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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

Slip Analysis in a Ni-base Superalloy

A Ni-base superalloy single crystal with Gamma/Gamma' structure was tested at room temperature along the , <110> and <111> directions. Consistent with previously reported investigations, this alloy did not obey the Schmid law and the CRSS (critical resolved shear stress) was noticeably lower for the <111>-oriented samples. Furthermore, the strain hardening rate decreased and the degree of deformation localization increased in the order of <111>, and <110> orientations. The appearance and orientation of deformation traces were found to depend on the loading orientation as well as the amount of strain. In general, when Gamma'-particles were sheared, the traces followed the expected octahedral shear planes. It is demonstrated that the wavy deformation traces that do not follow the {111} planes are associated with changes in the gamma-channels width and the falling off of the gamma-particles. In this paper the evolution of deformation bands are discussed in terms of plastic localization at microscopic, mesoscopic and macroscopic levels.

Westbrooke, Eboni F.↗
NASA NTRS2014

Theoretical Development of an Orthotropic Elasto-Plastic Generalized Composite Material Model

The need for accurate material models to simulate the deformation, damage and failure of polymer matrix composites is becoming critical as these materials are gaining increased usage in the aerospace and automotive industries. While there are several composite material models currently available within LSDYNA (Livermore Software Technology Corporation), there are several features that have been identified that could improve the predictive capability of a composite model. To address these needs, a combined plasticity and damage model suitable for use with both solid and shell elements is being developed and is being implemented into LS-DYNA as MAT_213. A key feature of the improved material model is the use of tabulated stress-strain data in a variety of coordinate directions to fully define the stress-strain response of the material. To date, the model development efforts have focused on creating the plasticity portion of the model. The Tsai-Wu composite failure model has been generalized and extended to a strain-hardening based orthotropic yield function with a nonassociative flow rule. The coefficients of the yield function, and the stresses to be used in both the yield function and the flow rule, are computed based on the input stress-strain curves using the effective plastic strain as the tracking variable. The coefficients in the flow rule are computed based on the obtained stress-strain data. The developed material model is suitable for implementation within LS-DYNA for use in analyzing the nonlinear response of polymer composites.

Polymer Matrix Composites↗
NASA NTRS2014

Theoretical Development of an Orthotropic Elasto-Plastic Generalized Composite Material Model

The need for accurate material models to simulate the deformation, damage and failure of polymer matrix composites is becoming critical as these materials are gaining increased usage in the aerospace and automotive industries. While there are several composite material models currently available within LS-DYNA (Registered), there are several features that have been identified that could improve the predictive capability of a composite model. To address these needs, a combined plasticity and damage model suitable for use with both solid and shell elements is being developed and is being implemented into LS-DYNA as MAT_213. A key feature of the improved material model is the use of tabulated stress-strain data in a variety of coordinate directions to fully define the stress-strain response of the material. To date, the model development efforts have focused on creating the plasticity portion of the model. The Tsai-Wu composite failure model has been generalized and extended to a strain-hardening based orthotropic material model with a non-associative flow rule. The coefficients of the yield function, and the stresses to be used in both the yield function and the flow rule, are computed based on the input stress-strain curves using the effective plastic strain as the tracking variable. The coefficients in the flow rule are computed based on the obtained stress-strain data. The developed material model is suitable for implementation within LS-DYNA for use in analyzing the nonlinear response of polymer composites.

Plasticity↗
NASA NTRS1987

Anisotropic constitutive modeling for nickel base single crystal superalloy Rene N4 at 982 C

A back stress/drag stress constitutive model based on a crystallographic approach to model single crystal anisotropy is presented. Experimental results demonstrated the need for the back stress variable in the inelastic flow equations. Experimental findings suggested that back stress is orientation dependent and controls both strain hardening and recovery characteristics. Due to the observed stable fatigue loops at 1800 F, drag stress is considered constant for this temperature. The constitutive model operated with constraints determined only from tensile data was extensively tested from simple tensile and fatigue to complicated strain hold tests. The model predicted very well under those conditions.

Stouffer, D. C.↗
NASA NTRSDate not supplied

Fracture Prediction of Epoxy Resin using Morse Bond Potential Embedded in GAFF using Molecular Dynamics Simulations

A generic method is developed to investigate the fracture behavior of an epoxy resin using atomistic molecular (MD) dynamics simulations. The epoxy system consisted of the stoichiometric mixture of tetra-/tri-functional epoxies and di-functional hardener molecules and was cured with a step-growth crosslinking algorithm. A hybrid force field (hFF) in which Morse bond function were added in second generation-general Amber Force Field (GAFF2) was used to capture bond breaking of the crosslinked epoxy system under uniaxial deformation. The Morse bond parameters for covalent bonds in the crosslinked backbone were fit to dissociation curves computed by CASPT2/6-311+G**. After the systems in various crosslink densities were obtained and equilibrated, the full length of stress-strain (𝜎-𝜀) curve was produced including initial elastic regime, yielding, plastic flow, strain hardening, and progressive failure. We found that the hFF was effective in studying the dissociation of crosslinked polymers and understanding the failure mechanism. In the results, the hFF delivered the same elastic property obtained from the unmodified force field GAFF. The stress-strain curve, however, began to be distinct in the plastic regime due to broken covalent bonds capable of altering the tensile behavior. Once the stress reached maximum, it progressively decreased until the ultimate failure (𝜎=0). Since hFF employed the existing GAFF functions with including a minimal change of its bond potential, this approach is easier to perform and computationally more efficient for unveiling the fracture behavior of polymer materials at the molecular level than other approaches with reactive force fields. We expect this approach will be utilized to accelerate the material-by-design process for thermosets by incorporate data from molecular models.

Changwoon Jang↗
NASA NTRS1989

Deformation modeling and constitutive modeling for anisotropic superalloys

A study of deformation mechanisms in the single crystal superalloy PWA 1480 was conducted. Monotonic and cyclic tests were conducted from 20 to 1093 C. Both (001) and near-(123) crystals were tested, at strain rates of 0.5 and 50 percent/minute. The deformation behavior could be grouped into two temperature regimes: low temperatures, below 760 C; and high temperatures, above 820 to 950 C depending on the strain rate. At low temperatures, the mechanical behavior was very anisotropic. An orientation dependent CRSS, a tension-compression asymmetry, and anisotropic strain hardening were all observed. The material was deformed by planar octahedral slip. The anisotropic properties were correlated with the ease of cube cross-slip, as well as the number of active slip systems. At high temperatures, the material was isotropic, and deformed by homogeneous gamma by-pass. It was found that the temperature dependence of the formation of superlattice-intrinsic stacking faults was responsible for the local minimum in the CRSS of this alloy at 400 C. It was proposed that the cube cross-slip process must be reversible. This was used to explain the reversible tension-compression asymmetry, and was used to study models of cross-slip. As a result, the cross-slip model proposed by Paidar, Pope and Vitek was found to be consistent with the proposed slip reversibility. The results were related to anisotropic viscoplastic constitutive models. The model proposed by Walter and Jordan was found to be capable of modeling all aspects of the material anisotropy. Temperature and strain rate boundaries for the model were proposed, and guidelines for numerical experiments were proposed.

Milligan, Walter W.↗
NASA NTRS1972

Metallic transfer between metals in sliding contact examined by auger emission spectroscopy

Metallic transfer between polycrystalline metals in sliding contact was examined. Hemispherical riders of iron, nickel, and cobalt were slid on tungsten, tantalum, niobium, and molybdenum disks in ultrahigh vacuum. Auger emission spectroscopy was used to monitor the elemental composition of the disk surfaces. Iron, nickel, and cobalt transferred to tungsten, whereas only cobalt transferred to tantalum, niobium, and molybdenum. The results of this investigation are discussed in terms of the cohesive energy and strain hardening characteristics of the specimen materials.

Pepper, S. V.↗
NASA NTRS1972

Mathematical characterization of mechanical behavior of porous frictional granular media

A new definition of loading and unloading along the yield surface of Roscoe and Burland is introduced. This is achieved by noting that the strain-hardening parameter in the plastic potential function is deduced from the yield locus equation of Roscoe and Burland. The analytical results are compared with the experimental results for plate-bearing and cone-penetrometer problems and close agreements are demonstrated. The wheel-soil interaction is studied under dynamic loading. The rate-dependent plasticity or viscoelastoplastic behavior is considered. This is accomplished by the internal (hidden) variables associated with time-dependent viscous properties directly superimposed with inelastic behavior governed by the yield criteria of Roscoe and Burland. Effects of inertia and energy dissipation are properly accounted for. Example problems are presented.

Chung, T. J.↗
NASA NTRS1972

Plasticity - Theory and finite element applications.

A unified presentation is given of the development and distinctions associated with various incremental solution procedures used to solve the equations governing the nonlinear behavior of structures, and this is discussed within the framework of the finite-element method. Although the primary emphasis here is on material nonlinearities, consideration is also given to geometric nonlinearities acting separately or in combination with nonlinear material behavior. The methods discussed here are applicable to a broad spectrum of structures, ranging from simple beams to general three-dimensional bodies. The finite-element analysis methods for material nonlinearity are general in the sense that any of the available plasticity theories can be incorporated to treat strain hardening or ideally plastic behavior.

Armen, H., Jr.↗
NASA NTRS1977

Stiffness and damping characteristics of aluminum in creep

Tensile creep tests conducted at 200 C were performed on annealed commercially pure aluminum specimens in order to measure the dominant elevated temperature dislocation processes. Testing consisted of applying small lateral loads to measure flexural stiffness, and vibrating the specimens laterally in order to measure dynamic modulus and internal damping. It was concluded that (1) the strain hardening increased static stiffness and decreased internal damping during early creep, and (2) the dynamic modulus remained essentially constant at the elastic value during creep. These results imply that primary creep may constitute a mechanism of recovery of dislocatory disorder induced by yielding the material during loading, and that the inelastic modulus utilized as a mathematical concept in several creep buckling theories is not a directly measurable material property.

Berkovits, A.↗
NASA NTRS1978

Creep of oxide dispersion strengthened materials (with special reference to TD nichrome)

It was shown that the creep behavior of oxide dispersion strengthened (ODS) alloys is controlled principally by the creep properties of the matrix of the alloy devoid of particles. Thus, diffusion controlled slip process determine the rate controlling step in such materials. The role of the particles is to stabilize a fine substructure which is invariant with the creep stress over a wide range of stress. This characteristic leads to negligible strain hardening during creep and suggests that creep relations developed for pure metals and many solid solution alloys at constant structure should be used to describe the creep of ODS alloys. A second characteristics of the ODS alloys is that a stress may exist below which creep will not occur (threshold stress).

Lin, J.↗
NASA NTRS1977

Finite-element analysis of crack growth under monotonic and cyclic loading

An elastic-plastic (incremental) finite-element analysis, in conjunction with a crack-growth criterion, was used to study crack-growth behavior under monotonic and cyclic loading. The crack-growth criterion was based on crack-tip strain. Whenever the crack-tip strain equals or exceeds a critical strain value, the crack grows. The effects of element-mesh size, critical strain, strain hardening, and specimen type (tension or bending) on crack growth under monotonic loading were investigated. Crack growth under cyclic loading (constant amplitude and simple variable amplitude) were also studied. A combined hardening theory, which incorporates features of both isotropic and kinematic hardening under cyclic loading, was also developed for smooth yield surfaces and was used in the analysis.

Newman, J. C., Jr.↗
NASA NTRS1985

A study of spectrum fatigue crack propagation in two aluminum alloys. 2: Influence of microstructures

The important metallurgical factors that influence both constant amplitude and spectrum crack growth behavior in aluminum alloys were investigated. The effect of microstructural features such as grain size, inclusions, and dispersoids was evaluated. It was shown that a lower stress intensities, the I/M 7050 alloy showed better fatigue crack propagation (FCP) resistance than P/M 7091 alloy for both constant amplitude and spectrum testing. It was suggested that the most important microstructural variable accounting for superior FCP resistance of 7050 alloy is its large grain size. It was further postulated that the inhomogenous planar slip and large grain size of 7050 limit dislocation interactions and thus increase slip reversibility which improves FCP performance. The hypothesis was supported by establishing that the cyclic strain hardening exponent for the 7091 alloy is higher than that of 7050.

Telesman, J.↗
NASA NTRS1985

On numerical integration and computer implementation of viscoplastic models

Due to the stringent design requirement for aerospace or nuclear structural components, considerable research interests have been generated on the development of constitutive models for representing the inelastic behavior of metals at elevated temperatures. In particular, a class of unified theories (or viscoplastic constitutive models) have been proposed to simulate material responses such as cyclic plasticity, rate sensitivity, creep deformations, strain hardening or softening, etc. This approach differs from the conventional creep and plasticity theory in that both the creep and plastic deformations are treated as unified time-dependent quantities. Although most of viscoplastic models give better material behavior representation, the associated constitutive differential equations have stiff regimes which present numerical difficulties in time-dependent analysis. In this connection, appropriate solution algorithm must be developed for viscoplastic analysis via finite element method.

Chang, T. Y.↗