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  • 1
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    Reliability of the finite element method for calculating free edge stresses in composite laminates (1982)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: The interlaminar normal stress distributions along the interface between the +45 deg and -45 deg plies of a graphite/epoxy laminate, obtained by various investigators, were found to disagree in both magnitude and sign. The reliability of the displacement-formulated finite element method in analyzing the edge-stress problem of a composite laminate is investigated. The history of the edge-stress problem is reviewed, and two well-known elasticity problems, one involving a stress discontinuity and one a singularity, are analyzed. The finite element analysis in these problems yields accurate stress distributions everywhere except in two elements closest to the stress discontinuity or singularity. Stress distributions for a + or -45 deg ply laminate near the singularity were similar to those of the two elasticity problems, demonstrating the methods, accuracy for calculating interlaminar stresses in composite laminates. The disagreement between the numerical methods was attributed to the unsymmetric stress tensor at singularity.
    Keywords: STRUCTURAL MECHANICS
    Type: Computers and Structures; 15; 1, 19; 1982
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  • 2
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    The accuracy of approximate solutions in the analysis of fracture of composites (1985)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: This paper concerns the accuracy of three related mathematical models (developed by Hedgepeth, Eringen and Sendeckyj and Jones) used in the stress analysis and in fracture studies of continuous-fiber composites. These models have particular application in the investigation of fiber and matrix stresses in unidirectional composites in the region near a crack tip. The interest in such models is motivated by the desire to be able to simplify the equations of elasticity to the point that they can be solved in a relatively easy manner.
    Keywords: STRUCTURAL MECHANICS
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  • 3
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    Vibration of a circular cylindrical elastic tank, partially filled with an incompressible fluid, undergoing an axial acceleration composed of a uniform and a periodic component technical memorandum no. 102 (1965)
    In:  CASI
    Details
    Publication Date: 2019-05-24
    Description: Forced vibration of circular cylindrical elastic shell partially filled with incompressible liquid and initially at rest in uniform gravitational field
    Keywords: STRUCTURAL MECHANICS
    Type: NASA-CR-67173 , E20-116
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  • 4
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    Shear-lag analysis of notched laminates with interlaminar debonding (1984)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The fracture behavior of a debonded zone of finite width with no longitudinal damage in the unidirectional ply is predicted and the solution is then extended to include longitudinal matrix yielding and splitting in the unidirectional ply at the crack tip. The shear-lag assumption is used to describe the shear transfer between fibers. The fracture behavior of the laminate is studied as a function of initial crack length, constraint ratio, and width of the debonded zone. Results indicate that debonding can reduce the maximum fiber stress at the crack tip on the order of ten percent. This effect is maximum for a debond width of two or three fiber spacings and is independent of the initial crack length. As the debond width grows beyond this point, the maximum stress increases. For widths of about ten fiber spacings or more, the maximum fiber stress is larger than for the fully bonded case. In the presence of longitudinal matrix damage the same general behavior is found; however, the location of the maximum fiber stress is quite complex.
    Keywords: COMPOSITE MATERIALS
    Type: NASA-CR-3798 , NAS 1.26:3798
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  • 5
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    Mathematical modeling of damage in unidirectional composites (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Extending the work of Goree and Gross (1979), solutions are given for a two-dimensional region of unidirectional fibers embedded in an elastic matrix whose initial flaw may take the form of a transverse notch, a rectangular cutout, or a circular hole. Subsequent flaw-induced damage is generated by remote stresses acting parallel to the fibers. For the case of such ductile matrix composites as boron/aluminum, present results indicate that both longitudinal matrix yielding and transverse notch extension must be included in order for the model to agree with experimental results. Little difference is found for the three types of initial damage considered. In all cases, the presence of additional damage changes the nature of stress distribution through the unbroken fibers.
    Keywords: COMPOSITE MATERIALS
    Type: Engineering Fracture Mechanics (ISSN 0013-7944); 17; 6 19; 1983
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  • 6
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    Fracture behavior of unidirectional boron/aluminum composite laminates (1983)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: An experiment was conducted to verify the results of mathematical models which predict the stresses and displacements of fibers and the amount of damage growth in a center-notched lamina as a function of the applied remote stress and the matrix and fiber material properties. A brittle lacquer coating was used to detect the yielding in the matrix while X-ray techniques were used to determine the number of broken fibers in the laminate. The notched strengths and the amounts of damage found in the specimens agree well with those predicted by the mathematical model. It is shown that the amount of damage and the crack opening displacement does not depend strongly on the number of plies in the laminate for a given notch width. By heat-treating certain laminates to increase the yield stress of the alumina matrix, the effect of different matrix properties on the fracture behavior was investigated. The stronger matrix is shown to weaken the notched laminate by decreasing the amount of matrix damage, thereby making the laminate more notch sensitive.
    Keywords: COMPOSITE MATERIALS
    Type: NASA-CR-3753 , NAS 1.26:3753
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  • 7
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    Opening of an interface flaw in a layered elastic half-plane under compressive loading (1984)
    In:  CASI
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    Publication Date: 2019-06-28
    Description: A static analysis is given of the problem of an elastic layer perfectly bonded, except for a frictionless interface crack, to a dissimilar elastic half-plane. The free surface of the layer is loaded by a finite pressure distribution directly over the crack. The problem is formulated using the two dimensional linear elasticity equations. Using Fourier transforms, the governing equations are converted to a pair of coupled singular integral equations. The integral equations are reduced to a set of simultaneous algebraic equations by expanding the unknown functions in a series of Jacobi polynomials and then evaluating the singular Cauchy-type integrals. The resulting equations are found to be ill-conditioned and, consequently, are solved in the least-squares sense. Results from the analysis show that, under a normal pressure distribution on the free surface of the layer and depending on the combination of geometric and material parameters, the ends of the crack can open. The resulting stresses at the crack-tips are singular, implying that crack growth is possible. The extent of the opening and the crack-top stress intensity factors depend on the width of the pressure distribution zone, the layer thickness, and the relative material properties of the layer and half-plane.
    Keywords: STRUCTURAL MECHANICS
    Type: NASA-TM-86282 , NAS 1.15:86282
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  • 8
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    Mathematical modeling of damage in unidirectional composites (1981)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A review of some approximate analytical models for damaged, fiber reinforced composite materials is presented. Using the classical shear lag stress displacement assumption, solutions are presented for a unidirectional laminate containing a notch, a rectangular cut-out, and a circular hole. The models account for longitudinal matrix yielding and splitting as well as transverse matrix yielding and fiber breakage. The constraining influence of a cover sheet on the unidirectional laminate is also modeled.
    Keywords: COMPOSITE MATERIALS
    Type: NASA-CR-3453
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  • 9
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    A new look at numerical analyses of free-edge stresses in composite laminates (1980)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The edge stress problem for a + or - 45 deg graphite/epoxy laminate was examined. The reliability of the displacement formulated finite element method in analyzing the edge stress problem was investigated. Analyses of two well known elasticity problems, one involving a stress discontinuity and one a singularity, showed that the finite element analysis yields accurate stress distributions everywhere except in two elements closest to the stress discontinuity of singularity. Stress distributions for a + or - 45 deg laminate showed the same behavior near the singularity found in the well known problems with exact solutions. The displacement formulated finite element method appears to be a highly accurate technique for calculating interlaminar stress in composite laminates. The disagreement among the numerical methods was attributed to the unsymmetric stress tensor at the singularity.
    Keywords: STRUCTURAL MECHANICS
    Type: NASA-TP-1751 , L-13777
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  • 10
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    Stresses in a three-dimensional unidirectional composite containing broken fibers (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: An approximate solution is developed for the determination of the interlaminar normal and shear stresses in the vicinity of a crack in a three dimensional composite containing unidirectional linearly elastic fibers in an infinite linearly elastic matrix. In order to reduce the complexity of the formulation, certain assumptions are made as to the physically significant stresses to be retained. These simplifications reduce the partial differential equations of elasticity to differential-difference equations which are tractable using Fourier transform techniques. This 'material modeling' approach is in contrast with solutions developed by considering each lamina as a homogeneous, orthotropic layer. The resulting solution does not contain the classical singular stress field for the fibers and the influence of broken fibers on unbroken fibers is felt by a change in stress concentration factors. The matrix stresses however, are unbounded as the fiber spacing vanishes and an equivalent fiber-matrix geometry is proposed which gives the correct singular behavior. The numerical solution is considered in detail and several specific examples are presented. The potential for damaged or debonded zones to be generated by an embedded crack is discussed, and stress concentration factors for fibers near the crack are given. Detailed comparisons are made between the present solution, the analogous two-dimensional problem, and corresponding shear-lag models.
    Keywords: STRUCTURAL MECHANICS
    Type: Engineering Fracture Mechanics; 13; 2, 19; 1980
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