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At least 217 records · Page 12

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NASA NTRS1989

SRB thermal environments

The objective was to utilize and expand the Solid Rocket Booster (SRB) orbital flight test data base for better predictions of future flight environments. There were five tasks associated with this effort: analyze the internal aft skirt wind tunnel data and incorporate it into a data base for generating design and preflight reeentry thermal environments; generate reentry design thermal environments for the SRB steel case with the nozzle extension off; generate reentry design thermal environments for the SRB Filament Wound Case with the nozzle extension off; develop an engineering tool to analyze the 3-D flowfield around the SRB aft skirt during reentry for the purpose of obtaining the frequency and severity of the belching gas intrusion internal to the aft skirt; and perform SRM transient joint flow analysis for subscale and full scale motor firing as well as determine the effects of debonds of the insulation on the fill time and heating within the field joint insulation. In addition, this work was extended to provide support for the 51L Shuttle SRB failure analysis.

Crain, W. K.↗

NASA NTRS1990

Open-mode delamination stress concentrations in horseshoe and elliptic composite curved bars subjected to end forces

The multilayer theory of anisotropic elasticity and a finite element method were used to analyze the open-mode delamination stress concentrations in horseshoe and elliptic laminated composite curved bars. Two types of laminations, solid laminations and sandwich laminations, were analyzed. It was found that the open-mode delamination stress concentration could be greatly increased in these two types of curved bars by decreasing their aspect ratios. The open-mode delamination stress concentration generated in the solid laminations was found to be far more severe than that generated in the sandwich laminations. The horseshoe curved bar may be used to determine both the open-mode delamination strength of solidly laminated composites and the open-mode debonding strength of sandwiched laminated composites. However, the elliptic curved bar is only good for determining the open-mode delamination strength of solidly laminated composites.

Ko, William L.↗

NASA NTRS1990

Observations of fatigue crack initiation and damage growth in notched titanium matrix composites

The purpose was to characterize damage initiation and growth in notched titanium matrix composites at room temperature. Double edge notched or center open hole SCS-6/Ti-15-3 specimens containing 0 deg plies or containing both 0 and 90 deg plies were fatigued. The specimens were tested in the as-fabricated (ASF) and in heat-treated conditions. A local strain criterion using unnotched specimen fatigue data was successful in predicting fatigue damage initiation. The initiation stress level was accurately predicted for both a double edge notched unidirectional specimen and a cross-plied center hole specimen. The fatigue produced long multiple cracks growing from the notches. These fatigue cracks were only in the matrix material and did not break the fibers in their path. The combination of matrix cracking and fiber/matrix debonding appears to greatly reduce the stress concentration around the notches. The laminates that were heat treated showed a different crack growth pattern. In the ASF specimens, matrix cracks had a more tortuous path and showed considerable more crack branching. For the same specimen geometry and cyclic stress, the (0/90/0) laminate with a hole had far superior fatigue resistance than the matrix only specimen with a hole.

Naik, R. A.↗

NASA NTRS1990

Elevated temperature crack growth in aluminum alloys: Tensile deformation of 2618 and FVS0812 aluminum alloys

Understanding the damage tolerance of aluminum alloys at elevated temperatures is essential for safe applications of advanced materials. The objective of this project is to investigate the time dependent subcritical cracking behavior of powder metallurgy FVS0812 and ingot metallurgy 2618 aluminum alloys at elevated temperatures. The fracture mechanics approach was applied. Sidegrooved compact tension specimens were tested at 175, 250, and 316 C under constant load. Subcritical crack growth occurred in each alloy at applied stress intensity levels (K) of between about 14 and 25 MPa/m, well below K (sub IC). Measured load, crack opening displacement and displacement rate, and crack length and growth rate (da/dt) were analyzed with several continuum fracture parameters including, the C-integral, C (sub t), and K. Elevated temperature growth rate data suggest that K is a controlling parameter during time dependent cracking. For FVS0812, da/dt is highest at 175 C when rates are expressed as a function of K. While crack growth rate is not controlled by C (sub t) at 175 C, da/dt appears to better correlate with C (sub t) at higher temperatures. Creep brittle cracking at intermediate temperatures, and perhaps related to strain aging, is augmented by time dependent transient creep plasticity at higher temperatures. The C (sub t) analysis is, however, complicated by the necessity to measure small differences in the elastic crack growth and creep contributions to the crack opening displacement rate. A microstructural study indicates that 2618 and FVS0812 are likely to be creep brittle materials, consistent with the results obtained from the fracture mechanics study. Time dependent crack growth of 2618 at 175 C is characterized by mixed transgranular and intergranular fracture. Delamination along the ribbon powder particle boundaries occurs in FVS0812 at all temperatures. The fracture mode of FVS0812 changes with temperature. At 175 C, it is characterized as dimpled rupture, and at 316 C as mixed matrix superplastic rupture and matrix-dipersoid debonding.

Leng, Yang↗

NASA NTRS1989

CMCs for the long run

The structural and environmental requirements for ceramic matrix composites (CMCs) are most demanding in aerospace applications involving human transport; this need for extreme reliability has led to intensive investigations of criteria for high CMC integrity. The ideal properties of a fiber-reinforced CMC have been found to depend on the use of high aspect ratio fibers capable of maximum strength retention during fabrication, uniform fiber infiltration by the matrix material, high matrix oxidation resistance in elevated temperature applications, and a fiber/matrix interface that accommodates debonding in the presence of matrix cracks.

Dicarlo, James A.↗

NASA NTRS1989

Investigation of interfacial shear strength in a SiC fibre/Ti-24Al-11Nb composite by a fibre push-out technique

A fiber push-out technique applied at several sample thicknesses was used to determine both the debond shear stress and the frictional shear stress at the fiber-matrix interface at room temperautre for a unidirectional SiC fiber-reinforced T-24Al-11Nb (in at. pct) composite prepared by a powder cloth technique. The push-out technique measures the separate contributions of bond strength and friction to the mechanical shear strength at the fiber-matrix interface. It was found that the fiber-matrix bond shear strength of this material is significantly higher than the fiber-matrix frictional shear stress (119.2 and 47.8 MPa, respectively).

Eldridge, J. I.↗

NASA NTRS1990

Effect of interleaves on fracture of laminated composites. I - Analysis. II - Solution and results

The effect of interleaves placed between the fiber-reinforced plies of multilayered composite laminates is investigated analytically, with the composite approximated by linearly elastic isotropic media consisting of half-planes separated from a finite-width layer by thin interleaves of low extensional and shear moduli. Integral equations are derived which describe the effect of interleaves on damage growth. Numerical solutions are then developed which are based on Gaussian integration techniques and Hadamard's concept of differentiation of Cauchy integrals. Critical stresses and displacements are calculated to investigate the influence of relative elastic properties and geometry of the interleafed composite on interface debonding, extent and suppression and delamination, and stress relieving in the undamaged plies.

Kaw, A. K.↗

NASA NTRS1990

Interfacial effects on the behavior of partially bonded metal matrix composite properties

A novel computational method developed at NASA-Lewis in order to predict the behavior of unidirectional composites has been used to explore the effects of partial debonding and fiber fracture on the behavior of room temperature and high temperature metal-matrix composites. Attention is presently given to the influence of disbonding, which occurs with fractured fibers, on the ply properties of metal-matrix composites with orthotropic fibers, in the case of a graphite fiber-reinforced copper-matrix composite. It is shown that, for small amounts of partial bonding on fractured fibers, composite material properties are not significantly affected.

Caruso, J. J.↗

NASA NTRS1990

Probabilistic failure assessment with application to solid rocket motors

A quantitative methodology is being developed for assessment of risk of failure of solid rocket motors. This probabilistic methodology employs best available engineering models and available information in a stochastic framework. The framework accounts for incomplete knowledge of governing parameters, intrinsic variability, and failure model specification error. Earlier case studies have been conducted on several failure modes of the Space Shuttle Main Engine. Work in progress on application of this probabilistic approach to large solid rocket boosters such as the Advanced Solid Rocket Motor for the Space Shuttle is described. Failure due to debonding has been selected as the first case study for large solid rocket motors (SRMs) since it accounts for a significant number of historical SRM failures. Impact of incomplete knowledge of governing parameters and failure model specification errors is expected to be important.

Jan, Darrell L.↗

NASA NTRS1989

Conjugate (solid/fluid) computational fluid dynamics analysis of the space shuttle solid rocket motor nozzle/case and case field joints

Three-dimensional, conjugate (solid/fluid) heat transfer analyses of new designs of the Solid Rocket Motor (SRM) nozzle/case and case field joints are described. The main focus was to predict the consequences of multiple rips (or debonds) in the ambient cure adhesive packed between the nozzle/case joint surfaces and the bond line between the mating field joint surfaces. The models calculate the transient temperature responses of the various materials neighboring postulated flow/leakpaths into, past, and out from the nozzle/case primary O-ring cavity and case field capture O-ring cavity. These results were used to assess if the design was failsafe (i.e., no potential O-ring erosion) and reusable (i.e., no excessive steel temperatures). The models are adaptions and extensions of the general purpose PHOENICS fluid dynamics code. A non-orthogonal coordinate system was employed and 11,592 control cells for the nozzle/case and 20,088 for the case field joints are used with non-uniform distribution. Physical properties of both fluid and solids are temperature dependent. A number of parametric studies were run for both joints with results showing temperature limits for reuse for the steel case on the nozzle joint being exceeded while the steel case temperatures for the field joint were not. O-ring temperatures for the nozzle joint predicted erosion while for the field joint they did not.

Doran, D.↗

NASA NTRS1991

Graphite/copper alloy interfacial energies determined using the sessile drop method

Graphite surfaces are not wet by pure copper. This lack of wetting is responsible for a debonding phenomenon that was found in continuous graphite fiber/copper matrix composites materials subjected to elevated temperatures. By suitably alloying copper, its capability to wet graphite surfaces can be enhanced. In situ measurements of graphite/copper alloy wetting angles were made using the sessile drop method. Interfacial energy values were calculated based upon these measurements.

Devincent, Sandra M.↗

NASA NTRS1991

Mechanical behavior of fiber reinforced SiC/RBSN ceramic matrix composites: Theory and experiment

The mechanical behavior of continuous fiber reinforced SiC/RBSN (Reaction Bonded Silicon Nitride) composites with various fiber contents is evaluated. Both catastrophic and noncatastrophic failures are observed in tensile specimens. Damage and failure mechanisms are identified via in-situ monitoring using NDE (nondestructive evaluation) techniques through the loading history. Effects of fiber/matrix interface debonding (splitting) parallel to fibers are discussed. Statistical failure behavior of fibers is also observed, especially when the interface is weak. Micromechanical models incorporating residual stresses to calculate the critical matrix cracking strength, ultimate strength, and work of pull-out are reviewed and used to predict composite response. For selected test problems, experimental measurements are compared to analytical predictions.

Chulya, Abhisak↗

NASA NTRS1991

In-situ x-ray monitoring of damage accumulation in SiC/RBSN tensile specimens

The room-temperature tensile testing of silicon carbide fiber reinforced reaction-bonded silicon nitride (SiC/RBSN) composite specimens was monitored by using in-situ x ray film radiography. Radiographic evaluation before, during, and after loading provided data on the effect of preexisting volume flaws (high density impurities, and local density variations) on the fracture behavior of composites. Results from (0)1, (0)3, (0)5, and (0)8 composite specimens, showed that x ray film radiography can monitor damage accumulations during tensile loading. Matrix cracking, fiber-matrix debonding, and fiber pullout were imaged throughout the tensile loading history of the specimens. Further, in-situ film radiography was found to be a helpful and practical technique for estimating interfacial shear strength between the SiC fiber and the RBSN matrix by the matrix crack spacing method. It is concluded that pretest, in-situ, and post-test radiography can provide for a greater understanding of ceramic matrix composite mechanical behavior, a verification of related experimental procedures, and a validation and development of related analytical models.

Baaklini, George Y.↗

NASA NTRS1991

Microfracture in high temperature metal matrix crossply laminates

Microfracture (fiber/matrix fracture, interphase debonding, and inter-ply delamination) in high temperature metal matrix composites (HTMMC), subjected to both mechanical and thermal loading, is computationally simulated. A crossply 0.3 fiber volume ratio SiC/Ti15 composite with 0/90/0 lay-up is evaluated for microfracture using a multicell finite element model. A computational simulation procedure based on strain energy release rates is used to predict the fracture process and establish the hierarchy of fracture modes. Microfracture results for various loading cases are presented and discussed.

Mital, Subodh K.↗

NASA NTRS1991

Development of graphite/copper composites utilizing engineered interfaces

In situ measurements of graphite/copper alloy contact angles were made using the sessile drop method. The interfacial energy values obtained from these measurements were then applied to a model for the fiber matrix interfacial debonding phenomenon found in graphite/copper composites. The formation obtained from the sessile drop tests led to the development of a copper alloy that suitably wets graphite. Characterization of graphite/copper alloy interfaces subjected to elevated temperatures was conducted using Scanning Electron Microscopy, Energy Dispersive Spectroscopy, Auger Electron Spectroscopy, and X Ray Diffraction analyses. These analyses indicated that during sessile drop tests conducted at 1130 C for 1 hour, copper alloys containing greater than 0.98 at pct chromium form continuous reaction layers of approx. 10 microns in thickness. The reaction layers are adherent to the graphite surface. The copper wets the reaction layer to form a contact angle of 60 deg or less. X ray diffraction results indicate that the reaction layer is Cr3C2.

Devincent, Sandra M.↗

NASA NTRS1991

Experimental and analytical analysis of stress-strain behavior in a (90/0 deg)2s, SiC/Ti-15-3 laminate

The nonlinear stress strain behavior of 90 degree/0 degree sub 2s, SiC/Ti-15-3 composite laminate was numerically investigated with a finite element, unit cell approach. Tensile stress-strain curves from room temperature experiments depicted three distinct regions of deformation, and these regions were predicted by finite element analysis. The first region of behavior, which was linear elastic, occurred at low applied stresses. As applied stresses increased, fiber/matrix debonding in the 90 degree plies caused a break in the stress-strain curve and initiated a second linear region. In this second region, matrix plasticity in the 90 degree plies developed. The third region, which was typified by nonlinear, stress-strain behavior occr red at high stresses. In this region, the onset of matrix plasticity in the 0 degree plies stiffened the laminate in the direction transverse to the applied load. Metallographic sections confirmed the existence of matrix plasticity in specific areas of the structure. Finite element analysis also predicted these locations of matrix slip.

Lerch, Bradley A.↗

NASA NTRS1991

Preliminary investigation of acousto-ultrasonic evaluation of metal-matrix composite specimens

Acoustoultrasonic (AU) measurements were performed on a series of (0)8, (90)8, (+/-30)2s, (+/-60)2s, (0/90)2s, and (90/0)2s tensile specimens composed of eight laminated layers of continuous, SiC-fiber-reinforced Ti-15-3 matrix. The frequency spectrum was dominated by frequencies of longitudinal wave resonance through the thickness of the specimen at the sending transducer; signal propagation from the sender to the receiver was by shear waves. The magnitude of the frequency spectrum of the AU signal was used for calculating a stress-wave factor (SWF) based upon integrating the spectral distribution function. The SWF was sensitive to fiber-matrix debonding caused by mechanical strain and also to unconstrained thermal cycling. Damage in this metal-matrix composite structure due to tensile loading or thermal cycling was detected by this change in the magnitude of the frequency spectrum.

Kautz, Harold E.↗

NASA NTRS1991

Bond verification of space shuttle tiles using time-domain techniques

The application of a high-resolution time-domain reflectometer (TDR) to the detection of faulty bonds in space shuttle tile structures is discussed. Experimental measurements on bonded and debonded tiles are included to demonstrate the technique. Computer simulations were used to verify the experimental results. A criterion for bond verification is proposed.

Bennia, Abdelhak↗