bluuebunny/crossref_metadata_2025 huggingface dataset:
Mesenchymal Stem Cells and Their Potential as Cardiac Therapeutics
Authors: Pittenger Mark F. , Martin Bradley J.
Published: July 2004

Mesenchymal stem cells (MSCs) represent a stem cell population present in adult tissues that can be isolated, expanded in culture, and characterized in vitro and in vivo. MSCs differentiate readily into chondrocytes, adipocytes, osteocytes, and they can support hematopoietic stem cells or embryonic stem cells in culture. Evidence suggests MSCs can also express phenotypic characteristics of endothelial, neural, smooth muscle, skeletal myoblasts, and cardiac myocyte cells. When introduced into the infarcted heart, MSCs prevent deleterious remodeling and improve recovery, although further understanding of MSC differentiation in the cardiac scar tissue is still needed. MSCs have been injected directly into the infarct, or they have been administered intravenously and seen to home to the site of injury. Examination of the interaction of allogeneic MSCs with cells of the immune system indicates little rejection by T cells. Persistence of allogeneic MSCs in vivo suggests their potential “off the shelf” therapeutic use for multiple recipients. Clinical use of cultured human MSCs (hMSCs) has begun for cancer patients, and recipients have received autologous or allogeneic MSCs. Research continues to support the desirable traits of MSCs for development of cellular therapeutics for many tissues, including the cardiovascular system. In summary, hMSCs isolated from adult bone marrow provide an excellent model for development of stem cell therapeutics, and their potential use in the cardiovascular system is currently under investigation in the laboratory and clinical settings.
Conductive High Temperature Nylon
Authors: King Julia A. , Weber Erik H. , Quan Congling , Tucker Kenneth W. , Vogt Bryan D.
Published: July 2004

Increasing the thermal and electrical conductivity of typically insulating polymers, such as nylons, would open new markets. A thermally conductive resin could be used for heat sink applications. An electrically conductive resin could be used in static dissipative and Electromagnetic Interference/Radio Frequency Interference shielding applications. This research focused on adding conductive fillers (namely carbon black and Thermocarb, a high quality synthetic milled graphite) and a chopped glass fiber to high temperature nylon. These materials were extruded and injection molded into test specimens. Tensile tests as well as in-plane electrical resistivity, in-plane thermal conductivity, and through-plane thermal conductivity tests were conducted.
Temperature Rise and Surface Roughness of Carbon Fiber Epoxy Composites during Cut-Off Grinding
Authors: Lee Dai Gil , Kim Po Jin , Choi Jin Kyung
Published: July 2004

Although near net-shape molding of advanced composites is desirable, machining operations such as cutting, drilling and grinding on the advanced composite materials are frequently required for the composite structures to be employed for precision machine elements. During the composite machining operations, the temperature at the cutting point may increase beyond the allowed limit due to the low thermal conductivity of composites, which might degrade the properties of the composite. Therefore, in this work, the temperature at the cutting point was measured during the cut-off grinding of carbon fiber epoxy composites. The grinding force and surface roughness under both dry and wet grinding conditions were also measured with respect to grinding speed and in-feed rate to investigate the optimum cut-off grinding characteristics of the composites.
Failure Mode Transition in Unidirectional E-Glass/Vinylester Composites under Multiaxial Compression
Authors: Oguni K. , Tan C. Y. , Ravichandran G.
Published: July 2004

Results from an experimental investigation on the mechanical behavior of a unidirectional fiber reinforced composite with 50% volume fraction E-glass/vinylester and the vinylester matrix under uniaxial and proportional multiaxial compression are presented. The stress-strain curve and the acoustic emission records together with the post-mortem observations on the specimen show the failure mode transition from axial splitting to kink band formation as the loading condition changes from uniaxial to proportional multiaxial compression. Axial splitting and "splitting induced" kink band formation were observed in the uniaxially loaded specimen and the multiaxially loaded specimen showed conjugate kink bands and no axial splitting.
Optimum Design of Laminated Composite Plates for Maximum Thermal Buckling Loads
Authors: Singha Maloy K. , Ramachandra L. S. , Bandyopadhyay J. N.
Published: July 2004

Buckling temperatures of graphite/epoxy laminated composite plates are maximized for a given total thickness considering fiber-directions and relative thicknesses of layers as design variables. Thermal buckling analysis is carried out using the finite element method with 4 node shear deformable plate element, while genetic algorithm (GA) is employed to optimize as many as ten variables for the five layered plates. In addition to traditional three-operator approach, i.e., reproduction, crossover and mutation, other variants of GAs such as the elitist model, two-point crossover models are also discussed. The study includes composite plates of three different aspect ratios, two support conditions and three different numbers of layers. The presented results reveal that the buckling loads can be increased significantly with appropriately orienting the fiber directions and varying the thickness of different layers. The authors also recommend the judicious selection of specific thicknesses and fiber orientations needed for the practical implementation highlighting the importance of their theoretical values so that they may also be made available in near future for fabrication.
Development of the Transient Simulated Laminate (TSL) Methodology for Moisture Ingression Studies Using Unsymmetric Laminates
Authors: Wu Yi-Jui , Takatoya Takeshi , Chung Kimo , Seferis James C. , Ahn Kyujong
Published: July 2004

This article presents a non-destructive method to examine a graphite/epoxy composite under hygrothermal conditions. By applying the transient simulated laminate (TSL) technique, a methodology was developed to evaluate the effects of water ingression on the laminates based on the changes in curvature of laminates. The absorption-desorption-resorption cycle of laminates was evaluated in situ by measuring both moisture content and curvatures of unsymmetric laminates [02/902] T. Curvatures of laminates decreased as more water diffused into the laminate until a high moisture content was reached. During hygrothermal cycling, the curvature of a laminate was affected not only by absorption temperature, but also desorption temperatures. This work demonstrated that curvature measurements in addition to moisture measurements could be an effective tool for relating physical state changes to hygrothermal cycling history in composite laminates.
Grinding Characteristics of Carbon Fiber Epoxy Composite Hollow Shafts
Authors: Kim Po Jin , Lee Dai Gil , Choi Jin Kyung
Published: July 2004

Since carbon fiber epoxy composite materials have excellent structural properties due to their high specific strength, high modulus, high damping and low thermal expansion, composite hollow shafts have been widely used for power transmission shafts for motor vehicles, spindles for machine tools, and rollers for thin film processing. However, the surfaces of molded composite shafts are not always satisfactory for precise mechanical elements, which often requires turning or grinding of surfaces.

The machining characteristics of composite hollow shafts are different from those of conventional metal hollow shafts because composite hollow shafts have anisotropic inhomogeneous material characteristics.

In this paper, the grinding characteristics of composite hollow flexible shafts were investigated experimentally and analytically with respect to stacking sequence and stiffness of carbon fiber epoxy hollow composite shafts.
Stitch Cracks in Constraint Plies Adjacent to a Cracked Ply
Authors: Lavoie J. AndrÉ , Adolfsson Erik
Published: July 2004

A high density form of matrixmicrocracking in constraint plies adjacent to a cracked ply, and here called stitch cracking, was found to form readily in awide variety of [+. n/–. n/902n] s laminates. Stitch cracks appear to form instead of interply delamination at the tip of the crack in the constrained ply, more typical of cross ply laminates, when the included angle is greater than 50°. Stitch cracks do not require fatigue loading to form, but appear due to thermal residual stress and monotonic loading. Stitch cracks were observed to instantly form in –. ° constraint plies next to a cracked 90° ply, apparently when the included angle of the plies was less than about 50°. In addition, they formed in –15° and –60° plies next to corresponding cracked +. ° surface plies. Data on stitch crack length and spacing is included. This paper seeks to raise awareness of stitch cracks by systematically demonstrating that they not only exist, but occur in awide variety of laminates. The data suggest that stitch cracks should not be overlooked during development of composite damage mechanics models.
Tackification of Textile Fiber Preforms in Resin Transfer Molding
Authors: Shih Chih-Hsin , Lee L. James
Published: July 2004

Tackified textile fiber preforms are used widely in resin transfer molding (RTM) to produce aerospace-grade composite parts. To optimize the preforming process, a thorough understanding of many important material and process issues, such as fabric formability, preform dimension control, preform permeability, fiber wetting, and tackifier compatibility with the matrix resin, is essential. These issues are investigated based on a commercial tackifier (PT 500 from 3M). The results show that tackifier concentration, tackifier powder size, and the application conditions all play important roles in governing the moldability of tackified fiber preforms.
Monte-Carlo Simulation and the Chain-of-Bundles Model for Titanium-Matrix Composite Materials
Authors: Laumakis P. J. , Bonacuse P. J.
Published: July 2004

A numerical simulation technique has been developed and is used to estimate the ultimate strength of several unidirectional metal-matrix composite systems. The results of multiple realizations of this technique, which explicitly accounts for the local nature of fiber load redistribution, are presented and show excellent agreement with experimental results. In addition to synthesizing some recent numerical modeling efforts for these types of composite systems, this simulation technique incorporates both the multi-axial stress state effects and the change in constituent residual stresses as the matrix plastically deforms. A thorough statistical analysis of the ultimate strength predictions is performed not only to assess the accuracy of the simulation model presented, but also to provide a measure of goodness of the various analytical models reviewed.
Impact Damage Tolerance Analysis of Stiffened Composite Panels
Authors: Chen Puhui , Shen Zhen , Wang Junyang
Published: July 2004

An approach based on a displacement compatibility model is presented for both riveted and bonded stiffened composite panels containing impact damage. This represents the first application of the displacement compatibility model to the failure analysis of impacted stiffened composite panels, and some newresults are obtained for the impact damage tolerance properties of stiffened composite panels. In the present analysis, the previous displacement compatibility model for a stringer/orthotropic skin panel is improved for a stringer/unbalanced skin panel. The impact damage is simplified as an elliptic hole based on the compressive failure mechanisms of impacted composite laminates and stiffened panels. Predictions for failure loads and damage arrest capability agree well with experimental results for several composite panel configurations. The important results obtained in this study are: (1) damage arrest capability is dominated by the strength of the skin/stringer attachment; (2) distinct two-stage failure shows that the riveted panels have significant damage arrest capability due to the high shear strength of the rivets; and (3) since the very low shear strength of the skin/stringer attachment, the co-cured panels have no damage arrest capability.
The Elastic Constants of Corrugated Board Panels
Authors: Lu T. J. , Zhu G.
Published: July 2004

Because corrugated board panels inevitably contain various processing- induced geometrical imperfections, to develop simple and yet reliable methods for measuring the elastic properties of orthotropic corrugated board panels is not only vitally important for the packaging industry, but also key for any analytical/numerical modeling of the deformation and failure behavior of the corrugated products. This paper focuses on developing and applying the Chladni-pattern based vibration testing method for corrugated panels. To ensure that the measured data from the non-destructive Chladni-pattern method are accurate, the elastic properties measured from traditional mechanical testing methods—lateral compression, three-point bending, and simple shear (including both singleand double-lap shear)—and predicted by the finite element method are presented and compared. Four different types of commercially available corrugated board are examined. It is found that the vibration testing method is ideal for quality control of the boards during manufacturing.
Failure of Delaminated Carbon/Epoxy Composite Plates under Compression
Authors: Hwang Shun-fa , Mao Ching-ping
Published: July 2004

The purpose of this work is to study the buckling loads, buckling modes, postbuckling behavior, and critical loads of delamination growth for delaminated unidirectional carbon/epoxy composites. Nonlinear buckling analysis, which is a finite-element method including contact elements to prevent the overlapping situation, was applied to predict the delamination buckling loads. In order to predict the delamination growth loads, the total strain-energy release rate criterion, the strain-energy release rate component criterion, and the inerlaminar stress criterion were used in the postbuckling analysis. Experimental results were also provided to compare with the prediction. The results indicate that nonlinear buckling analysis has very good agreement on the buckling loads with the experiments except for the global buckling case. For global buckling mode, the buckling loads are suggested to represent the failure loads of delaminated composites, and postbuckling analysis may not be necessary. For mixed and local buckling modes, the postbuckling behavior is in the mixed mode, and the failure of the delaminated composite is due to the growth of delamination. The errors of the three different criteria in predicting delamination growth loads are from 10% to 22% for different cases.
Aspects of the Compaction of Composite Angle Laminates: An Experimental Investigation
Authors: Hubert Pascal , Poursartip Anoush
Published: July 2004

To date, a fundamental knowledge of flow and compaction mechanisms has been gained by the study of laminates with simple geometries. Since most of the applications of composites involve complex shapes, it is necessary to study their compaction behaviour to develop more accurate theories. In this paper, the flow and compaction of angle laminates, manufactured with the autoclave process, was studied for two carbonepoxy composites (AS4/3501-6 and AS4/8552) under a wide range of bagging and moulding conditions. The variation of the laminate thickness, mass and local fibre volume fraction was investigated. The final thickness of the laminate was uniform in the flat section. It was found that laminate defects (voids, wrinkles, etc.) are localized at the corner. The low resin viscosity AS4/3501-6 laminates exhibit more resin loss than the high resin viscosity AS4/8552 laminates. In the flat section, the total compaction strain under bleed conditions is principally caused by percolation flow for AS4/3501-6. For AS4/8552 the total compaction strain is a combination of percolation and compaction strain caused by the collapse of voids introduced during lay-up. At the corner, high strains due to shear flow are observed for a [90°] lay-up, creating corner thinning for a convex tool and corner thickening for a concave tool. Finally, the fibre volume fraction gradient measured through the thickness and in the longitudinal direction confirms that under bleed conditions, net percolation of the resin occurs from the tool to the bleeder. Under no-bleed conditions, a small amount of internal percolation can be observed from the corner to the flat section of the angle.
Impact Characteristics of Glass Fiber Composites with Respect to Fiber Volume Fraction
Authors: Lee Dai Gil , Cheon Seong Sik
Published: July 2004

The impact energy absorption characteristics of glass epoxy composites and glass polyester composites with respect to fiber volume fraction were investigated by the instrumented Charpy impact test method. Also, the interlaminar shear properties were measured by the short beam shear test to investigate the correlation between the interlaminar shear properties and the impact energy absorption characteristics. Appropriate curing cycle was suggested for polyester resin by measuring the dissipation factor. Using the experimental results, the impact absorption characteristics of glass fiber composites such as fiber breakage and fiber pull-out and delamination were discussed.
Demonstration of Bond Quality Improvement for Closed Loop Control of Thermoplastic Tape-Laying
Authors: Sun Wei-Ching , Mantell Susan C. , Stelson Kim A.
Published: July 2004

In thermoplastic tape-laying, the temperature at the interface between the top ply and substrate is critical to achieving interlaminar bonding. Based on a three lump process model, a Kalman filter and a state feedback controller were built to successfully estimate and control the interlaminar bonding temperature during the process. A practical advantage of this controller is that the only measurement required is the heater temperature. In this work, two methods are used to examine the bond quality when applying this feedback control method. One method is a microscopy study and the other is a lap shear strength test. Specimens were manufactured using the developed controller to regulate the bonding temperature. The microscopy pictures show that the interlaminar bond width is reduced as the bonding temperature is increased. The lap shear strength tests show that the bond strength is significantly improved as the bonding temperature is raised. Both test results have consistently demonstrated the causal relationship between interlaminar bonding temperature and bond quality. The control method produces high quality laminates by using temperature feedback from the heater to estimate the interlaminar bonding temperature.
Improvement of Interfaces by Tethered Polymer Chains. Part I: Evaluation of the Interface in a Single-Fiber Composite at Low and High Strain Rates
Authors: Greenfield M. J. , Hunter T. F. , Kalika D. S. , Penn L. S.
Published: July 2004

A study of the effect of tethered chains on the fiber-matrix interface in a single-fiber composite at low and high strain rates was conducted. The materials system studied was composed of glass fiber, polysulfone matrix, and polysulfone tethered chains. Single fiber fragmentation tests demonstrated that the tethered chains were beneficial to the interface not only at a typical low strain rate, but also under impact at an estimated strain rate >2000 times higher. It is believed that the tethered chains, by entangling with the matrix chains, provide energy dissipation mechanisms that operate over a wide range of time scales to improve the performance of the interface.
Analytical and Experimental Behaviour of Laminated Panels with Rectangular Opening under Biaxial Tension, Compression and Shear Loads
Authors: Romeo Giulio
Published: July 2004

The strain distribution around a rectangular opening of an infinite anisotropic plate has been derived by using Lekhnitskii’s theory and using the complex variable method. The testing machine was implemented in order to simultaneously apply uni and biaxial tension or compression combined with shear loading. The strain distribution was recorded in several points of a composite panel, with the presence of the rectangular cutout, in order to emphasize the effect of stress concentrations. A good agreement has been obtained between the analytical and the experimental results; several original results are reported in the paper for laminated panels under combined biaxial and shear loading. Under uniaxial load, a very high strain concentration was measured in the transverse direction along the cutout end; the introduction of the transverse and shear load greatly reduced the longitudinal and transverse strain levels along both the sides and ends of the cutout; a very high strain level was recorded in the cutout corner when the three loads were applied simultaneously. A FEM analysis was carried out, obtaining a good agreement with the experimental results also as far as the buckling behaviour is concerned. The buckling load was not affected very much by the presence of the opening, except when shear load was applied.
Off-Axis Fatigue Behavior of Unidirectional Carbon Fiber-Reinforced Composites at Room and High Temperatures
Authors: Kawai M. , Yajima S. , Hachinohe A. , Takano Y.
Published: July 2004

High-temperature off-axis fatigue behavior of unidirectional T800H/Epoxy composite has been studied. Effects of different temperature on off-axis fatigue strength are also discussed. Tension-tension fatigue tests are performed at room temperature and 100°C, respectively, on six kinds of off-axis plain coupon specimen. The absolute off-axis fatigue strengths are significantly reduced as the test temperature becomes higher. Irrespective of test temperature, almost linear S-N relationships are obtained for all off-axis angles over the range of fatigue life up to 106 cycles. Normalizing the maximum fatigue stress with respect to the static tensile strength at test temperature, the fatigue data for all off-axis angles eventually fall on a single S-N relationship. It is shown that a theoretical fatigue strength ratio can be defined as a non-dimensional effective stress based on the classical static failure theory and it succeeds in coping with the directional nature of off-axis fatigue strength. A fatigue damage mechanics model is developed using the non-dimensional effective stress, and its capability to describe the off-axis S-N relationship for the unidirectional T800H/Epoxy is demonstrated. Comparisons with the classical fatigue failure models for composites are also presented.
Generalised Mori-Tanaka Scheme to Model Anisotropic Damage Using Numerical Eshelby Tensor
Authors: Desrumaux F. , Meraghni F. , Benzeggagh M. L.
Published: July 2004

This work presents a new micromechanical model to describe the 3-D elastic and damaged behaviour of randomly oriented fibre composite. The damage mechanisms are described by the progressive introduction of oriented microcracks in a random discontinuous fibre composite. In this work, the homogenisation is performed using a Mori-Tanaka scheme. Whereas, symmetry is not obtained in estimates of the overall stiffness for multiphase systems with heterogeneity of different shapes or orientations. For this reason, in this work two levels of homogenisation are considered. The Mori-Tanaka method is applied on the matrix with microcracks to give an equivalent damaged matrix, which can be anisotropic. The Mori-Tanaka scheme is used for the second time to homogenise fibres embedded in the new equivalent damage matrix. Generally, this new matrix is anisotropic and for this case no explicit formula for the Eshelby tensor ( S) has been obtained. Thus, it is necessary to compute numerically the components of this fourth order tensor. Moreover damage mechanisms can be modelled using statistical considerations. The damage onset and growth can appear in the matrix, fibres or interface. Hence, overall damaged behaviour can be modelled and predicted as well as the microscopic damage accumulation process.
Buckling of Rectangular Orthotropic Plates Subjected to Biaxial Normal Forces
Authors: Kollar Laszlo P. , Veres Istvn A.
Published: July 2004

Closed form approximate formulas are presented for the calculation of the buckling load of rectanglar orthotropic plates with clamped and/or simply supported edges. The solution is based on the Ritz method assuming one term approximation.
A Three-Dimensional Crack Tip Element for Energy Release Rate Determination in Layered Elastic Structures
Authors: Yu Lijie , Davidson Barry D.
Published: July 2004

A three-dimensional crack tip element is presented and used to determine energy release rate and mode mix for different types of laminated plates containing delaminations. These results are then compared to predictions by three-dimensional finite element analyses. For all cases, total energy release rates as predicted by the two methods are in good agreement. For delaminations between plies at the same orientation, energy release rate components are determined by the crack tip element using both classical and non-classical definitions. The non-classical definition is one that has been derived in a previous work and, compared to the classical result, is shown to provide superior capability for predicting delamination growth in some graphite/epoxy composites. The classical energy release rate components predicted by the crack tip element are compared to results by three-dimensional finite element analyses and the virtual crack closure technique and good agreement is obtained. For delaminations between plies at different orientations, the crack tip element is used to predict non-classical energy release rate components as well as components based on a finite amount of crack closure. These latter quantities are compared to results by three dimensional finite element analyses and the virtual crack closure technique, and good correlation is obtained. These results indicate that the crack tip element may be used to accurately determine energy release rate components in practical problems of delamination where these components are defined classically, defined by commonly used approaches such as finite crack closure, or defined using a non-classical approach. As such, the element provides a powerful yet adaptable technique for predicting delamination growth in a wide variety of materials.
Evolution of Residual Stresses Induced during Curing Processing Using a Viscoelastic Micromechanical Model
Authors: Chen Yu , Xia Zihui , Ellyin Fernand
Published: July 2004

The effect of viscoelasticity of matrix material on the evolution of processing- induced residual stresses in [0/90] glass fiber/epoxy cross-ply laminate has been investigated by a finite element micromechanical model. The micromechanical model is based on a periodic array of continuous fibers embedded in an epoxy matrix. The epoxy matrix is represented by a nonlinear viscoelastic model. The finite element residual stress analysis indicates that a higher cooling rate results in higher initial residual stresses in the laminate. However, the residual stress relaxes with time and tends to an asymptotic small value independent of the cooling rate. The effect of free edge surface on the generation of residual stresses is also investigated, and it is found that they are significantly different from those in the interior region. Although the coefficients of thermal expansion (CTE) for the individual constituents (glass fiber and epoxy matrix) are constant, the CTE for composite, on the other hand, is initially time-dependent due to the mismatch constraint and approaches an asymptotic value after a long stress relaxation period.
A 3-D Finite Deformation Anisotropic Visco-Plasticity Model for Fiber Composites
Authors: Espinosa H. D. , Lu H-C. , Zavattieri P. D. , Dwivedi S.
Published: July 2004

A 3-D finite deformation anisotropic visco-plasticity model is presented for fiber composites in total Lagrangian co-ordinates. The plastic potential function is given by a quadratic function in stresses in the local co-ordinates system of the lamina. The model is used to derive the anisotropic plastic constitutive relation of a woven composite made of S-2 glass fibers embedded in polyester resinwith approximately 60% by weight of fibers. The coefficients of the constitutive model are experimentally determined through off-axis tension tests and out-of-plane shear tests. Off-axis tension tests are carried out by varying the angle between the fiber orientation and loading direction. The measured stress-strain curves are used to derive a master effective stress–effective plastic strain curve, which is described by two power laws. Amodified Arcan fixture is used to carry out pure shear tests to determine the out-of-plane shear coefficient. Compression tests are carried out to establish the material compressive response in the plane of the lamina and along the fiber direction. The anisotropic plasticity model is integrated into the in-house finite element code FEAP98. Numerical analyses are carried out for the off-axis tension tests and compression tests. These analyses show that the model reasonably predicts the constitutive response of wovenGRPcomposites in confirmation with the experimental data. The model further incorporates strain rate and temperature dependence on the anisotropic plastic flow constitutive law. Ballistic penetration simulations are carried out using the integrated code. The velocity at the back surface of the composite target, obtained by analyses, is compared with the data measured experimentally using interferometry. Insight into the failure process is obtained through analysis of different energy dissipation mechanisms.
Thermo-Oxidative Stability of High Performance Composites under Thermal Cycling Conditions
Authors: Lee Sang-Ho , Nam Jae-Do , Ahn Kyujong , Chung Ki-Mo , Seferis James C.
Published: July 2004

Thermo-oxidative stability of high performance composites was investigated under thermal cycling conditions between room and 177°C curing temperature. For the analysis of thermal cycling experimental results, an equivalent cycle time (ECT) was developed by applying degradation-reaction kinetic theories to thermal-cycling conditions. Applying this methodology to the weight loss measurements of composite specimens, thermal cycling was found to exhibit a slower weight-loss degradation rate than isothermal conditions. This observation seemed to result from a retarded diffusion rate of oxygen through the viscoelastic relaxation of the epoxy matrix exposed to the thermal cycling conditions. In the later stage of thermal cycling, the through-thickness microcracks were induced at the exposed surfaces and then propagated into the core of the composite laminates. Subsequently, the degradation rate measured by the composite weight loss was accelerated with the thermal cycles seemingly because themicrocracks increased the effective degradation-reaction area and enhanced the diffusion rate of oxygen into the unreacted core.
Modeling Strength of Multidirectional Laminates under Thermo-Mechanical Loads
Authors: Huang Zheng-Ming
Published: July 2004

This paper investigates the progressive failure process in a multidirectional composite laminate subjected to an arbitrary thermo-mechanical load condition. A micromechanics modeling approach is employed to predict each-ply failure strength of the laminate and to identify the corresponding failure mode. The analysis has incorporated both material non-linearity and stiffness reduction. The load shared by each lamina in the laminate is determined based on the classical laminate theory, whereas the internal stresses in the constituent fibers and matrix of the lamina are explicitly related to this load by making use of combining the bridging model and the Benveniste and Dvorak’s formula. The lamina failure is assumed whenever any constituent material attains its ultimate stress state. A generalized maximum normal stress criterion is employed to detect the constituent failure. In this approach, all the simulation equations are given explicitly, no iteration is involved, and a minimum number of input data are required. Comparison of the predicted thermo-mechanical strengths of several laminates with available experimental data is favorable.
Volatile-Induced Void Formation in Amorphous Thermoplastic Polymeric Materials: I. Modeling and Parametric Studies
Authors: Roychowdhury S. , Gillespie J. W. , Advani S. G.
Published: July 2004

Void formation in polymeric materials is of critical importance in a wide range of process technologies such as composites manufacturing, polymer devolatilization and foam molding. A predictive model for volatile-induced void formation and growth during the processing of thermoplastic polymeric materials is presented. This model is comprehensive, incorporating the effect of material properties, such as surface tension and moisture solubility and diffusivity in these materials, and processing conditions and part thickness on the spatial nucleation and growth of voids. Fickian diffusion of volatiles during polymer processing provides concentration profiles that are coupled with classical nucleation theory to predict nucleation density through the part thickness as a function of process temperature, heating rate and applied pressure. Void growth is predicted at each material point based on local vapor pressure, material properties and process conditions. Parametric studies to elucidate the effect of various processing parameters and material properties on moisture-driven Void Formation in Amorphous Thermoplastic Polymeric Materials: I 341 void formation are conducted for amorphous thermoplastic engineering polymers such as polyetherimide (PEI). Further, the role of material and processing parameters in the generation of a spatial distribution of void content within the part being processed is also demonstrated.
Cure Simulation of Thick Composite Structures Using the Finite Element Method
Authors: Park Hoon Cheol , Lee Sung W
Published: July 2004

A finite element code is developed for the two-dimensional cure simulation of thick composite structures. In contrast to one or two-dimensional codes based on the finite difference method, the present finite element method can easily model composite structures with arbitrary shapes including the mandrel used for the cure set-up. For the two-dimensional cure simulation, a degeneration method to build the thermal conductivity matrix is proposed. The present finite element method is verified by two numerical examples. The numerical results show good agreement with the measured data appeared in the literatures. However, a full three-dimensional cure simulation is needed for more accurate cure simulation.
Reliability of Composite Laminates through a Damage Tolerance Approach: Applications into Carbon-Epoxy and Glass-Epoxy Composite Shells
Authors: Ikonomopoulos G , Perreux D
Published: July 2004

In the present paper, the damaged mechanical behaviour of carbon-epoxy and glass-epoxy composite shells has been characterised into quasi-static conditions, under different uniaxial or biaxial loadings. The experimental data obtained through different tests upon [±55°]3 glass-epoxy laminates and upon [±55°]3, [±35°]3, [±75°]3 carbon-epoxy shells have been compared with the numerical results based on a damage-elastoplastic theoretical model. This damage model has been developed by introducing damage and elastoplastic parameters into layer constitutive equations. In particular, a tensor quantity has been introduced, able to simulate damage in the elastoplastic field and to describe the new stress distribution into layers. The resulting strongly non-linear set of equations requires numerical solution, since generally, the mechanical characteristics of the layers result function of their position into laminate shell, and depending on damage through its characteristic tensor variable.
Impact Response of a Composite Laminate Bonded to a Metal Substrate
Authors: Helms Jack E , Li Guoqiang , Pang Su-Seng
Published: July 2004

Based on a spring-mass model developed by Pang et al. (1995) for a single-lap joint under impact load, the low-velocity impact responses of a composite laminate adhesively-bonded to a metal substrate have been studied. In this quasi-static model, the impact response is derived for a time-dependent force. The target structure is modeled as a single plate with an equivalent stiffness. Using an energy method, an analytical model has also been developed to determine the equivalent stiffness of the composite laminate/metal substrate hybrid structure. Advanced structural mechanics were used in the derivation of the governing equations of the structure. A FORTRAN program was developed on an IBM compatible personal computer to implement the solution algorithm. With the spring-mass system and the developed equivalent stiffness model, a relationship between the impact response and the impact duration has been established. Impact tests have been conducted to verify the proposed model. The analytical model results and the experimental data match reasonably well.
Effect of Thermal Deformation and Chemical Shrinkage on the Process Induced Distortion of Carbon/Epoxy Curved Laminates
Authors: Yoon K. J. , Kim Ju-Sik
Published: July 2004

Effect of anisotropy of the thermal expansion and the chemical shrinkage of epoxy on the process induced distortion of carbon/epoxy composite stiffener structures was investigated and analyzed. The coefficients of thermal expansion (CTE) and elastic properties in the material principal directions were characterized for temperature variation. By applying the characterized properties to the classical lamination theory, the change of CTE of general laminates was calculated for the temperature variation. A computational method to predict the distortion of curved laminate due to the difference of CTE and the chemical shrinkage of epoxy resin was proposed. The spring-forward angles of L-section laminates of various angle plies were measured and compared with those predicted. The predicted values from the thermal and the chemical shrinkage analysis show that the difference of CTE and the chemical shrinkage strain are the major sources of the process induced deformation of curved section laminates.
Equilibrium Water Uptake of Epoxy/Carbon Fiber Composites in Hygrothermal Environmental Conditions
Authors: Suh Dae-Won , Ku Mi-Kyung , Nam Jae-Do , Kim Byung-Sun , Yoon Sung-Chul
Published: July 2004

Equilibrium water absorption behavior of epoxy/carbon fiber composites was investigated under hygrothermal conditions. The temperature dependence of the equilibrium water uptake was quantitatively determined by assuming two distinctly different physical states of bound water and free water. The bound water was viewed as a reversible thermodynamic quantity and thus its temperature dependence was described by a negative value of the exothermic heat of sorption in Henry’s law. Ascribed to the Langmuir nonequilibrium kinetic process, the free water was assumed to be immobilized by irreversible microcavitational damage or crazing in hygrothermal environmental conditions. Accordingly, the amount of free water increased with hygrothermal temperature in an irreversible way and its temperature dependence was expressed by the polymer-diluent affinity parameter in Langmuir equation. For various hygrothermal conditions, the total amount of equilibrium water uptake was successfully predicted by the sum of the bound water at the last hygrothermal temperature and the free water at the highest hygrothermal temperature.
A Novel Double Cantilever Beam Test for Stitched Composite Laminates
Authors: Chen Leishan , Ifju Peter G. , Sankar Bhavani V.
Published: July 2004

Through-the-thickness stitching dramatically improves the interlaminar fracture toughness of laminated composites. Currently theDouble Cantilever Beam (DCB) test is being used for determining the Mode I interlaminar fracture toughness of unstitched and lightly stitched composite laminates. But the standard DCB test method is not suitable for laminates with medium to high density stitches. Due to the high bending moment at the crack-tip, the specimens fail due to compressive stresses before the delamination could propagate. In order to overcome this limitation, a new fixture has been developed and used to perform Mode I fracture toughness tests on laminates with high density stitches. An analysis method has been developed to determine the load-deflection and the load-energy release rate relationships in the new test. The new test method has been found to be successful in testing graphite/epoxy laminates containing Kevlar stitches with a density of 64 stitches per square inch. The apparent fracture toughness of these specimens is about 45 times that of unstitched specimens.
Nondestructive Evaluation of Laminated Composites Using Rayleigh Waves
Authors: Zurn Benjamin , Mantell Susan C.
Published: July 2004

The design and implementation of a nondestructive method for evaluating interlaminar bonding during the in-situ thermoplastic tow placement process was developed. This method utilizes two piezoelectric transducers in a “pitch-catch” arrangement.Each transducer is mounted on an acrylic wedge which, in turn, is mounted on a steel mediator wedge. This “double wedge” assembly produces Rayleigh surface waves in a composite specimen without a gel couplant. The received signals are analyzed for changes in the Rayleigh wave speed. Poorly bonded top most plies cause the Rayleigh wave velocity to decrease by as much as 30%. Verification of this bond quality sensor is demonstrated under various tape laying conditions. In each specimen, the Rayleigh wave velocity was measured.The bond quality was assessed quantitatively with a peel test and qualitatively by visual inspection (photomicrographs). The Rayleigh wave velocity was compared with the strength and thickness of the bond between the top ply and substrate. The Rayleigh wave speed increases linearly with the top ply peel strength. Photomicrographs indicated that poorly bonded specimens have a resin rich region at the top ply/substrate interface. These tests demonstrate that the double wedge sensor can measure bond quality during in-situ consolidation of thermoplastic matrix composites.
Uniform Processing of V-Shaped and Tri-Planar Composite Parts Using Microwaves
Authors: Qiu Yunchang , Hawley Martin
Published: July 2004

Uniform processing of V-shaped and tri-planar graphite/epoxy composite parts has been achieved in a single mode cavity using variable frequency microwave energy. A computer process control system was used to achieve uniform heating and stable curing temperature. The control parameters were microwave frequency and power. The control algorithm was mainly composed of two parts: intelligent mode selection for uniform heating, and power control for constant curing temperature. Experimental results were compared between single mode heating, mode sweeping heating, and intelligent mode switching heating. The comparison showed significant improvement of temperature uniformity using the intelligent mode switching technique.
On the Interaction Between Transformation Toughening and Crack Bridging by Ductile Layers in Hybrid Composites
Authors: Li M. , Katsube N. , Soboyejo W. O.
Published: July 2004

This paper presents the results of a combined theoretical and experimental study of toughening due to the combined effects of crack bridging ductile layers and particles that undergo stress-induced martensitic phase transformations in brittle matrix composites. The synergism that arises due to interactions between crack bridging and transformation toughening is discussed within the context of a micromechanics model. The synergism is shown to result in the potential for the design of hybrid composites with greater overall toughening levels than what can be achieved individually from each of the toughening mechanisms, i.e., crack bridging and transformation toughening. The theoretical model is validated for hybrid NiAl composites reinforced with ductile Vlayers and partially stabilized zirconia particles. The implications of the results are then discussed for the design of toughened brittle matrix composites.
Evaluation of Stress Corrosion Properties of Pultruded Glass Fiber/Polymer Composite Materials
Authors: Ely T. , Armentrout D. , Kumosa M.
Published: July 2004

A series of stress corrosion experiments were performed on 12 pultruded glass fiber composite materials (two fibers in six different matrices) commonly used in high voltage composite insulators. In this study two types of glass fibers were investigated, namely E-glass and ECR-glass. The tests were performed in nitric acid solutions (pH 1.2) using constant KI specimens that were specifically designed for the stress corrosion testing of unidirectional fiber/polymer matrix composites. The effect that the magnitude of the applied load had on the stress corrosion fracture process was investigated using acoustic emission (AE) methods. Post-test analysis of the test samples revealed that when the specimens were subjected to static loads ranging from 71.2 to 124.6 N in the presence of nitric acid, planar cracks formed and propagated perpendicular to the fiber direction without generating a significant degree of fiber debonding or pullout. The results showed that the E-glass/polymer unidirectional composites were not immune to stress corrosion cracking in nitric acid. On the other hand, the ECR-glass/polymer composites showed no evidence of stress corrosion formation under any of the loading conditions considered. Furthermore, the test results clearly demonstrate that the constant KI stress corrosion tests are a suitable experimental method for evaluating stress corrosion properties of unidirectional glass fiber/polymer matrix composites. In particular, by using the constant KI specimen geometry, the crack propagation rates in the composites under the stress corrosion conditions can be very accurately determined.
Post-Buckling of Composite I-Sections. Part 1: Theory
Authors: Papila Melih , Akgn Mehmet A.
Published: July 2004

Atheoretical investigation of initial local buckling and post-buckling behavior of composite I-sections is presented. The equilibrium equation for initial buckling is solved both exactly and with an approximate method, namely, Galerkin’s method. In Galerkin’s method, the out-of-plane deflection of each plate element is approximated by a weighted sum of polynomial functions. The post-buckling response is studied as an extension of the approximate analysis with Galerkin’s method where now both equilibrium and compatibility equations must be solved. The bending deflection in the post-buckling regime is assumed to be a magnification of the deflection function used in initial buckling analysis. No mode-shape change is thus allowed in the post-buckling region. A polynomial type of function is also adopted for stress distribution in order to take into account the deviation from uniform in-plane load distribution in the plate elements following the onset of local buckling. The paper provides an efficient and accurate method for predicting the post-buckling behavior of composite structural sections composed of plate elements. Galerkin’s methodwas previously applied to isotropic flat plates only. The present approach is tested against a commercial code, STAGS, with a very good agreement in results and a very large saving in computer time for post-buckling analysis of an I-section.