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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Interfacial strength development in thermoplastic resins and fiber-reinforced thermoplastic composites

An experimental program to develop test methods to be used to characterize interfacial (autohesive) strength development in polysulfone thermoplastic resin and graphite-polysulfone prepreg during processing is reported. Two test methods were used to examine interfacial strength development in neat resin samples. These included an interfacial tension test and a compact tension (CT) fracture toughness test. The interfacial tensile test proved to be very difficult to perform with a considerable amount of data scatter. Thus, the interfacial test was discarded in favor of the fracture toughness test. Interfacial strength development was observed by measuring the refracture toughness of precracked compact tension specimens that were rehealed at a given temperature and contact time. The measured refracture toughness was correlated with temperature and contact time. Interfacial strength development in graphite-polysulfone unidirectional composites was measured using a double cantilever beam (DCB) interlaminar fracture toughness test. The critical strain energy release rate of refractured composite specimens was measured as a function of healing temperature and contact time.

Howes, Jeremy C.↗
NASA NTRS1988

A fiber-resin micromechanics analysis of the delamination front in a DCB specimen

A 3-D finite element model was developed to analyze the fiber-resin behavior near the delamination front in a graphite-epoxy double cantilever beam (DCB) specimen. The specimen interior was analyzed using a typical one-fiber slice, represented by a local 3-D fiber-resin model. The resin stresses were computed for the resin-rich layer at the ply interface as well as for the regions between the fibers close to the delamination front. However, the computed strain energy release rate G sub I along the delamination front varied by less than two percent, and was within about four percent of the plane-strain value. The von Mises yield criterion was used to estimate the extent of yielding near the delamination front. The yielding extended ahead of the delamination and also developed between the fibers. Although the fibers had only a negligible effect on G sub I, they caused yielding within the ply and therefore could influence delamination fracture toughness. The normal and shear stresses at the fiber-resin interface were computed near the delamination front. These results suggest that multi-axial stress criteria may be required to analyze fiber-resin interfaces.

Crews, J. H.↗
NASA NTRS1988

Interlaminar fracture toughness of thermoplastic composites

Edge delamination tension and double cantilever beam tests were used to characterize the interlaminar fracture toughness of continuous graphite-fiber composites made from experimental thermoplastic polyimides and a model thermoplastic. Residual thermal stresses, known to be significant in materials processed at high temperatures, were included in the edge delamination calculations. In the model thermoplastic system (polycarbonate matrix), surface properties of the graphite fiber were shown to be significant. Critical strain energy release rates for two different fibers having similar nominal tensile properties differed by 30 to 60 percent. The reason for the difference is not clear. Interlaminar toughness values for the thermoplastic polyimide composites (LARC-TPI and polyimidesulfone) were 3 to 4 in-lb/sq in. Scanning electron micrographs of the EDT fracture surfaces suggest poor fiber/matrix bonding. Residual thermal stresses account for up to 32 percent of the strain energy release in composites made from these high-temperature resins.

Hinkley, J. A.↗
NASA NTRS1988

Characterization of Mode 1 and Mode 2 delamination growth and thresholds in graphite/peek composites

Composite materials often fail by delamination. The onset and growth of delamination in AS4/PEEK, a tough thermoplastic matrix composite, was characterized for mode 1 and mode 2 loadings, using the Double Cantilever Beam (DCB) and the End Notched Flexure (ENF) test specimens. Delamination growth per fatigue cycle, da/dN, was related to strain energy release rate, G, by means of a power law. However, the exponents of these power laws were too large for them to be adequately used as a life prediction tool. A small error in the estimated applied loads could lead to large errors in the delamination growth rates. Hence strain energy release rate thresholds, G sub th, below which no delamination would occur were also measured. Mode 1 and 2 threshold G values for no delamination growth were found by monitoring the number of cycles to delamination onset in the DCB and ENF specimens. The maximum applied G for which no delamination growth had occurred until at least 1,000,000 cycles was considered the threshold strain energy release rate. Comments are given on how testing effects, facial interference or delamination front damage, may invalidate the experimental determination of the constants in the expression.

Martin, Roderick H.↗
NASA NTRS1985

Fuel containment and damage tolerance in large composite primary aircraft structures. Phase 2: Testing

Technical issues associated with fuel containment and damage tolerance of composite wing structures for transport aircraft were investigated. Material evaluation tests were conducted on two toughened resin composites: Celion/HX1504 and Celion/5245. These consisted of impact, tension, compression, edge delamination, and double cantilever beam tests. Another test series was conducted on graphite/epoxy box beams simulating a wing cover to spar cap joint configuration of a pressurized fuel tank. These tests evaluated the effectiveness of sealing methods with various fastener types and spacings under fatigue loading and with pressurized fuel. Another test series evaluated the ability of the selected coatings, film, and materials to prevent fuel leakage through 32-ply AS4/2220-1 laminates at various impact energy levels. To verify the structural integrity of the technology demonstration article structural details, tests were conducted on blade stiffened panels and sections. Compression tests were performed on undamaged and impacted stiffened AS4/2220-1 panels and smaller element tests to evaluate stiffener pull-off, side load and failsafe properties. Compression tests were also performed on panels subjected to Zone 2 lightning strikes. All of these data were integrated into a demonstration article representing a moderately loaded area of a transport wing. This test combined lightning strike, pressurized fuel, impact, impact repair, fatigue and residual strength.

Sandifer, J. P.↗
NASA NTRS1987

The role of damage-softened material behavior in the fracture of composites and adhesives

The failure mechanism of polymer composites and adhesives with high strain in the zone near and ahead of the crack tip is investigated analytically, summarizing the findings of Ungsuwarungsri (1986). A double-cantilever-beam specimen with the nonlinear material confined to a thin strip between two wedge-loaded elastic beams is modeled as a beam on a nonlinear foundation; the two-point boundary-value problem for stationary and propagating cracks is solved numerically; and an FEM approach is applied to study the behavior of the nonlinear strip in detail. Typical results are presented in extensive graphs, and the accuracy and efficiency of the present method are shown to be superior to those of the Berry (1963) procedure.

Ungsuwarungsri, T.↗
NASA NTRS1987

Rate effects on delamination fracture toughness of graphite/epoxy composites

The effects of loading rate on interlaminar fracture toughness were investigated for two graphite/epoxy materials, one with a brittle matrix and the other with a toughened one. Mode I delamination fracture toughness was studied by means of uniform-width, width-tapered, and height-tapered double cantilever beam specimens. The specimens were loaded at various crosshead rates corresponding to crack extension rates up to 26 m/s. It was found that for the brittle-matrix composite the energy release rate increases up to a certain crack velocity and thereafter it decreases. In the case of the toughened-matrix composite the energy release rate decreases monotonically with increasing crack velocity.

Daniel, I. M.↗
NASA NTRS1988

Autohesive strength development in polysulfone resin and graphite-polysulfone composites

The effects of bonding temperature and contact time on autohesive strength development in thermoplastic polysulfone resin and graphite-polysulfone composites were investigated. Two test methods were examined to measure autohesion in the neat resin samples. These included an interfacial tension test and a compact tension fracture toughness test. Autohesive strength development in fiber-reinforced composites was measured using a double cantilever beam interlaminar fracture toughness test. The results of the tests were compared with current diffusion theories explaining crack healing and welding of glassy polymers. Discrepancies between the results of the present investigation and the diffusion theories are discussed.

Howes, Jeremy C.↗
NASA NTRS1987

A microscopy study of composite delamination

Double cantilever beam specimens of carbon fiber-epoxy composites were tested for interlaminar fracture energy. While still under load, the specimens were potted in a clear epoxy polymer. The potted specimens were cut longitudinally into sections 0.6 mm (0.025 in.) or less in thickness and examined using reflection and transmission light microscopy. Massive fiber bridging was observed especially at the crack tip.

Bascom, W. D.↗
NASA NTRS1988

A mixed-mode bending apparatus for delamination testing

A mixed-mode delamination test procedure was developed combining double cantilever beam mode I loading and end notch flexure mode II loading on a split unidirectional laminate. By loading the specimen with a lever, a single applied load simultaneously produces mode I and II bending loads on the specimen. This mixed mode bending (MMB) test was analyzed using both finite element procedures and beam theory to calculate the mode I and II components of strain energy release rate, G sub I and G sub II, respectively. The analyses showed that a wide range of G sub I/G sub II ratios could be produced by varying the applied load position on the loading lever. As the delamination extended, the G sub I/G sub II ratios varied by less than 5 percent. The simple beam theory equations were modified to account for the elastic interaction between the two arms of the specimen and to account for shear deformations. The resulting equations agreed closely with the finite element results and provide a basis for selection of G sub I/G sub II test ratios and a basis for computing the mode I and II components of measured delamination toughness. The MMB specimen analysis and test procedures were demonstrated using unidirectional laminates.

Crews, John H., Jr.↗
NASA NTRS1988

Adhesively bonded joints: Testing, analysis, and design; Proceedings of the International Symposium, Baltimore, MD, Sept. 10-12, 1986

The papers contained in this volume provide an overview of the state of the art of testing, analysis, and design of adhesively bonded joints. The papers are grouped in the following sections: mechanical testing, stress analysis, failure mechanisms, and design and durability. Specific topics discussed include determination and verification of elastic parameters for adhesives; factors influencing elastic stresses in double cantilever beam specimens; special mixed finite elements for interfacial stress analysis of adhesively bonded joints; debonding characteristics of adhesively bonded woven Kevlar composites; and environmental durability of adhesively bonded joints.

Johnson, W. S.↗
NASA NTRS1989

Effect of bond thickness on fracture and fatigue strength of adhesively bonded composite joints

An experimental investigation of composite to composite bonded joints was undertaken to study the effect of bond thickness on debond growth rate under cyclic loading and critical strain energy release rate under static loading. Double cantilever beam specimens of graphite/epoxy adherends bonded with EC 3445 were tested under mode I loading. A different behavior of fracture and fatigue strength was observed with variation of bondline thickness.

Mall, S.↗
NASA NTRS1989

The mixed-mode bending method for delamination testing

A mixed-mode bending (MMB) test procedure is presented which combines double cantilever beam mode-I loading and end-notch flexure mode II loading on a split, unidirectional laminate. The MMB test has been analyzed by FEM and by beam theory in order to ascertain the mode I and mode II components' respective strain energy release rates, G(I) and G(II); these analyses indicate that a wide range of G(I)/G(II) ratios can be generated by varying the applied load's position on the loading lever. The MMB specimen analysis and test procedures are demonstrated for the case of AS4/PEEK unidirectional laminates.

Reeder, James R.↗
NASA NTRS1989

Anisotropic thermal electron distributions in the solar wind

This paper presents data on ISEE 3 observations of unusually anisotropic core electrons detected in the solar wind near 1 AU, which exhibited temperature (T) ratios T-parallel/T-perpendicular of 1.5 to 4. Distinct anisotropy events are described, together with the observed correlations between core temperature anisotropy and various other solar wind parameters. It was found that the periods of extreme electron anisotropy tended to be coincident with intervals of double ion beam, suggesting similar causal mechanisms for the two phenomena.

Phillips, J. L.↗
NASA NTRS1989

The interlaminar fracture toughness of woven graphite/epoxy composites

The interlaminar fracture toughness of 2-D graphite/epoxy woven composites was determined as a function of stacking sequence, thickness, and weave pattern. Plain, oxford, 5-harness satin, and 8-harness satin weaves of T300/934 material were evaluated by the double cantilever beam test. The fabric material had a G (sub Ic) ranging from 2 to 8 times greater than 0 degrees unidirectional T300/934 tape material. The interlaminar fracture toughness of a particular weave style was dependent on whether the stacking sequence was symmetric or asymmetric and, in some cases, on the fabric orientation.

Funk, Joan G.↗
NASA NTRS1989

Effects of T-tabs and large deflections in DCB specimen tests

A simple strength of materials analysis was developed for a double-cantilever beam (DCB) specimen to account for geometric nonlinearity effects due to large deflections and T-tabs. A new DCB data analysis procedure was developed to include the effects of these nonlinearities. The results of the analysis were evaluated by DCB tests performed for materials having a wide range of toughnesses. The materials used in the present study were T300/5208, IM7/8551-7, and AS4/PEEK. Based on the present analysis, for a typical deflection/crack length ratio of 0.3 (for AS4/PEEK), T-tabs and large deflections cause a 15 percent and 3 percent error, respectively, in the computer Mode 1 strain energy release rate. Design guidelines for DCB specimen thickness and T-tab height were also developed in order to keep errors due to these nonlinearities within 2 percent. Based on the test results, for both hinged and tabbed specimens, the effects of large deflection on the Mode 1 fracture toughness (G sub Ic) were almost negligible (less than 1 percent) in the case of T300/5208 and IM7/8551-7; however, AS4/PEEK showed a 2 to 3 percent effect. The effects of T-tabs G sub Ic were more significant for all the materials with T300/5208 showing a 5 percent error, IM7/8551-7 a 15 percent error, and, AS4/PEEK a 20 percent error.

Naik, R. A.↗
NASA NTRS1989

Toughness properties of a three-dimensional carbon-epoxy composite

The three-dimensional (3D) orthogonal interlocked fabric contains through-the-thickness reinforcement in order to enhance the interlaminar fracture toughness of the composite. The interlaminar fracture toughness of a carbon-epoxy orthogonal interlocked fabric composite was experimentally determined by use of the recently developed tabbed double cantilever beam specimen. The data reduction methods applicable to these tests and materials and the interpretation of the results were discussed. The results of critical strain energy release rate, G(IC), were compared to those of a two-dimensional (2D) laminate having the same in-plane structure. The energy-dissipating crack propagation processes were described. The in-plane fracture toughness of the three-dimensional fabric was experimentally measured and compared to that of the two-dimensional laminate. The through-the-thickness fibers were found to create a tenfold increase in interlaminar toughness, and a 25-percent improvement in the in-plane fracture toughness.

Guenon, Valerie↗
NASA NTRS1989

Interlaminar fracture toughness of thermoplastic composites

Edge delamination tension and double cantilever beam tests were used to characterize the interlaminar fracture toughness of continuous graphite-fiber composites made from experimental thermoplastic polyimides and a model thermoplastic. Residual thermal stresses, known to be significant in materials processed at high temperatures, were included in the edge delamination calculations. In the model thermoplastic system (polycarbonate matrix), surface properties of the graphite fiber were shown to be significant. Cricital strain energy release rates for two different fibers having similar nominal tensile properties differed by 30 to 60 percent. The reason for the difference is not clear. Interlaminar toughness values for the thermoplastic polyimide composites (LARC-TPI and polyimidesulfone) were 3 to 4 in-lb/sq in. Scanning electron micrographs of the EDT fracture surfaces suggest poor fiber/matrix bonding. Residual thermal stresses account for up to 32 percent of the strain energy release in composites made from these high-temperature resins.

Hinkley, Jeffrey A.↗