bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Moisture Absorption and Test Method on the Properties of E-glass/Polyester Hull Laminates
Authors: Miller Paul H.
Published: July 2003

Effects of moisture absorption on the tensile, compressive, shear, bending and bending fatigue properties of an iso-polyester/E-glass hull laminate were studied. Fickian diffusion constants were found that closely modeled moisture absorption. Comparisons were made of standard moisture experiments using boiling water versus long-term exposure. Results indicated that the boiling water test yielded significantly conservative values. Fatigue endurance limits were found to be near 25% of static load, indicating a fatigue design factor of four is required for infinite service with this material. Coupon results were used with finite element analysis (FEA) to predict panel deformations and strains. Good correlation was achieved.
Micromechanics of Moisture Diffusion in Composites with Impermeable Fibers
Authors: Whitcomb John , Tang Xiaodong
Published: July 2003

The objective of this paper is to determine the effect of fiber volume fraction on effective diffusivity of composites with impermeable fibers using finite elements. Both hexagonal and square fiber arrangements were studied. It is found that for low to moderate fiber fractions (Vf< 40%)the effective diffusivity was essentially the same for both fiber arrangements and the calculated values agreed very well with Hashin’s formula. For high fiber fractions (Vf> 40%)there were significant differences, but the behavior was still quite simple. A very simple closedform formula, which consists of Hashin’s formula for part of the range and a straight line for the rest, is proposed for calculating the effective diffusivity as a function of fiber volume fraction. The simple formula predictions matched the finite element predictions extremely well.
Mode II Mesoscopic Fracture Toughness of CFRP by Raman Coating Method
Authors: Miyagawa Hiroaki , Sato Chiaki , Ikegami Kozo
Published: July 2003

Mesoscopic fracture toughness of epoxy matrix phase in unidirectional Carbon Fiber Reinforced Plastics (CFRP) in Mode II loading was investigated using End Notched Flexure (ENF) specimens and Raman spectroscopy. The distribution of shear strain near the crack tip in the epoxy matrix phase was measured experimentally. A lead oxide (PbO) thin film was deposited on the measured surface of the ENF specimen by Physical Vapor Deposition (PVD) as the pretreatment for Raman spectroscopy. The Mesoscopic fracture toughness of the epoxy matrix phase in CFRP in Mode II was determined using the measured results. It was also investigated using the Finite Element Method (FEM). The macroscopic fracture toughness measured by conventional ENF testing was four times as great as the mesoscopic fracture toughness of the epoxy matrix phase.
A Virtual Containment Strategy for Filament-Wound Composite Flywheel Rotors with Damage Growth
Authors: Shiue Fuh-Wen , Lesieutre George A. , Bakis Charles E.
Published: July 2003

This paper presents a method for settinga safe operatingspeed for a filament-wound composite flywheel with an idealized delamination-type flaw. Fracture mechanics concepts were used to establish the severity and growth rate of such a flaw. The strain energy release rates of various-sized delaminations were evaluated usinga 2-D finite element method, and the effects of flaw radial location and press-fittingwere investigated. Once a flaw attains a certain size, the strain energy release rate is found to decrease, thus stabilizing further growth. Flaw growth rates under cyclic loadingwere estimated usingthe Paris law. Several virtual containment strategies were also investigated. Once a flaw is detected during flywheel operation, the maximum operatingspeed can be reduced to prevent catastrophic failure, achieve a specific design life, and maximize energy storage capacity over the remaining life. With a speed control strategy for energy maximization following the detection of damage, a numerical example showed an improvement of more than four times the energy capacity over a system without virtual containment.
A Three-Dimensional Model of the Resin Film Infusion Process
Authors: Loos A. C. , Rattazzi D. , Batra R. C.
Published: July 2003

A finite element code, in modular form, has been developed to model the complete three-dimensional resin film infusion (RFI) process. The problem formulation and its analysis incorporate compaction of the anisotropic elastic porous preform, elastic deformations of the tooling components, heat transfer in the resin, flow of resin through the preform, cure kinetics of the resin, and the heat transfer between the tools and the surrounding environment in the autoclave. The inertia effects and the transfer of heat by convection have been neglected. Two techniques, namely the slideline algorithm and a compliant layer interface, are used to model the possible sliding of the tool over the preform at their common interfaces. Weak forms are derived for (a) the initial-boundary-value problem corresponding to the transient thermal problem, (b) the boundary-value problem for the fluid through an elastic porous medium, and (c) the boundary-value problems for the quasistatic deformations of the tooling components and for a partially or fully saturated porous elastic preform. The finite element method is used to solve these equations, and the flow front is located by using a control volume technique. Computed results are presented for a stiffened T-panel and a two-stiffener panel.
Energy Approach for Prediction of Mechanical Behavior of 2-D Triaxially Braided Composites Part II: Parameter Analysis
Authors: Yan Yong , Van Hoa Suong
Published: July 2003

A 2-arcsmodel of 2-D braided composite is developed. Fiber volume fraction is evaluated by this model and compared with experimental results. Predicted valuesof Exx, Eyy, νxy, and νyx by energy approach are compared with results of experiment and other approaches [Reference 1]. Effective stiffness of 2-D braided composite is obtained in a closed form by the analysis of elastic deformation energy in repeat-unit-cell (RUC) of braided composites. Independent parameters of 2-D braided composite are separated from dependent parameters. There are three groups of independent parameters. The first group is yarn geometrical parameters which include filament packing fraction, axial yarn and braiding yarn count number ma, mb, filament diameter df, yarn thickness to width ratio ξ. The second group is composite geometric parameters which include braiding angle Θ and yarn space L. The last group is material constants which include material constants of both resin and yarn. Three dimensional parameter analysis is done by changing any two parametersof yarn space L, braiding angle Θ, count number of axial yarn ma while all other parametersare kept unchanged.
Thick-Sectioned RTM Composite Manufacturing: Part I – In Situ Cure Model Parameter Identification and Sensing
Authors: Michaud D. J. , Beris A. N. , Dhurjati P. S.
Published: July 2003

In the present work, we validate experimentally the cure simulation of a thick-sectioned composite processed by resin transfer molding (RTM). The simulation was based on an improved version of the model equations presented previously [Michaud, D.J., Beris, A.N. and Dhurjati, P.S. (1998). Curing behavior of thick-sectioned RTM composites. J. Compos. Mat., 32(14): 1273–1296.]. The presence of fiber reinforcement was found to significantly impact the curing behavior of the resin, leading to significant changes from the neat resin kinetic parameters. Thus, experimental processing data from seven 2.54 cm thick laminates were used to characterize the composite’s heat transfer and kinetic model parameters not readily computable from pure component values. The apparent initial concentration of inhibitor additives within the resin system decreased almost 80%, possibly due to absorption on the fiber surfaces. The presence of fibers was also found to reduce the extent of polymerization within the system. Key heat transfer model parameters, for both uncured and cured states, were also identified from experimental data. The importance of considering batch to batch variations and the temperature dependence of the resulting model parameters is discussed. In addition to the development and validation of the RTM cure simulation, heat flux sensors (HFSs)were evaluated as a nonintrusive sensor to replace internal thermocouples as a means of measuring internal cure behavior within a thick-sectioned composite.
Micropolar in-Plane Shear and Rotation Moduli of Unidirectional Fiber Composites with Fiber–Matrix Interfacial Debonding
Authors: Hutapea Parsaoran , QIAO PIZHONG
Published: July 2003

The mechanical properties of fiber-reinforced composites are significantly affected by the bond between the fiber and the matrix. The objective of this research is to determine the micropolar in-plane shear and rotation moduli of unidirectional brittle matrix composites with fiber–matrix interfacial debonding. Traction and displacement continuity conditions are imposed along the boundary of adjacent representative elements. In addition to a shear test (i.e., symmetric shear loading) for classical in-plane shear modulus, a center rotation test of a nine-cell model is proposed to introduce the asymmetric shear loading and numerically determine the micropolar in-plane shear and rotation moduli by means of finite element analysis. A doubly symmetric fiber–matrix interfacial debonding is utilized in this study. For the shear test, the composites are modeled using a quarter cell whereas for the rotation test, a nine-cell model is proposed to simulate the nonsymmetric nature of micropolar theory. The cell boundaries, in general, do not remain straight for the composites under loading. Parametric studies assessing the effects of debonding angle and the fiber volume fraction on the micropolar in-plane shear and rotation moduli of unidirectional composites are presented, and some basic characteristics of the moduli with interfacial debonding are discussed. Due to matrix domination in determining the shear properties of composites, the effects of an increase in interfacial debonding and fiber volume fraction are notably identified by the reduction of properties under both symmetric and asymmetric shear loading.
Manufacture of Thick Cross-Section Composites Using a Pre-Catalyzed Fabric Technique
Authors: Reuss Jonathan D. , Fowler Alex J. , Kim Yong K. , Lewis Armand
Published: July 2003

The development of a technique is reported for the manufacture of thick unsaturated polyester matrix resin composites using pre-catalyzed glass fabric. Composites over 2 cm thick using glass fabric that was pre-catalyzed (sized) with benzoyl peroxide were compared to composites made using conventional premixed catalyst and resin compounds. The pre-catalyzed fabric is shown to result in reduced internal temperatures during curing. This allows for the construction of thick fiber reinforced composite materials that do not suffer thermal degradation during the curing process. Mechanical testing of prepared thick composites demonstrated that the pre-catalyzed glass fabric composites were significantly stronger than conventionally prepared composites. The center regions of pre-catalyzed composites were about 80% stronger under tension, 25% stronger in compression and 35% stronger under shear than those of conventionally prepared composites of the same thickness. The pre-catalyzed composites were also more uniformly consolidated. Previous work aimed at the manufacture of thick composites has utilized controlled curing by use of expert systems, local resistance heating within the composite or the addition of chemical inhibitors. A method for staged curing has also shown considerable promise. None of these methods, however, has enabled high volume production of high quality thick composites. This paper presents the results of a new method for manufacture of thick composites based on a pre-catalyzed fabric technique.
Characterization of Fiber Optic Sensors for Structural Health Monitoring
Authors: Lee Dong Gun , Mitrovic Milan , Friedman Andrew , Carman Greg P. , Richards Lance
Published: July 2003

This paper presents experimental results on thermomechanical behavior of Extrinsic Fabry-Perot Interferometric fiber optic strain sensors (EFPI-FOSS). The objective of this study was to determine the accuracy, strength characteristics, and durability properties of both bare (nonembedded) EFPI sensors, and embedded optical fiber sensors in either a neat resin or aerospace grade composite laminate. Experimental results suggest that the embedded EFPI sensors provide reliable strain measurements for values exceeding 10,000 μ∊ under static loading conditions. A major portion of this study focused on evaluating the long term tension–tension fatigue behavior of optical fiber sensors. Test data suggest the EFPI sensors provide reliable data up to 1 million cycles at fatigue strain levels below 3,000 μ∊. For fatigue strain levels above this value, failure of the fiber optic sensor was observed. While the sensor failed it did not influence the strength and fatigue life of the composite coupons. Considering the design strains used in aerospace components, these results provide evidence that the EFPI sensors will survive during the life of typical aerospace structures.
Stress Model of Composite Pipe Joints under Bending
Authors: Yang Chihdar , Huang Hai , Guan Zhidong
Published: July 2003

Recently, composite pipe is becomingpopular in the offshore oil and gas industry due to the new “floating” designs of various platforms which have made deepwater oil and gas exploration and production more economical and affordable. The limited space in platforms emphasizes the needs for composite pipe joints in order to accommodate turns and bends. It has been estimated that there is one joint for every 4 ft of composite pipe installed for marine applications. The joints are the weakest link in a composite pipingsystem. Bending, one of the most common loads in a pipingsystem, was applied to the joint system and analyzed. An analytical model was developed usingthe first-order laminated anisotropic plate theory. Due to the asymmetric nature of the bendingload about the pipe central axis, a twodimensional model is necessary to simulate the system response. In this developed model, a three-component joint system consistingof coupling, adhesive, and pipe was used to model different types of composite pipe joints such as adhesive-bonded socket joints, butt-and-strap joints, and heat-activated couplingjoints. Good correlation was found between results from the developed model and finite element model including adhesive peel stress and shear stress distributions. This investigation has shown the effectiveness of the first-order laminated plate theory in modelingtwo-dimensional tubular geometries, includingthe surface displacements which are important in determiningthe adhesive peel and shear stresses in this study.
Determination of the Elastic Constants of a Transversely Isotropic Lamina Using Laminate Coefficients of Thermal Expansion
Authors: Melo José Daniel D. , Radford D. W.
Published: July 2003

Three-dimensional elastic constants for a fiber-reinforced lamina are often difficult to obtain from traditional mechanical tests. The relationships developed in thiswork enable the determination of any three of the five independent elastic constants, of a transversely isotropic lamina, from measurements of the remaining two elastic constants and the coefficients of thermal expansion (CTE’s) of [(+θ/−θ)n]s and unidirectional laminates. The approach is used to predict the elastic propertiesof a carbon fiber reinforced epoxy material and the predicted properties are shown to be in good agreement with quoted values. Further, the developed relationships can be used to determine elastic constants over a range of temperatures and, potentially, to study the effects of manufacturing on elastic properties. Therefore, this technique constitutes an additional method for evaluating the elastic constants that characterize a transversely isotropic, fiber-reinforced lamina.
Effect of Stitching on In-Plane and Interlaminar Properties of Sisal/Epoxy Laminates
Authors: Rong Min Zhi , Zhang Ming Qiu , Liu Yuan , Zhang Zhi Wei , Yang Gui Cheng , Zeng Han Min
Published: July 2003

The factors that influence in-plane mechanical responses and mode I interlaminar fracture toughness of stitched unidirectional sisal/epoxy laminates were studied in this paper. It was found that in-plane strength and stiffness of the laminates were not significantly affected by the stitching threads, while the interlaminar toughness was greatly improved, especially at higher sisal content. Unlike glass fiber reinforced polymer composites, sisal laminates have a rather high tolerance against the damages induced by stitching process. In addition, stitching expanded the fiber bridging zone and determined the R-curve profiles of sisal laminates. Factors including stitching density, diameter and species of stitching thread, continuity of stitching, and modification methods of sisal, were proved to greatly influence the delamination resistance of the laminates.
Effects of Geometric and Material Factors on Mechanical Response of Laminated Composites Due to Low Velocity Impact
Authors: Cho Chongdu , Zhao Guiping
Published: July 2003

In this paper, the geometric and material parameters of Graphite/Epoxy composite laminates such as span-to-thickness ratio, out-of-plane stiffness, orthotropy, stacking sequence are considered to investigate their influence on the mechanical behavior of composite plates under a low speed impact. Investigation is performed by means of two- and three-dimensional finite element methods combined with the modified Hertzian contact law, the failure criterion, and the degeneration of damaged elements. It is demonstrated that two-dimensional finite element analysis concerning transverse shear deformation due to the low speed impact by a solid object is acceptable when the minimum aspect ratio ( b/h) is greater than 30. Also it is found that the shape of damage zone may not be peanut-shaped and determined by the in-plane orthotropy of laminates. Besides, the effect of stacking sequence on the impact response is discussed.
Parametric Study on the Failure of Fiber-Reinforced Composite Laminates under Biaxial Tensile Load
Authors: Lin Wen-Pin , Hu Hsuan-Teh
Published: July 2003

A numerical constitutive model for a single layer of fiber-reinforced composite laminates, including nonlinear stress–strain relations, mixed failure criterion and post failure response, is used to predict the ultimate strengths of the composite laminates under biaxial tensile loads. The nonlinear constitutive law uses a variable shear parameter to model the nonlinear behavior of the in-plane shear. The onset of failure for individual lamina is determined by a mixed failure criterion composed of the Tsai-Wu and the maximum stress criteria. After the initial damage occurs, the response of the lamina is described by brittle or degrading modes up to the collapse of the entire laminate. The constitutive model is tested against experimental data and satisfactory results are obtained. In addition, parametric studies for composite laminates with different laminate layups and under various biaxial tensile load ratios are presented.
Identification of the Most Significant Processing Parameters on the Development of Fiber Waviness in Thin Laminates
Authors: Kugler Danielle , Moon Tess J.
Published: July 2003

This paper identifies the material and processing parameters which most significantly influence the development of in-plane waviness in laminates. Thin laminates of unidirectional, T300 carbon-fiber/polysulfone matrix prepreg were processed in an autoclave and a custom-made water-cooled chamber, which allowed fast cooling rates. Multivariate regression analysis of process-induced waviness was performed for combinations of the select process variables and their interactions to identify those factors responsible for waviness development. Of the eight parameters investigated – hold temperature, hold time, pressure, length, width, thickness, cooling rate, and tool plate material – only three affected the development of fiber waviness: length, cooling rate, and tool plate material. Length affects not only the number of wrinkles and wrinkle distribution, but also the average amplitude of the waviness. Cooling rate affects the wavelength and amplitude of the waviness, as well as the number of wrinkles. Tool plate material primarily affects the number of wrinkles, without showing a significant effect on the average wave geometry. There is also an interaction between tool plate material and cooling rate in producing fiber waviness. For the three relevant parameters, the possible waviness-inducing mechanisms are tool plate/part coefficient of thermal expansion (CTE) mismatch, temporal temperature gradients (or cooling rates), and spatial temperature gradients. The tool plate/part CTE mismatch proved to be the most important mechanism driving fiber waviness in plates, although changes in cooling rates also dramatically affected the quantity of waviness which developed. Spatial temperature gradients were negligible for this study. The tool plate/part CTE mismatch-driven axial buckling loads on the fibers were substantial in the outermost laminate plies, or skin, but negligible in the laminate core. Waviness was limited to the surface or skin plies, even in identically-processed thick laminates. This study confirmed that if the fibers experience axial loads – albeit a small fraction of their Young’s modulus – while the matrix is unable to provide some level of transverse fiber support, the fibers will microbuckle resulting in waviness (in-plane or out-of-plane depending upon the laminate constraint).
Saturated and Unsaturated Polymer Flows: Microphenomena and Modeling
Authors: Dungan F. D. , Sastry A. M.
Published: July 2003

Fabrication of large composite parts via resin impregnation necessarily involves both saturated and unsaturated flow. While Darcy’s law assumptions classically model only the saturated flow, the idea of a Darcy-type “permeability has been widely used to describe unsaturated flows as well. Differences between saturated and unsaturated resin flow through fibrous preforms have long been reported in the literature, however, and selection of permeability values for use in numerical simulation requires a clear understandingof the differences between these two flow types. Here, we present a new model for use in prediction of the ratios of these two permeabilities. Capillary effect and the fiber roughness are shown not to be dominant phenomena, as previously assumed by other workers. Instead, transverse flow into tows, which occurs after the fluid has permeated the intertow gaps, is found to produce significant differences in saturated and unsaturated flow velocities. Our model, which is based on Berman’s flow in a channel with porous walls, additionally accounts for walls reaching saturation as the flow moves axially through the reinforcement phase, and is shown to be in agreement with earlier experimental data. We also present analyses and experimental results based on atomic force microscopy on fiber roughness, showing its effects on flow to be of less importance than the tow permeation process. The investigation on roughness shows convincingly that for practical materials, roughness is not a concern in flow retardation at any scale (fiber or tow).
Towards Automatic Designing of 2D Biaxial Woven and Braided Fabric Reinforced Composites
Authors: Huang Zheng-Ming , Ramakrishna S.
Published: July 2003

This paper presents a general methodology to automatically analyze the stiffness and strength of a textile composite reinforced with any 2D (twodimensional)biaxial woven or braided preform only using its constituent fiber and matrix properties and limited number of independent textile geometric parameters. The textile composite under consideration can be subjected to an arbitrary load condition. Based on the assumption of a perfect contact between the fill and warp yarns and a microphotographic observation, a geometric model is developed to describe the yarn architecture of the 2D biaxial woven or braided fabric structure. The textile composite is eventually subdivided into small elements, each of which is considered as a unidirectional (UD)fibrous composite in its local coordinate system. The bridging micromechanics model is applied to determine the 3D mechanical properties of the UD composite at each load level up to failure based on the information of the current constituent properties and the fiber volume fraction. The matrix properties have been defined according to the averaged value of the current three principal stresses in the matrix. If this value is positive, the properties obtained from a tensile experiment should be used; otherwise, those from a compression test are employed. This is because the matrix material in the textile composite is usually subjected to 3D stress state, and is because a polymer matrix generally has different properties in a uniaxial tension from those in a uniaxial compression. The resulting properties of all the UD composites are assembled by means of a volume average based on either iso-strain or iso-stress approach, to give the overall stiffness and strength of the textile composite. One significant advantage of the present method is that only minimal input data are required, which can be easily measured before composite fabrication or taken as designing variables. Thus, an automatic design for the required properties of a textile composite can be achieved. Applications have been made to those composites reinforced with plain, 5-harness and 8-harness satin weaves and diamond and regular braids. While limited test results for the woven fabric composites are available from the literature, an extensive experimental program has been carried out to measure stiffnesses, strengths, and stress–strain curves of the braid composites under uniaxial tensile load. Two different material systems, i.e., glass/epoxy and carbon/epoxy systems, each with three different braiding angles have been studied. It has been found that the iso-strain assemblage scheme gives grossly better correlation for both the stiffness and strength of all the composites with the experiments.
Effect of Thermal Conductivity Mismatch on the Thermal Stresses in a Dispersed Phase-Continuous Matrix Composite Material Undergoing Steady-State Heat Flow
Authors: Hasselman D. P. H.
Published: July 2003

Under conditions of heat flow a mismatch in the thermal conductivity of the components in a composite material gives rise to localized stresses and displacements not present under isothermal conditions. These localized displacements result in additional bending displacements (i.e., additional curvature) at the continuum level. When such curvature is constrained, additional thermal stresses will arise which are superposed on those based on conventional thermoelastic theory and the isothermal properties of the composite material. This phenomenon is referred to as the “thermal conductivity mismatch effect.” For any given composite material, this effect can be analyzed by numerical means. It is the purpose of this study to present an analytical approach to this problem based on an approximate analytical expression for the effective coefficient of thermal expansion in thermal bending under conditions of steady-state heat flow. The specific geometry selected for this analysis consisted of a short ring subjected to steady-state radial heat flow. The approach taken was to first calculate the thermal stresses based on isothermal thermal properties and then superpose the stresses resulting from the change in the coefficient of thermal expansion in thermal bending due to the presence of heat flow. Numerical examples for an aluminum oxide-aluminum composite material indicated that the thermal conductivity mismatch effect can play a significant role in the magnitude of the thermal stresses and can be positive or negative depending on the direction of mismatch. This latter finding suggests a solution for materials selection for engineering design involving high magnitudes of thermal stress.
A 3-D Connectivity Model for Effective Piezoelectric Properties of Yarn Composites
Authors: Ruan Xiaoping , Chou Tsu-Wei , Safari Ahmad , Danforth Stephen C.
Published: July 2003

The purpose of this paper is to develop a 3-D connectivity model that enables better understanding, at the micro-structural level, the effect of fiber orientation, matrix properties, and inter-fiber contact on the effective piezoelectric properties of a composite consisting of a unidirectional piezoceramic fiber yarn in a polymeric matrix. It is assumed in the 3-D connectivity model that the piezoceramic fiber and matrix are connected in parallel in the fiber direction and in series transverse to the fiber direction. The poling direction is at an arbitrary angle from the fiber axis. Using these assumptions and coordinate transformation, the effective piezoelectric properties of yarn composites can be determined. It is concluded that the effective piezoelectric constants are very sensitive to the fiber orientation. There is a significant effect of the dielectric permittivity of the matrix material on the effective piezoelectric constants, and the effective piezoelectric constants obtained from the 3-D connectivity model can change dramatically depending on the non-contact and contact assumptions of the fibers.
The Influence of Internal Stresses on the Microbond Test II: Physical Aging and Adhesion
Authors: Mendelsy D.-A. , Leterrier Y. , Manson J.-A. E. , Nairn J. A.
Published: July 2003

The present work investigated the effects of physical aging on interfacial adhesion and internal stresses using single-fibre model composites. First, the stress state of the interface and its evolution upon aging were studied for the microbond geometry. This stress analysis was used with experiments to determine an intrinsic interfacial shear strength. Second, this stress analysis, including aging effects, was used in a fracture mechanics or energy release rate analysis of the microbond test, which involves the energy necessary to propagate a crack instead of the minimum stress to initiate a crack. These methods were tested for both freshly cured and aged epoxy droplets on glass fibres, and were shown to yield similar results. It was found that neither the interfacial shear strength nor the interfacial toughness were changed by physical aging of the polymer matrix when the relaxation of internal stresses and shift of relaxation times of the matrix due to physical aging were accounted for. Thus, the intrinsic shear strength and toughness of the fibre– matrix interface were determined.
Effective Mass Diffusivity in Composites
Authors: Li Shoujie , Lee L. James , Castro Jose
Published: July 2003

Gas diffusion is important in composite manufacturing, while moisture diffusion is important for composite performance. In this study, analytical models are developed to predict the mass diffusivity of both square and hexagonal arrays of cylindrical cylinders. These models are able to bridge the gap between continuum calculations via finite differencing of finite element analysis and simple/trivial models. By breaking the liquid region into zones and making some general assumptions about the diffusion in those zones, simplified models have been developed, which rely solely on the geometry and fiber volume fraction. The models are fairly accurate for a broad porosity range, but specially at low porosity values. A single weight factor is proposed to compensate for the error introduced by the model assumptions. When the weight factor is incorporated, the modified models agree extremely well with the experimental data for the whole porosity range.
Modulus Graded Composite Adherends for Single-Lap Bonded Joints
Authors: Ganesh V. K. , Choo T. S.
Published: July 2003

The effect of spatial grading of adherend elastic modulus on the peak stress and stress distribution in the single-lap bonded joint is studied. Single-lap joint with various modulus grading profiles were studied and the results compared with the traditional uniform modulus adherends using linear elastic finite element analysis. Braided preform with continuously varying braid angle was fabricated and the variation of the braid angle measured to realistically evaluate the performance of adherend modulus grading in single-lap bonded joint. The peak stress, stress distribution and transverse displacement at the adhesive mid-thickness were used to compare between the different adherend modulusgrading. The maximum shear stress reduced by about 20% and the shear stress was more uniformly distributed in the adhesive for an actual case of adherend modulus grading. It was also noticed that the joint rotation wasminimum for thisactual case of adherend grading.
An Uneven Strain Model for Analysis of Residual Stress and Interface Stress in Laminate Composites
Authors: Bao Yiwang , Su Shengbiao , Huang Jow-Lay
Published: July 2003

An uneven strain model was proposed to analyze the residual stress and interfacial stress in laminates. Results suggested that the residual stress caused by the difference of thermal expansion coefficient between two adjoining layers is a function of the relative location in the laminates, instead of being a constant value. In addition, both shear stress and normal stress existed in the interface, and they varied with the distance to the interfacial boundaries. Calculations based on experiments containing different material types of laminate composites and indentation crack sizes were conducted. Results indicated that the maximum residual stress and interfacial tensile stress occurred at the center of a laminate, while the maximum interfacial shear stress and compressive stress normal to the interface occurred near the boundary of the laminates.
Cure Cycle Effect on Composite Structures Manufactured by Resin Transfer Molding
Authors: Kim Yeong K. , Daniel Isaac M.
Published: July 2003

An experimental investigation was conducted of the effect of curing cycle on the development of material properties, residual strains and stresses in composite parts during the resin transfer molding (RTM) process. The material investigated consisted of AS4 carbon fibers as the preform anda three-part epoxy system. Unidirectional and crossply carbon/epoxy laminates were prepared by the RTM process in aluminum molds. Several different curing cycles were designed by changing the peak cure temperature andheat up rate basedon a cure kinetic model. Strains were measured in the composite laminates during curing using embedded fiber optic strain sensors andelectrical resistance strain gages. It was foundthat significant strain was developed by interference between composite and mold during cure, resulting in constraint-induced strain. The magnitude of residual stresses was also assessedby measuring the warpage curvature of asymmetric cross ply laminates. The data showed that the residual stress was significantly dependent on the cure cycle andthe interference.
Modeling and Characterization of Fatigue Strength of Laminated Composites with Knitted Fabric Reinforcement
Authors: Huang Zheng-Ming , Ramakrishna S. , Thwe Aye Aye
Published: July 2003

Results from both theoretical and experimental investigations on laminated epoxy composites reinforced with plain weft-knitted carbon fiber fabrics of different arrangements subjected to fatigue loads are presented in this paper. The stacking sequences used are [0/0/0/0], [0/45/−45/0], [90/90/90/90], [0/90/90/0], and [90/45/−45/90] where 0 refers to the fabric wale direction and 90 to the course direction. Tensile fatigue with stress control was applied to the laminates as well as to the monolithic matrix material, with a stress ratio of R = 0.1 and a cycling frequency of 5 Hz. The theoretical prediction for the laminate fatigue strength was made using a general methodology, which is a combination of the classical lamination theory and the bridging micromechanics model. The stresses sustained by each lamina layer in the laminate were determined according to the classical lamination theory, whereas the internal fatigue stresses generated in the fiber and matrix materials of the lamina were obtained by virtue of the bridging model. The internal stresses were detected against the constituent fatigue strengths at the similar loading conditions as applied to the laminate. Thus, the laminate fatigue strength was predicted only using the constituent fatigue data and the laminate geometric parameters, which were measured/determined independently. Reasonably good agreement has been found between the predicted and experimental S–N curves of all the five laminates.
Observations of Fiber Fracture and Interfacial Debonding Phenomena Using the Fragmentation Test in Single Fiber Composites
Authors: Kim Ben W. , Nairn John A.
Published: July 2003

Fiber-reinforced polymer composites are being studied for many applications because of their potential for outstanding mechanical properties. To be used in new applications, composites have to be more fully understood. The fiber, the matrix, and the fiber/matrix interface are the building blocks that need study. This research has concentrated on the interface where debonds, or damage need to be studied more thoroughly. The experimental work here focused on the fragmentation test. The specimens used were AS4-carbon/Epoxy and E-glass/Epoxy composites. The fragmentation process was continuously monitored with careful attention to debonding at each break and debonding growth at higher strain as a function of applied tensile strain. Photoelastic birefringence patterns in the specimen were observed in more detail than ever to accurately monitor interfacial debonding and damage. The fiber fracture and the debonding length between the fiber and the matrix were measured only by visual observation using the photoelastic patterns rather than the Laser Raman Spectroscopy (LRS) which has usually been used in measuring interfacial debonding length or damage. This visual measurement technique provides the much simpler and more precise method than traditional LRS in measuring the fiber fracture and the interfacial damage in fiber-embedded composites. From the experimental data of fiber breaks and interfacial debond length, the interfacial toughness between the fiber and the matrix was analyzed for each composite system using the energy method. The interfacial debond energy, Γd, of AS4-Carbon/Epoxy was 220 J/m2 and 130 J/m2 for instantaneous debond and whole debond, respectively. For E-glass/Epoxy, Γd was obtained as 105 J/m2 and 98 J/m2 for instantaneous debond and whole debond, respectively.
Laser Ultrasonic Array System for Real-Time Cure Monitoring of Polymer-Matrix Composites
Authors: Fomitchov Pavel A. , Kim Yeong K. , Kromine Alexei K. , Krishnaswamy Sridhar
Published: July 2003

A laser ultrasonic system for real-time cure monitoring of a graphite-epoxy composite is proposed. The system contains an array of fiberized laser ultrasonic sources, and an embedded fiber optic Sagnac ultrasonic sensor, and is integrated with a Resin Transfer Molding machine. The use of an optical switch allows ultrasonic generation at several locations of the composite part. Ultrasound generated by the laser source is transmitted into the composite and is detected by the embedded sensor. The cure state is inferred from measurements of ultrasonic velocity. The results of ultrasonic measurements during manufacturing of a composite specimen are presented. The laser ultrasonic cure monitoring system described in this paper has the ability tooperate in a high temperature and high pressure environment; is small enough to be incorporated into molds; and measures the cure state at several locations within the composite part.
Effect of Matrix Cracks on Damping in Unidirectional and Cross-Ply Ceramic Matrix Composites
Authors: Birman Victor , Byrd Larry W.
Published: July 2003

The paper elucidates the methodsof estimating damping in ceramic matrix composites (CMC) with matrix cracks. Unidirectional composites with bridging matrix cracks and cross-ply laminates with tunneling cracks in transverse layers and bridging cracks in longitudinal layers are considered. It is shown that bridging matrix cracksdra matically increase damping in unidirectional CMC due to a dissipation of energy along damaged sections of the fiber–matrix interface (interfacial friction). Such friction is absent in the case of tunneling cracks in transverse layers of cross-ply laminates where the changes in damping due to a degradation of the stiffness remain small. However, damping in cross-ply laminates abruptly increases, if bridging cracks appear in the longitudinal layers.
Validation of the Quadratic Composite Failure Criteria with Out-of-Plane Shear Terms
Authors: Huybrechts Steven , Maji Arup , Lao Jennifer , Wegner Peter , Meink Troy
Published: July 2003

The Tsai–Wu Quadratic Failure Criterion is a modified tensor polynomial criterion, that is widely used and the most readily accepted failure criterion for orthotropic materials. While it is an excellent criterion for the majority of composite structures, there is limited experimental validation of this criterion when predicting the failure envelope for “combined shear” failures or, better stated, failures that involve both 2-D (in-plane) and 3-D (out-of-plane) shear stresses in an orthotropic material or laminate. Since several common types of material failure involve complex stress states, the ability to incorporate the effect of 3-D shear stress into failure prediction is attractive.

The 3-D Tsai–Wu quadratic failure criterion that incorporates multiple shear stresses is presented in this text and tests were conducted on several laminate specimens in an effort to validate this criterion. Failure load predictions were made to validate the modified 3-D criterion and compare the results with the 2-D Tsai–Wu criterion for specialized cases where multiple shear stresses were present at failure. This testing clearly shows strong agreement between the 3-D criterion’s predictions and experiments for these cases, implying that the standard 2-D Tsai–Wu quadratic failure criterion can be safely extended, through the addition of terms, to 3-D cases.
The Dynamic Stress Field in the Matrix Surrounding a Spheroidal Particle
Authors: Paskaramoorthy R. , Kienhöfer F. , Meguid S. A.
Published: July 2003

In this study, the steady-state stresses resulting from a dynamic loading in a composite reinforced by a single spheroidal particle are determined and the stress concentration factors within the matrix obtained. A hybrid technique that combines the finite element method with an eigenfunction expansion technique is used to determine the stresses. The stress concentrations within the matrix of the composite are found to be dependent on the frequency of excitation, the mismatch of elastic properties and density between the particle and the matrix, the aspect-ratio of the particle and Poisson’s ratio of the particle and matrix. In particular, the study reveals that the matrix would experience dynamic stresses up to 100% greater than the static values when the particle density is greater than that of the matrix. The results also indicate that composites with brittle matrices will experience crack initiation at the pole of the particle where the principal stresses are the largest. For ductile matrices, on the other hand, the region of maximum von Mises equivalent stress within the matrix varied along the particle–matrix interface, but also occurred at interior points away from the interface under certain conditions.
On-Line Processing of Unidirectional Fiber Composites Using Radiative Heating: II. Radiative Properties, Experimental Validation and Process Parameter Selection
Authors: Chern Bih-Cherng , Moon Tess J. , Howell John R.
Published: July 2003

Experimental validation is presented for a detailed thermal model (described in Paper I) for on-line processing of unidirectional fiber composites by surface or volumetric radiative heating. Surface and volumetric radiative properties of unidirectional graphite/epoxy and glass/epoxy are presented: measurements of the complex refractive index of an uncured and cured 3501-6 epoxy resin as a function of wavelength; semi-empirical extinction and scattering coefficients and phase functions for graphite/epoxy and glass/epoxy as a function of wavelength and incident angle, assuming independent scattering; model predictions of the effects of dependent scattering (i.e. electromagnetic wave interference) in graphite/epoxy and glass/epoxy; and measurements of the directional-hemispherical reflectance of AS4/3501-6 as a function of wavelength, incident angle, unidirectional composite orientation, and degree of cure. Experimental temperature histories for in-situ (or continuous) curing of graphite/epoxy, hoop-wound cylinders using infrared (IR) heating at power levels of 5 and 7 kW and mandrel winding speeds of 0.1 and 0.15 m/s are presented. Good qualitative agreement is found between the experimental results and model predictions for AS4/3501-6. Recommended manufacturing process windows for graphite/epoxy and glass/epoxy are presented for several process parameters, such as radiant-source emissive power level and winding speed. Due to their higher radiation absorptivity and lower heat capacity, graphite composites generally have narrower process windows.
Probability-Based Modelling of Composites Manufacturing and Its Application to Optimal Process Design
Authors: Li Hong , Foschi Ricardo , Vaziri Reza , Fernlund Göran , Poursartip Anoush
Published: July 2003

The control of process-induced deformations in composite structures is important for cost-effective manufacturing. In recent years, significant advances have been made in predicting the average deformation behaviour, but little work has been done in predicting the variability, which results from uncertainties in both the raw material properties and the manufacturing process conditions. A probability-based approach is presented in this paper for predicting the variability of process-induced deformations. A two-dimensional finite element code, which deterministically simulates the various physical phenomena during processing of composite structures, is integrated with a first-order reliability analysis method to calculate the probability of the deformations exceeding a specified allowable tolerance. The methodology is demonstrated through two case studies. In the first study, a probabilistic description of the process-induced spring-in of a channel section is achieved and the effect of variability in material properties on the final channel angle is studied. In the second study, the optimal tool-shape for the channel section is determined by coupling reliability analysis with a simple cost model of the manufacturing process.