Showing papers on "Composite laminates published in 1984"

Composite laminates with negative through-the-thickness Poisson's ratios

Abstract: A simple analysis using two dimensional lamination theory combined with the appropriate three dimensional anisotropic constitutive equation is presented to show some rather surprising results for the range of values of the through-the-thickness effective Poisson's ratio nu sub xz for angle ply laminates. Results for graphite-epoxy show that the through-the-thickness effective Poisson's ratio can range from a high of 0.49 for a 90 laminate to a low of -0.21 for a + or - 25s laminate. It is shown that negative values of nu sub xz are also possible for other laminates.

Failure of Composite Laminates Containing Pin Loaded Holes—Method of Solution

Abstract: A method is presented for predicting the failure strength and the failure mode of fiber reinforced composite laminates containing one or two pin loaded holes. The method involves two steps. First, the stress distribution in the laminate is calculated by the use of a finite element method. Second, the failure load and failure mode are predicted by means of a proposed failure hypothesis together with the Yamada-Sun failure criterion. A computer code was developed which can be used to calculate the maximum load and the mode of failure of laminates with different ply orientations, different material properties, and different geometries. Tests were also conducted measuring the failure strengths and failure modes of Fiberite T300/1034-C laminates containing a pin-loaded hole or two pin-loaded holes in parallel or in series. Com parisons were made between the data and the results of the model. Good agreement was found between the analytical and the experimental results.

Fracture Mechanics of Sublaminate Cracks in Composite Materials

Abstract: This paper presents an overview of a fracture-mechanics approach to some of the most frequently encountered matrix-dominated sublaminate cracks in epoxy-based composite laminates. By “sublaminate,” it is meant that the cracks are internal to the laminate, generally invisible macroscopically, but are much larger in size than those microcracks considered in the realm of micromechanics. The origin of sublaminate cracks is assumed to stem from the coalesence of natural material flaws (also microcracks), which occur under a certain favorable laminate stress condition. Thus, the modeling of the mechanisms of sublaminate crack initiation and propagation is essentially mechanistic and probabilistic in nature. Some specific results from several analytical/experimental investigations using graphite-epoxy laminates are presented and discussed in this paper.

A Stochastic Model for the Growth of Matrix Cracks in Composite Laminates

Abstract: This paper presents a stochastic simulation model for the growth of multiple matrix cracks in composite laminates subjected to both static and fatigue loads. Working within the premise of ply-elasticity, a new concept of effective flaws is introduced which replaces the conventional constant ply strength criterion. Thus, the model con sists of an application of fracture mechanics and a rational representation of material flaw distributions. Simulation examples are presented on [02/902]s and [0 2/903]s graphite-epoxy laminates which undergo characteristic transverse cracking under uniaxial tension.

Failure Strength of Nonlinearly Elastic Composite Laminates Containing a Pin Loaded Hole

Abstract: A method is presented for calculating the failure strengths and failure modes of composite laminates containing a pin loaded hole for materials exhibiting nonlinearly elastic behavior. The analysis follows that proposed by Chang, Scott, and Springer for linearly elastic composites with two notable exceptions: a) the shear stress-shear strain relation in each ply is taken to be nonlinear with the form proposed by Hahn and Tsai, and b) the Yamada-Sun failure criterion is modified to include nonlinear effects. Numerical results, generated using a nonlinear finite element scheme, were compared to data. These comparisons show that for laminates exhibiting nonlinear behavior the present analysis provides the failure strengths and failure modes more accurately than the previous method employing a linear stress-strain analysis.

Effects of Orthotropy and Width on the Compression Strength of Graphite-Epoxy Panels with Holes

Abstract: An experimental study was conducted to evaluate the effect of laminate orthotropic properties and panel width on the compression strength of 48-ply graphite-epoxy laminates with drilled holes. The test results were evaluated on the basis of hole size and specimen width and were used to determine parameters necessary to predict trends using the point-stress failure criterion. Good agreement was obtained between experimental and predicted values of failure for panels fabricated from two quasi-isotropic laminates and one orthotropic laminate. The results indicate that panels of different widths that have large holes in relation to the failure prediction parameter should be included in any test program conducted to develop predictions. Panel failures are shown to have an upper limit given by the reduction in specimen cross-sectional area and a lower limit given by the effect of stress concentration. Limited tests of a quasi-isotropic laminate were conducted on panels with machined cracks and panels with punched holes and the results compared to those with drilled holes. The sequence of events that occurs in the vicinity of a hole during failure was identified and provides insight into the mechanisms associated with failure. a d

Edge delamination in angle-ply composite laminates

Abstract: ion has caused severe concern in the design and analysis of advanced composite materials and structures. Due to its complex nature, very limited knowledge for the problem is currently available. It involves not only geometric and material discontinuities b ut also inherently coupled mode I, I1 and 111 fracture in the layered anisotropic system. Based on complex-variable stress potentials in the a nisotropic elasticity theory and eigenfunction expansion, exact o rders of the cracktip stress singularity and complete field solutions are obtained. Results are given for edge delaminated, angle-ply c omposites subjected to uniform axial extension for illustrative purposes. Effects of geometric, lamination, and crack variables are determined.

The Nonlinear Viscoelastic Response of Resin Matrix Composite Laminates

Abstract: 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.

Analysis of multi-layer laminates using three-dimensional super-elements

Abstract: Analysis of multi-layer composite laminates using three-dimensional finite element models often results in high computational cost. This communication reports a three-dimensional super-element scheme which can reduce the computational cost.

Effect of microcracks on the thermal expansion of composite laminates

Abstract: A finite element analysis was used to quantitatively predict the effect of matrix microcracks in the 90 deg plies of graphite/epoxy composites on the coefficient of thermal expansion in the 0 deg direction, alpha (y) (perpendicular to the cracks) Results were generated for (0m/90n)s, (0/+ or 45/90)s and (0/90/ + or - 45)s graphite/epoxy laminate configurations Analytical predictions were compared with experimental results for the two quasi-isotropic laminate configurations Both analytical and experimental results showed that microcracks reduced the effective stiffness of the 90 deg plies, thus causing the laminates, thermal response to be more like that of a (0) laminate The change in alpha(y) was a function of lamina material properties, microcrack density, fiber orientation, and stacking sequence A combination of classical lamination theory and finite element analysis was used to predict the effect of microcracks in both the 90 deg and 0 deg plies Analytical results showed that the addition of microcracks in the 0 deg plies do affect alpha(y), but to a lesser extent than those in the 90 deg plies

The Effect of Geometry on Interlaminar Stresses of [0/90]s Composite Laminates with Circular Holes

Abstract: The effect of the radius to thickness ratio R/t on the interlaminar stresses around the hole in a [0/90]s composite laminate containing a circular hole and loaded in uniaxial tension was investigated. Three dimensional finite elements with a technique that is similar to substructuring were used for the analysis. The results show that as the ratio R/t increases, the magnitude of the tangential interlaminar shear stress on the interface between the crossed plies and near the hole edge increases. Also, as the ratio R/t in creases, the maximum of the interlaminar normal stress on the midplane near the hole increases and its position on the hole edge shifts towards the transverse axis where other stresses are also maximum.

Design of Composite Laminates Containing Pin Loaded Holes

Abstract: Methods are presented for sizing composite laminates containing one, two, or several pin loaded holes, the holes being arranged either in a single row or in two parallel rows. It is shown how the design parameters (number of holes, hole diameter, hole positions) can be found, which result in the maximum failure load and in the max imum failure load per unit weight.

Computer sizing of composite laminates for strength

Abstract: This report presents a procedure to design laminated composite panels subjected to bending loads and in plane loads. The procedure takes advantage of the directionality of the mechanical properties of the composite material using the laminated plates theory.

Comparison of toughened composite laminates using NASA standard damage tolerance tests

Abstract: The proposed application of composite materials to transport wing and fuselage structures prompted the search for tougher materials having improved resistance to impact damage and delamination. Several resin/graphite fiber composite materials were subjected to standard damage tolerance tests and the results were compared to ascertain which materials have superior toughness. In addition, test results from various company and NASA laboratories were compared for repeatability.

Strength of Bolted Joints in Laminated Composites

Abstract: : A method is presented for predicting the failure strength and failure mode of pin-loaded holes in fiberite reinforced composite laminates. The method includes two steps. First, the stress distribution in the laminate is calculated by the use of a finite element method. Second, the failure load and failure mode are predicted by means of a proposed failure hypothesis together with the Yamada-Sun failure criterion. A computer code was developed, which can be used to calculate the maximum load and the mode of failure of joints involving laminates with different ply orientations, different material properties, and different geometries. Tests were performed, measuring the rail-shear strength and the characteristic lengths for Fiberite composites. Tests were also conducted, measuring the failure strengths and failure modes of Fiberite laminates containing a pin-loaded hole or two pin-loaded holes in parallel or in series. Comparisons were made between the data and the results of the model. Good agreement was found between the analytical and the experimental results. Using the computer code, parametric studies were performed, illustrating the procedures which can be used to size composites containing pin-loaded holes.

Strain-energy-release rate analysis of delamination around an open hole in composite laminates

Abstract: A simple technique was developed for calculating the strain-energy-release rate, G, for delamination around an open hole in a laminate. Discrete locations around the hole boundary were modeled as straight edges, with the ply orientations rotated by an appropriate angle. The circumferential strain, calculated from an elasticity solution, was substituted into a simple equation, derived from the rule of mixtures and laminated plate theory, to generate G distributions around the hole boundary. These G distributions were plotted for delamination in each unique interface of two quasi-isotropic laminates to identify critical regions for delamination onset at the hole boundary. Previous work had indicated that delamination onset in brittle epoxy composites under static loading was controlled by the critical mode I G component. In order to compare to existing experimental data, distributions of approximate interlaminar normal stresses, a,,, at the hole boundary were generated from a simple analysis based on laminated plate theory. Then, a quasi-3D finite-element analysis was performed in the regions that had both high G values and tensile ozz stresses to calculate the various G components due to interlaminar tension and shear (GI, GII, and GIII). Static delamination onset strains were predicted at locations where GI was a maximum and were compared to measured values for T300/ 5208 laminates. Curves relating interlaminar GIc to delamination onset strains were drawn to evaluate the influence of a tough matrix on laminates with open holes.

The effects of thermal cycling on matrix cracking and stiffness changes in composite tubes

Abstract: The study investigated the accumulation of transverse matrix cracks and the resultant loss of torsional, extensional, and bending stiffnesses in 8 layer 0.5 in. diameter crossply tubes subjected to thermal cycling. The tubes were graphite-epoxy and the temperature range during cycling was -250 to 200 F. The effect of fiber and matrix properties was investigated through the use of T300 and P75S fibers and 934 and CE339 resins. The study considered 0, 10, 50, 100, 300, and 500 thermal cycles. Photomicrographs, X-rays, and edge replication were used to evaluate cracking. Special loading fixtures were used to measure stiffness changes. An important finding was that for the tubes studied, even with extensive cracking, the bending and extensional stiffnesses were not affected. The torsional stiffness, however, was strongly affected.

Method of and apparatus for blind tufting composite laminated joints

Abstract: A self-threading stitching device for joining uncured composite laminates which comprises a vertically mounted needle shaft adapted for reciprocal motion, a thread feeding means operating synchronously with the needle shaft and self-threading needle means adapted to pick up the thread from the thread feeding means and carry the thread through uncured composite laminate being penetrated by the needle and release the thread in the laminate as the needle withdraws therefrom.

Residual stiffness properties of cracked composite laminates

Abstract: Cracks in composite laminates are characterized by a set of vectors, each representing an individual cracking mode. The vector components are taken as internal variables in the elastic strain energy function and the elastic constitutive equations are derived for the inplane loading condition. For low concentration of cracks in laminates, the residual stiffness properties are related to the initial elastic constants and the magnitude of the damage vectors. These equations are then used to predict stiffness reductions of composite laminates from the observed crack densities. Very good agreement with the experimental values is found.

A survey of strength analysis methods for laminates with holes

Abstract: The methods for predicting the strength of composite laminates with fastener holes are reviewed for the cases of unloaded as well as loaded holes. Three categories of methods are discussed: (1) the hole-boundary stress (HBS) methods which compare the peak stresses and the laminate strength; (2) the linear elastic fracture (LEFM) methods which assume cracks emanating from the hole and predict failure when the stress intensity factor for a crack equals the laminate fracture toughness; and (3) two different two-parameter (TP) methods: the average-stress (ASTP) method, which predicts failure when the average stress over a distance from the hole equals the laminate strength; and the point-stress (PSTP) Whitney-Nuismer (1974) approach, which predicts failure when the stress near the hole equals the material strength. The PSTP is relatively accurate and is by far the most widely used of all the prediction methods. However, for large ranges of variables and cases, the current strength predictions may not always be accurate.

Delamination growth analysis in quasi-isotropic laminates under loads simulating low-velocity impact

Abstract: A geometrically nonlinear finite-element analysis was developed to calculate the strain energy released by delamination plates during impact loading. Only the first mode of deformation, which is equivalent to static deflection, was treated. Both the impact loading and delamination in the plate were assumed to be axisymmetric. The strain energy release rate in peeling, G sub I, and shear sliding, G sub II, modes were calculated using the fracture mechanics crack closure technique. Energy release rates for various delamination sizes and locations and for various plate configurations and materials were compared. The analysis indicated that shear sliding (G sub II) was the primary mode of delamination growth. The analysis also indicated that the midplane (maximum transverse shear stress plane) delamination was more critical and would grow before any other delamination of the same size near the midplane region. The delamination growth rate was higher (neutrally stable) for a low toughness (brittle) matrix and slower (stable) for high toughness matrix. The energy release rate in the peeling mode, G sub I, for a near-surface delamination can be as high as 0.5G sub II and can contribute significantly to the delamination growth.

Prediction of thermal and mechanical properties of glass-epoxy composite laminates

Abstract: This paper provides an experimental test of the validity of laminate theory used for calculating thermoelastic properties of a unidirectional composite laminate. Theoretically calculated values of elastic moduli and coefficients of thermal expansion for unidirectional glass/epoxy laminates were compared with the measured values. Laminates (8″ × 8″ × 1/4″) as a function of ply orientation were made and their elastic moduli, thermal expansion coefficients, fiber volume, and void contents were measured.

Analyses of quasi-isotropic composite plates under quasi-static point loads simulating low-velocity impact phenomena

Abstract: In thin composite laminates, the first level of visible damage occurs in the back face and is called back face spalling. A plate-membrane coupling model, and a finite element model to analyze the large deformation behavior of eight-ply quasi-isotropic circular composite plates under impact type point loads are developed. The back face spalling phenomenon in thin composite plates is explained by using the plate-membrane coupling model and the finite element model in conjunction with the fracture mechanics principles. The experimental results verifying these models are presented. Several conclusions concerning the deformation behavior are reached and discussed in detail.