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At least 109 records · Page 6

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NASA NTRS1986

Interlaminar analysis of laminated composites using a sublaminate approach

A simple analysis method based upon a transverse shear deformation theory and a sublaminate approach is utilized to analyze a Mixed-Mode edge delamination specimen. The analysis provides a closed form distribution of the interlaminar shear stresses ahead of the crack, and the parameters controlling the behavior are identified. The effect of specimen stacking sequence and delamination interface on the strain energy release rate components is investigated. Results are compared with a finite element simulation for reference. The simple nature of the method makes it suitable for preliminary design analyses which require a large number of configurations to be evaluated quickly and economically.

Armanios, E. A.↗

NASA NTRS1986

Fracture of thick graphite/epoxy laminates with part-through surface flaws

Fracture behavior of tensile specimens containing through-the-thickness flaws and part-through semi-elliptic surface flaws was investigated. The material was T300/5208 graphite/epoxy, with a stacking sequence 0/+ or - 45/90 sub ns. Laminate thicknesses ranged from 8 to 120 plies. Fracture toughness for specimens with through-the-thickness flaws was a function of laminate thickness. Fracture toughness was also independent of crack size. Fracture toughness for the thick laminates was also independent of specimen geometry (center-cracked tension specimen, compact tension specimen, and three-point bend specimen). Using the asymptotic value of fracture toughness and linear elastic isotropic fracture mechanics, the notched strength of tensile specimens with semi-elliptic surface flaws was predicted. The strength ratio is a function of the flaw depth (a/t) and flaw aspect (a/c) ratios. Comparisons between predicted and experimental strengths are good for large values of a/t. The notched strength of filament wound graphite/epoxy tensile specimens with part-through surface flaws was also predicted.

Harris, C. E.↗

NASA NTRS1984

Sizing-stiffened composite panels loaded in the postbuckling range

Stiffened panels are widely used in aircraft structures such as wing covers, fuselages, control surfaces, spar webs, bulkheads, and floors. The detailed sizing of minimum-weight stiffened panels involves many considerations. Use of composite materials introduces additional complexities. Many potential modes of failure exist. Analyses for these modes are often not trivial, especially for those involving large out-of-plane displacements. Accurate analyses of all potential failure modes are essential. Numerous practical constraints arise from manufacturing/cost considerations and from damage tolerance, durability, and stiffness requirements. The number of design variables can be large when lamina thicknesses and stacking sequence are being optimized. A significant burden is placed on the sizing code due to the complex analyses, practical constraints, and number of design variables. On the other hand, sizing weight-efficient panels without the aid of an automated procedure is almost out of the question. The sizing code postbuckled Open-Stiffener Optimum Panels (POSTOP) has been developed to aid in the design of minimum-weight panels subject to the considerations mentioned above. Developed for postbuckled composite panels, POSTOP may be used for buckling resistant panels and metallic panels as well. The COPES/CONMIN optimizer is used in POSTOP although other options such as those in the ADS system could be substituted with relative ease. The basic elements of POSTOP are shown. Some of these elements and usage of the program are described.

Biggers, S. B.↗

NASA NTRS1986

Observation of the initiation and progression of damage in compressively loaded composite plates containing a cutout

A series of experiments was carried out to determine the mechanism of failure in compressively loaded laminated plates with a circular cutout. Real time holographic interferometry and photomicrography are used to observe the progression of failure. These observations together with post experiment plate sectioning and deplying for interior damage observation provide useful information for modelling the failure process. It is revealed that the failure is initiated as a localised instability in the zero layers, at the hole surface. With increasing load extensive delamination cracking is observed. The progression of failure is by growth of these delaminations induced by delamination buckling. Upon reaching a critical state, catastrophic failure of the plate is observed. The levels of applied load and the rate at which these events occur depend on the plate stacking sequence.

Waas, A.↗

NASA NTRS1987

A parametric study of fracture toughness of fibrous composite materials

Impacts to fibrous composite laminates by objects with low velocities can break fibers giving crack-like damage. The damage may not extend completely through a thick laminate. The tension strength of these damage laminates is reduced much like that of cracked metals. The fracture toughness depends on fiber and matrix properties, fiber orientations, and stacking sequence. Accordingly, a parametric study was made to determine how fiber and matrix properties and fiber orientations affect fracture toughness and notch sensitivity. The values of fracture toughness were predicted from the elastic constants of the laminate and the failing strain of the fibers using a general fracture toughness parameter developed previously. For a variety of laminates, values of fracture toughness from tests of center-cracked specimens and values of residual strength from tests of thick laminates with surface cracks were compared to the predictions to give credibility to the study. In contrast to the usual behavior of metals, it is shown that both ultimate tensile strength and fracture toughness of composites can be increased without increasing notch sensitivity.

Poe, C. C., Jr.↗

NASA NTRS1987

A structural model for composite rotor blades and lifting surfaces

Composite material systems are currently candidates for aerospace structures, primarily for the design flexibiity they offer i.e., it is possible to tailor the material and manufacturing approach to the application. Two notable examples are the wing of the Grumman/USAF/DARPA X-29 and rotor blades under development by the U.S.A. Aerostructures Directorate (AVSCOM), Langley Research Center. A working definition of elastic or structural tailoring is the use of structural concept, fiber orientation, ply stacking sequence, and a blend of materials to achieve specific performance goals. In the design process, choices of materials and dimensions are made which produce specific response characteristics which permit the selected goals to be achieved. Common choices for tailoring goals are preventing instabilities or vibration resonances or enhancing damage tolerance. An essential, enabling factor in the design of tailored composite structures is structural modeling that accurately, but simply, characterizes response. The objective of this paper is to improve the single-cell beam model for composite rotor blades or lifting surfaces and to demonstrate its usefullness in applications.

Rehfield, Lawrence W.↗

NASA NTRS1987

Structural modeling for multicell composite rotor blades

Composite material systems are currently good candidates for aerospace structures, primarily for the design flexibility they offer, i.e., it is possible to tailor the material and manufacturing approach to the application. A working definition of elastic or structural tailoring is the use of structural concept, fiber orientation, ply stacking sequence, and a blend of materials to achieve specific performance goals. In the design process, choices of materials and dimensions are made which produce specific response characteristics, and which permit the selected goals to be achieved. Common choices for tailoring goals are preventing instabilities or vibration resonances or enhancing damage tolerance. An essential, enabling factor in the design of tailored composite structures is structural modeling that accurately, but simply, characterizes response. The objective of this paper is to present a new multicell beam model for composite rotor blades and to validate predictions based on the new model by comparison with a finite element simulation in three benchmark static load cases.

Rehfield, Lawrence W.↗

NASA NTRS1986

The role of fiber and matrix in crash energy absorption of composite materials

Static crushing tests were conducted on tube specimens fabricated from graphite/epoxy, Kevlar/epoxy and hybrid combinations of graphite-Kevlar/epoxy to examine the influence the fiber and matrix constitutive properties and laminate architecture have on energy absorption. Fiber and matrix ultimate failure strain were determined to significantly effect energy absorption. The energy absorption capability of high ultimate failure strain materials (AS-6/F185 and AS-6/HST-7) was less than materials having lower ultimate failure strain. Lamina stacking sequence had up to a 300 percent change in energy absorption for the materials tested. Hybridizing with graphite and Kevlar reinforcements resulted in materials with high energy absorption capabilities that have postcrushing integrity.

Farley, G. L.↗

NASA NTRS1987

An integrated finite element model of edge-delamination analysis for laminates due to tension, bending, and torsion loads

A two-dimensional finite-element model has been developed to calculate interlaminar stresses and strain energy release rates for the study of delamination in composite laminates subjected to various loads. This paper addresses the formulation, implementation, and verification of a suitable 'element for uniaxial tension, bending, and torsion', as well as parametric studies of the interlaminar stresses and strain energy release rates for various composite laminates. The study concentrates on establishing relationships of GI, GII, and GIII as functions of crack length and stacking sequence in laminates due to bending. Preliminary results of laminates subjected to torsional loads are included to increase the applicability and completeness of the present formulation.

Chan, W. S.↗

NASA NTRS1987

Thermal expansion of selected graphite-reinforced polyimide-, epoxy-, and glass-matrix composites

The thermal expansion of continuous carbon-fiber reinforced composites with epoxy-, polyimide-, and borosilicate glass-matrices has been measured and compared. The expansion of a rubber-toughened epoxy-matrix/P75S carbon-fiber composite was very different from the expansion of two different single-phase epoxy-matrix/P75S composites, although all three had the same stacking sequence. Reasonable agreement was obtained between measured thermal expansion data and results from classical laminate theory. Microdamage in the graphite/polyimide laminate, induced by 250 cycles between -156 and 121 C, caused a 53 percent decrease in the coefficient of thermal expansion. The thermal expansion of the graphite/glass laminate was not changed after 100 thermal cycles from -129 to 38 C; however, a residual strain of about 10 x 10 to the -6 was observed for the laminate tested.

Tompkins, S. S.↗

NASA NTRS1988

An analytical study of the effects of transverse shear deformation and anisotropy on natural vibration frequencies of laminated cylinders

Natural vibration frequencies of orthotropic and anisotropic simply supported right circular cylinders are predicted using a higher-order transverse-shear deformation theory. A comparison of natural vibration frequencies predicted by first-order transverse-shear deformation theory and the higher-order theory shows that an additional allowance for transverse shear deformation has a negligible effect on the lowest predicted natural vibration frequencies of laminated cylinders but significantly reduces the higher natural vibration frequencies. A parametric study of the effects of ply orientation on the natural vibration frequencies of laminated cylinders indicates that while stacking sequence affects natural vibration frequencies, cylinder geometry is more important in predicting transverse-shear deformation effects. Interaction curves for cylinders subjected to axial compressive loadings and low natural vibration frequencies indicate that transverse shearing effects are less important in predicting low natural vibration frequencies than in predicting axial compressive buckling loads. The effects of anisotropy are more important than the effects of transverse shear deformation for most strongly anisotropic laminated cylinders in predicting natural vibration frequencies. However, transverse-shear deformation effects are important in predicting high natural vibration frequencies of thick-walled laminated cylinders. Neglecting either anisotropic effects or transverse-shear deformation effects leads to non-conservative errors in predicted natural vibration frequencies.

Jegley, Dawn C.↗

NASA NTRS1987

A parametric study of fracture toughness of fibrous composite materials

Poe, C. C., Jr.↗

NASA NTRS1988

Residual thermal and moisture influences on the free-edge delamination of laminated composites

A simple delamination analysis method is developed. It is based on a shear-type deformation theory and includes hygrothermal effects. The method is applied to mixed-mode edge delamination specimens made of T300/5208 graphite/epoxy. It is found that residual thermal influences were alleviated when moisture stresses were included. Thermal effects on the interlaminar shear stress and total energy release rate were totally alleviated for the same specific moisture content. Moreover, this value of moisture content was not significantly affected by the stacking sequence for the laminates considered.

Armanios, Erian A.↗

NASA NTRS1988

Combined mechanical loading of composite tubes

An analytical/experimental investigation was performed to study the effect of material nonlinearities on the response of composite tubes subjected to combined axial and torsional loading. The effect of residual stresses on subsequent mechanical response was included in the investigation. Experiments were performed on P75/934 graphite-epoxy tubes with a stacking sequence of (15/0/ + or - 10/0/ -15), using pure torsion and combined axial/torsional loading. In the presence of residual stresses, the analytical model predicted a reduction in the initial shear modulus. Experimentally, coupling between axial loading and shear strain was observed in laminated tubes under combined loading. The phenomenon was predicted by the nonlinear analytical model. The experimentally observed linear limit of the global shear response was found to correspond to the analytically predicted first ply failure. Further, the failure of the tubes was found to be path dependent above a critical load level.

Derstine, Mark S.↗

NASA NTRS1988

A study on the thermal and moisture influences on the free-edge delamination of laminated composites

A simple delamination analysis method is presented here. It is based on a shear-type deformation theory and includes hygrothermal effects. These environmental conditions are applied to the strain energy release rate and interlaminar shear stresses. The method is applied to mixed mode edge delamination specimens made of T300/5208 graphite/epoxy material. Residual thermal and moisture stresses significantly influenced the strain energy release rate and interlaminar stresses. Both experienced large increases when thermal conditions were added to the mechanical strains. These effects were alleviated when moisture stresses were included. Thermal effects on the interlaminar shear stress and total energy release rate were totally alleviated for the same specific moisture content. Moreover, the value of the moisture content was not significantly affected by the stacking sequence for the laminates considered.

Mahler, Mary A.↗

NASA NTRS1989

A study of material damping in large space structures

A constitutive model was developed for predicting damping as a function of damage in continuous fiber reinforced laminated composites. The damage model is a continuum formulation, and uses internal state variables to quantify damage and its subsequent effect on material response. The model is sensitive to the stacking sequence of the laminate. Given appropriate baseline data from unidirectional material, and damping as a function of damage in one crossply laminate, damage can be predicted as a function of damage in other crossply laminates. Agreement between theory and experiment was quite good. A micromechanics model was also developed for examining the influence of damage on damping. This model explicitly includes crack surfaces. The model provides reasonable predictions of bending stiffness as a function of damage. Damping predictions are not in agreement with the experiment. This is thought to be a result of dissipation mechanisms such as friction, which are not presently included in the analysis.

Highsmith, A. L.↗

NASA NTRS1989

An analytical and experimental investigation of edge delamination in laminates subjected to tension, bending, and torsion

An integrated two-dimensional finite element was developed to calculate interlaminar stresses and strain energy release rates for the study of delamination in composite laminates subjected to uniaxial tension, bending, and torsion loads. Addressed are the formulation, implementation, and verification of the model. Parametric studies were conducted on the effect of Poisson's ratio mismatch between plies and the stacking sequence on interlaminar stress, and on the effect of delamination opening height and delamination length, due to bending, on strain energy release rate for various laminates. A comparison of strain energy release rates in all-graphite and graphite/glass hybrid laminates is included. The preliminary results of laminates subjected to torsion are also included. Fatigue tension tests were conducted on Mode 1 and mixed mode edge-delamination coupons to establish the relationship between fatigue load vs. onset of delamination cycle. The effect on the fatigue delamination onset of different frequencies (1 and 5 Hz) was investigated for glass, graphite,and their hybrid laminates. Although a 20 percent increase in the static onset-of-delamination strength and a 10 percent increase in ultimate strength resulted from hybridizing the all-graphite laminate with a 90 deg glass ply, the fatigue onset is lower in the hybrid laminate than in the all-graphite laminate.

Chan, Wen S.↗

NASA NTRS1989

Scaling effects in the static large deflection response of graphite-epoxy composite beams

Scaling effects in the large deflection response of graphite-epoxy composite beams was investigated. Eight different scale model beams ranging from 1/6 to full-scale were subjected to an eccentric axial compressive load to promote large bending deformations and failures. Beams having laminate stacking sequences including unidirectional, angle ply, cross ply, and quasi-isotropic were tested to examine a wide variety of composite response and failure modes. The model beams were loaded under scaled test conditions until catastrophic failure. Data acquired included load, end displacement, and strain measurements, and qualitative failure measurements. The experimental data is compared to a large rotation beam analysis and a finite element model analysis. Results from the tests indicate that the beam response becomes nonlinear. Failure modes are consistent between scale models within a laminate family, however, a significant scale effect is observed in strength of the scaled beams.

Jackson, Karen E.↗