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NASA NTRS1989

Markov reliability models for digital flight control systems

The reliability of digital flight control systems can often be accurately predicted using Markov chain models. The cost of numerical solution depends on a model's size and stiffness. Acyclic Markov models, a useful special case, are particularly amenable to efficient numerical solution. Even in the general case, instantaneous coverage approximation allows the reduction of some cyclic models to more readily solvable acyclic models. After considering the solution of single-phase models, the discussion is extended to phased-mission models. Phased-mission reliability models are classified based on the state restoration behavior that occurs between mission phases. As an economical approach for the solution of such models, the mean failure rate solution method is introduced. A numerical example is used to show the influence of fault-model parameters and interphase behavior on system unreliability.

Mcgough, John↗

NASA NTRS1989

Computations of turbulent evaporating sprays

A computational study of turbulent evaporating sprays is reported. The major focus is to examine the sensitivity of the vaporization behavior of turbulent sprays to the transient liquid-phase processes. Three models considered to represent these processes are the thin skin, infinite diffusion, and diffusion limit models. Favre-averaged equations with k-epsilon-g turbulence model are employed for the gas phase. The Lagrangian approach with a stochastic separated flow method is used for the liquid phase where the effects of gas turbulence on droplet trajectories and interphase transport rates are considered using random-walk computations. Also the variable-property effects are considered in detail. Results indicate that, depending upon the boiling temperature and heat of vaporization of the fuel considered, the vaporization behavior of turbulent sprays may be quite sensitive to the modeling of transient liquid-phase processes. Thus, it is important that for most hydrocarbon fuels these processes be adequately represented in any comprehensive spray computations. The present results also provide further support to the conclusions of earlier studies which have been based on simplified spray configurations.

Aggarwal, S. K.↗

NASA NTRS1990

Demonstration of capabilities of high temperature composites analyzer code HITCAN

The capabilities a high temperature composites analyzer code, HITCAN which predicts global structural and local stress-strain response of multilayered metal matrix composite structures, are demonstrated. The response can be determined both at the constituent (fiber, matrix, and interphase) and the structure level and includes the fabrication process effects. The thermo-mechanical properties of the constituents are considered to be nonlinearly dependent on several parameters including temperature, stress, and stress rate. The computational procedure employs an incremental iterative nonlinear approach utilizing a multifactor-interactive constituent material behavior model. Various features of the code are demonstrated through example problems for typical structures.

Singhal, Surendra N.↗

NASA NTRS1990

On the formation and expansion of H II regions

The evolution of H II regions in spherical clouds with small, constant-density cores and power-law density distributions r exp -w outside the core is described analytically. It is found that there is a critical exponent above which the cloud becomes completely ionized. Its value in the formation phase depends on the initial conditions, but it has a well-defined value w(crit) = 3/2 during the expansion phase. For w less than w(crit), the radius of the H II region grows at a given rate, while neutral mass accumulates in the interphase between the ionization and shock fronts. For w = w(crit), the fronts move together without mass accumulation. Cases with w greater than w(crit) lead to the champagne phase: once the cloud is fully ionized, the expansion becomes supersonic. For self-gravitating disks without magnetic fields, the main features include a new 'variable-size' stage. The initial shape of the H II region has a critical point beyond which the disk becomes completely ionized.

Franco, Jose↗

NASA NTRS1990

Hypergolic bipropellant spray combustion and flow modelling in rocket engines

A predictive tool for hypergolic bipropellant spray combustion and flow evolution in small rocket combustion chambers is described. It encompasses a computational technique for the gas-phase governing equations, a discrete particle method for liquid bipropellant sprays, and constitutive models for combustion chemistry, interphase exchanges, and unlike impinging hypergolic spray interactions. Emphasis is placed on the phenomenological modeling of the hypergolic liquid bipropellant gasification processes. Sample computations with the N2H4-N2O4 propellant system are given in order to show some of the capabilities and inadequacies of this tool.

Larosiliere, Louis M.↗

NASA NTRS1990

Understanding the interdiffusion behavior and determining the long term stability of tungsten fiber reinforced niobium-base matrix composite systems

The long term interdiffusional stability of tungsten fiber reinforced niobium alloy composites is addressed. The matrix alloy that is most promising for use as a high temperature structural material for reliable long-term space power generation is Nb1Zr. As an ancillary project to this program, efforts were made to assess the nature and kinetics of interphase reaction between selected beryllide intermetallics and nickel and iron aluminides.

Tien, John K.↗

NASA NTRS1991

HITCAN for actively cooled hot-composite thermostructural analysis

A computer code, high temperature composite analyzer (HITCAN), was developed to analyze/design hot metal matrix composite structures. HITCAN is a general purpose code for predicting the global structural and local stress-strain response of multilayered (arbitrarily oriented) metal matrix structures both at the constituent (fiber, matrix, and interphase) and the structural level, including the fabrication process effects. The thermomechanical properties of the constituents are considered to be nonlinearly dependent on several parameters, including temperature, stress, and stress rate. The computational procedure employs an incremental iterative nonlinear approach utilizing a multifactor-interaction material behavior model, i.e., the material properties are expressed in terms of a product of several factors that affect the properties. HITCAN structural analysis capabilities (static, load stepping - a multistep static analysis with material properties updated at each step, modal, and buckling) for cooled hot structures are demonstrated through a specific example problem.

Chamis, C. C.↗

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 NTRS1990

Computational simulation of high-temperature metal matrix composites cyclic behavior

A procedure was developed and is described that can be used to computationally simulate the cyclic behavior of high-temperature metal matrix composites (HTMMC) and its degradation effects on the structural response. This procedure consists of HTMMC mechanics coupled with a multifactor-interaction constituent material relationship and with an incremental iterative nonlinear analysis. The procedure is implemented in a computer code that can be used to computationally simulate the thermomechanical behavior of HTMMC starting from the fabrication process and proceeding through thermomechanical cycling, accounting for the interface/interphase region. Results show that combined thermal/mechanical cycling, the interphase, and in situ matrix properties have significant effects on the structural integrity of HTMMC.

Chamis, Christos C.↗

NASA NTRS1986

Behavior of centrosomes during fertilization and cell division in mouse oocytes and in sea urchin eggs

The behavior of centrosomes during the stages of fertilization and cell division in mouse oocytes and in sea urchin eggs was monitored in an immunofluorescence microscope, using autoimmune centrosomal antiserum derived from a patient with scleroderma to label the centrosomal material. These observations showed that centrosomes reproduce during the interphase and aggregate and separate during cell mitosis. Results supported the hypothesis of Mazia (1984), who proposed that centrosomes are 'flexible bodies'. It was also found that, while the sea urchin centrosomes are paternally inherited as was initially proposed by Bovery (1904), the mouse centrosomes are of maternal origin.

Schatten, Heide↗

NASA NTRS1991

Nonlinear analysis of longitudinal-mode liquid propellant rocket combustion instability

A computational technique for the nonlinear analysis of longitudinal-mode liquid propellant rocket combustion instability is examined based on the unsteady, quasi-one-dimensional Euler equations with appropriate source terms introduced to account for interphase transport coupling with the spray. The method is first assessed for unsteady, nonreacting, isentropic duct flow with specified admittances at the outflow and inflow boundaries. For small amplitude disturbances, numerical results based on a sufficient number of grid points compare favorably with predictions from a small-disturbance linearized analysis.

Jeng, San-Mou↗

NASA NTRS1988

Particle-laden weakly swirling free jets: Measurements and predictions

A theoretical and experimental investigation of particle-laden, weakly swirling, turbulent free jets was conducted. Glass particles, having a Sauter mean diameter of 39 microns, with a standard deviation of 15 microns, were used. A single loading ratio (the mass flow rate of particles per unit mass flow rate of air) of 0.2 was used in the experiments. Measurements are reported for three swirl numbers, ranging from 0 to 0.33. The measurements included mean and fluctuating velocities of both phases, and particle mass flux distributions. Measurements were also completed for single-phase non-swirling and swirling jets, as baselines. Measurements were compared with predictions from three types of multiphase flow analysis, as follows: (1) locally homogeneous flow (LHF) where slip between the phases was neglected; (2) deterministic separated flow (DSF), where slip was considered but effects of turbulence/particle interactions were neglected; and (3) stochastic separated flow (SSF), where effects of both interphase slip and turbulence/particle interactions were considered using random sampling for turbulence properties in conjunction with random-walk computations for particle motion. Single-phase weakly swirling jets were considered first. Predictions using a standard k-epsilon turbulence model, as well as two versions modified to account for effects of streamline curvature, were compared with measurements. Predictions using a streamline curvature modification based on the flux Richardson number gave better agreement with measurements for the single-phase swirling jets than the standard k-epsilon model. For the particle-laden jets, the LHF and DSF models did not provide very satisfactory predictions. The LHF model generally overestimated the rate of decay of particle mean axial and angular velocities with streamwise distance, and predicted particle mass fluxes also showed poor agreement with measurements, due to the assumption of no-slip between phases. The DSF model also performed quite poorly for predictions of particle mass flux because turbulent dispersion of the particles was neglected. The SSF model, which accounts for both particle inertia and turbulent dispersion of the particles, yielded reasonably good predictions throughout the flow field for the particle-laden jets.

Bulzan, Daniel L.↗

NASA NTRS1991

Thermally-driven microfracture in high temperature metal matrix composites

Microfracture (fiber/matrix fracture, interphase debonding and interply delamination) in high temperature metal matrix composites (HTMMC), subjected to thermal loading, is computationally simulated. Both unidirectional and crossply SiC/Ti15 composites are evaluated for microfracture driven by thermal loads, using multicell finite element models. Results indicate that under thermal loads alone, microfracture propagation is not as sensitive as it is under mechanical loads.

Mital, Subodh K.↗

NASA NTRS1992

Towards the development of micromechanics equations for ceramic matrix composites via fiber substructuring

A generic unit cell model which includes a unique fiber substructuring concept is proposed for the development of micromechanics equations for continuous fiber reinforcement ceramic composites. The unit cell consists of three constituents: fiber, matrix, and an interphase. In the present approach, the unit cell is further subdivided into several slices and the equations of micromechanics are derived for each slice. These are subsequently integrated to obtain ply level properties. A stand alone computer code containing the micromechanics model as a module is currently being developed specifically for the analysis of ceramic matrix composites. Towards this development, equivalent ply property results for a SiC/Ti-15-3 composite with 0.5 fiber volume ratio are presented and compared with those obtained from customary micromechanics models to illustrate the concept. Also, comparisons with limited experimental data for the ceramic matrix composite, SiC/RBSN (Reaction Bonded Silicon Nitride) with a 0.3 fiber volume ratio are given to validate the concepts.

Murthy, P. L. N.↗

NASA NTRS1992

Direct numerical simulation of a combusting droplet with convection

The evaporation and combustion of a single droplet under forced and natural convection was studied numerically from first principles using a numerical scheme that solves the time-dependent multiphase and multispecies Navier-Stokes equations and tracks the sharp gas-liquid interface cutting across an arbitrary Eulerian grid. The flow fields both inside and outside of the droplet are resolved in a unified fashion. Additional governing equations model the interphase mass, energy, and momentum exchange. Test cases involving iso-octane, n-hexane, and n-propanol droplets show reasonable comparison rate, and flame stand-off distance. The partially validated code is, thus, readied to be applied to more demanding droplet combustion situations where substantial drop deformation render classical models inadequate.

Liang, Pak-Yan↗

NASA NTRS1992

Computational simulation of matrix micro-slip bands in SiC/Ti-15 composite

Computational simulation procedures are used to identify the key deformation mechanisms for (0)(sub 8) and (90)(sub 8) SiC/Ti-15 metal matrix composites. The computational simulation procedures employed consist of a three-dimensional finite-element analysis and a micromechanics based computer code METCAN. The interphase properties used in the analysis have been calibrated using the METCAN computer code with the (90)(sub 8) experimental stress-strain curve. Results of simulation show that although shear stresses are sufficiently high to cause the formation of some slip bands in the matrix concentrated mostly near the fibers, the nonlinearity in the composite stress-strain curve in the case of (90)(sub 8) composite is dominated by interfacial damage, such as microcracks and debonding rather than microplasticity. The stress-strain curve for (0)(sub 8) composite is largely controlled by the fibers and shows only slight nonlinearity at higher strain levels that could be the result of matrix microplasticity.

Mital, S. K.↗

NASA NTRS1992

Improving the low temperature ductility of NiAl

The intermetallic NiAl exhibits excellent potential as a structural material for application in air-breathing and single-stage-to-orbit engines. Unfortunately NiAl is brittle at ambient temperature, which is partly attributed to the lack of five independent slip systems as required by von Mises criterion for uniform, volume conserving deformation since the operative group of zone axes (100) slip offers only three independent slip systems. Isostructural FeAl, however, deforms by group of zone axes (111) slip at room temperature which provides five independent slip systems. Thus, it was decided to macroalloy NiAl with Fe to promote group of zone axes (111) slip thereby possibly improving the ductility. For Ni - 30 at. (percent) Al - 20 at. (percent) Fe having essentially bcc microstructure indicated up to 6 percent tensile elongation combined with high yield strength (about 800 MPa). The results compare favorably with those of stoichiometric Ni - 50 at. (percent) Al which, however, deforms by group of zone axes (100) slip at room temperature. The multi-phase alloy approach has proven even more successful for Ni - 20 at. (percent) Al - 30 at. (percent) Fe. The high temperature ductility of the multi-phase alloy was attributed to deformation transfer across interphase boundaries and the crack stopping action of the constituent gamma/gamma' phase. While the constituent phase became increasingly brittle, the beta' phase became more ductile at elevated temperatures. Thus the multi-phase alloy did not exhibit any elevated temperature embrittlement. Similarly, while the beta' phase became weak at high temperatures, the gamma/gamma' phase exhibited better strength retention leading to the improved elevated temperature strength of the multi-phase alloy. Thus, the multi-phase alloy benefits from both its constituent phases, with each phase alleviating the disadvantages associated with the other phase over any temperature range. The multi-phase alloy approach is suggested as a possible approach to designing intermetallic-based alloys.

Guha, Sumit↗

NASA NTRS1991

Computational simulation of microfracture in high temperature metal matrix composites

A computational simulation procedure, using three-dimensional finite element method and global strain energy release rates, is described to predict the microfracture process and identify/quantify the hierarchy of respective fracture modes in metal matrix composites. The procedure is used to predict the microfracture in unidirectional composites under longitudinal loads and accounts for the interphase strength in the in situ state. A novel procedure to computationally simulate the fiber pushthrough process is also described. In this simulation, the interface material is replaced by an anisotropic material with greatly reduced shear modulus in order to simulate the fiber pushthrough process using a linear analysis. Such a procedure is easily implemented and is computationally very effective.

Mital, Subodh K.↗