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  • 1
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    Thermoplastic Ribbon-Ply Bonding Model (1996)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The aim of the present work was to identify key variables in rapid weldbonding of thermoplastic tow (ribbon) and their relationship to matrix polymer properties and to ribbon microstructure. Theoretical models for viscosity, establishment of ply-ply contact, instantaneous (Velcro) bonding, molecular interdiffusion (healing), void growth suppression, and gap filling were reviewed and synthesized. Consideration of the theoretical bonding mechanisms and length scales and of the experimental weld/peel data allow the prediction of such quantities as the time and pressure required to achieve good contact between a ribbon and a flat substrate, the time dependence of bond strength, pressures needed to prevent void growth from dissolved moisture and conditions for filling gaps and smoothing overlaps.
    Keywords: Nonmetallic Materials
    Type: NASA-TM-110203 , NAS 1.15:110203
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  • 2
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    How Molecular Structure Affects Mechanical Properties of an Advanced Polymer (2000)
    In:  CASI
    Details
    Publication Date: 2018-06-05
    Description: density was performed over a range of temperatures below the glass transition temperature. The physical characterization, elastic properties and notched tensile strength all as a function of molecular weight and test temperature were determined. For the uncrosslinked SI material, it was shown that notched tensile strength is a strong function of both temperature and molecular weight, whereas stiffness is only a strong function of temperature. For the crosslinked PETI-SI material, it was shown that the effect of crosslinking significantly enhances the mechanical performance of the low molecular weight material; comparable to that exhibited by the high molecular weight material.
    Keywords: Nonmetallic Materials
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  • 3
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    Effects of Molecular Structure in Macroscopic Mechanical Properties of an Advanced Polymer (LARC(sup TM)-SI) (2004)
    In:  CASI
    Details
    Publication Date: 2018-06-02
    Description: Mechanical testing of an advanced polymer resin with known variations in molecular weight was performed over a range of temperatures below the glass transition temperature. The elastic properties, inelastic elongation behavior, and notched tensile strength all as a function of molecular weight and test temperature were determined. It was shown that notched tensile strength is a strong function of both temperature and molecular weight, whereas stiffness is only a strong function of temperature.
    Keywords: Nonmetallic Materials
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  • 4
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    Stretch-Orientation of LaRC(TM) RP 50 Polyimide Film (2006)
    In:  CASI
    Details
    Publication Date: 2018-06-11
    Description: An addition-curable poly(amic acid) film was subjected to various thermal pretreatments and then to uniaxial or biaxial drawing. Hot-stretching to 300% of the initial length produced better than twofold increases in tensile modulus and strength of partially-cured films. Most of the improvement was maintained upon completion of the curing at constant length.
    Keywords: Nonmetallic Materials
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  • 5
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    Tensile Properties of Hydrogels and of Snake Skin (2002)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Stimulus-responsive or 'smart' gels are of potential interest as sensors and actuators, in industrial separations, and as permeable delivery systems. In most applications, a certain degree of mechanical strength and toughness will be required, yet the large-strain behavior of gels has not been widely reported. Some exceptions include work on gelatin and other food gels, some characterization of soft gels applicable for in-vitro cell growth studies, and toughness determinations on commercial contact lens materials. In general, it can be anticipated that the gel stiffness will increase with increasing degree of crosslinking, but the tensile strength may go through a maximum. Gel properties can be tailored by varying not only the degree of crosslinking, but also the polymer concentration and the nature of the polymer backbone (e.g. its stiffness or solubility). Polypeptides provide an especially interesting case, where secondary structure affects trends in moduli and conformational transitions may accompany phase changes. A few papers on the tensile properties of responsive gels have begun to appear. The responsive hydrogel chosen for the present study, crosslinked hydroxypropylcellulose, shrinks over a rather narrow temperature range near 44 C. Some vertebrate skin is also subject to substantial strain. Among reptiles, the morphologies of the skin and scales show wide variations. Bauer et al. described the mechanical properties and histology of gecko skin; longitudinal tensile properties of snake skin were examined by Jayne with reference to locomotion. The present measurements focus on adaptations related to feeding, including the response of the skin to circumferential tension. Tensile properties will be related to interspecific and regional variation in skin structure and folding.
    Keywords: Nonmetallic Materials
    Type: 1st World Congress on Biomim. and Artificial Muscles; Dec 09, 2002 - Dec 11, 2002; Albuquerque, NM; United States
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  • 6
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    Impression Testing of Self-Healing Polymers (2005)
    In:  CASI
    Details
    Publication Date: 2019-07-11
    Description: As part of the BIOSANT program (biologically-inspired smart nanotechnology), scientists at NASA-Langley have identified a "self-healing" plastic that spontaneously closes the hole left by the passage of a bullet. To understand and generalize the phenomenon in question, the mechanical properties responsible for this ability are being explored. Low-rate impression testing was chosen to characterize post-yield material properties, and it turned out that materials that heal following ballistic puncture also show up to 80% healing of the low-rate impression. Preliminary results on the effects of temperature and rate of puncture are presented.
    Keywords: Nonmetallic Materials
    Type: NASA/TM-2005-213532 , L-19086
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  • 7
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    Coarse-Grained and Atomistic Modeling of Polyimides (2004)
    In:  CASI
    Details
    Publication Date: 2019-07-10
    Description: A coarse-grained model for a set of three polyimide isomers is developed. Each polyimide is comprised of BPDA (3,3,4,4' - biphenyltetracarboxylic dianhydride) and one of three APB isomers: 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene or 1,3-bis(3-aminophenoxy)benzene. The coarse-grained model is constructed as a series of linked vectors following the contour of the polymer backbone. Beads located at the midpoint of each vector define centers for long range interaction energy between monomer subunits. A bulk simulation of each coarse-grained polyimide model is performed with a dynamic Monte Carlo procedure. These coarsegrained models are then reverse-mapped to fully atomistic models. The coarse-grained models show the expected trends in decreasing chain dimensions with increasing meta linkage in the APB section of the repeat unit, although these differences were minor due to the relatively short chains simulated here. Considerable differences are seen among the dynamic Monte Carlo properties of the three polyimide isomers. Decreasing relaxation times are seen with increasing meta linkage in the APB section of the repeat unit.
    Keywords: Nonmetallic Materials
    Type: NASA/TM-2004-213030 , L-19026
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  • 8
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    Computational Materials Research (1996)
    In:  CASI
    Details
    Publication Date: 2019-08-13
    Description: Computational Materials aims to model and predict thermodynamic, mechanical, and transport properties of polymer matrix composites. This workshop, the second coordinated by NASA Langley, reports progress in measurements and modeling at a number of length scales: atomic, molecular, nano, and continuum. Assembled here are presentations on quantum calculations for force field development, molecular mechanics of interfaces, molecular weight effects on mechanical properties, molecular dynamics applied to poling of polymers for electrets, Monte Carlo simulation of aromatic thermoplastics, thermal pressure coefficients of liquids, ultrasonic elastic constants, group additivity predictions, bulk constitutive models, and viscoplasticity characterization.
    Keywords: Nonmetallic Materials
    Type: NASA-CP-10190 , NAS 1.55:10190 , Jan 04, 1996 - Jan 05, 1996; Williamsburg, VA; United States
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  • 9
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    Crystallization of Stretched Polyimides: A Structure-Property Study (2002)
    In:  CASI
    Details
    Publication Date: 2019-07-10
    Description: A simple rotational isomeric state model was used to detect the degree to which polyimide repeat units might align to give an extended crystal. It was found experimentally that the hallmarks of stretch-crystallization were more likely to occur in materials whose molecules could readily give extended, aligned conformations. A proposed screening criterion was 84% accurate in selecting crystallizing molecules.
    Keywords: Nonmetallic Materials
    Type: NASA/TM-2002-211418 , NAS 1.15:211418 , L-18142
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  • 10
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    Stretch-Oriented Polyimide Films (2000)
    In:  CASI
    Details
    Publication Date: 2019-07-10
    Description: Two thermoplastic polyimides - one amorphous, the other crystallizable -- were subjected to isothermal stretching just above their glass transition temperatures. Room-temperature strengths in the stretch direction were greatly improved and, moduli increased up to 3.6-fold. Optimum stretching conditions were determined.
    Keywords: Nonmetallic Materials
    Type: NASA/TM-2000-210094 , NAS 1.15:210094 , L-17953
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