Showing papers on "Composite laminates published in 1981"

Fatigue of Composite Materials: Damage Mechanisms and Fatigue Life Diagrams

Abstract: The basic fatigue damage mechanisms in composite laminates are reviewed. Based on these mechanisms a pattern in the fatigue-life diagrams is proposed. Several experimental data are shown to agree with this basic pattern. Fatigue ratio is defined in terms of strains, and fatigue limit is shown to exist for unidirectional, cross-plied and angle-plied laminates. The limitations to the fatigue performance of composite laminates are pointed out and suggestions for improving the fatigue resistance are made.

Indentation Law for Composite Laminates.

Abstract: 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.

On the relationship between engineering properties and delamination of composite materials

Abstract: The influence of the coefficient of mutal influence, Poisson's ratio and coefficients of thermal and moisture expansion on delamination is studied. Engineering theories are compared to finite element and experimental results. It is shown that the mismatch in coefficients of mutual influence can have a strong influence on delamination with fiber angles in the 10-15 degree range being critical for adjacent layer combinations. The mismatch in coefficient of mutual influence is reduced by a factor of two and the interlaminar shear stress is reduced significantly when the + or - adjacent layers are interspersed between 0 and 90 degree layers. It is shown how the results can be used for design of composite laminates.

Analysis of the Viscoelastic Response of Composite Laminates During Hygrothermal Exposure

Abstract: The linear viscoelastic response of polymeric matrix laminated com posites to hygrothermal exposure is investigated within the framework of lamination theory. Experimentally measured curvature changes in non- symmetric GY70/339 composite laminates during long term exposures to elevated hygrothermal conditions are shown to be accurately predicted by the linear viscoelastic analysis. Application of the analysis is made to prob lems involving 1) dimensional stability and alteration of residual stresses, 2) the strain rate dependence of tensile response, and 3) the effect of residual thermal stress relaxation upon creep in laminated composite plates under mechanically applied loads.

Insulating apparatus and composite laminates employed therein

Abstract: Transaction processing enclosures for containment of electronic data processing equipment, banking apparatus and the like incorporating a housing comprising a composite laminate composed of reflective, refractory and structural elements to provide an enclosure of unique structural integrity, and protection to the environment within and outside said enclosure against adverse factors such as impact and fire, and insulation against wide variations in temperature exterior to said enclosure while sustaining a stable environment within said enclosure.

Failure Analysis of Composite Laminates with a Fastener Hole.

Abstract: : Stress and strength analysis of composite laminates with a through the thickness circular hole, under uniaxial tensile load has been conducted. The hole is filled with a rigid core simulating a bolt. As a representation of the force at the bolted joint, displacement boundary conditions are applied to the rigid core. The stress analysis is based on treating the laminate as a homogeneous plate using finite element technique. The strength analysis is based on the tensor polynomial failure criterion applied to each constituent ply orientations. The ultimate laminate failure strength is based on the last ply failure stress. The resulting predictions of strength are found to be conservative as compared to experimental values. (Author)

Load sequence effects on the fatigue of unnotched composite materials

Abstract: A more comprehensive version of an earlier fatigue and residual strength degradation model is proposed to predict the effect of load sequence on the statistical fatigue behavior of composite laminates. The model, which reduces to various fatigue models proposed in the literature by means of approximations, is verified by a survey of experiments on glass/epoxy laminates. It is shown that the correlation between the model and the test results under dual stress levels is reasonable, and that a simplified version of the model is verified by experiments on graphite/epoxy laminates in which the correlation between theoretical predictions and results under dual stress levels is satisfactory. The model is also shown capable of predicting the effect of proof loads on the fatigue behavior of composite materials.

Fatigue behavior of graphite-epoxy laminates at elevated temperatures

Abstract: An examination and analysis is presented of the elevated temperature fatigue behavior of multidirectional graphite-epoxy laminates in terms of single lamina behavior. The laminate strength is predicted by considering the cyclic stress field in each lamina, the interlaminar stresses, and the experimentally determined temperature-effect 'shifting factors'. The initial failure of a lamina in a laminate is examined first in terms of stress redistribution, and then in terms of total failure and final laminate fracture; these analytical results are then compared with the actual fatigue behavior of T300/5208 graphite-epoxy composite laminates. In view of the agreement obtained, it is concluded that the temperature 'shifting factors' introduced here enable one to predict long-term behavior at a given temperature from short-time testing at elevated temperatures.

Wave Propagation Experiments On Ballistically Impacted Composite Laminates

Abstract: The details of the time history of the elastic waves in ballistically im pacted composite laminates have been obtained by using surface and em bedded strain gages. Records show that a very low-amplitude in-plane tensile wave arrives first followed by a medium-amplitude flexural wave and then a high-amplitude flexural wave. Measured and calculated values of wave speeds agree well.

Edge delamination in angle-ply composite laminates

Abstract: Edge delamination 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 but also inherently coupled mode I, II and III fracture in the layered anisotropic system. Based on complex-variable stress potentials in the anisotropic elasticity theory and eigenfunction expansion, exact orders of the crack-tip stress singularity and complete field solutions are obtained. Results are given for edge delaminated, angle-ply composites subjected to uniform axial extension for illustrative purposes. Effects of geometric, lamination, and crack variables are determined.

Calculation of Interlaminar Stress Concentration in Composite Laminates

Abstract: A new method for analyzing the free edge stress field in composite laminates is developed based on a perturbation and assumed stress approach. Convergence of the interlaminar shear stress in angle-ply laminates is studied.

Postbuckling Behavior of Composite Shear Webs

Abstract: In this paper the postbuckling behavior of multibay composite shear webs is explored. Several test specimens are designed through the use of advanced state of the art design methods taking into account material anisotropy. The panels are subjected to static as well as fatigue loading in a series of well instrumented and carefully conducted tests to determine various failure modes as well as failure loads. The experimentally ob- served failure modes and failure loads are compared with design loads and failure modes. Large deflection analysis is used to examine failure modes typical of composite panels. Composite shear webs are shown to have significant postbuckling strength and exhibit failure modes that are quite different from metal shear webs. TRINGER-STIFFENED shear panels are used extensively in many metal aircraft applications. In many of these applications buckling of the skin between the stringers is permitted to occur below limit load—sometimes well below. Composite materials are undergoing a rapid use escalation in new and projected systems. In order to make composite components weight efficient as well as cost competitive with their metal counterparts, the postbuckling strength of composite materials must be exploited. The available data in the literature indicate that composite materials have significant postbuckling strength. Postbuckling strength of boron-epoxy shear webs is demonstrated by Kaminski and Ashton.) In a similar study by Bhatia,2 graphite-epoxy shear webs are shown to have considerable postbuckling strength. In metal structures, the principal structural concern for postbuckled shear webs is that significant permanent set not occur for loading up to limit loads. Fatigue and ''wear out" are not of concern. For advanced composite stiffened panels, the concern about postbuckling behavior is much greater. Catastrophic failure can occur in the buckled skin due to high local compressive stresses or tension stresses; the relieving effects of yielding, as found in most metal structures, are not available in most practical composite layups. These com- pressive stresses, in addition to tension stresses present in shear webs, may result in severe strength degradation of composite laminates when subjected to repeated fatigue cycles. The purpose of this paper is to evaluate the behavior of realistically configured multibay panels- operating well into the postbuckled regime, to disclose specific failure modes, to assess the adequacy (or inadequacy) of available buckling and strength prediction methodology, to provide direction for future research, and to provide confidence in the viability of this useful type of construction. The main components of a shear beam are the web, uprights, and chords. The web transfers and/or resists the applied shear. The uprights are used to increase the buckling load of the web and to resist compression loads that are in- troduced from the tendency of the tension field forces in the web to pull the chords together. The chords together with the uprights prevent the structure from collapsing and are sub- jected to primary axial compression and tension loads due to primary bending of the beam as well as to secondary bending about their own axes due to the vertical component of the

Fatigue degradation in compressively loaded composite laminates

Abstract: The effect of imbedded delaminations on the compression fatigue behavior of quasi-isotropic, T300/5208, graphite/epoxy laminates was investigated. Teflon imbedments were introduced during panel layup to create delaminations. Static and constant amplitude (R=10, omega = 10 Hz) fatigue tests were conducted. S-N data and half life residual strength data were obtained. During static compression loading, the maximum deflection of the buckled delaminated region was recorded. Under compression fatigue, growth of the imbedded delamination was identified as the predominant failure mode in most of the test cases. Specimens that exhibited others failures had a single low stiffness ply above the Teflon imbedment. Delamination growth during fatigue was monitored using DIB enhanced radiography. In specimens with buried delaminations, the dye penetrant (DIB) was introduced into the delaminated region through a minute laser drilled hole, using a hypodermic needle. A low kV, microfocus, X-ray unit was mounted near the test equipment to efficiently record the cyclic growth of buried delaminations on Polaroid film.

A model of passive thermal nondestructive evaluation of composite laminates

Abstract: : A numerical model of passive thermal nondestructive evaluation of composite laminates was developed. This model, based on a transient, three-dimensional, finite difference solution to the heat conduction equations, can be used to characterize in-plane defects in the laminates and to predict the laminate response to the thermal test. The model was experimentally verified using two material systems. A parametric study was then conducted to ascertain which variables were critical to the success of passive thermal NDE, and how flaw resolution might be improved. jg p.4

Compression Fatigue Analysis of Fiber Composites

Abstract: A macromechanics model, based on the delamination propagation between the plies of a composite laminate, has been developed for compression fatigue analysis of fiber composites. The model is based on the assumption that initial defects exist in composites between plies. These defects propagate due to the interlaminar stresses produced by applied fatigue loads. Existing compression fatigue data have been analyzed, using the model, and analytically predicted fatigue life compared with experimental data. Test data have been generated under constant amplitude loading on composite laminates with four different stacking sequences. Good correlations between experimentally observed fatigue data and analytical predictions have been found.

Moisture and Thermal Expansion of Composite Materials

Abstract: : An experimental technique is presented for determining the moisture and thermal expanbsion coefficients of polymers, and polymer-matrix composite materials. Materials tested included Hercules 3501-6 neat epoxy resin, Hercules AS/3501-6 graphite/epoxy composite and Owens-Corning S2 glass fibers in the same Hercules 3501-6 epoxy matrix. Correlations of the experimentally determined moisture and thermal expansion properties with a nonlinear finite element micromechanics analysis are presented. Thermal expansion results for all three materials, both dry and moisture-conditioned, were obtained. Diffusivity constants were also experimentally determined. It is demonstrated that the moisture and thermal expansion of composite materials can be determined experimentally, and predicted numerically with generally good results. (Author)

Failure of Composite Joints under Combined Tension and Bolt Loads

Abstract: By observing results of experimental program for testing static strength of composite laminates under combined by-pass tension and bolt load, a tension failure factor is proposed herein to determine the failure modes of the composites. Comparison with experimental results shows that the proposed tension failure factor is a reasonable indicator. Possible application for estimating failure loads by interpolation of available data is illustrated with a numerical example.

Stress and Strength Analysis of Bolted Joints in Composite Laminates

Abstract: Composite laminates with a through-the-thickness fastener hole, free or loaded, have been treated for stress and strength analysis under uniaxial tensile loading conditions. This investigation has been carried out within the framework of laminated plate theory. A finite element method has been utilized to conduct the stress analysis of the laminate with a loaded fastener hole. For this case the hole is assumed to be filled with a rigid core, simulating a bolt, and displacement boundary conditions are applied at the semicircular contact surface. The strength analysis is based on the tensor polynomial failure criterion applied to each ply. The results for a free hole boundary condition are obtained by using closed form solutions. An approximation procedure is suggested to calculate stress levels for multidirectional laminates using the stress fields in individual ply laminate systems. This procedure works for both the loading conditions. The strength predictions for the loaded hole case are close to the experimental results.

Boundary layer thermal stresses in angle-ply composite laminates, part 1. [graphite-epoxy composites

Abstract: Abstract : Thermal boundary-layer stresses (near free edges) and displacements are determined by a unique method developed under this grant. This method consists of an eigenfunction expansion technique and the establishment of an appropriate particular solution. Current solutions in the region away from the singular domain (free edge) are found to be excellent agreement with existing approximate numerical results. As the edge is approached, the singular term controls the near field behavior of the boundary layer. Results are presented for cases of various angle-ply graphite/epoxy laminates with (e/-e/-e/9) configurations rough-the-thickness) stresses. Thermal boundary-layer thicknesses of different composite systems are determined by examining the strain energy density distribution in composites. It is shown that the boundary-layer thickness depends on the degree of anisotropy of each individual lamina, thermomechanical properties of each ply, and the relative thickness of adjacent layers. Also, the interlaminar thermal stresses are compressive with increasing temperature. The corresponding residual stresses are tensile and may enhance interply delaminations. (MM)

The effect of resin on the impact damage tolerance of graphite-epoxy laminates

Abstract: The effect of the matrix resin on the impact damage tolerance of graphite-epoxy composite laminates was investigated. The materials were evaluated on the basis of the damage incurred due to local impact and on their ability to retain compression strength in the presence of impact damage. Twenty-four different resin systems were evaluated. Five of the systems demonstrated substantial improvements compared to the baseline system including retention of compression strength in the presence of impact damage. Examination of the neat resin mechanical properties indicates the resin tensile properties influence significantly the laminate damage tolerance and that improvements in laminate damage tolerance are not necessarily made at the expense of room temperature mechanical properties. Preliminary results indicate a resin volume fraction on the order of 40 percent or greater may be required to permit the plastic flow between fibers necessary for improved damage tolerance.

A single fracture toughness parameter for fibrous composite laminates

Abstract: A general fracture toughness parameter Qc was previously derived and verified to be a material constant, independent of layup, for centrally cracked boron aluminum composite specimens The specimens were made with various proportions of 0 and + or - 45 degree plies A limited amount of data indicated that the ratio Qc/epsilon tuf' where epsilon tuf is the ultimate tensile strain of the fibers, might be a constant for all composite laminates, regardless of material and layup In that case, a single value of Qc/epsilon tuf could be used to predict the fracture toughness of all fibrous composite laminates from only the elastic constants and epsilon tuf Values of Qc/epsilon tuf were calculated for centrally cracked specimens made from graphite/polyimide, graphite/epoxy, E glass/epoxy, boron/epoxy, and S glass graphite/epoxy materials with numerous layups Within ordinary scatter, the data indicate that Qc/epsilon tuf is a constant for all laminates that did not split extensively at the crack tips or have other deviate failure modes

Environmental Effects on Defect Growth in Composite Materials

Abstract: : The objective of this program was to derive data for evaluating the effects of moisture and temperature on the integrity of fiber composite components. In particular, the static and cyclic performance of three composite laminates containing flaws was investigated at room temperature and 422K (300 deg F) in wet and dry conditions. The test results evaluate the following for each environmental condition: (a) Effect of defect type and size on static fracture; (b) Influence of compression loadings on fatigue; (c) Description of the effects of static and cyclic loadings on damage accumulation; (d) The effect of preloading on damage growth, static strength, and cyclic load behavior; and (e) The comparison of wet and dry data at room and 422K (300 deg F) temperatures. Static fracture data were obtained for three different composite systems that had been moisture soaked to near the maximum moisture absorption potential. The fracture test data were obtained at room temperature and immediately after the application of an abrupt temperature transient that caused the specimen to attain 422K (3000F). Three different 20-ply layups were tested. Laminate L1 represents general purpose structure, laminate L2 is typical of polar and hoop wound pressure vessels, and laminate L3 is representative of turbine engine fan blades or tubular strut support structure. Half the test specimens were preloaded to 90% of their ultimate static load prior to moisture conditioning. Intermittent nondestructive inspection, including visual and ultrasonic techniques, were used to detect defect growth.

Research Study to Define the Critical Failure Mechanisms in Notched Composites under Compression Fatigue Loading.

Abstract: : Analytical and experimental studies of identify the dominant failure mechanisms for compression fatigue of notched composite laminates have been conducted. The effects of stacking sequence and matrix material on damage growth as well as failure mechanisms under compression fatigue loading are reported. Two types of resin materials and four kinds of laminate layups have been considered. Two of the stacking sequences are fiber dominated and two are quasi-isotropic. Ultrasonic C-scans and photomicrographic studies have been utilized for experimental observations. An analytical methodology developed for the study of tension fatigue of notched laminates has been modified to obtain patterns of damage growth under quasi-static load by increasing it gradually in steps. For each step increase in load, a finite element stress analysis is performed and intralaminar and/or interlaminar failures in the elements are predicted.

NDT of Composites by Thermography

Abstract: This paper describes ongoing research eHorts to evaluate thermographic teci1niques for locating flaws or damage in structural fiber composite laminates. An infra-red camera with video isotherm readout is used to identify perturbations in uniform or linear thermal fields which may be caused by presence of flaws or damage such as matrix cracks, delaminations, blind side impact damage, and partial through holes. This procedure has potential for rapid qualitative screening of large surface areas. Potential defective areas may then be analyzed by a more accurate (but more time consuming) method . Two techniques are discussed; externally applied thermal field (EATF) and stress-generated thermal field (SGTF). The EATF technique involves applying heat to a composite structure and observing theresulting transient thermal pattern. The SGTF technique requires stress cycling to create hot spots in regions of high stress concentrations adjacent to flaws or damage sites.

Fatigue mechanisms in metal matrix composite laminates

Abstract: Certain aspects of the damage process are described. The shakedown phenomenon in laminates is first explained. This is followed by a presentation of matrix test data used in evaluating the shakedown limits of the laminates used in the subsequent experiments. Test results suggesting that there is no loss of elastic stiffness in laminates loaded at constant amplitudes within their shakedown limits are presented. Damage accumulation and the development of the saturation damage state are illustrated by experimental results on specimens loaded outside the shakedown range. Attention is also given to the behavior of laminates subjected to variable cyclic loading.