Results for “Cell Size” — Page 15

NASA NTRS2008

A Modeling Technique and Representation of Failure in the Analysis of Triaxial Braided Carbon Fiber Composites

Quasi-static tests have been performed on triaxially braided carbon fiber composite materials with large unit cell sizes. The effects of different fibers and matrix materials on the failure mode were investigated. Simulations of the tests have been performed using the transient dynamic finite element code, LS-DYNA. However, the wide range of failure modes observed for the triaxial braided carbon fiber composites during tests could not be simulated using composite material models currently available within LS-DYNA. A macroscopic approach has been developed that provides better simulation of the material response in these materials. This approach uses full-field optical measurement techniques to measure local failures during quasi-static testing. Information from these experiments is then used along with the current material models available in LS-DYNA to simulate the influence of the braided architecture on the failure process. This method uses two-dimensional shell elements with integration points through the thickness of the elements to represent the different layers of braid along with a new analytical method for the import of material stiffness and failure data directly. The present method is being used to examine the effect of material properties on the failure process. The experimental approaches used to obtain the required data will be described, and preliminary results of the numerical analysis will be presented.

Littell, Justin D.↗

NASA NTRS2008

A Modeling Technique and Representation of Failure in the Analysis of Triaxial Braided Carbon Fiber Composites

Quasi-static tests have been performed on triaxially braided carbon fiber composite materials with large unit cell sizes. The effects of different fibers and matrix materials on the failure mode were investigated. Simulations of the tests have been performed using the transient dynamic finite element code, LS-DYNA. However, the wide range of failure modes observed for the triaxial braided carbon fiber composites during tests could not be simulated using composite material models currently available within LS-DYNA. A macroscopic approach has been developed that provides better simulation of the material response in these materials. This approach uses full-field optical measurement techniques to measure local failures during quasi-static testing. Information from these experiments is then used along with the current material models available in LS-DYNA to simulate the influence of the braided architecture on the failure process. This method uses two-dimensional shell elements with integration points through the thickness of the elements to represent the different layers of braid along with a new analytical method for the import of material stiffness and failure data directly. The present method is being used to examine the effect of material properties on the failure process. The experimental approaches used to obtain the required data will be described, and preliminary results of the numerical analysis will be presented.

Littell, Justin D.↗

NASA NTRS2008

Full-Field Strain Methods for Investigating Failure Mechanisms in Triaxial Braided Composites

Recent advancements in braiding technology have led to commercially viable manufacturing approaches for making large structures with complex shape out of triaxial braided composite materials. In some cases, the static load capability of structures made using these materials has been higher than expected based on material strength properties measured using standard coupon tests. A more detailed investigation of deformation and failure processes in large-unit-cell-size triaxial braid composites is needed to evaluate the applicability of standard test methods for these materials and to develop alternative testing approaches. This report presents some new techniques that have been developed to investigate local deformation and failure using digital image correlation techniques. The methods were used to measure both local and global strains during standard straight-sided coupon tensile tests on composite materials made with 12- and 24-k yarns and a 0 /+60 /-60 triaxial braid architecture. Local deformation and failure within fiber bundles was observed and correlations were made between these local failures and global composite deformation and strength.

Littell, Justin D.↗

NASA NTRS2008

Effect of Microscopic Damage Events on Static and Ballistic Impact Strength of Triaxial Braid Composites

In previous work, the ballistic impact resistance of triaxial braided carbon/epoxy composites made with large flat tows (12k and 24k) was examined by impacting 2 X2 X0.125" composite panels with gelatin projectiles. Several high strength, intermediate modulus carbon fibers were used in combination with both untoughened and toughened matrix materials. A wide range of penetration thresholds were measured for the various fiber/matrix combinations. However, there was no clear relationship between the penetration threshold and the properties of the constituents. During some of these experiments high speed cameras were used to view the failure process, and full-field strain measurements were made to determine the strain at the onset of failure. However, these experiments provided only limited insight into the microscopic failure processes responsible for the wide range of impact resistance observed. In order to investigate potential microscopic failure processes in more detail, quasi-static tests were performed in tension, compression, and shear. Full-field strain measurement techniques were used to identify local regions of high strain resulting from microscopic failures. Microscopic failure events near the specimen surface, such as splitting of fiber bundles in surface plies, were easily identified. Subsurface damage, such as fiber fracture or fiber bundle splitting, could be identified by its effect on in-plane surface strains. Subsurface delamination could be detected as an out-of-plane deflection at the surface. Using this data, failure criteria could be established at the fiber tow level for use in analysis. An analytical formulation was developed to allow the microscopic failure criteria to be used in place of macroscopic properties as input to simulations performed using the commercial explicit finite element code, LS-DYNA. The test methods developed to investigate microscopic failure will be presented along with methods for determining local failure criteria that can be used in analysis. Results of simulations performed using LS-DYNA will be presented to illustrate the capabilities and limitations for simulating failure during quasi-static deformation and during ballistic impact of large unit cell size triaxial braid composites.

Littell, Justin D.↗

NASA NTRS2008

Elastic Distribution of Microshutters, Measurements Obtainable on James Web Space Telescope

Spectrographic astronomy measurements in the near-infrared region will be done by functional two-dimensional microshutter arrays that are being fabricated at the NASA Goddard Space Flight Center for the James Webb Space Telescope (JWST). These micro-shutter arrays will represent the first mission-critical MEMS devices to be flown in space. JWST will use microshutter arrays to select focal plane object. 2-D programmable aperture masks of more than 200,000 elements select such space object. The use of silicon wafer material promises high efficiency and high contrast. Microshutter operation temperature is around 35K. Microshutter arrays are fabricated as close-packed silicon nitride membranes with a unit cell size of 105 x 204 micrometers. A layer of magnetic material is deposited onto each shutter. Individual shutters are equipped with a torsion flexure. Reactive ion etching (RIE) releases the shutters so they can open up to 90 degrees using the torsion flexure. Shutter rotation is initiated into a silicon support structure via an external magnetic field. Two electrically independent aluminum electrodes are deposited, one onto each shutter and another onto the support structure side-wall, permitting electrostatic latching and 2-D addressing to hold specific shutters open via external electronics.

Kletetschka, Gunther↗

NASA NTRS2009

Characterization of Damage in Triaxial Braid Composites Under Tensile Loading

Carbon fiber composites utilizing flattened, large tow yarns in woven or braided forms are being used in many aerospace applications. Their complex fiber architecture and large unit cell size present challenges in both understanding deformation processes and measuring reliable material properties. This report examines composites made using flattened 12k and 24k standard modulus carbon fiber yarns in a 0 /+60 /-60 triaxial braid architecture. Standard straight-sided tensile coupons are tested with the 0 axial braid fibers either parallel with or perpendicular to the applied tensile load (axial or transverse tensile test, respectively). Nonuniform surface strain resulting from the triaxial braid architecture is examined using photogrammetry. Local regions of high strain concentration are examined to identify where failure initiates and to determine the local strain at the time of initiation. Splitting within fiber bundles is the first failure mode observed at low to intermediate strains. For axial tensile tests splitting is primarily in the 60 bias fibers, which were oriented 60 to the applied load. At higher strains, out-of-plane deformation associated with localized delamination between fiber bundles or damage within fiber bundles is observed. For transverse tensile tests, the splitting is primarily in the 0 axial fibers, which were oriented transverse to the applied load. The initiation and accumulation of local damage causes the global transverse stress-strain curves to become nonlinear and causes failure to occur at a reduced ultimate strain. Extensive delamination at the specimen edges is also observed.

Littell, Justin D.↗

NASA NTRS2010

Experimental Techniques for Evaluating the Effects of Aging on Impact and High Strain Rate Properties of Triaxial Braided Composite Materials

An experimental program is underway to measure the impact and high strain rate properties of triaxial braided composite materials and to quantify any degradation in properties as a result of thermal and hygroscopic aging typically encountered during service. Impact tests are being conducted on flat panels using a projectile designed to induce high rate deformation similar to that experienced in a jet engine fan case during a fan blade-out event. The tests are being conducted on as-fabricated panels and panels subjected to various numbers of aging cycles. High strain rate properties are being measured using a unique Hopkinson bar apparatus that has a larger diameter than conventional Hopkinson bars. This larger diameter is needed to measure representative material properties because of the large unit cell size of the materials examined in this work. In this paper the experimental techniques used for impact and high strain rate testing are described and some preliminary results are presented for both as-fabricated and aged composites.

Pereira, J. Michael↗

NASA NTRS2010

Analysis of Photospheric Convection Cells with SDO/HMI

Supergranulation is a component of solar convection that assists in the outward transportation of internal energy. Supergranule cells are approximately 35 Mm across, have lifetimes on the order of a day and have divergent horizontal velocities of around 300 m/s, a factor of 10 higher than their central radial components. While they have been observed using Doppler methods for around half a century, their existence is also observed in other datasets such as magnetograms and Ca II K images. These datasets clearly show the influence of supergranulation on solar magnetism and how the local field is organized by the flows of supergranule cells. The Heliospheric and Magnetic Imager (HMI) aboard SDO is making fresh observations of convection phenomena at a higher cadence and a higher resolution that should make granular features visible. Granulation and supergranulation characteristics can now be compared within the same datasets, which may lead to further understanding of any mutual influences. The temporal and spatial enhancements of HMI will also reduce the noise level within studies of convection so that more detailed studies of their characteristics may be made. We present analyses of SDO/HMI Dopplergrams that provide new estimates of convection cell sizes, lifetimes, and velocity flows, as well as the rotation rates of the convection patterns across the solar disk. We make comparisons with previous data produced by MDI, as well as from data simulations.

Williams, Peter E.↗

NASA NTRS2011

Axisymmetric Implementation for 3D-Based DSMC Codes

The primary objective in developing NASA s DSMC Analysis Code (DAC) was to provide a high fidelity modeling tool for 3D rarefied flows such as vacuum plume impingement and hypersonic re-entry flows [1]. The initial implementation has been expanded over time to offer other capabilities including a novel axisymmetric implementation. Because of the inherently 3D nature of DAC, this axisymmetric implementation uses a 3D Cartesian domain and 3D surfaces. Molecules are moved in all three dimensions but their movements are limited by physical walls to a small wedge centered on the plane of symmetry (Figure 1). Unfortunately, far from the axis of symmetry, the cell size in the direction perpendicular to the plane of symmetry (the Z-direction) may become large compared to the flow mean free path. This frequently results in inaccuracies in these regions of the domain. A new axisymmetric implementation is presented which aims to solve this issue by using Bird s approach for the molecular movement while preserving the 3D nature of the DAC software [2]. First, the computational domain is similar to that previously used such that a wedge must still be used to define the inflow surface and solid walls within the domain. As before molecules are created inside the inflow wedge triangles but they are now rotated back to the symmetry plane. During the move step, molecules are moved in 3D but instead of interacting with the wedge walls, the molecules are rotated back to the plane of symmetry at the end of the move step. This new implementation was tested for multiple flows over axisymmetric shapes, including a sphere, a cone, a double cone and a hollow cylinder. Comparisons to previous DSMC solutions and experiments, when available, are made.

Stewart, Benedicte↗

NASA NTRS2004

Further Improvement in 3DGRAPE

3DGRAPE/AL:V2 denotes version 2 of the Three-Dimensional Grids About Anything by Poisson's Equation with Upgrades from Ames and Langley computer program. The preceding version, 3DGRAPE/AL, was described in Improved 3DGRAPE (ARC-14069) NASA Tech Briefs, Vol. 21, No. 5 (May 1997), page 66. These programs are so named because they generate volume grids by iteratively solving Poisson's Equation in three dimensions. The grids generated by the various versions of 3DGRAPE have been used in computational fluid dynamics (CFD). The main novel feature of 3DGRAPE/AL:V2 is the incorporation of an optional scheme in which anisotropic Lagrange-based trans-finite interpolation (ALBTFI) is coupled with exponential decay functions to compute and blend interior source terms. In the input to 3DGRAPE/AL:V2 the user can specify whether or not to invoke ALBTFI in combination with exponential-decay controls, angles, and cell size for controlling the character of grid lines. Of the known programs that solve elliptic partial differential equations for generating grids, 3DGRAPE/AL:V2 is the only code that offers a combination of speed and versatility with most options for controlling the densities and other characteristics of grids for CFD.

Alter, Stephen↗

NASA NTRS2012

Design, Analysis and Fabrication of Secondary Structural Components for the Habitat Demonstration Unit-Deep Space Habitat

In support of NASA s Habitat Demonstration Unit - Deep Space Habitat Prototype, a number of evolved structural sections were designed, fabricated, analyzed and installed in the 5 meter diameter prototype. The hardware consisted of three principal structural sections, and included the development of novel fastener insert concepts. The articles developed consisted of: 1) 1/8th of the primary flooring section, 2) an inner radius floor beam support which interfaced with, and supported (1), 3) two upper hatch section prototypes, and 4) novel insert designs for mechanical fastener attachments. Advanced manufacturing approaches were utilized in the fabrication of the components. The structural components were developed using current commercial aircraft constructions as a baseline (for both the flooring components and their associated mechanical fastener inserts). The structural sections utilized honeycomb sandwich panels. The core section consisted of 1/8th inch cell size Nomex, at 9 lbs/cu ft, and which was 0.66 inches thick. The facesheets had 3 plys each, with a thickness of 0.010 inches per ply, made from woven E-glass with epoxy reinforcement. Analysis activities consisted of both analytical models, as well as initial closed form calculations. Testing was conducted to help verify analysis model inputs, as well as to facilitate correlation between testing and analysis. Test activities consisted of both 4 point bending tests as well as compressive core crush sequences. This paper presents an overview of this activity, and discusses issues encountered during the various phases of the applied research effort, and its relevance to future space based habitats.

Smith, Russell W.↗

NASA NTRS2011

Are the World's Oceans Optically Different?

Regional differences in the Sea-viewing Wide Field-of-view Sensor chlorophyll algorithm uncertainty were observed in a large global data set containing coincident in situ measurements of chlorophyll a concentration (Chla) and spectral radiometry. The uncertainty was found to be systematic when the data were sorted by ocean: Atlantic, Pacific, Southern, and Indian Oceans. Artifacts associated with different instrumentation and analytical methods had been previously ruled out. Given these oceanic biases in the chlorophyll algorithm, we hypothesized that the oceans may be optically different, and their optical differences may be intrinsically related to regional differences in phytoplankton community structure or biogeochemical processes. The oceanic biases, originally observed using radiometric measurements, were independently verified using total absorption measurements in a subset of the data. Moreover, they were explained through oceanic differences in the absorption of colored detrital matter (CDM) and phytoplankton. Both effects were considered together in explaining the ocean biases through a stepwise linear regression analysis. Significant oceanic differences in the amount of CDM and in phytoplankton cell sizes and pigmentation would give rise to optical differences, but we raise a concern for the spatial coverage of the data. We do not suggest the application of ocean-based algorithms but rather emphasize the importance of consolidating regional data sets before reaching this conclusion.

Szeto, M.↗

NASA NTRS2011

Properties of Supergranulation During the Solar Minima of Cycles 22/23 and 23/24

The solar minimum at the transition from cycle 23 to 24 was notable for its low level of activity and its extended duration. Among the various fields of study, the evolution of the solar convection zone may provide insight into the causes and consequences of this recent minimum. This study continues previous investigations of the characteristics of solar supergranulation, a convection component strongly linked to the structure of the magnetic field, namely the time-evolution of the global mean of supergranule cell size, determined from spectral analysis of MDI Dopplergrams from the two previous solar minima. Analyses of the global mean of supergranule sizes show a quasi-oscillatory nature to the evolution of this particular supergranule characteristic. Performing similar analyses on realistic, synthetic Doppler images show similar time-dependent characteristics. We conclude that the observed fluctuations are not observational artifacts, and that an underlying trend exists within the evolution of the supergranulation network.

Williams, Peter E.↗

NASA NTRS2013

Next Generation Lightweight Mirror Modeling Software

The advances in manufacturing techniques for lightweight mirrors, such as EXELSIS deep core low temperature fusion, Corning's continued improvements in the Frit bonding process and the ability to cast large complex designs, combined with water-jet and conventional diamond machining of glasses and ceramics has created the need for more efficient means of generating finite element models of these structures. Traditional methods of assembling 400,000 + element models can take weeks of effort, severely limiting the range of possible optimization variables. This paper will introduce model generation software developed under NASA sponsorship for the design of both terrestrial and space based mirrors. The software deals with any current mirror manufacturing technique, single substrates, multiple arrays of substrates, as well as the ability to merge submodels into a single large model. The modeler generates both mirror and suspension system elements, suspensions can be created either for each individual petal or the whole mirror. A typical model generation of 250,000 nodes and 450,000 elements only takes 5-10 minutes, much of that time being variable input time. The program can create input decks for ANSYS, ABAQUS and NASTRAN. An archive/retrieval system permits creation of complete trade studies, varying cell size, depth, and petal size, suspension geometry with the ability to recall a particular set of parameters and make small or large changes with ease. The input decks created by the modeler are text files which can be modified by any editor, all the key shell thickness parameters are accessible and comments in deck identify which groups of elements are associated with these parameters. This again makes optimization easier. With ANSYS decks, the nodes representing support attachments are grouped into components; in ABAQUS these are SETS and in NASTRAN as GRIDPOINT SETS, this make integration of these models into large telescope or satellite models possible.

Arnold, William↗

NASA NTRS2013

Next-Generation Lightweight Mirror Modeling Software

The advances in manufacturing techniques for lightweight mirrors, such as EXELSIS deep core low temperature fusion, Corning's continued improvements in the Frit bonding process and the ability to cast large complex designs, combined with water-jet and conventional diamond machining of glasses and ceramics has created the need for more efficient means of generating finite element models of these structures. Traditional methods of assembling 400,000 + element models can take weeks of effort, severely limiting the range of possible optimization variables. This paper will introduce model generation software developed under NASA sponsorship for the design of both terrestrial and space based mirrors. The software deals with any current mirror manufacturing technique, single substrates, multiple arrays of substrates, as well as the ability to merge submodels into a single large model. The modeler generates both mirror and suspension system elements, suspensions can be created either for each individual petal or the whole mirror. A typical model generation of 250,000 nodes and 450,000 elements only takes 5-10 minutes, much of that time being variable input time. The program can create input decks for ANSYS, ABAQUS and NASTRAN. An archive/retrieval system permits creation of complete trade studies, varying cell size, depth, and petal size, suspension geometry with the ability to recall a particular set of parameters and make small or large changes with ease. The input decks created by the modeler are text files which can be modified by any editor, all the key shell thickness parameters are accessible and comments in deck identify which groups of elements are associated with these parameters. This again makes optimization easier. With ANSYS decks, the nodes representing support attachments are grouped into components; in ABAQUS these are SETS and in NASTRAN as GRIDPOINT SETS, this make integration of these models into large telescope or satellite models possible

Arnold, William R., Sr.↗

NASA NTRS2013

Next Generation Lightweight Mirror Modeling Software

The advances in manufacturing techniques for lightweight mirrors, such as EXELSIS deep core low temperature fusion, Corning's continued improvements in the Frit bonding process and the ability to cast large complex designs, combined with water-jet and conventional diamond machining of glasses and ceramics has created the need for more efficient means of generating finite element models of these structures. Traditional methods of assembling 400,000 + element models can take weeks of effort, severely limiting the range of possible optimization variables. This paper will introduce model generation software developed under NASA sponsorship for the design of both terrestrial and space based mirrors. The software deals with any current mirror manufacturing technique, single substrates, multiple arrays of substrates, as well as the ability to merge submodels into a single large model. The modeler generates both mirror and suspension system elements, suspensions can be created either for each individual petal or the whole mirror. A typical model generation of 250,000 nodes and 450,000 elements only takes 5-10 minutes, much of that time being variable input time. The program can create input decks for ANSYS, ABAQUS and NASTRAN. An archive/retrieval system permits creation of complete trade studies, varying cell size, depth, and petal size, suspension geometry with the ability to recall a particular set of parameters and make small or large changes with ease. The input decks created by the modeler are text files which can be modified by any editor, all the key shell thickness parameters are accessible and comments in deck identify which groups of elements are associated with these parameters. This again makes optimization easier. With ANSYS decks, the nodes representing support attachments are grouped into components; in ABAQUS these are SETS and in NASTRAN as GRIDPOINT SETS, this make integration of these models into large telescope or satellite models easier.

Arnold, William R., Sr.↗

NASA NTRS2015

Advances in Distance-Based Hole Cuts on Overset Grids

An automatic and efficient method to determine appropriate hole cuts based on distances to the wall and donor stencil maps for overset grids is presented. A new robust procedure is developed to create a closed surface triangulation representation of each geometric component for accurate determination of the minimum hole. Hole boundaries are then displaced away from the tight grid-spacing regions near solid walls to allow grid overlap to occur away from the walls where cell sizes from neighboring grids are more comparable. The placement of hole boundaries is efficiently determined using a mid-distance rule and Cartesian maps of potential valid donor stencils with minimal user input. Application of this procedure typically results in a spatially-variable offset of the hole boundaries from the minimum hole with only a small number of orphan points remaining. Test cases on complex configurations are presented to demonstrate the new scheme.

Advances↗

NASA NTRS2018

Influence of Core and Hexapod Geometry, and Local Reinforcement on the Performance of Ultra-Lightweight ULE Mirror

The Habitable Exoplanet Observatory (HabEx) mission has unique optical performance requirements which drive the mirror design process beyond the traditional criteria. While mass and stiffness are still important, the response to inertia loading (expressed in terms of Zernike coefficients) to omni-directional excitation dominates the effort. While a Zerodur mirror is the current baseline, as mass budgets change, a ULE design is being studied as a potential alternative. This trade study looked at over 264 design variations using the Arnold Mirror Modeler and ANSYS(c) to investigate the influence of various design elements, including: substrate thickness, core cell size, hexapod geometry and local reinforcement. Design 'goodness' was evaluated based on the mirror's inertial deformation response to omni-directional input. This response was calculated via RSSing Zernike polynomial responses to (XYZ) accelerations.

Space Telescope Technology↗