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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 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1998

Accelerated Strength Testing of Thermoplastic Composites

Constant ramp strength tests on unidirectional thermoplastic composite specimens oriented in the 90 deg. direction were conducted at constant temperatures ranging from 149 C to 232 C. Ramp rates spanning 5 orders of magnitude were tested so that failures occurred in the range from 0.5 sec. to 24 hrs. (0.5 to 100,000 MPa/sec). Below 204 C, time-temperature superposition held allowing strength at longer times to be estimated from strength tests at shorter times but higher temperatures. The data indicated that a 50% drop in strength might be expected for this material when the test time is increased by 9 orders of magnitude. The shift factors derived from compliance data applied well to the strength results. To explain the link between compliance and strength, a viscoelastic fracture model was investigated. The model, which used compliance as input, was found to fit the strength data only if the critical fracture energy was allowed to vary with temperature reduced stress rate. This variation in the critical parameter severely limits its use in developing a robust time-dependent strength model. The significance of this research is therefore seen as providing both the indication that a more versatile acceleration method for strength can be developed and the evidence that such a method is needed.

Reeder, J. R.↗
NASA NTRS2005

Deformation Response and Life of Metallic Composites

The project was initially funded for one year (for $100,764) to investigate the potential of particulate reinforced metals for aeropropulsion applications and to generate fatigue results that quantify the mean stress effect for a titanium alloy matrix material (TIMETAL 21S). The project was continued for a second year (for $85,000) to more closely investigate cyclic deformation, especially ratcheting, of the titanium alloy matrix at elevated temperature. Equipment was purchased (for $19,000) to make the experimental program feasible; this equipment included an extensometer calibrator and a multi-channel signal conditioning amplifier. The project was continued for a third year ($50,000) to conduct cyclic relaxation experiments aimed at validating the elastic-viscoelastic-viscoplastic model that NASA GRC had developed for the titanium alloy. Finally, a one-year no cost extension was granted to enable continued analysis of the experimental results and model comparisons.

Lissenden, Cliff J.↗
NASA NTRS2007

Post-glacial Sediment Load and Subsidence in Coastal Louisiana

Sea level rise in the Gulf of Mexico has occurred at a rate of 1.8-2.2 mm/yr during the 20th century, or nearly the same as observed globally due to combined steric and water mass changes. Tide gauges in coastal Louisiana, however, record a substantially larger rate of rise and while a number of causal mechanisms may be responsible, their specific contribution is poorly understood. Using a realistic viscoelastic Earth model, detailed geologic parameters for south Louisiana and new GPS data, we demonstrate that Holocene sedimentary loading in the Gulf and Mississippi River delta is capable of contributing to 1-8 mm/yr of subsidence over areas of 30-0.75 x 10(exp 3) km(exp 2).

rheology mantle↗
NASA NTRS1992

Postseismic deformation and stress diffusion due to viscoelasticity and comments on the modified Elsasser model

Finite element techniques have been used to investigate the far-field deformation and stress changes due to asthenospheric viscoelastic relaxation following a dip-slip earthquake. The diffusion of extensional stress toward the subduction zone following a thrust earthquake on land is qualitatively consistent with the modified Elsasser model as proposed by Rydelek and Sacks (1988, 1990) to explain the coupled occurrence of land and sea earthquakes near Japan. However, the magnitude of the diffusing tensional signal is significantly smaller. The nominal model consists of a partially faulted elastic lithosphere overlying a viscoelastic substrate. Other models consider thin channel flow, rupturing of the entire elastic lithosphere, and changes in the depth of faulting. While some of these changes have significant impact on the magnitude and spatial features of the stress and deformation field, the far-field stress remains small. Numerical experiments demonstrate that the assumptions of the modified Elsasser model accentuate uniaxial deformation. When these assumptions are replicated in the finite element calculations, reasonable agreement between the models is achieved.

Cohen, Steven C.↗
NASA NTRS2017

Non-Linear Analysis of the NASA Super Pressure Balloons: Whole Flight Simulations

Tensys have a long-established background in the shape generation and load analysis of architectural stressed membrane structures. Founded upon their inTENS finite element analysis suite, these activities have broadened to encompass 'lighter than air' structures such as aerostats, airships, hybrid air vehicles and stratospheric balloons. Tensys have acted as consultants to the NASA Super Pressure Balloon (SPB) Program since 2004. Previous papers have focussed upon the application of inTENS to the overall structural and stability analysis of pumpkin type balloons as used in the SPB Program. Particular emphasis has been placed upon the ability to study both stress and stability at all stages of a flight. As the program has developed, increasing modelling fidelity has been introduced as design refinement has moved emphasis from the overall shape to the performance of individual details. Examples include the introduction of a contact capability into inTENS to better represent the separate cap layer of film. Localised investigations have considered the consequence of local geometric anomalies introduced during fabrication and the effects of debonding of the PBO tendon within its sleeve. Analysis to date has used a material model for the polyethylene shell film developed by Dr Rand of Winzen Engineering, supported by a program of fine resolution material tests at the Balloon Research and Development Laboratory (BRDL) at the GSFC Wallops Island Facility. Based upon work first reported by Schapery, this model has been incorporated into so-called 'snapshot' analyses within inTENS. For a given elapsed time point and temperature distribution, non-linear material properties are iteratively updated for the current stress state in each individual element until changes in those material properties are insignificant. This process has to be iterative as the film stresses and material properties are interdependent. Attempts to derive a time-stepping incremental viscoelastic capability using the Schapery Rand model encountered problems when dealing with the varying temperatures associated with balloon deployment, pressurisation and diurnal behaviour. An alternative approach has been developed by Pellegrino et al at Caltech, again with support from the NASA Balloon Program Office. This is a large strain non-linear viscoelastic model that includes film out-of-plane mechanical and thermal effects. Working from a unified base, this model can be utilised for time-stepping analyses in both a modulus or compliance mode, or a combination of both. This new capability is intended to enhance the current NASA balloon design process. This paper presents the implementation of the Caltech model into a specialist finite element suite, inTENS and its application to whole flight simulations.

Wakefield, David↗
NASA NTRS1991

Models of recurrent strike-slip earthquake cycles and the state of crustal stress

Numerical models of the strike-slip earthquake cycle, assuming a viscoelastic asthenosphere coupling model, are examined. The time-dependent simulations incorporate a stress-driven fault, which leads to tectonic stress fields and earthquake recurrence histories that are mutually consistent. Single-fault simulations with constant far-field plate motion lead to a nearly periodic earthquake cycle and a distinctive spatial distribution of crustal shear stress. The predicted stress distribution includes a local minimum in stress at depths less than typical seismogenic depths. The width of this stress 'trough' depends on the magnitude of crustal stress relative to asthenospheric drag stresses. The models further predict a local near-fault stress maximum at greater depths, sustained by the cyclic transfer of strain from the elastic crust to the ductile asthenosphere. Models incorporating both low-stress and high-stress fault strength assumptions are examined, under Newtonian and non-Newtonian rheology assumptions. Model results suggest a preference for low-stress (a shear stress level of about 10 MPa) fault models, in agreement with previous estimates based on heat flow measurements and other stress indicators.

Lyzenga, Gregory A.↗
NASA NTRS1988

SRM (Solid Rocket Motor) propellant and polymer materials structural modeling

The following investigation reviews and evaluates the use of stress relaxation test data for the structural analysis of Solid Rocket Motor (SRM) propellants and other polymer materials used for liners, insulators, inhibitors, and seals. The stress relaxation data is examined and a new mathematical structural model is proposed. This model has potentially wide application to structural analysis of polymer materials and other materials generally characterized as being made of viscoelastic materials. A dynamic modulus is derived from the new model for stress relaxation modulus and is compared to the old viscoelastic model and experimental data.

Moore, Carleton J.↗
NASA NTRS2014

Broadscale Postseismic Gravity Change Following the 2011 Tohoku-Oki Earthquake and Implication for Deformation by Viscoelastic Relaxation and Afterslip

The analysis of GRACE gravity data revealed post-seismic gravity increase by 6 micro-Gal over a 500 km scale within a couple of years after the 2011 Tohoku-Oki earthquake, which is nearly 40-50% of the co-seismic gravity change. It originates mostly from changes in the isotropic component corresponding to the M(sub rr) moment tensor element. The exponential decay with rapid change in a year and gradual change afterward is a characteristic temporal pattern. Both viscoelastic relaxation and afterslip models produce reasonable agreement with the GRACE free-air gravity observation, while their Bouguer gravity patterns and seafloor vertical deformations are distinctly different. The post-seismic gravity variation is best modeled by the bi-viscous relaxation with a transient and steady state viscosity of 10(exp 18) and 10(exp 19) Pa s, respectively, for the asthenosphere. Our calculated higher-resolution viscoelastic relaxation model, underlying the partially ruptured elastic lithosphere, yields the localized post-seismic subsidence above the hypocenter reported from the GPS-acoustic seafloor surveying.

GRACE↗
NASA NTRSDate not supplied

Variational Asymptotic Homogenization of Finitely Deformed Viscoelastic-Viscoplastic Composites

The objective of this paper is to develop a constitutive model for finitely deformed viscoelastic-viscoplastic materials and a micromechanics approach to homogenizing composites consisting of such materials. The development of the constitutive model involves establishing a thermodynamic framework based on finite strain theory, developing a viscoelasticity and a viscoplasticity model based on the thermodynamic framework, developing a radial return algorithm based on a classic framework, and deriving a closed-from incremental constitutive relation in the spatial configuration. The development of the micromechanics approach involves pulling-back the above constitutive relation to the material configuration, formulating a variational statement with the resulting constitutive relation, discretizing variational statement in a finite-dimensional space, and solving the discretized variational statement using an Euler–Newton predictor–corrector method. The constitutive model is calibrated via monotonic uniaxial compression tests on a polymer, and the calibrated model is validated by comparing its predictions with the cyclic test data. It is shown capable of characterizing viscoelasticity, viscoplasticity, and complex loading paths. The micromechanics approach’s capabilities are demonstrated through homogenizing a unidirectional fiber-reinforced composite, subjected to uniaxial, biaxial, and shear loading, at different strain rates. It is demonstrated to be capable of handling rate dependence and complex loading paths. The present framework can be further improved by implementing more sophisticated viscoelasticity and viscoplasticity models in future work.

Finite element analysis↗
NASA NTRS1988

Tether damping in space

This paper analyzes the dynamics of a long tether connecting two spacecraft in earth orbit, one of the spacecraft having dominant mass. In particular, it considers the material damping of the tether. The nominal position of the tether is stabilized by the gravity gradient such that it is aligned with the local vertical. The tether is modeled as a viscoelastic flexible continuum. Modal frequencies are derived in an analytical approximation form. Damping ratios are estimated according to the linear model calibrated by ground measurements. The results show that, with properly chosen tether material and braiding structure, longitudinal vibrations of the tethered system are well damped.

Powell, J. David↗
NASA NTRS1993

Establishing the relationship between manufacturing and component performance in stretch formed thermoplastic composites

Flexible manufacturing methods are needed to reduce the cost of using advanced composites in structural applications. One method that allows for this is the stretch forming of long discontinuous fiber materials with thermoplastic matrices. In order to exploit this flexibility in an economical way, a thorough understanding of the relationship between manufacturing and component performance must be developed. This paper reviews some of the recent work geared toward establishing this understanding. Micromechanics models have been developed to predict the formability of the material during processing. The latest improvement of these models includes the viscoelastic nature of the matrix and comparison with experimental data. A finite element scheme is described which can be used to model the forming process. This model uses equivalent anisotropic viscosities from the micromechanics models and predicts the microstructure in the formed part. In addition, structural models have been built to account for the material property gradients that can result from the manufacturing procedures. Recent developments in this area include the analysis of stress concentrations and a failure model each accounting for the heterogeneous material fields.

Santare, Michael H.↗
NASA NTRS1979

Frequency effect on the fatigue life of a laminated composite

Effects of load frequency on the fatigue behavior of notched (+ or - 45 deg)2s laminated graphite/epoxy composites are investigated. Experimental results are obtained using four frequencies at three load levels. Temperature rises near the center hole are also measured. It is found that when temperature increase during the fatigue test is small, fatigue life increases as the load frequency increases. However, an increase in the near hole temperature results in a reduction in fatigue life. A theoretical model based upon crack propagation in viscoelastic media is presented. The model is modified to account for the temperature effect.

Sun, C. T.↗
NASA NTRS1995

The Singularity Mystery Associated with a Radially Continuous Maxwell Viscoelastic Structure

The singularity problem associated with a radially continuous Maxwell viscoclastic structure is investigated. A special tool called the isolation function is developed. Results calculated using the isolation function show that the discrete model assumption is no longer valid when the viscoelastic parameter becomes a continuous function of radius. Continuous variations in the upper mantle viscoelastic parameter are especially powerful in destroying the mode-like structures. The contribution to the load Love numbers of the singularities is sensitive to the convexity of the viscoelastic parameter models. The difference between the vertical response and the horizontal response found in layered viscoelastic parameter models remains with continuous models.

Fang, Ming↗
NASA NTRS1988

Numerical solution methods for viscoelastic orthotropic materials

Numerical solution methods for viscoelastic orthotropic materials, specifically fiber reinforced composite materials, are examined. The methods include classical lamination theory using time increments, direction solution of the Volterra Integral, Zienkiewicz's linear Prony series method, and a new method called Nonlinear Differential Equation Method (NDEM) which uses a nonlinear Prony series. The criteria used for comparison of the various methods include the stability of the solution technique, time step size stability, computer solution time length, and computer memory storage. The Volterra Integral allowed the implementation of higher order solution techniques but had difficulties solving singular and weakly singular compliance function. The Zienkiewicz solution technique, which requires the viscoelastic response to be modeled by a Prony series, works well for linear viscoelastic isotropic materials and small time steps. The new method, NDEM, uses a modified Prony series which allows nonlinear stress effects to be included and can be used with orthotropic nonlinear viscoelastic materials. The NDEM technique is shown to be accurate and stable for both linear and nonlinear conditions with minimal computer time.

Gramoll, K. C.↗
NASA NTRS1998

High Strain Rate Deformation Modeling of a Polymer Matrix Composite: Matrix Constitutive Equations - Part 1

Recently applications have exposed polymer matrix composite materials to very high strain rate loading conditions, requiring an ability to understand and predict the material behavior under these extreme conditions. In this first paper of a two part report, background information is presented, along with the constitutive equations which will be used to model the rate dependent nonlinear deformation response of the polymer matrix. Strain rate dependent inelastic constitutive models which were originally developed to model the viscoplastic deformation of metals have been adapted to model the nonlinear viscoelastic deformation of polymers. The modified equations were correlated by analyzing the tensile/ compressive response of both 977-2 toughened epoxy matrix and PEEK thermoplastic matrix over a variety of strain rates. For the cases examined, the modified constitutive equations appear to do an adequate job of modeling the polymer deformation response. A second follow-up paper will describe the implementation of the polymer deformation model into a composite micromechanical model, to allow for the modeling of the nonlinear, rate dependent deformation response of polymer matrix composites.

Goldberg, Robert K.↗
NASA NTRS1985

Thermoplastic matrix composite processing model

The effects the processing parameters pressure, temperature, and time have on the quality of continuous graphite fiber reinforced thermoplastic matrix composites were quantitatively accessed by defining the extent to which intimate contact and bond formation has occurred at successive ply interfaces. Two models are presented predicting the extents to which the ply interfaces have achieved intimate contact and cohesive strength. The models are based on experimental observation of compression molded laminates and neat resin conditions, respectively. Identified as the mechanism explaining the phenomenon by which the plies bond to themselves is the theory of autohesion (or self diffusion). Theoretical predictions from the Reptation Theory between autohesive strength and contact time are used to explain the effects of the processing parameters on the observed experimental strengths. The application of a time-temperature relationship for autohesive strength predictions is evaluated. A viscoelastic compression molding model of a tow was developed to explain the phenomenon by which the prepreg ply interfaces develop intimate contact.

Dara, P. H.↗