NASA Scientific and Technical Aerospace Reports

20040070823 NASA Langley Research Center, Hampton, VA, USA
Combined In-Plane and Through-the-Thickness Analysis for Failure Prediction of Bolted Composite Joints
Kradinov, V.; Madenci, E.; Ambur, D. R.; [2004]; 24 pp.; In English; 45th AIAA/ASME/AHS/ASC Structures, Structural
Dynamics and Materials Conference, 19-22 Apr. 2004, Palm Springs, CA, USA; Original contains black and white
illustrations
Contract(s)/Grant(s): 23-762-55-LE
Report No.(s): AIAA Paper 2004-1703; Copyright; Avail: CASI; A03, Hardcopy
Although two-dimensional methods provide accurate predictions of contact stresses and bolt load distribution in bolted
composite joints with multiple bolts, they fail to capture the effect of thickness on the strength prediction. Typically, the plies
close to the interface of laminates are expected to be the most highly loaded, due to bolt deformation, and they are usually
the first to fail. This study presents an analysis method to account for the variation of stresses in the thickness direction by
augmenting a two-dimensional analysis with a one-dimensional through the thickness analysis. The two-dimensional in-plane
solution method based on the combined complex potential and variational formulation satisfies the equilibrium equations
exactly, and satisfies the boundary conditions and constraints by minimizing the total potential. Under general loading
conditions, this method addresses multiple bolt configurations without requiring symmetry conditions while accounting for the
contact phenomenon and the interaction among the bolts explicitly. The through-the-thickness analysis is based on the model
utilizing a beam on an elastic foundation. The bolt, represented as a short beam while accounting for bending and shear
deformations, rests on springs, where the spring coefficients represent the resistance of the composite laminate to bolt
deformation. The combined in-plane and through-the-thickness analysis produces the bolt/hole displacement in the thickness
direction, as well as the stress state in each ply. The initial ply failure predicted by applying the average stress criterion is
followed by a simple progressive failure. Application of the model is demonstrated by considering single- and double-lap
joints of metal plates bolted to composite laminates.
Author
Composite Structures; Failure Analysis; Lap Joints; Thickness; Bolts; Two Dimensional Models
20040073434 NASA Langley Research Center, Hampton, VA, USA
Evaluation of Double-Vacuum-Bag Process For Composite Fabrication
Hou, T. H.; Jensen, B. J.; January 16, 2004; 13 pp.; In English; SAMPE 2004 Symposium and Exhibition, 16-20 May 2004,
Long Beach, CA, USA
Contract(s)/Grant(s): 23-794-40-4G; No Copyright; Avail: CASI; A03, Hardcopy
A non-autoclave vacuum bag process using atmospheric pressure alone that eliminates the need for external pressure
normally supplied by an autoclave or a press is an attractive method for composite fabrication. This type of process does not
require large capital expenditures for tooling and processing equipment. In the molding cycle (temperature/pressure profile)
for a given composite system, the vacuum application point has to be carefully selected to achieve the final consolidated
laminate net shape and resin content without excessive resin squeeze-out. The traditional single-vacuum- bag (SVB) process
is best suited for molding epoxy matrix based composites because of their superior flow and the absence of reaction
by-products or other volatiles. Other classes of materials, such as polyimides and phenolics, generate water during cure. In
addition, these materials are commonly synthesized as oligomers using solvents to facilitate processability. Volatiles (solvents
and reaction byproducts) management therefore becomes a critical issue. SVB molding, without additional pressure, normally
fails to yield void-free quality composites for these classes of resin systems. A double-vacuum- bag (DVB) process for volatile
management was envisioned, designed and built at the NASA Langley Research Center. This experimental DVB process
affords superior volatiles management compared to the traditional SVB process. Void-free composites are consistently
fabricated as measured by C-scan and optical photomicroscopy for high performance polyimide and phenolic resins.
Author
Epoxy Matrix Composites; Fabrication; Vacuum; Autoclaves; By-Products; Phenolic Resins
20040073454 NASA Langley Research Center, Hampton, VA, USA
Polyimide/Carbon Nanotube Composite Films for Electrostatic Charge Mitigation
Delozier, D. M.; Tigelaar, D. M.; Watson, K. A.; Smith, J. G., Jr.; Lillehei, P. T.; Connell, J. W.; [2004]; 13 pp.; In English;
SAMPE 2004 Symposium and Exhibition, 16-20 May 2004, Long Beach, CA, USA
Contract(s)/Grant(s): 23-319-03-01
Report No.(s): LAR-16589-1; No Copyright; Avail: CASI; A03, Hardcopy
Low color, space environmentally durable polymeric films with sufficient electrical conductivity to mitigate electrostatic
charge build-up have been under investigation as part of a materials development activity. In the work described herein,
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