Showing papers on "Composite laminates published in 1977"

Some New Results on Edge Effect in Symmetric Composite Laminates

Abstract: An improved numerical (finite element) procedure is developed to investigate the stress field in symmetrically laminated composites of finite dimensions Emphasis is placed on assessing the singular behavior of stresses in regions close to ply-interfaces and the exposed free edge An accurate evaluation of these stresses will help to better understand the failure process of composite laminates Mechanical load in the form of uniaxial tension and environmental variables such as temperature and humidity exposures are considered in the formulation Results tor lam inates under uniaxial tension are presented in this paper; thermal and hygroscopic stresses will be reported in a subsequent paper

Edge Effects in Angle-Ply Composite Laminates*:

Abstract: This paper presents the results of a zeroth-order solution for edge effects in angle-ply composite laminates obtained using perturbation techniques and a limiting free body approach. The general solution for edge effects in laminates of arbitrary angle ply is applied to the special case of a (+ or - 45)s graphite/epoxy laminate. Interlaminar stress distributions are obtained as a function of the laminate thickness-to-width ratio and compared to finite difference results. The solution predicts stable, continuous stress distributions, determines finite maximum tensile interlaminar normal stress and provides mathematical evidence for singular interlaminar shear stresses in (+ or - 45) graphite/epoxy laminates.

Edge Effects on Thermally Induced Stresses in Composite Laminates

Abstract: Thermally induced stresses in fiber composite laminates play an important role in the strength and failure of such laminates as load carrying members. When laminates have free edges, cutouts or holes, interlaminar stress concentration will usually develop near the free-edge region under service loads. An accurate evaluation of thermally induced edge stresses will further the understanding of the laminates' behavior. The present paper examines the stress field of a finite-width, symmetrical laminate when subjected to a uniform temperature change. A finite-element proce dure employing advanced sparse matrix solution technique is employed to study the singular nature of stresses near the free-edge region. Edge effects on the overall response of the laminate under thermal loading are discussed.

Some fundamental aspects of the fatigue and fracture response of composite materials

Abstract: A simple analytical model, based on the concept that the constraining plies surrounding cracks through the thickness of a given ply or plies control the spacing of those cracks by controlling the surrounding stress state, appears to agree well with experimental data Such a model can be used to evaluate critically various constraint situations including the prediction of the ability of a given constraint layer to arrest flaw development The analytical model, which uses an equilibrium element approach in the present case, is sensitive to lamina and laminate elastic properties, flaw size, stacking sequence, and an interlaminar stress transfer distance which is estimated from experimental observations The analysis is easily extended to study flaw growth and plastic region formation

Moisture Absorption and Desorption in Epoxy Composite Laminates

Abstract: Epoxy composite laminates absorb moisture from the atmosphere via a diffusion controlled process and undergo changes in both mechanical properties and dimensional shapes. This paper reviews the nature of moisture absorption by epoxy composites and the resultant effects upon the laminate.

A New Static Compression Fixture for Advanced Composite Materials

Abstract: Advanced composite compressive data are generally less abundant than the corresponding tension data for the same material. There are two reasons for this paucity of data: (1) the difficulties associated with compressive testing of specimens prepared from thin sheets of advanced composite materials such as end splitting, buckling of the specimens, and load alignment problems; and (2) the expense associated with generally accepted workable but complex specimens such as the sandwich beam compressive specimen. This paper describes a method of compression testing of specimens which alleviates the problem areas described above. In addition, several associated areas which are adjunct to the method such as elevated temperature testing are also reported and detailed. Basically, a fixture with linear, nearly frictionless bearings is employed with a straight-sided tab-ended specimen to determine compressive strength, yield, ultimate strain, modulus, and Poisson's ratio. A comparison is made with the test results of other methods. Finally, recommendations for additional improvement on this fixture are set forth.

Interlaminar Stresses around Circular Cutouts in Composite Plates under Tension

Abstract: 1~5 From these results, the static failure strengths are obtained with good quantitative prediction compared to experiments. Delamination at free edges is another failure mode of laminated composites, especially under fatigue loadings. Extensive delamination at free edges is reported on static strength as well as fatigue strength, with and without notches.6"10 Hence, the determination of interlaminar shear stresses and the normal or "peel" stress is a current research topic. However, all the previous studies are restricted to straight boundaries11"27 except for finite element solutions in Refs. 14 and 28-30. There is no analytical closed-form solution available to predict interlaminar stresses for curvilinear boundaries. Therefore, the present paper attempts to study the interlaminar stresses at a circular cutout in an "infinite" composite laminate under uniform tensile load. The boundary-layer theory for composite laminates in Ref. 20 is extended here to the formulation of polar coordinates, and the title problem is investigated for the case of orthotropic composite plates. The laminate considered here is under inplane loading symmetric about the midplane. The construction of the laminate is also midplane symmetric so that there is no stretching/bending coupling of the plate due to external load. The force resultants around the hole of an orthotropic or anisotropic plate are given by Refs. 31 and 32. The radial and shear force resultants at the edge of the hole are zero. From the plane stress solution, the strains at the edge of the hole can be calculated by the force resultant-strain relation. Due to the compatibility of deformation, the strains of individual layers are the same as the laminate. Therefore, the stresses of each layer may then be computed from the layer stress-strain law. However, the computed radial and shear stresses of each layer along the contour of the hole are in general not zero. Because there exists a three-dimensio nal state of stresses at the free edge of each layer where the plane stress solution cannot predict. The region of the plate adjacent to and including the edge, where the plane stress solution may not be adequate, is called the boundary layer. To obtain the governing equations in this

Fatigue Damage: Stiffness/Strength Comparisons for Composite Materials

Abstract: Boron/epoxy laminates in a [0-deg/ + 45-deg/ - 45-deg/ 0-deg/0-deg/ + 45 -deg/ - 45-deg/0-deg] layup were tested statically for initial strength and fracture strain and were tested in strain-controlled fatigue loading at 30 Hz, with a strain-amplitude ratio of one tenth for five different testing sequences. A reduced stiffness analysis was used in conjunction with laminated plate theory to predict stiffness changes based on observed debonding and fiber breakage. Specimens used for strength analysis either fractured during fatigue or were loaded statically to failure after fatigue. All specimens were leached with a heated acid, and fiber breakage was recorded. Correlations are noted between matrix damage and stiffness changes as well as between fiber breakage and strength reduction.

Analysis of shear test method for composite laminates

Abstract: An elastic plane stress finite element analysis of the stress distributions in four flat test specimens for in-plane shear response of composite materials subjected to mechanical or thermal loads is presented. The shear test specimens investigated include: slotted coupon, cross beam, losipescu, and rail shear. Results are presented in the form of normalized shear contour plots for all three in-plane stess components. It is shown that the cross beam, losipescu, and rail shear specimens have stress distributions which are more than adequate for determining linear shear behavior of composite materials. Laminate properties, core effects, and fixture configurations are among the factors which were found to influence the stress distributions.

Designing for Integrity in Long-Life Composite Aircraft Structures

Abstract: Results of test programs conducted to obtain structural performance information essential to the design of long-life composite aircraft structure are presented. Static, fatigue, and fracture tests were conducted on T300/5208 and T300/ 5209 graphite/epoxy laminate specimens which were representative of full-scale strength-designed laminates, Results showed that static tensile and compressive strength are significantly degraded by discontinuities and impact damage. Laminates with representative fiber damage are relatively insensitive to cyclic tensile loadings; however, cyclic compressive loading results in finite fatigue lives. Impact damage which caused barely visible fiber failure was found to reduce fatigue performance below that defined for an open hole. The graphite/epoxy laminates tested were found to be fracture sensitive. Laminate stacking sequence was found to have a significant influence on fracture-strength and failure mode.

Delamination in Quasi-Isotropic Graphite-Epoxy Laminates

Abstract: The results of a study of delamination in two quasi-isotropic graphite-epoxy laminates are reported. Experimental observations are compared with predictions of laminate theory using the approach of Pagano and Pipes. The influence of stacking sequence is discussed. The relationship of delamination to other damage development is described, and the influence of gripping methods is indicated.

Characterization of Composite Laminates Using Tubular Specimens

Abstract: : Properties of a unidirectional graphite/epoxy (T300/5208) composite are characterized using tubular specimens The elastic compliances based on the average invariants are found to describe fairly accurately the elastic behavior under various combined loadings The equality between the tension and compression compliances and the symmetry of the compliance tensor are also established to within the experimental scatter The matrix/interface-controlled failure is characterized by a second-order polynomial including a first-order term This failure criterion agrees with the experimental observation that the compressive loading perpendicular to the fibers can increase the longitudinal shear stress required for failure (Author)

Evaluation of Flawed Composite Structure Under Static and Cyclic Loading

Abstract: This paper presents the results of a program investigating the effects of initial defects on the fatigue and fracture response of composite laminates. The structural laminates investigated were a typical angle-ply laminate, a polar/hoop-wound pressure vessel laminate, and a typical engine fan blade laminate. Defects investigated were full- and half-penetration circular holes, full- and half-penetration slits, and countersink holes. Results are presented showing the effects of the defect size and type on the static fracture strength, fatigue performance, and residual static strength. The results of inspection procedures are shown, describing the effect of cyclic and static loadings on damage propagation in composite laminates. The data in this study were used to define proof test levels as a qualification procedure in composite structure subjected to cyclic loading.

Static and Impact Behavior of Graphite/Epoxy Composite Laminates Containing Third-Phase Reinforcement Materials

Abstract: A series of graphite/epoxy composite laminates containing four different types of third-phase reinforcement materials were fabricated, incorporating an anisotropic ply lay-up pattern. Both thick and thin laminates were used. Static tensile, flexure, and short beam shear specimens and Charpy impact specimens were tested, the latter with an instrumented impact tester. The facture surfaces of the static flexure and the Charpy impact specimens were then examined in detail with a scanning electron microscope. Observed similarities and differences between the static flexure and impact fracture characteristics of a given composite were noted and referred to the corresponding energies absorbed to failure. Similar comparisons were made between the various material combinations.

A Comparison of the Static and Fatigue Strengths of Formed and Drilled Holes in Composite Laminates.

Abstract: : Holes in structural members introduce areas of high stress concentration When "laying-up" a laminate composite, it is possible to form the holes before cure, instead of cutting the fibers by drilling the holes after cure. The diverted fibers maintain their continuity and provide added strength in the highly stressed region around the hole. Accordingly, it was hypothesized that a laminate with formed holes would be stronger than a similar laminate with drilled holes.

Influence of temperature change on optimum laminate design

Abstract: Results of a laminate optimization study, which includes prescribed temperature change and mechanical loading conditions, are presented. Minimum-weight designs for balanced symmetric laminates are obtained subject to strength, membrane stiffness, and minimum thickness requirements, while including and omitting temperature change effects. Based on a first ply failure design philosophy and employing a linear thermoelastic analysis, it is shown that taking temperature decrease effects into account leads to substantial weight penalties in strength critical fiber composite laminates. Numerical results are presented for representative fiber composite materials based on three commonly used combined stress failure criteria.

Evaluation of the Cost Effectiveness of Hybrid Composite Laminates

Abstract: Theme I this paper, a methodology is presented for the evaluation of cost performance of hybrid composite laminates. The optimum cost and weight configurations are obtained for simple structural configurations which are representative of "real world" structures, over a practical range of loadings. Aluminum and hybrid and single-fiber laminates are the candidate materials. Measures of the optimum weight and cost for the representative structural elements are compared for these various candidate materials. Weight differences are incorporated into the cost comparisons through the use of a premium cost to quantify the value of weight savings. Material costs, manufacturing costs, and premiums are all treated as variables in this design study. Hence, the results can be applied to a broad range of problems of interest for the dual purposes of 1) establishing cost-performance ranking of a particular hybrid system in a preliminary fashion for a specific structural application and loading and 2) providing guidance for developing additional improved material systems and hybrid laminate configurations.

Stress-Strain State of Multilayer Composite Laminates

Abstract: N ACCORDANCE WITH current data, the strength of heterophase composites depends upon the adhesion between the component phases [1-~4] . This dependence was observed in studies of filled resins [ 1 ] and plastics [2] , glass-reinforced laminates [3], PMMA [4, 5] , and multilayer composites [6] . In the last-named case, a strengthening effect was noted, described as follows: When a multilayer composite comprises several layers (films) of a base material

An Investigation of Edge Damage Development in Quasi-Isotropic Graphite Epoxy Laminates.

Abstract: : This investigation describes and documents in detail the initiation, growth, and interactions of damage along the free edges of two types of graphite/epoxy fiber-reinforced composite laminates. The damage is initiated by tensile static loading of flat coupons from each type to three different stress levels. Growth of the damage was caused by tension-tension fatigue loading. The laminates that were investigated differed only by stacking sequence. The observations were made through the use of the replication technique, a method developed for the purpose of this investigation. This technique allows for three-dimensional, instantaneous recordings of the entire specimen edge while it is under maximum load. The recordings or impressions can be studied microscopically and photographed. This method provides for detail and an overall field of vision that has not been obtained in previous studies. The damage development was also monitored by two non-destructive techniques; vibrothermography and an ultrasonic-pulse echo method.

Process for manufacturing synthetic resinnmetal composite laminates

Abstract: PURPOSE:To manufacture thermoplastic resin-metal composite laminates with thick core layers by laminating synthetic resin sheets with thin metal plates using a drying adhesive having a softening point lower than that of the resin.

Tensile and compressive behavior of Borsic/aluminum

Abstract: The results of an experimental investigation of the mechanical behavior of Borsic/aluminum are presented. Composite laminates were tested in tension and compression for monotonically increasing load and also for variable loading cycles in which the maximum load was increased in each successive cycle. It is shown that significant strain-hardening, and corresponding increase in yield stress, is exhibited by the metal matrix laminates. For matrix dominated laminates, the current yield stress is essentially identical to the previous maximum stress, and unloading is essentially linear with large permanent strains after unloading. For laminates with fiber dominated behavior, the yield stress increases with increase in the previous maximum stress, but the increase in yield stress does not keep pace with the previous maximum stress. These fiber dominated laminates exhibit smaller nonlinear strains, reversed nonlinear behavior during unloading, and smaller permanent strains after unloading. Compression results from sandwich beams and flat coupons are shown to differ considerably. Results from beam specimens tend to exhibit higher values for modulus, yield stress, and strength.

Influence of residual stresses on the tensile strength of composite-metal sandwich laminates

Abstract: The tensile strength of boron-epoxy/aluminum sandwich laminates is discussed relative to the residual thermal stresses generated by curing and bonding at elevated temperatures. It is shown that the sandwich laminates investigated exhibit three dinstinct modes of failure, depending upon the fiber orientation of the composite. Sandwich laminates with moderate to high percentage of 0-deg fibers exhibit early failures initiated by edge effect; laminates with moderate to high percentage of 90-deg fibers fail according to a first-ply failure criterion; laminates with moderate to high percentages of plus or minus 45 deg plies fail at strains equal to or greater than the failure strain of the corresponding all-composite laminate.