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At least 937 records · Page 52

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NASA NTRS1984

The nonlinear viscoelastic response of resin matrix composite laminates

Possible treatments of the nonlinear viscoelastic behavior of materials are reviewed. A thermodynamic based approach, developed by Schapery, is discussed and used to interpret the nonlinear viscoelastic response of a graphite epoxy laminate, T300/934. Test data to verify the analysis for Fiberite 934 neat resin as well as transverse and shear properties of the unidirectional T300/934 composited are presented. Long time creep characteristics as a function of stress level and temperature are generated. Favorable comparisons between the traditional, graphical, and the current analytical approaches are shown. A free energy based rupture criterion is proposed as a way to estimate the life that remains in a structure at any time.

Hiel, C.↗

NASA NTRS1984

A Shear Deformable Shell Element for Laminated Composites

A three-dimensional element based on the total Lagrangian description of the motion of a layered anisotropic composite medium is developed, validated, and used to analyze layered composite shells. The element contains the following features: geometric nonlinearity, dynamic (transient) behavior, and arbitrary lamination scheme and lamina properties. Numerical results of nonlinear bending, natural vibration, and transient response are presented to illustrate the capabilities of the element.

Chao, W. C.↗

NASA NTRS1984

Hole interaction and load introduction effects for compression-loaded laminates with holes

The effects of hole interaction and load introduction for (+ or - 30/90) sub 8 (s) and (+ or - 45) sub 12 (s) laminates with holes was conducted. A hole interaction region was identified in some specimens, and this region was ineffective for carrying load. The membrane stiffness of a specimen with a hole interaction region is less than the membrane stiffness of a similar specimen without a hole interaction region. A load introduction effect that occurs on the sublaminate level was also observed. This effect can cause a nonuniform load diffusion into the interior of the specimen.

Shuart, M. J.↗

NASA NTRS1984

Fracture modes in notched angleplied composite laminates

The Composite Durability Structural Analysis (CODSTRAN) computer code is used to determine composite fracture. Fracture modes in solid and notched, unidirectional and angleplied graphite/epoxy composites were determined by using CODSTRAN. Experimental verification included both nondestructive (ultrasonic C-Scanning) and destructive (scanning electron microscopy) techniques. The fracture modes were found to be a function of ply orientations and whether the composite is notched or unnotched. Delaminations caused by stress concentrations around notch tips were also determined. Results indicate that the composite mechanics, structural analysis, laminate analysis, and fracture criteria modules embedded in CODSTRAN are valid for determining composite fracture modes.

Irvine, T. B.↗

NASA NTRS1983

Fire- and heat-resistant laminating resins based on maleimido-substituted aromatic cyclotriphosphazenes

A novel class of flame- and heat-resistant polymers has been synthesized by the thermal polymerization of maleimido-substituted aromatic cyclotriphosphazenes. The polymer obtained from tris-(aminophenoxy)tris(maleimidophenoxy)cyclotriphosphazene has good thermal stability and is noteworthy for its high char yield, viz., 82 percent at 800 C in nitrogen and 81 percent at 700 C in air. Graphite-fabric laminates prepared with this polymer did not burn in pure oxygen, even at 300 C, and were tested for mechanical properties. Hexakis(4-maleimidophenoxy)cyclotriphosphazene and some fluorine-containing monomers have also been synthesized. The structures of these cyclic phosphazene precursors and polymers were characterized by FT IR spectrophotometry, H-1 NMR, F-19 NMR, and P-31 solid-state magic angle spinning NMR spectroscopy, and mass spectrometry. The curing behavior of the polymer precursors and the thermal stabilities of the polymers were evaluated by differential scanning calorimetry (DSC) and thermogravimetric analyses (TGA).

Kumar, D.↗

NASA NTRS1983

Low velocity transverse impact behavior of 8-ply, graphite-epoxy laminates

This work discusses the behavior of eight-ply, quasi-isotropic, graphite epoxy laminates subjected to low velocity transverse impact loading. Large deflection theory of plates was used to predict the load-deflection characteristics during the impact event. The impact model considered that the indentation, flexural and shear stiffnesses could be represented by three-equivalent springs in series. The analysis of static and dynamic impact loading test-data concluded that the membrane parameter, beta, used in flexural stiffness relation was proportional to the square of the coefficient of restitution, e, of the impactor. Use of beta=0.443 e-squared was made in load-deflection relations to consider the drop in plate stiffness due to delamination. The results of impact tests up to impact velocity 5.3 m/sec. were found in close agreement with the predictions.

Lal, K. M.↗

NASA NTRS1983

Environmental effects and viscoelastic behavior of laminated graphite/epoxy composites

Primarily because of the polymeric matrix, graphite/epoxy composites are viscoelastic materials which exhibit creep and delayed failures. Guided by the time-temperature superposition principle, the authors are developing accelerated characterization techniques to predict the long term compliance behavior and creep rupture strength of composite materials based on short term tests at elevated temperatures. The effect of a post-cure cycle on the compliance and creep rupture strength of composite materials is discussed. The Zhurkov type failure law is applied to experimental creep rupture data of a typical laminate and is shown to correlate the data. A beneficial mechanical strengthening effect is described which significantly affects the failure strengths of specimens which have been under creep loading for a period of time. This aging effect is reversible if the specimen is allowed to recover before being loaded to failure. Other environmental aspects of composite materials are also reviewed.

Dillard, D. A.↗

NASA NTRS1983

Bolt clampup relaxation in a graphite/epoxy laminate

A simple bolted joint was analyzed to calculate bolt clampup relaxation for a graphite/epoxy (T300/5208) laminate. A viscoelastic finite element analysis of a double-lap joint with a steel bolt was conducted. Clampup forces were calculated for various steady-state temperature-moisture conditions using a 20-year exposure duration. The finite element analysis predicted that clampup forces relax even for the room-temperature-dry condition. The relaxations were 8, 13, 20, and 30 percent for exposure durations of 1 day, 1 month, 1 year, and 20 years, respectively. As expected, higher temperatures and moisture levels each increased the relaxation rate. The combined viscoelastic effects of steady-state temperature and moisture appeared to be additive. From the finite-element analysis, a simple equation was used to calculate clampup forces for the same temperature-moisture conditions as used in the finite-element analysis. The two sets of calculated results agreed well. Previously announced in STAR as N82-19320

Shivakumar, K. N.↗

NASA NTRS1983

Load sequence effects on graphite/epoxy /+, -35/2s laminates

Load sequence effects on composite material fatigue behavior are due to the difference in residual strength when fatigue occurs, or the 'boundary effect', and the 'material money effect' due to previous loads. The present model, which allows the isolation of the effects of memory from those of boundary, has been investigated through a test program for the case of /+, -35/2s graphite/epoxy laminates under high-low and low-high load sequences which generated statistical residual strength data. As predicted by the model, the memory effect is present and a good correlation is obtained between test results and the theoretical predictions for both median residual strength and the expected percentage of fatigue failures under dual loading. It is noted, however, that the statistical dispersion of the residual strength test results is much smaller than that of the static ultimate strength.

Yang, J. N.↗

NASA NTRS1982

Indentation law for composite laminates

Static indentation tests are described for glass/epoxy and graphite/epoxy composite laminates with steel balls as the indentor. Beam specimens clamped at various spans were used for the tests. Loading, unloading, and reloading data were obtained and fitted into power laws. Results show that: (1) contact behavior is not appreciably affected by the span; (2) loading and reloading curves seem to follow the 1.5 power law; and (3) unloading curves are described quite well by a 2.5 power law. In addition, values were determined for the critical indentation, alpha sub cr which can be used to predict permanent indentations in unloading. Since alpha sub cr only depends on composite material properties, only the loading and an unloading curve are needed to establish the complete loading-unloading-reloading behavior. Previously announced in STAR as N82-15123

Yang, S. H.↗

NASA NTRS1983

Natural vibrations of laminated anisotropic plates using three-dimensional elasticity theory

The paper contains a description of the three-dimensional elasticity equations and the associated finite element model for natural vibrations of laminated anisotropic rectangular plates. The numerical results for natural frequencies are compared with those obtained by a shear deformable plate theory. A number of cross-ply and angle-ply rectangular plates are analyzed for natural frequencies. For relatively thick plates, the shear deformable-plate theory element predicts frequencies higher than those predicted by the three-dimensional elasticity theory element.

Reddy, J. N.↗

NASA NTRS1983

Effect of load introduction in compression testing of composite laminates

Compression testing of composite materials is affected by the manner in which the compressive load is introduced. Two such effects are studied in this paper: (a) the constrained edge effect, in which transverse expansion of the edges is prevented while the axial load is introduced, and (b) nonuniform gripping, as manifested by inplane bending of the test specimen. The principle of minimum complementary energy is used to develop an analytical model that quantifies these two effects upon the measured elastic properties of laminated composites. Numerical results are presented for selected high-strength graphite/epoxy composites.

Reiss, R.↗

NASA NTRS1984

Failure analysis for composite laminates

A discussion of the tensor polynomial failure criterion is presented together with a description of a biaxial compression test procedure used in the evaluation of the interaction strength parameters. In addition, a formulation for predicting the fatigue life of laminates is given based on the experimental evaluation of fatigue functions which are also utilized in a form of the tensor polynomial failure criterion as well. Experimental results are provided to compare with the fatigue life predictions and to demonstrate the effect of flaws and environment. Both bond-line defects in sandwich beam construction and inter-laminar disbond flaws are studied. Up to present, compressive strength test data have been obtained for ambient and elevated temperature, moisture-saturated conditions, including results from thermal-spike cycling simulating supersonic flight.

Tennyson, R. C.↗

NASA NTRS1984

Large deflection, large amplitude vibrations and random response of symmetrically laminated rectangular plates

An analytical method is presented for determining large-deflection static bending, large-amplitude free and forced vibrations, and large-amplitude random response of a clamped, symmetrically laminated, rectangular, thin plate subjected to a uniformly distributed transverse loading. Both movable and immovable inplane boundary conditions are considered. Numerical results for bending deflections and strains, frequency ratios, mean-square center deflections and mean-square maximum strains are presented showing the parametric effects of plate length-to-width ratio, orientation of layers, and intensities of applied force for both the linear and nonlinear cases. The analytical results for large-deflection random response are verified through comparison with experimental data.

Gray, C. E., Jr.↗

NASA NTRS1983

Damping mechanisms in fiber reinforced laminates

Low strain damping in fiber reinforced composite materials is due to material loss factors in both fibers and matrix materials. The high modulus of the fibers makes up for the low damping factor. Strains high enough to cause transverse layer cracking in laminates with organic matrix materials causes a large permanent increase in the damping factor. This increase is not due to the transverse cracks but rather to short microcracks in the high shear strain regions. Other damping mechanisms at high strains are also discussed.

Plunkett, R.↗

NASA NTRS1984

Eigenvalue similarity rules for symmetric cross-ply laminated plates

Symmetric cross-ply laminated plates are analyzed by employing a previously presented affine stretching process, some material constant definitions from Tsai (1964), and by gneralizing existing similarity rules. As examples, similarity rules are employed to find the frequency spectrum of a cross-ply symmetric plate from the corresponding isotropic solution, and a similarity solution is presented for the vibration of an axially loaded, simply supported cross-ply symmetric plate. Buckling similarity rules are also presented.

Brunelle, E. J.↗

NASA NTRS1984

Natural vibrations of laminated anisotropic plates

This paper contains a description of the three-dimensional elasticity equations and the associated finite element model for natural vibrations of laminated rectangular plates. The numerical results for natural frequencies are compared with those obtained by a shear deformable plate theory. A number of cross-ply and angle-ply rectangular plates are analyzed for natural frequencies. For relatively thick plates, the plate element predicts frequencies higher than those predicted by the 3-D element.

Reddy, J. N.↗

NASA NTRS1984

A mixed shear flexible finite element for the analysis of laminated plates

A mixed shear flexible finite element based on the Hencky-Mindlin type shear deformation theory of laminated plates is presented and their behavior in bending is investigated. The element consists of three displacements, two rotations, and three moments as the generalized degrees of freedom per node. The numerical convergence and accuracy characteristics of the element are investigated by comparing the finite element solutions with the exact solutions. The present study shows that reduced-order integration of the stiffness coefficients due to shear is necessary to obtain accurate results for thin plates.

Putcha, N. S.↗