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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
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Presentations were made concerning the development of design analysis tools for combustor liners, turbine vanes, and turbine blades. Presentations were divided into six sections: instrumentation, combustion, turbine heat transfer, structural analysis, fatigue and fracture, surface protective coatings, constitutive behavior of materials, stress-strain response and life prediction methods.
The objective of this paper is to evaluate, both experimentally and analytically, the appropriate forms of the hardening evolution equations in unified constitutive models for conditions involving nonisothermal loading. Critical experiments were performed for a cast nickel-base superalloy by using variable temperature tensile, creep, and cyclic tests in the 538-982 C temperature range. These experimental results were compared with both isothermal data and predictions of the Bodner-Partom-Bodner, 1987 elastic-viscoplastic theory to assess the effects of thermal history on constitutive behavior. The results indicate that the hardening evolution equations based on isothermal data are applicable for nonisothermal loading of these precipitation strengthened alloys. Additional thermal history effect terms in the hardening evolution equations were not required beyond those accounting for the variation of material constants with temperature. Using material constants determined solely from isothermal data, the inelastic deformation behavior of B1900 + Hf subject to thermomechanical loading were adequately predicted by the Bodner-Partom model.
Failure of a two layer plasma deposited thermal barrier coating is caused by cyclic thermal exposure and occurs by spallation of the outer ceramic layer. Spallation life is quantitatively predictable, based on the severity of cyclic thermal exposure. This paper describes and attempts to explain unusual constitutive behavior observed in the insulative ceramic coating layer, and presents details of the ceramic cracking damage accumulation process which is responsible for spallation failure. Comments also are offered to rationalize the previously documented influence of interfacial oxidation on ceramic damage accumulation and spallation life.
The volume-averaged constitutive behavior of a polymer sustaining void growth under dilational deformation is addressed. Polyvinylacetate is deformed between two aluminum beams in a double cantilever geometry and the deformations, recorded with the aid of optical interferometry, are used to deduce the stress-strain behavior of the material as it passes from small strain behavior, through the voiding process, to near-failure under exclusion of rate-sensitivity considerations.
A mathematical model based on the Euler-Bermoulli beam theory is proposed for predicting the effective Young's moduli of piecewise isotropic composite laminates with local ply curvatures in the main load-carrying layers. Strains in corrugated layers, in-phase layers, and out-of-phase layers are predicted for various geometries and material configurations by assuming matrix layers as elastic foundations of different spring constants. The effective Young's moduli measured from corrugated aluminum specimens and aluminum/epoxy specimens with in-phase and out-of-phase wavy patterns coincide very well with the model predictions. Moire fringe analysis of an in-phase specimen and an out-of-phase specimen are also presented, confirming the main assumption of the model related to the elastic constraint due to the matrix layers. The present model is also compared with the experimental results and other models, including the microbuckling models, published in the literature. The results of the present study show that even a very small-scale local ply curvature produces a noticeable effect on the mechanical constitutive behavior of a laminated composite.
The results of the present experimental study of fiber microbuckling indicates that free surfaces, ply orientations, and resin-rich regions affect the strain level for the initiation of in-plane fiber microbuckling. Nonlinear matrix constitutive behavior, in conjunction with initial fiber waviness, cause significant reductions of fiber microbuckling initiation strain level; these two factors generate nonlinear global stress-strain responses in the models. The fiber microbuckling initiation strains correlate well with the applied strain associated with asymptotic local shear strains.
The effects of the stacking sequence (orientation of plies adjacent to the 0-deg plies), free surfaces, fiber/matrix interfacial bond strength, initial fiber waviness, resin-rich regions, and nonlinear shear constitutive behavior of the resin on the initiation of fiber microbuckling in thermoplastic composites were investigated using nonlinear geometric and nonlinear 2D finite-element analyses. Results show that reductions in the resin shear tangent modulus, large amplitudes of the initial fiber waviness, and debonds each cause increases in the localized matrix shear strains; these increases lead in turn to premature initiation of fiber microbuckling. The numerical results are compared to experimental data obtained using three thermoplastic composite material systems: (1) commercial APC-2, (2) QUADRAX Unidirectional Interlaced Tape, and AU4U/PEEK.
This report summarizes the result of research done under NASA NAG3-882 Nonlinear Mechanics of Composites with Periodic Microstructure. The effort involved the development of non-finite element methods to calculate local stresses around fibers in composite materials. The theory was developed and some promising numerical results were obtained. It is expected that when this approach is fully developed, it will provide an important tool for calculating local stresses and averaged constitutive behavior in composites. NASA currently has a major contractual effort (NAS3-24691) to bring the approach developed under this grant to application readiness. The report has three sections. One, the general theory that appeared as a NASA TM, a second section that gives greater details about the theory connecting Greens functions and Fourier series approaches, and a final section shows numerical results.
When amorphous materials are quenched below their glass transition temperature, excess enthalpy is trapped in the glassy material because the viscosity is too great to allow the material to remain in volumetric equilibrium. Over time, this excess free volume is reduced as the material slowly approaches its equilibrium configuration. This process, known as physical aging, leads to substantial changes in the constitutive behavior of polymers, as has been widely discussed in the literature. Less is known about the effects of this physical aging process on fracture and fatigue properties of aged materials. The original goal of the summer was to investigate the effects of physical aging on the fracture and fatigue behavior of LaRC-TPI, a thermoplastic polyimide developed at NASA-Langley. Preliminary results are reported, although a lack of equipment availability prevented completion of this task. In the process of making specimens, the current LaRC-TPI was observed to be extremely susceptible to environmental stress cracking. A study of the unique failure patterns resulting from this degradation process in bonded joints was conducted and is also reported herein.
Polymer materials retrieved from LDEF exhibit nonlinear constitutive behavior; thus the authors present a method to characterize nonlinear viscoelastic behavior using measurements from dynamic (oscillatory) mechanical tests. Frequency-derived measurements are transformed into time-domain properties providing the capability to predict long term material performance without a lengthy experimentation program. Results are presented for thin-film high-performance polymer materials used in the fabrication of high-altitude scientific balloons. Predictions based upon a linear test and analysis approach are shown to deteriorate for moderate to high stress levels expected for extended applications. Tests verify that nonlinear viscoelastic response is induced by large stresses. Hence, an approach is developed in which the stress-dependent behavior is examined in a manner analogous to modeling temperature-dependent behavior with time-temperature correspondence and superposition principles. The development leads to time-stress correspondence and superposition of measurements obtained through dynamic mechanical tests. Predictions of material behavior using measurements based upon linear and nonlinear approaches are compared with experimental results obtained from traditional creep tests. Excellent agreement is shown for the nonlinear model.
This study examines the delamination evolution, under quasi-static conditions, of laminated polymeric composites with mechanically nonlinear resin rich interfaces. The constitutive behavior of the interface is represented by two models developed by Needleman and Tvegaard. These models assumed that the interfacial tractions, a function of only the interfacial displacement, will behave similarly to the interatomic forces generated during the interatomic seperation. The interface material's parameters control the load at which the delamination growth initiates and the final delamination size. A wide range of damage accumulation responses have been obtained by varying the model parameters. These results show that Tvergaard's model has been found to be better suited of the two models in predicting damage evolution for the configurations examined.
The axisymmetric electrohydrodynamic interaction between two spherical emulsion drops has been examined, using the leaky dielectric model to represent the constitutive behavior of the liquid phases. The results follow from the general solutions in bispherical coordinates to the Laplace equation for the electric potential and the Stokes equations for the velocity field. For drops of similar composition, the electrical interactions induced between the drops by the imposition of the electric field are always attractive, meaning they favor coalescence of the drop pair. The hydrodynamic interactions, however, are not always favorable and, indeed, are shown in certain circumstances to drive the drops apart.
A comprehensive experimental investigation was conducted to investigate the effects of loading condition and confining pressure on strength properties and instability phenomena in sands. A uniform sub-rounded to rounded natural silica sand known as F-75 Ottawa sand was used in the investigation. The results of a series on Conventional Triaxial Compression (CTC) experiments tested under very low confining pressures (0.05 - 1.30) kPa tested in a Microgravity environment abroad the NASA Space Shuttle are presented in addition to the results similar specimens tested in terrestrial laboratory to investigate the effect of confining pressure on the constitutive behavior of sands. The behavior of the CTC experiments is compared with the results of Plane Strain (PS) experiments. Computed tomography and other digital imaging techniques were used to study the development and evolution of shear bands.
Carbon nanotubes naturally tend to form crystals in the form of hexagonally packed bundles or ropes that should exhibit a transversely isotropic constitutive behavior. Although the intratube axial stiffness is on the order of 1 TPa due to a strong network of delocalized bonds, the intertube cohesive strength is orders of magnitude less controlled by weak, nonbonding van der Waals interactions. An accurate determination of the effective mechanical properties of nanotube bundles is important to assess potential structural applications such as reinforcement in future composite material systems. A direct method for calculating effective material constants is developed in the present study. The Lennard-Jones potential is used to model the nonbonding cohesive forces. A complete set of transverse moduli are obtained and compared with existing data.
A non-linear, continuum-based constitutive model is developed for carbon nanotube materials in which bundles of aligned carbon nanotubes have varying amounts of crosslinks between the nanotubes. The model accounts for the non-linear elastic constitutive behavior of the material in terms of strain, and is developed using a thermodynamic energy approach. The model is used to examine the effect of the crosslinking on the overall mechanical properties of variations of the crosslinked carbon nanotube material with varying degrees of crosslinking. It is shown that the presence of the crosslinks has significant effects on the mechanical properties of the carbon nanotube materials. An increase in the transverse shear properties is observed when the nanotubes are crosslinked. However, this increase is accompanied by a decrease in axial mechanical properties of the nanotube material upon crosslinking.
Enhanced finite elements are elements with an embedded analytical solution which can capture detailed local fields, enabling more efficient, mesh-independent finite element analysis. In the present study, an enhanced finite element is applied to generate a general framework capable of modeling an array of joint types. The joint field equations are derived using the principle of minimum potential energy, and the resulting solutions for the displacement fields are used to generate shape functions and a stiffness matrix for a single joint finite element. This single finite element thus captures the detailed stress and strain fields within the bonded joint, but it can function within a broader structural finite element model. The costs associated with a fine mesh of the joint can thus be avoided while still obtaining a detailed solution for the joint. Additionally, the capability to model non-linear adhesive constitutive behavior has been included within the method, and progressive failure of the adhesive can be modeled by using a strain-based failure criteria and re-sizing the joint as the adhesive fails. Results of the model compare favorably with experimental and finite element results.
A novel and precise characterization of the constitutive behavior of solid and laminated research-grade, polycrystalline Galfenol (Fe81:6Ga18:4) under under quasi-static (1 Hz) and dynamic (4 to 1000 Hz) stress loadings was recently conducted by the authors. This paper summarizes the characterization by focusing on the experimental design and the dynamic sensing response of the solid Galfenol specimen. Mechanical loads are applied using a high frequency load frame. The dynamic stress amplitude for minor and major loops is 2.88 and 31.4 MPa, respectively. Dynamic minor and major loops are measured for the bias condition resulting in maximum, quasi-static sensitivity. Three key sources of error in the dynamic measurements are accounted for: (1) electromagnetic noise in strain signals due to Galfenol's magnetic response, (2) error in load signals due to the inertial force of fixturing, and (3) time delays imposed by conditioning electronics. For dynamic characterization, strain error is kept below 1.2 % of full scale by wiring two collocated gauges in series (noise cancellation) and through lead wire weaving. Inertial force error is kept below 0.41 % by measuring the dynamic force in the specimen using a nearly collocated piezoelectric load washer. The phase response of all conditioning electronics is explicitly measured and corrected for. In general, as frequency increases, the sensing response becomes more linear due to an increase in eddy currents. The location of positive and negative saturation is the same at all frequencies. As frequency increases above about 100 Hz, the elbow in the strain versus stress response disappears as the active (soft) regime stiffens toward the passive (hard) regime.
Galfenol has the potential to transform the smart materials industry by allowing for the development of multifunctional, load-bearing devices. One of the primary technical challenges faced by this development is the very limited experimental data on Galfenol's frequency-dependent response to dynamic stress, which is critically important for the design of such devices. This report details a novel and precise characterization of the constitutive behavior of polycrystalline Galfenol (Fe81.6Ga18.4) under quasi-static (1 Hz) and dynamic (4 to 1000 Hz) stress loadings. Mechanical loads are applied using a high-frequency load frame. Quasi-static minor and major hysteresis loop measurements of magnetic flux density and strain are presented for constant electromagnet currents (0 to 1.1 A) and constant magnetic fields 0 to 14 kA/m (0 to 180 Oe). The dynamic stress amplitude for minor and major loops is 2.88 and 31.4 MPa (418 and 4550 psi), respectively. Quasi-static material properties closely match published values for similar Galfenol materials. Quasi-static actuation responses are also measured and compared to quasi-static sensing responses; the high degree of reversibility seen in the comparison is consistent with published measurements and modeling results. Dynamic major and minor loops are measured for dynamic stresses up to 31 MPa (4496 psi) and 1 kHz, and the bias condition resulting in maximum, quasi-static sensitivity. Eddy current effects are quantified by considering solid and laminated Galfenol rods. Three key sources of error in the dynamic measurements are accounted for: (1) electromagnetic noise in strain signals due to Galfenol's magnetic response, (2) error in load signals due to the inertial force of fixturing, and (3) phase misalignment between signals due to conditioning electronics. For dynamic characterization, strain error is kept below 1.2 percent of full scale by wiring two collocated gauges in series (noise cancellation) and through leadwire weaving. Inertial force error is kept below 0.41 percent by measuring the dynamic force in the specimen using a nearly collocated piezoelectric load washer. The phase response of all conditioning electronics is explicitly measured and corrected for. In general, as frequency is increased, the sensing response becomes more linear because of an increase in eddy currents. As frequency increases above approximately 100 Hz, the elbow in the strain-versus-stress response disappears as the active (soft) regime stiffens toward the passive (hard) regime. Under constant-field conditions, the loss factors of the solid rod peak between 200 and 600 Hz, rather than exhibiting a monotonic increase. Compared to the solid rod, the laminated rod exhibits much slower increases in hysteresis with frequency, and its quasi-static behavior extends to higher frequencies. The elastic modulus of the laminated rod decreases between 100 and 300 Hz; this trend is currently unexplained.