Showing papers on "Composite laminates published in 1982"

Stiffness-reduction mechanisms in composite laminates

Abstract: It now is recognized widely that stiffness changes during the service loading of composite laminates can be significantly large, especially as those changes affect deflections, dimensional changes, vibration characteristics, and load or stress distributions. Several generic sources of stiffness change can be identified, in various degrees, in fibrous composite materials. The source which occurs quite early in the life of a specimen or component is matrix cracking, the subject of this paper. While most laboratories now report stiffness changes, very little systematic philosophy has been developed to account for and explain such stiffness changes. The complexity of this situation requires systematic study, and motivates the search for a model, or models, which can describe the behavior and predict unfamiliar response. The present paper reports the results of an experimental program and an analytical modelling exercise which indicate that much of the observed matrix cracking can be predicted and the effects on stiffness calculated with various degrees of accuracy depending upon the sophistication of the model used.

Experimental Determination of the In Situ Transverse Lamina Strength in Graphite/Epoxy Laminates

Abstract: The uniaxial tensile load at which transverse cracking initiated in the 90 deg. laminae of (0 2/90n)s, (±30/90n) s and (±60/90n)s, n =1,2,4,8, T300/934 composite laminates was determined experimentally using DIB enhanced x-radiography. "In situ" transverse strengths of the 90 deg. laminae corresponding to these loads were then calculated based upon lamination theory predictions including the effects of residual thermal stresses. These in situ strengths, which were as high as 2.5 times the unidirec tional transverse strength, were found to depend strongly on both the 90 deg. laminae thickness and orientation of adjacent laminae suggesting that transverse strength should not be considered as an intrinsic lamina property.

The Dependence of Transverse Cracking and Delamination on Ply Thickness in Graphite/Epoxy Laminates

Abstract: (25/-25/90 n ) s , n = ½, 1, 2, 3, 4, 6, 8, and (25 2 /-25 2 /90 2 ) s T300/934 graphite/epoxy laminate coupons were tested in tension and examined periodically by di-iodobutane (DIB) enhanced X-radiography following progressively larger increments of applied load. The tensile strain required to initiate transverse cracking in the 90-deg ply was found to depend on the thickness of the 90-deg layer. For n ≤ 3 delamination was initiated at the edge of the coupon. The onset of delamination occurred before or after transverse cracking, depending on the laminate construction. For n ≥ 4 delamination at the 25/90 interface was initiated across the entire width of the coupon by the formation of a transverse crack in the 90-deg ply. In some laminate constructions the location of specimen separation during final fracture was associated with the growth and coalescence of edge delaminations under either incrementally increasing or constant strain cyclic loading. Comparisons of experiments to the fracture sequence predictions by stress and energy analysis are made to assess our ability to account for the ply thickness dependence of fracture in these laminates.

An Investigation of Cumulative Damage Development in Quasi-Isotropic Graphite/Epoxy Laminates

Abstract: Results of an experimental investigation of cumulative damage development in two unnotched quasi-isotropic graphite/epoxy laminates subjected to quasi-static tension and tension-tension fatigue are presented. Damage development in the form of transverse cracking in all off-axis laminae, longitudinal cracking, and delamination was monitored via the surface replication technique. Results of the study include a detailed description of the chronology of damage development prior to failure. Evidence also is presented in support of a damage model based on the concept of a characteristic damage state.

Plasticity Analysis of Laminated Composite Plates

Abstract: Elastic-plastic behavior of symmetric metal-matrix composite laminates is analyzed for the case of in-plane mechanical loading. The overall response of the laminate at each instant is derived from the elastic-plastic deformation of the individual fibrous layers, and from their mutual constraints. Constitutive equations of the laminated plates are presented in terms of initial yield conditions, hardening rules, and instantaneous compliances. Local stresses, hardening parameters, and strains are found in each lamina and in the fiber and matrix phases within each lamina. Specific results are obtained with the continuum model of elastic-plastic fibrous composites [1] which has been recently developed by the authors. Comparisons of analytical results with experimental measurements are made for certain laminated plates.

Effect of resin on impact damage tolerance of graphite/epoxy laminates

Abstract: Twenty-four different epoxy resin systems were evaluated by a variety of test techniques to identify materials that exhibited improved impact damage tolerance in graphite/epoxy composite laminates. Forty-eight-ply composite panels of five of the material systems were able to sustain 100 m/s impact by a 1.27-cm-diameter aluminum projectile while statically loaded to strains of 0.005. Of the five materials with the highest tolerance to impact, two had elastomeric additives, two had thermoplastic additives, and one had a vinyl modifier; all the five systems used bisphenol A as the base resin. An evaluation of test results shows that the laminate damage tolerance is largely determined by the resin tensile properties, and that improvements in laminate damage tolerance are not necessarily made at the expense of room-temperature mechanical properties. The results also suggest that a resin volume fraction of 40 percent or greater may be required to permit the plastic flow between fibers necessary for improved damage tolerance.

Metal-thermoplastic-metal laminates

Abstract: Lightweight metal-thermoplastic-metal laminates exhibiting an improved combination of good formability and bending strength comprise a core layer of a thermoplastic material selected from certain partly crystalline polyamides and polyesters, and a metal layer of certain aluminum alloys laminated on each side of the core layer, said thermoplastic and metal layers having certain requirements with respect to thickness and tensile elongation, and, additionally for the metal, tensile offset yield strength.

A Note on Specially Orthotropic Laminates

Abstract: A particular class of composite laminates is presented, where coupling ef fects are rigorously zero.

Microscopic Observations of Cross Sections of Impacted Composite Laminates

Abstract: Microscopic observations of cross sections of impacted [(0°)5/(90°)5/(0°)5] glass/epoxy composite laminates have been made using a scanning electron microscope. The microphotos obtained have confirmed macroscopic observations such as the initiation and propagation of the transverse cracks and delamination cracks and also given some details that cannot be obtained macroscopically. The transverse cracks are perpendicular to lamina interfaces when there is no delamination nearby, while transverse cracks grow obliquely to interfaces when they are accompanied by a delamination crack. Delamination cracking appears to interact with the observed transverse cracks.

Effect of stacking sequence on damage propagation and failure modes in composite laminates

Abstract: The effect of stacking sequence on damage growth and failure modes in composite laminates is discussed. Tests were conducted on AS/3501-6 graphite/epoxy coupons with four different stacking sequences. All laminates had the same percentage of 0, 90, and ′45 degree plies. Constant amplitude fatigue tests were performed at R (minimum to maximum stress ratio) = -1.0 and -∞. The specimens were periodically taken out of the testing machine and subjected to nondestructive inspection using X-ray radiography with diiodobutane. The results of nondestructive inspection indicate that the direction of damage propagation depends on the stacking sequence in the composite laminate. The damage may propagate predominantly along the loading direction or in a direction at some angle to the loading direction, depending on the stacking sequence. Interlaminar stress analysis of four laminates was performed to correlate the direction of damage propagation with the nature of interlaminar stresses.

Effects of moisture, residual thermal curing stresses and mechanical load on the damage development in quasi-isotropic laminates

Abstract: This investigation demonstrates how moisture absorbed in (0/+ or - 45/90)s and (0/90/+ or - 45)s graphite epoxy laminates significantly alters the stress state and chronology of damage development along the laminate edge during static tension and tension-tension cyclic loading. Emphasis is placed on using reasonable approximations for wet and dry elastic properties, including out-of-plane properties (nu sub 23 and G sub 23), since these properties are required by finite element and shear lag models to predict the stress state at the laminate edge. Moisture was observed to alter the dry edge stress state in the 90-deg plies of the (0/+ or - 45/90)s laminate such that delaminations occurred at a lower load and transverse cracks occurred at a higher load. A model was developed which predicted the differences in loads required to initiate damage in the 90-deg plies of the two laminates in the wet and dry conditions. Although moisture can alter the chronology of damage development, the damage state in each laminate observed prior to fracture appeared to be independent of moisture content.

Boundary-layer effects in composite laminates. I - Free-edge stress singularities. II - Free-edge stress solutions and basic characteristics

Abstract: The fundamental nature of the boundary-layer effect in fiber-reinforced composite laminates is formulated in terms of the theory of anisotropic elasticity. The basic structure of the boundary-layer field solution is obtained by using Lekhnitskii's stress potentials (1963). The boundary-layer stress field is found to be singular at composite laminate edges, and the exact order or strength of the boundary layer stress singularity is determined using an eigenfunction expansion method. A complete solution to the boundary-layer problem is then derived, and the convergence and accuracy of the solution are analyzed, comparing results with existing approximate numerical solutions. The solution method is demonstrated for a symmetric graphite-epoxy composite.

The Relationship of Stiffness Changes in Composite Laminates to Fracture-Related Damage Mechanisms

Abstract: The monotonic or cyclic loading of continuous fiber laminated composite materials produces various types of micro-damage prior to fracture. Each time one of these micro-events occurs, the local and global stiffness of the material is altered, causing several types of variations in the tensor modulus components of the laminae and laminate. This paper attempts to establish specific relationships between these variations in stiffness and the factors which control the laminate fracture event.

Damage mechanism and life prediction of graphite/epoxy composites

Abstract: This paper describes observation of fatigue damage mechanism in composites subjected to compression dominated fatigue loading, formulation of a damage-indicating parameter based on the intralaminar microcracking of the resin, and its application in conjunction with a linear fatigue damage model to predict spectrum life of graphite/epoxy composite laminates.

Boundary-layer hygroscopic stresses in angle-ply composite laminates

Abstract: An investigation on hygroscopically- induced stress fields near g eometric b oundaries of finitewidth, a ngle-ply composite laminates is presented. The study is formulated on the b asis of the t heory of anisotropic hygroelasticity. By the i ntroduction of Lehknitskii's complex stress functions, governing partial differential equations of the sixth-order are established for composite laminates subjected to uniformly distributed moisture absorption. The hygroscopic stress singularity at the edges of a c omposite laminate is determined b y an eigenfunction expansion technique recently developed by the authors. Distributions of stresses and associated strain energy densities in the boundarylayer region of the composite laminate are obtained by using a boundary-colloca tion method. A symmetric, angle-ply (45°/-450/-450/4 50) graphitel epoxy is chosen for illustrative purposes in this study. The solution for the (+45), composite is presented to elucidate t he fundamental behavior of hygroscopic edge stresses in composite laminates. Of particular i nterest are the p resently introduced hygroscopic b oundary-layer stress intensity factors and their implications. Details of these are reported in the paper.

Prediction of residual tensile strength of transversely impacted composite laminates

Abstract: The response to low velocity impact of graphite-epoxy T300/5208 composite laminates is discussed. Steel balls of 3/8 inch, 5/8 inch, and 1 inch diameter were the projectiles. Impact energy was limited to 1.2 joules. Impacted specimens were ultrasonically C scanned to determine the impact damaged region. The threshold value of impact energy for impact damage was found to be approximately 0.3 joules. A model was developed to predict the tensile residual strength of impact damaged specimens from fracture mechanics concepts. Impacted specimens were tested in tension to provide a fracture data base. The experimental results agreed well with the predictions from fracture mechanics. In this study, the maximum impact velocity used to simulate the low velocity transverse impact from common objects like tool drops was 10 m/s.

Composite Defect Significance.

Abstract: : State of the art of defect criticality assessment in structural composite laminates has been summarized and significance of other ongoing programs has been reviewed through literature survey and organization of a limited attendance symposium on the subject. A number of tests have been performed to determine the range of validity of criticality criteria for disbonds in laminated beam and plate type structures, which were developed in previous related programs. Data correlation studies have shown the usefulness of linear elastic fracture mechanics approach, methods of stress analysis based on 2-D elasticity and modified laminated plate theories as well as semi-empirical growth laws for cyclic loading for assessing growth of disbonds under transverse shear. (Author)

Interply Behavior Exhibited in Compliant Filamentary Composite Laminates

Abstract: Cord-rubber composite systems allow a visualization of interply shear strain effects because of the compliant nature of the matrix material. A technique termed the pin test was developed to aid this visualization of interply shear strain. The pin test performed on both flat pads and radial tires shows that interlaminar shear strain behavior in both types of specimens is similar, most of the shear strain being confined to a region approximately 10 interly rubber thicknesses from the edge. The observed shear strain is approximately an order of magnitude greater than the applied extensional strain. A simplified mathematical model, called the Kelsey strip, for describing such behavior for a two-ply (±θ) cord-rubber strip has been formulated and demonstrated to be qualitatively correct. Furthermore, this model is capable of predicting trends in both compliant and rigid matrix composites and allows for simplified idealizations. A finite-element code for dealing with such interply effects in a simple bu...

Influence of Fiber Orientation and Ply Thickness on Hygroscopic Boundary-Layer Stresses in Angle-Ply Composite Laminates

Abstract: The influence of fiber orientation and ply thickness on hygroscopic boundary-layer stresses in angle-ply composite laminates is studied. Based on the recently developed hygroelasticity theory for anisotropic composites, complete solutions are obtained for the hygroscopic stresses in [θ/-θ/-θ/θ] graphite-epoxy laminates with various fiber orientations and ply thicknesses. Both variables are shown to have significant effects on the develop ment of in-plane and interlaminar hygroscopic stresses. Important parameters characterizing the boundary-layer behavior such as hygroscopic edge stress singularities and stress intensity factors are presented. Hygroscopic boundary-layer widths are determined also for various cases by considering strain energy density distributions along the ply interface.

Compression and compression fatigue testing of composite laminates

Abstract: The effects of moisture and temperature on the fatigue and fracture response of composite laminates under compression loads were investigated. The structural laminates studied were an intermediate stiffness graphite-epoxy composite (a typical angle ply laimna liminate had a typical fan blade laminate). Full and half penetration slits and impact delaminations were the defects examined. Results are presented which show the effects of moisture on the fracture and fatigue strength at room temperature, 394 K (250 F), and 422 K (300 F). Static tests results show the effects of defect size and type on the compression-fracture strength under moisture and thermal environments. The cyclic tests results compare the fatigue lives and residual compression strength under compression only and under tension-compression fatigue loading.

The relationship between tensile and bending properties of non-linear composite materials

Abstract: The effect of a Weibull distribution of flaws on the strength on non-linear composite materials is considered. The tensile strength applying in bending is compared with that in direct tension. This leads to an estimate of the ratio modulus of rupture to ultimate tensile strength for composite materials.

Double torsion fracture toughness test for evaluating transverse cracking in composites

Abstract: An important failure mode of composite laminates is transverse cracking in the plies as a result of an applied load perpendicular to the fibre direction. This is a matrix-dominated mode. Transverse cracking in a composite can take place at loads below the failure load of the fibres. The damage sometimes leads to catastrophic failure and to property degradation of the laminates. A better understanding of this failure mode is particularly desirable. There has been a fair amount of work [1-7] on the mechanical behaviour of transverse cracking. It has been shown that the transverse crack process is controlled by the fracture toughness, defined by the critical strain energy release rate Gc, associated with this type of failure. The measurement of G e provides basic information about the transverse crack resistance of the laminates. In addition, it provides the materials scientist with an index for selecting resin matrix materials for composites to better resist transverse cracking. There are numerous test methods for measuring the fracture toughness of composite laminates. Most of them are based on tests originally used for homogeneous materials. The application of these techniques to angle-ply and cross-ply composites has not been all that satisfactory because of the complex damage phenomena involved. These composites do not usually fail with a single crack as is usually the case with homogeneous materials. Different methods have been developed to test laminates for transverse cracking parallel to the fibres. For example, Sanford and Stonesifer [8] used double edge notched (DEN) and single edge notched (SEN) specimens for evaluating the fracture toughness of (0 °) fibreglass laminates. Parvizi et al. [5] also used DEN and notched three point bend specimens for fracture toughness measurement on (0 °) fibreglass laminates. Slepetz and Carlson [9] employed compact tension specimens to test (0 °) S-glass/epoxy and (0 °) graphite/epoxy composites. The compliance calibration technique has been most conveniently employed for measuring the Gc of composites. The specimen compliance, C, against crack length, a, relationship is first established. In order to obtain an accurate C-a curve, a large enough population of specimens must be tested. In this respect, a test method which yields a linear C-a relationship is certainly more convenient to use, for example, the double torsion (DT) test [10, 11 ]. In addition to the advantage of rapid calibration of the C-a relationship, the test requires very simple specimen geometries which greatly minimize the machining operation involved. In this study, the double torsion test was investigated for its applicability to characterize transverse cracking in unidirectional fibre composites. This method has been successful for measuring fracture toughness of brittle homogeneous materials such as glass [10], ceramics [11] and glassy polymers [12-14]. When the technique was applied here to laminates, the load-displacement curves of the specimens were found to be different from those of the homogeneous materials. A correct interpretation of the results, however, led to the determination of meaningful fracture toughness related to transverse cracking. The original double torsion test technique was suggested by Outwater and developed by Kies and Clark [10]. The specimen is a rectangular plate supported along its two longer edges. In our case, the fibre direction was along the longer axis of the specimen. The specimen can be supported by either two rollers (Fig. 1) [11-14] or four steel spheres [15, 16]. In this study, we have experimented with test fixtures built with both of these supporting conditions. The load was applied to the specimen through two steel spheres as shown in Fig. 1.

Compression fatigue behaviour of notched composite laminates

Abstract: The influence of compression fatigue on graphite/epoxy laminates containing circular holes of 0.25 inch diameter was investigated. The laminate stacking sequences were [O2/±45]5S, [0/45/0/-45]5S, [0/±45/90]5S, and [90/0/±45]5S. Specifically this study examined the nature and extent of induced compression fatigue damage and the determination of the effects of this damage on the laminate residual failure mechanisms. Two modes of compressive failure were found to occur: the diagonal shear and the net compression modes. Both failure modes were characterized by local instability of individual lamina or small laminae subgroupings, with diagonal shear predominant in the fiber-dominated laminates, and net compression predominant in the quasi-isotropic laminates. The mode and direction of failure were dependent upon the nature of the specimen delamination. It was also found that the laminate stacking sequence influenced the intraply crack development in the laminates as well as the failure mode. The failure mechanisms were essentially the same for two different material systems which were studied (Narmco 5208 and 5209).