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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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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Inelastic work and thermomechanical coupling in viscoplasticity

The inelastic deformation of engineering alloys was recently shown to be rate (time)-dependent at room temperature. Experiments reported herein demonstrate that inelastic strain path length is a suitable measure for the modeling of work hardening for these materials, whereas inelastic work is not appropriate. The measured deformation induced temperature change is well predicted by a previously proposed theory of thermomechanical coupling. It has been derived by neglecting the stored energy of cold work and by assuming tha the internal energy expression for thermoelasticity is valid even when the mechanical deformation is inelastic.

Krempl, E.↗
NASA NTRS1985

Viscoplasticity based on overstress with a differential growth law for the equilibrium stress

Two coupled, nonlinear differential equations are proposed for the modeling of the elastic and rate (time) dependent inelastic behavior of structural metals in the absence of recovery and aging. The structure of the model is close to the unified theories but contains essential differences. It is shown that the model reproduces almost elastic regions upon initial loading and in the unloading regions of the hysteresis loop. Under loading, unloading and reloading in strain control the model simulated the experimentally observed sharp transition from nearly elastic to inelastic behavior. When a formulation akin to existing unified theories is adopted the almost elastic regions reduce the points and the transition upon reloading is very gradual. For different formulations the behavior under sudden in(de)creases of the strain rate by two orders of magnitude is simulated by numerical experiments and differences are noted. The model represents cyclically neutral behavior and contains three constants and two positive, decreasing functions. The determination of constants and functions from monotonic loading with strain rate changes and relaxation periods is described.

Krempl, E.↗
NASA NTRS1986

Viscoplasticity based on overstress with a differential growth law for the equilibrium stress

Two coupled, nonlinear differential equations are proposed for the modeling of the elastic and rate (time) dependent inelastic behavior of structural metals in the absence of recovery and aging. The structure of the model is close to the unified theories but contains essential differences. It is shown that the model reproduces almost elastic regions upon initial loading and in the unloading regions of the hysteresis loop. Under loading, unloading and reloading in strain control the model simulated the experimentally observed sharp transition from nearly elastic to inelastic behavior. When a formulation akin to existing unified theories is adopted the almost elastic regions reduce the points and the transition upon reloading is very gradual. For different formulations the behavior under sudden in(de)creases of the strain rate by two orders of magnitude is simulated by numerical experiments and differences are noted. The model represents cyclically neutral behavior and contains three constants and two positive, decreasing functions. The determination of constants and functions from monotonic loading with strain rate changes and relaxation periods is described.

Krempl, E.↗
NASA NTRS1985

Experimental determination of flow potential surfaces supporting a multiaxial formulation of viscoplasticity

Details of the experimental procedure followed in a preliminary set of experiments conducted on the representative alloy type 316 stainless steel are given. Although primary interest is in the determination of initial and subsequent surfaces of constant inelastic strain rate (SCISR) at high homologous temperatures (e.g., approx. 0.5). The preliminary tests were conducted at lower temperatures (approx. 0.2) largely because of experimental convenience. Nevertheless, significant time-dependent response at the lower temperature allowed SCISR measurements to be carried out and an evaluation of the experimental technique made. Results of the preliminary tests are presented showing three of a family of initial SCISR's, including strain rate vectors for assessing the condition of normality. Conclusions are drawn concerning the feasibility of the experimental technique, the nature of the measured SCISR's and an assesssment of the normality condition. Finally, a discussion is given of future research.

Ellis, J. R.↗
NASA NTRS1988

Applications of elastic-viscoplastic constitutive models in dynamic analyses of crack run-arrest events

Applications of nonlinear techniques to the first series of six HSST wide-plate crack-arrest tests that were performed are described. The experiments include crack initiations at low temperatures and relatively long (20 cm) cleavage propagation phases which are terminated by arrest in high temperature regions. Crack arrest are then followed by ductile tearing events. Consequently, the crack front regions are exposed to wide ranges of strain rates and temperatures.

Bass, B. R.↗
NASA NTRS1988

Some implications for cyclic plastic and viscoplastic equations based on nonproportional loading experiments

From cyclic, strain controlled, nonproportional tests on type 304 stainless steel and Hastelloy-X, the following statements may be made: (1) A dynamic recovery term is essential to properly model the backstress evolution. (2) From analysis of Hastelloy-X data obtained at 649 C, the inelastic strain rate appears to be a satisfactory directional index for direct hardening, but the backstress appears to be an inappropriate directional index of dynamic recovery. (3) Sinusoidal, 90 deg out-of-phase axial torsional tests can be very useful in aiding determination of backstress evolution functions, including both directional indices and scalar hardening functions, by virtue of the associated approximately constant magnitudes of overstress, inelastic strain rate, and effective stress. Such tests have previously been associated with the study of nonproportional hardening effects but have more far ranging applications.

Mcdowell, D. L.↗
NASA NTRS1987

Unified viscoplastic constitutive equations and their applications

Unified constitutive equations for time- and temperature-dependent metallic plastic deformation have been applied in FEM simulations of forming processes; increasingly powerful computational tools and physical models are being used to numerically model complex engineering problems. Once confidence has been gained through adequate verification, these numerical models will increasingly replace experimental models. Attention is presently given to the contributions made by physical metallurgy, continuum mechanics, and computational mechanics.

Lindholm, U. S.↗
NASA NTRS1988

Considerations in development and implementation of elasto-viscoplastic constitutive model for high temperature applications

The prediction of inelastic behavior of metallic materials at elevated temperatures has increased in importance in recent years. The operating conditions within the hot section of a rocket motor or a modern gas turbine engine present an extremely harsh thermomechanical environment. Large thermal transients are induced each time the engine is started or shut down. Additional thermal transients from an elevated ambient occur whenever the engine power level is adjusted to meet flight requirements. The structural elements employed in such hot sections, as well as any engine components located therein, must be capable of withstanding such extreme conditions. Failure of a component would, due to the critical nature of the hot section, lead to an immediate and catastrophic loss in power. Consequently, assuring satisfactory long term performance for such components is a major concern. Nonisothermal loading of structures often causes excursion of stress well into the inelastic range. Moreover, the influence of geometry changes on the response is also significant in most cases. Therefore, both material and geometric nonlinear effects are considered.

Riff, Richard↗
NASA NTRS1990

Experimental and computational study of the viscoplastic response of high temperature structures

The first nine months of experimental and computational studies of the thermal-structural behavior of thin panels subjected to localized heating are described. Initial experimental studies have focused on developing an experimental set-up with well-defined thermal-structure boundary conditions. Preliminary tests with a Heldenfels panel have demonstrated out of plane bending (thermal buckling) due to panel initial imperfections. Initial computational studies have focused on: (1) validation of a thermoviscoplastic code to predict thermal stresses in the unbuckled panel, and (2) investigating in-plane stresses for test panels under transient thermal loading. Plans for future research are described.

Thornton, Earl A.↗
NASA NTRS1991

Experimental study of the viscoplastic response of high temperature structures

Computational and experimental studies of the thermal structural behavior of thin panels subjected to localized heating are described. Three research tasks are described: (1) development of a finite element thermoviscoplastic computational approach; (2) experimental determination of material parameters for Bodner-Partom constitutive models of panel materials; and (3) experimental study of 'Heldenfels' panels subjected to intense local heating. Recent progress in each task is reviewed. Development of a new experimental set-up for the panel tests is described in detail and preliminary test results are presented.

Coyle, Marshall F.↗
NASA NTRS1991

Viscoplastic response of structures for intense local heating

A thermoviscoplastic finite element method employing the Bodner-Partom constitutive model is used to investigate the response of simplified thermal-structural models to intense local heating. With rapid rises of temperature, the nickel alloy structures display initially higher yield stresses due to strain rate effects. As temperatures approach elevated values, yield stress and stiffness degrade rapidly and pronounced plastic deformation occurs.

Thornton, Earl A.↗
NASA NTRS1991

Effects of elevated temperature on the viscoplastic modeling of graphite/polymeric composites

To support the development of new materials for the design of next generation supersonic transports, a research program is underway at NASA to assess the long term durability of advanced polymer matrix composites (PMC's). One of main objectives of the program was to explore the effects of elevated temperature (23 to 200 C) on the constitutive model's material parameters. To achieve this goal, test data on the observed nonlinear, stress-strain behavior of IM7/5260 and IM7/8320 composites under tension and compression loading were collected and correlated against temperature. These tests, conducted under isothermal conditions using variable strain rates, included such phenomena as stress relaxation and short term creep. The second major goal was the verification of the model by comparison of analytical predictions and test results for off axis and angle ply laminates. Correlation between test and predicted behavior was performed for specimens of both material systems over a range of temperatures. Results indicated that the model provided reasonable predictions of material behavior in load or strain controlled tests. Periods of loading, unloading, stress relaxation, and creep were accounted for.

Gates, Thomas S.↗
NASA NTRS1991

On the thermodynamic framework of generalized coupled thermoelastic-viscoplastic-damage modeling

A complete potential based framework using internal state variables is put forth for the derivation of reversible and irreversible constitutive equations. In this framework, the existence of the total (integrated) form of either the (Helmholtz) free energy or the (Gibbs) complementary free energy are assumed a priori. Two options for describing the flow and evolutionary equations are described, wherein option one (the fully coupled form) is shown to be over restrictive while the second option (the decoupled form) provides significant flexibility. As a consequence of the decoupled form, a new operator, i.e., the Compliance operator, is defined which provides a link between the assumed Gibb's and complementary dissipation potential and ensures a number of desirable numerical features, for example the symmetry of the resulting consistent tangent stiffness matrix. An important conclusion reached, is that although many theories in the literature do not conform to the general potential framework outlined, it is still possible in some cases, by slight modifications of the used forms, to restore the complete potential structure.

Arnold, S. M.↗
NASA NTRS1994

Viscoplastic response of structures for intense local heating

A thermoviscoplastic finite element method employing the Bodner-Partom constitutve model is used to investigate the response of simplified thermal-structural models to intense local heating. The computational method formulates the problem in rate and advances the solution in time by numerical integration. The thermoviscoplastic response of simplified structures with prescribed temperatures is investigated. With rapid rises of temperature, the nickel alloy structures display initially higher yield stresses due to strain rate effects. As temperatures approach elevated values, yield stress and stiffness degrade rapidly and pronounced plastic deformation occurs.

Thornton, Earl A.↗
NASA NTRS1999

Viscoplastic Constitutive Theory Demonstrated for Monolithic Ceramic Materials

Development of accurate three-dimensional (multiaxial) inelastic stress-strain models is critical in utilizing advanced ceramics for challenging 21st century high-temperature structural applications. The current state of the art uses elastic stress fields as a basis for both subcritical crack growth and creep life prediction efforts aimed at predicting the time dependent reliability response of ceramic components subjected to elevated service temperatures. However, to successfully design components that will meet tomorrow's challenging requirements, design engineers must recognize that elastic predictions are inaccurate for these materials when subjected to high-temperature service conditions such as those encountered in advanced heat engine components. Analytical life prediction methodologies developed for advanced ceramics and other brittle materials must employ accurate constitutive models that capture the inelastic response exhibited by these materials at elevated service temperatures. A constitutive model recently developed at the NASA Lewis Research Center helps address this issue by accounting for the time-dependent (inelastic) material deformation phenomena (e.g., creep, rate sensitivity, and stress relaxation) exhibited by monolithic ceramics exposed to high-temperature service conditions. In addition, the proposed formulation is based on a threshold function that is sensitive to hydrostatic stress and allows different behavior in tension and compression, reflecting experimental observations obtained for these material systems.

Janosik, Lesley A.↗
NASA NTRS1985

Thermodynamically consistent constitutive equations for nonisothermal large strain, elasto-plastic, creep behavior

The paper is concerned with the development of constitutive relations for large nonisothermal elastic-viscoplastic deformations for metals. The kinematics of elastic-plastic deformation, valid for finite strains and rotations, is presented. The resulting elastic-plastic uncoupled equations for the deformation rate combined with use of the incremental elasticity law permits a precise and purely deductive development of elastic-viscoplastic theory. It is shown that a phenomenological thermodynamic theory in which the elastic deformation and the temperature are state variables, including few internal variables, can be utilized to construct elastic-viscoplastic constitutive equations, which are appropriate for metals. The limiting case of inviscid plasticity is examined.

Riff, R.↗