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At least 163 records · Page 9

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NASA NTRS1992

Modeling of crack bridging in a unidirectional metal matrix composite

The effective fatigue crack driving force and crack opening profiles were determined analytically for fatigue tested unidirectional composite specimens exhibiting fiber bridging. The crack closure pressure due to bridging was modeled using two approaches: the fiber pressure model and the shear lag model. For both closure models, the Bueckner weight function method and the finite element method were used to calculate crack opening displacements and the crack driving force. The predicted near crack tip opening profile agreed well with the experimentally measured profiles for single edge notch SCS-6/Ti-15-3 metal matrix composite specimens. The numerically determined effective crack driving force, Delta K(eff), was calculated using both models to correlate the measure crack growth rate in the composite. The calculated Delta K(eff) from both models accounted for the crack bridging by showing a good agreement between the measured fatigue crack growth rates of the bridged composite and that of unreinforced, unbridged titanium matrix alloy specimens.

Ghosn, Louis J.↗

NASA NTRS1990

Intermetallic matrix composites; Proceedings of the MRS Symposium, San Francisco, CA, Apr. 18-20, 1990

The present volume on intermetallic matrix composites discusses the modeling, processing, microstructure/property relationships, and compatibility of intermetallic matrix composites. Attention is given to models for the strength of ductile matrix composites, innovative processing techniques for intermetallic matrix composites, ductile phase toughening of brittle intermetallics, and reactive synthesis of NbAl3 matrix composites. Topics addressed include solidification processing of NbCr2 alloys, Ta and Nb reinforced MoSi2, the microstructure and mechanical behavior of Ni3Al-matrix composites, and ductile-phase toughening of Cr3Si with chromium. Also discussed are dislocation morphologies in TiB2/NiAl, the development of highly impact resistant NiAl matrix composites, the effect of notches on the fatigue life of the SCS-6Ti3Al composite, and the chemical stability of fiber-metal matrix composites.

Anton, D. L.↗

NASA NTRS1992

Reduction of Thermal Residual Stresses in Advanced Metallic Composites Based upon a Compensating/Compliant Layer Concept

High residual stresses within metal and intermetallic matrix composite systems can develop upon cooling from the processing temperature to room temperature due to the coefficient of thermal expansion (CTE) mismatch between the fiber and matrix. As a result, within certain composite systems, radial, circumferential, and/or longitudinal cracks have been observed to form at the fiber-matrix interface region. The compliant layer concept (insertion of a compensating interface material between the fiber and matrix) has been proposed to reduce or eliminate the residual stress buildup during cooling, and thus minimize cracking. The present study investigates elastic-plastically the viability of the proposed compliant layer concept. A detailed parametric study was conducted utilizing a unit cell model consisting of three concentric cylinders to determine the required character (i.e., thickness and mechanical properties) of the compliant layer as well as its applicability. The unknown compliant layer mechanical properties were expressed as ratios of the corresponding temperature dependent Ti-24Al-11Nb (a/o) matrix properties. The fiber properties taken were those corresponding to SCS-6 (SiC). Results indicated that the compliant layer can be used to reduce, if not eliminate, radial and circumferential residual stresses within the fiber and matrix and therefore also reduce or eliminate the radial cracking. However, with this decrease in in-plane stresses, one obtains an increase in longitudinal stress, thus potentially initiating longitudinal cracking. Guidelines are given for the selection of a specific compensating/ compliant material, given a perfectly bonded system.

S. M. Arnold↗

NASA NTRS1992

Palladium-chromium static strain gages for high temperatures

An electrical resistance strain gage that can provide accurate static strain measurement to a temperature of 1500 F or above is being developed both in fine wire and thin film forms. The gage is designed to be temperature compensated on any substrate material. It has a dual element: the gage element is a special alloy, palladium-13wt percent chromium (PdCr), and the compensator element is platinum (Pt). Earlier results of a PdCr based wire gage indicated that the apparent strain of this gage can be minimized and the repeatability of the apparent strain can be improved by prestabilizing the gage on the substrate for a long period of time. However, this kind of prestabilization is not practical in many applications and therefore the development of a wire gage which is prestabilized before installation on the substrate is desirable. This paper will present our recent progress in the development of a prestabilized wire gage which can provide meaningful strain data for the first thermal cycle. A weldable PdCr gage is also being developed for field testing where conventional flame-spraying installation can not be applied. This weldable gage is narrower than a previously reported gage, thereby allowing the gage to be more resistant to buckling under compressive loads. Some preliminary results of a prestabilized wire gage flame-sprayed directly on IN100, an engine material, and a weldable gage spot-welded on IN100 and SCS-6/(beta)21-S Titanium Matrix Composite (TMC), a National Aero-Space Plane (NASP) structure material, will be reported. Progress on the development of a weldable thin film gage will also be addressed. The measurement technique and procedures and the lead wire effect will be discussed.

Lei, Jih-Fen↗

NASA NTRS1992

Inelastic deformation of metal matrix composites: Plasticity and damage mechanisms, part 2

The inelastic deformation mechanisms for the SiC (SCS-6)/Ti-15-3 system were studied at 538 C (1000 F) using a combination of mechanical measurements and detailed microstructural examinations. The objectives were to evaluate the contributions of plasticity and damage to the overall MMC response, and to compare the room temperature and elevated temperature deformation behaviors. Four different laminates were studied: (0)8, (90)8,(+ or -45)2s, and (0/90)2s, with the primary emphasis on the unidirectional (0)8, and (90)8 systems. The elevated temperature responses were similar to those at room temperature, involving a two-stage elastic-plastic type of response for the (0)8 system, and a characteristic three-stage deformation response for the (90)8 and (+ or -45)2s systems. The primary effects of elevated temperatures included: (1) reduction in the 'yield' and failure strengths; (2) plasticity through diffused slip rather than concentrated planar slip (which occurred at room temperature); and (3) time-dependent deformation. The inelastic deformation mechanism for the (0)8 MMC was dominated by plasticity at both temperatures. For the (90)8 and (+ or -45)2s MMCs, a combination of damage and plasticity contributed to the deformation at both temperatures.

Majumdar, B. S.↗

NASA NTRS1993

Fatigue of continuous fiber reinforced metallic materials

The complex damage mechanisms that occur in fiber reinforced advanced metallic materials are discussed. As examples, results for several layups of SCS-6/Ti-15-3 composites are presented. Fatigue tests were conducted and analyzed for both notched and unnotched specimens at room and elevated temperatures. Test conditions included isothermal, non-isothermal, and simulated mission profile thermomechanical fatigue. Test results indicated that the stress in the 0 degree fibers is the controlling factor for fatigue life for a given test condition. An effective strain approach is presented for predicting crack initiation at notches. Fiber bridging models were applied to crack growth behavior.

Johnson, W. S.↗

NASA NTRS1993

Damage development in titanium metal-matrix composites subjected to cyclic loading

Several layups of SCS-6/Ti-15-3 composites were investigated. Fatigue tests were conducted and analyzed for both notched and unnotched specimens at room temperature and elevated temperatures. Thermo-mechanical fatigue results were analyzed. Test results indicated that the stress in the 0 degree fibers is the controlling factor in fatigue life. The static and fatigue strength of these materials is shown to be strongly dependent on the level of residual stresses and the fiber/matrix interfacial strength. Fatigue tests of notched specimens showed that cracks can initiate and grow many fiber spacings in the matrix materials without breaking fibers. Fiber bridging models were applied to characterize the crack growth behavior. The matrix cracks are shown to significantly reduce the residual strength of notched composites. The notch strength of these composites was accurately predicted using a micromechanics based methodology.

Johnson, W. S.↗

NASA NTRS1991

Creep and stress relaxation modeling of polycrystalline ceramic fibers

A variety of high performance polycrystalline ceramic fibers are currently being considered as reinforcement for high temperature ceramic matrix composites. However, under mechanical loading above 800 C, these fibers display creep-related instabilities which can result in detrimental changes in composite dimensions, strength, and internal stress distributions. As a first step toward understanding these effects, this study examines the validity of mechanistic-based empirical model which describes primary stage tensile creep and stress relaxation of polycrystalline ceramic fibers as independent functions of time, temperature, and applied stress or strain. To verify these functional dependencies, a simple bend test is used to measure stress relaxation for four types of commercial ceramic fibers for which direct tensile creep data are available. These fibers include both nonoxide (SCS-6, Nicalon) and oxide (PRD-166, FP) compositions. The results of the bend stress relaxation (BSR) test not only confirm the stress, time, and temperature dependencies predicted by the model but also allow measurement of model empirical parameters for the four fiber types. In addition, comparison of model predictions and BSR test results with the literature tensile creep data show good agreement, supporting both the predictive capability of the model and the use of the BSR test as a simple method for parameter determination for other fibers.

Dicarlo, James A.↗

NASA NTRS1991

Probabilistic micromechanics for metal matrix composites

A probabilistic micromechanics-based nonlinear analysis procedure is developed to predict and quantify the variability in the properties of high temperature metal matrix composites. Monte Carlo simulation is used to model the probabilistic distributions of the constituent level properties including fiber, matrix, and interphase properties, volume and void ratios, strengths, fiber misalignment, and nonlinear empirical parameters. The procedure predicts the resultant ply properties and quantifies their statistical scatter. Graphite copper and Silicon Carbide Titanlum Aluminide (SCS-6 TI15) unidirectional plies are considered to demonstrate the predictive capabilities. The procedure is believed to have a high potential for use in material characterization and selection to precede and assist in experimental studies of new high temperature metal matrix composites.

Engelstad, S. P.↗

NASA NTRS1991

Fatigue of continuous fiber reinforced titanium matrix composites

Several lay-ups of SCS-6/Ti-15-3 composites were investigated. Static and fatigue tests were conducted for both notched and unnotched specimens at room and elevated temperatures. Test results indicated that the stress in the 0 fibers is the controlling factor in fatigue life. The static and fatigue strength of these materials is shown to be dependent on the level of residual stresses and the fiber/ matrix interfacial strength. Fatigue tests of notched specimens showed that cracks can initiate and grow many fiber spacings in the matrix material without breaking fibers. These matrix cracks can significantly reduce the residual strength of notched composite.

Johnson, W. S.↗

NASA NTRS1993

Residual strain gradient determination in metal matrix composites by synchrotron X-ray energy dispersive diffraction

An X-ray technique for the measurement of internal residual strain gradients near the continuous reinforcements of metal matrix composites has been investigated. The technique utilizes high intensity white X-ray radiation from a synchrotron radiation source to obtain energy spectra from small (0.001 cu mm) volumes deep within composite samples. The viability of the technique was tested using a model system with 800 micron Al203 fibers and a commercial purity titanium matrix. Good agreement was observed between the measured residual radial and hoop strain gradients and those estimated from a simple elastic concentric cylinders model. The technique was then used to assess the strains near (SCS-6) silicon carbide fibers in a Ti-14Al-21Nb matrix after consolidation processing. Reasonable agreement between measured and calculated strains was seen provided the probe volume was located 50 microns or more from the fiber/matrix interface.

Kuntz, Todd A.↗

NASA NTRS1993

Evolution of damage and plasticity in titanium-based, fiber-reinforced composites

The inelastic deformation mechanisms were evaluated for a model titanium-based, fiber-reinforced composite: a beta titanium alloy (Ti-15V-3Al-3Cr-3Sn) reinforced with SiC (SCS-6) fibers. The primary emphasis of this article is to illustrate the sequence in which damage and plasticity evolved for this system. The mechanical responses and the results of detailed microstructural evaluations for the 0(8), 90(8), and +/- 45(2s) line oriented laminates are provided. It is shown that the characteristics of the reaction zone around the fiber play a very important role in the way damage and plasticity evolve, particularly in the microyield regime of deformation, and must be included in any realistic constitutive model. Fiber-matrix debonding was a major damage mode for the off-axis systems. The tension test results are also compared with the predictions of a few constitutive models.

Majumdar, B. S.↗

NASA NTRS1993

Assessment of damage in ceramics and ceramic matrix composites using ultrasonic techniques

This paper addresses the application of ultrasonic sensing to damage assessment in ceramics and ceramic matrix composites. It focuses on damage caused by thermal shock or oxidation at elevated temperatures, which often results in elastic anisotropy. This damaged-induced anisotropy is determined by measuring the velocities of ultrasonic waves in different propagation directions. Thermal shock damage is assessed in ceramic samples of reaction bonded silicon nitride (RBSN). Thermal shock treatment from different temperatures up to 1000 C is applied to produce the microcracks. Results indicate that most microcracks produced by thermal shock are located near sample surfaces. Ultrasonic measurements using the surface wave method are found to correlate well with measurements of degradation of mechanical properties obtained independently by other authors using destructive methods. Oxidation damage is assessed in silicon carbide fiber/reaction bonded silicon nitride matrix (SCS-6/RBSN) composites. The oxidation is done by exposing the samples in a flowing oxygen environment at elevated temperatures, up to 1400 C, for 100 hr. The Youngs' modulus in the fiber direction as obtained from ultrasonic measurements decreases significantly at 600 C but retains its original value at temperatures above 1200 C. This agrees well with the results of destructive tests by other authors. On the other hand, the transverse moduli obtained from ultrasonic measurements decrease continually until 1200 C. Measurements on the shear stiffnesses show behavior similar to the transverse moduli. The results of this work show that the damage-induced anisotropy in both ceramics and ceramic matrix composites can be determined successfully by ultrasonic methods. This suggests the possibility of assessing damage severity using ultrasonic techniques.

Chu, Y. C.↗

NASA NTRS1993

Thermal stress effects in intermetallic matrix composites

Intermetallic matrix composites develop residual stresses from the large thermal expansion mismatch (delta-alpha) between the fibers and matrix. This work was undertaken to: establish improved techniques to measure these thermal stresses in IMC's; determine residual stresses in a variety of IMC systems by experiments and modeling; and, determine the effect of residual stresses on selected mechanical properties of an IMC. X ray diffraction (XRD), neutron diffraction (ND), synchrotron XRD (SXRD), and ultrasonics (US) techniques for measuring thermal stresses in IMC were examined and ND was selected as the most promising technique. ND was demonstrated on a variety of IMC systems encompassing Ti- and Ni-base matrices, SiC, W, and Al2O3 fibers, and different fiber fractions (Vf). Experimental results on these systems agreed with predictions of a concentric cylinder model. In SiC/Ti-base systems, little yielding was found and stresses were controlled primarily by delta-alpha and Vf. In Ni-base matrix systems, yield strength of the matrix and Vf controlled stress levels. The longitudinal residual stresses in SCS-6/Ti-24Al-llNb composite were modified by thermomechanical processing. Increasing residual stress decreased ultimate tensile strength in agreement with model predictions. Fiber pushout strength showed an unexpected inverse correlation with residual stress. In-plane shear yield strength showed no dependence on residual stress. Higher levels of residual tension led to higher fatigue crack growth rates, as suggested by matrix mean stress effects.

Wright, P. K.↗

NASA NTRS1993

Analysis of thermal mechanical fatigue in titanium matrix composites

Titanium metal matrix composites are being evaluated for structural applications on advanced hypersonic vehicles. These composites are reinforced with ceramic fibers such as silicon carbide, SCS-6. This combination of matrix and fiber results in a high stiffness, high strength composite that has good retention of properties even at elevated temperatures. However, significant thermal stresses are developed within the composite between the fiber and the matrix due to the difference in their respective coefficients of thermal expansion. In addition to the internal stresses that are generated due to thermal cycling, the overall laminate will be subjected to considerable mechanical loads during the thermal cycling. In order to develop life prediction methodology, one must be able to predict the stresses and strains that occur in the composite's constituents during the complex loading. Thus the purpose is to describe such an analytical tool, VISCOPLY.

Johnson, W. Steven↗

NASA NTRS1994

Matrix Fatigue Cracking Mechanisms of Alpha(2) TMC for Hypersonic Applications

The objective of this work was to understand matrix cracking mechanisms in a unidirectional alpha(sub 2) TMC in possible hypersonic applications. A (0)(sub 8) SCS-6/Ti-24Al-11Nb (at. percent) TMC was first subjected to a variety of simple isothermal and nonisothermal fatigue cycles to evaluate the damage mechanisms in simple conditions. A modified ascent mission cycle test was then performed to evaluate the combined effects of loading modes. This cycle mixes mechanical cycling at 150 and 483 C, sustained loads, and a slow thermal cycle to 815 C. At low cyclic stresses and strains more common in hypersonic applications, environment-assisted surface cracking limited fatigue resistance. This damage mechanism was most acute for out-of-phase nonisothermal cycles having extended cycle periods and the ascent mission cycle. A simple linear fraction damage model was employed to help understand this damage mechanism. Time-dependent environmental damage was found to strongly influence out-of-phase and mission life, with mechanical cycling damage due to the combination of external loading and CTE mismatch stresses playing a smaller role. The mechanical cycling and sustained loads in the mission cycle also had a smaller role.

Gabb, Timothy P.↗

NASA NTRS1994

Implementation of thermal residual stresses in the analysis of fiber bridged matrix crack growth in titanium matrix composites

In this research, thermal residual stresses were incorporated in an analysis of fiber-bridged matrix cracks in unidirectional and cross-ply titanium matrix composites (TMC) containing center holes or center notches. Two TMC were investigated, namely, SCS-6/Timelal-21S laminates. Experimentally, matrix crack initiation and growth were monitored during tension-tension fatigue tests conducted at room temperature and at an elevated temperature of 200 C. Analytically, thermal residual stresses were included in a fiber bridging (FB) model. The local R-ratio and stress-intensity factor in the matrix due to thermal and mechanical loadings were calculated and used to evaluate the matrix crack growth behavior in the two materials studied. The frictional shear stress term, tau, assumed in this model was used as a curve-fitting parameter to matrix crack growth data. The scatter band in the values of tau used to fit the matrix crack growth data was significantly reduced when thermal residual stresses were included in the fiber bridging analysis. For a given material system, lay-up and temperature, a single value of tau was sufficient to analyze the crack growth data. It was revealed in this study that thermal residual stresses are an important factor overlooked in the original FB models.

Bakuckas, John G., Jr.↗

NASA NTRS1994

Process/composite concurrent tailoring of SiC/Ti composites

Recent work on the prediction of optimal processing and material characteristics for improved fatigue behavior of metal and intermetallic matrix composites (MMC's/IMC's) is summarized. The method is incorporated into the MMLT (Metal Matrix Laminate Tailoring) code. Excellent correlations between predictions for the isothermal fatigue life of the SCS-6/Ti-24Al-11Nb composite and experimental data are obtained at various temperatures and stress ranges. Finally, the optimal processing conditions for improved isothermal fatigue life of the composite are evaluated and the attained isothermal fatigue life improvements are shown.

Chamis, C. C.↗