast, taken from a part of the halo ring. The location of partial melting can be identified by a dark contrast along the matrix grain boundaries and the matrix- reinforcement interfaces. Above this temperature, grain-boundary corners become a rounded shape caused by the formation of the liquid phase at triple grain junctions. Figure 3 shows a concentration profile across a matrix-reinforcement interface. The left side is the aluminum matrix and the right is a Si<sub>3</sub>N<sub>4</sub> particle. In the ...
Date: February 1, 1999
Creator: Koike, J.; Mitchell, T.E. & Sickafus, K.E.
Partner: UNT Libraries Government Documents Department

Analyses of interfacial shear debonding in fiber-reinforced ceramic composites

Description: An important toughening mechanism in fiber-reinforced ceramic composites is pullout of fibers from the matrix during matrix cracking. This relies on mode II (i.e., shear) debonding at the fiber/matrix interface which can be analyzed using either the strength-based or the energy-based criterion. In the strength-based approach, debonding occurs when the maximum interfacial shear stress induced by the applied load reaches the interfacial shear strength, {tau}{sub s}. In the energy-based approach, a mode II crack propagating along the interface is considered, and debonding occurs when the energy release rate due to crack propagation reaches the interface debond energy, {Gamma}{sub 1}. Based on the above two criteria, the applied stress on the fiber to initiate debonding (i.e., the initial debond stress), {sigma}{sub d}, can be derived. The first issue considered in the present study is the relation between {tau}{sub s} and {Gamma}{sub 1}. Also, for a monolithic ceramic, the tensile strength can be related to its defect size based on the Griffith theory. A question is hence raised as to whether the initial debond stress for fiber pullout in a fiber-reinforced ceramic composite can be related to any defect at the interface.
Date: February 1, 1998
Creator: Hsueh, C.H. & Becher, P.F.
Partner: UNT Libraries Government Documents Department

Structure-property relationships in silica-siloxane nanocomposite materials

Description: The simultaneous formation of a filler phase and a polymer matrix via in situ sol-gel techniques provides silica-siloxane nanocomposite materials of high strength. This study concentrates on the effects of temperature and relative humidity on a trimodal polymer system in an attempt to accelerate the reaction as well as evaluate subtle process- structure-property relations. It was found that successful process acceleration is only viable for high humidity systems when using the tin(IV) catalyst dibutyltin dilaurate. Processes involving low humidity were found to be very temperature and time dependent. Bimodal systems were investigated and demonstrated that the presence of a short-chain component led to enhanced material strength. This part of the study also revealed a link between the particle size and population density and the optimization of material properties.
Date: March 1, 1997
Creator: Ulibarri, T.A.; Derzon, D.K. & Wang, L.C.
Partner: UNT Libraries Government Documents Department

R-curve behavior of whisker-reinforced ceramic composites

Description: Ceramics have attracted great interest for advanced heat engines and industrial applications as a result of their stiffness, light weight, corrosion resistance, and superior performance at high temperatures. However, their utilization in engineering applications is severely limited by their brittleness. One approach toward substantially reducing this brittleness is by incorporating strong whiskers into the ceramic matrix. Studies of whisker-reinforced ceramic composites have shown both rising fracture resistance with crack extension (i.e. R-curve behavior) and a three- to four-fold increase in steady-state toughness. The primary toughening mechanism is bridging of the crack surface by whiskers. This paper discusses the bridging mechanism in detail and presents R-curves for SiC whisker-reinforced Al{sub 2}O{sub 3} composites.
Date: June 1, 1995
Creator: Hsueh, Chun-Hway & Becher, P.F.
Partner: UNT Libraries Government Documents Department

On the distinction between large deformation and large distortion for anisotropic materials

Description: A motion involves large distortion if the ratios of principal stretches differ significantly from unity. A motion involves large deformation if the deformation gradient tensor is significantly different from the identity. Unfortunately, rigid rotation fits the definition of large deformation, and models that claim to be valid for large deformation are often inadequate for large distortion. An exact solution for the stress in an idealized fiber-reinforced composite is used to show that conventional large deformation representations for transverse isotropy give errant results. Possible alternative approaches are discussed.
Date: February 24, 2000
Partner: UNT Libraries Government Documents Department

Imaging radar for bridge deck inspection

Description: Lawrence Livermore National Laboratory (LLNL)l is developing a prototype imaging radar for inspecting steel reinforced concrete bridge decks. The system is designed to acquire Synthetic Aperture Radar (SAR) data and provide high-resolution images of internal structure, flaws, and defects enabling bridge inspectors to nondestructively evaluate and characterize bridge deck condition. Concrete delamination resulting from corrosion of steel reinforcing bars (rebars) is an important structural defect that the system is designed to detect. The prototype system uses arrays of compact, low-cost Micropower Impulse Radar (MIR) modules, supported by appropriate data acquisition and storage subsystems, to generate and collect the radar data, and unique imaging codes to reconstruct images of bridge deck internals. In this paper, we provide an overview of the prototype system concept, discuss its expected performance, and present recent experimental results showing the capability of this approach to detect thin delamination simulations embedded in concrete.
Date: April 13, 1995
Creator: Warhus, J.; Mast, J. & Nelson, S.
Partner: UNT Libraries Government Documents Department