bluuebunny/crossref_metadata_2025 huggingface dataset:
Interlaminar Shear Fatigue Damage Evolution of 2-D Carbon-Carbon Composites
Authors: Williams J. C. , Yurgartis S. W. , Moosbrugger J. C.
Published: April 2011

Low-cycle and high-cycle bending fatigue experiments were conducted on 2-D woven carbon-carbon composites at room temperature to evaluate the possible evolution of damage within their complex microstructure. Since carbon-carbons are known to be weak in interlaminar shear, a 3-point bending geometry was chosen to produce high interlaminar shear stresses. This paper extends previous work by focusing on interlaminar shear fatigue, and by using a specially developed technique for the in-situ examination of distributed damage. For the low-cycle tests the load was cycled between 10% and 90% of the material's monotonic failure load; the range was 8% to 82% for the high-cycle experiments. A load-frame mounted optical microscope allowed the collection of digital images of the surface microstructure at various intervals up to 100,000 cycles for the low-cycle and up to 20 × 106 cycles for the high-cycle experiments. Microcrack lengths and orientations were extracted from these images, which permitted defining a discrete damage function as a measure of the distributed microcrack damage. Within the uncertainty of the measurements, distributed fatigue damage evolution was not discovered. A slight increase in residual static strength was detected with no change in stiffness.
Effects of Material Nonlinearity on the Three-Dimensional Stress State of Pin-Loaded Composite Laminates
Authors: Shokrieh Mahmood M. , Lessard Larry B.
Published: April 2011

A three-dimensional nonlinear finite element code is established to analyze the effects of material nonlinearity on the state of stress and failure prediction near the stress concentrations of a pin-loaded laminated composite plate. Theoretically, stress singularities may occur at the interface between two layers of different ply orientation on the free edges. High magnitudes of stresses at the free edge of the ply interface are responsible for failure initiation at those locations. Therefore the state of stress near the edge of the hole and at the ply interface between two different layers is important and has been studied in detail. An existing nonlinear material model, for in-plane behaviour, has been modified to be applicable for three-dimensional cases. Nonlinear shear stress-strain behaviour of a unidirectional ply, in the x-y and z-x planes, is the source of nonlinearity. Three different configurations, namely, cross-ply [04/904] s, and [904/04] s and an angle-ply [+454/-454] s, were considered. A failure analysis, using stress based failure criteria, is performed to predict the failure initiation load of different configurations. The effect of material nonlinearity on the predicted failure initiation load is studied. The results of prediction of failure initiation load by considering the material nonlinearity is in excellent agreement with experimental results. The results obtained from failure analysis emphasize that considering the material nonlinearity, especially for highly shear induced cases like [+454/−454] s is important in failure analysis of composite laminates using stress based failure criteria.
Effect of Fiber Orientation on the Energy Absorption Capability of Carbon Fiber/PEEK Composite Tubes
Authors: Hamada H. , Ramakrishna S. , Sato H.
Published: April 2011

Previous research works identified the effects of cooling rate, testing speed and tube geometry on the energy absorption behavior of carbon fiber reinforced polyether-ether-ketone (carbon/PEEK) composite tubes. One of the objectives of this work is to optimize the energy absorption capability of these tubes by changing the fiber orientation. Composite tubes with fiber orientations 0°, ±5°, ±10°, ±15°, ±20°, ±25° and ±30° with respect to the axis of the tube have been investigated. The energy absorption capability as well as the crush zone morphology were dependent on the fiber orientation. Efforts have been made to relate the variations of specific energy to the changes in the crush zone morphology. Tubes with ±15° fiber orientation displayed a specific energy of 225 kJ/kg, the highest ever reported in the literature. The fracture mechanisms that resulted in the superior energy absorption performance of these tubes have been identified.
Influence of Water Absorption on Transverse Tensile Properties and Shear Fracture Toughness of Glass/Polypropylene
Authors: Davies Peter , Pomies Frederic , Carlsson Leif A.
Published: April 2011

Water absorption characteristics, and developments of transverse tensile modulus and strength, and mode II fracture toughness with water absorption of a unidirectional glass/polypropylene (PP) composite have been examined. Immersion times up to 5 months at 50°C were employed. Overall, very small amounts of water were absorbed. The absorption of water was more rapid in distilled water than in seawater. No significant change in the transverse tensile modulus was observed after immersion, which is attributed to negligible softening of the highly crystalline PP matrix. The transverse tensile strength, very low initially, was also virtually unaffected by water absorption. The mode II fracture toughness, GIIc, decreased significantly after water absorption. The reduction of GIIc is attributed to degradation of the matrix due to water absorption as observed in the scanning electron microscope.
A Methodology to Derive the Implicit Equation of Failure Criteria for Fibrous Composite Laminates
Authors: Echaabi James , Trochu François
Published: April 2011

Strength failure criteria of fibrous composite laminates have been proposed up till now in two different forms: (1) parametric formulations, in which the failure envelope is described by a parametric equation in the stress or strain space and (2) implicit formulations, when the failure surface is defined by an implicit equation between failure stresses and strains. A systematic connection between these two types of formulations has not yet been elaborated. Such a connection would permit the combination of the flexibility of the parametric formulation with the simplicity of the implicit criterion. In previous papers [8,10] a methodology to derive failure envelopes of composite laminates based on uniaxial failure stresses, physical considerations and/or failure modes has been elaborated. Up till now, only the parametric expression of the criterion has been derived. In this paper, a procedure is proposed to calculate the equivalent implicit equation of a criterion from its parametric expression. Although different failure modes may be described simultaneously by the failure criterion, with dual kriging only one analytical equation is necessary in the stress or in the strain space. This procedure is general and describes the failure of various kinds of composite laminates. The practical relationship with tensorial failure criteria is also exposed. Finally, the methodology is applied to a unidirectional graphite-epoxy and to [0/90] graphite-epoxy fabrics.
Effects of Compaction on the Stiffness and Strength of Plain Weave Fabric RTM Composites
Authors: Falzon Paul J. , Herszberg Israel , Karbhari Vistasp M.
Published: April 2011

Changes in fiber architecture which result from the compaction of fabric plies can be a major cause for scatter and inconsistencies in measured mechanical properties. The need to quantify and provide insight into the influence of compaction on both the stiffness and strength of plain weave fabric composites, has led to the development of a model. The proposed model extends an existing modelling technique, a point-wise lamination approach using Classical Laminate Theory, to analyse the compaction problem. The analysis, which is valid for crimp angles less than 20°, is applied in both the warp and weft directions, with ply nesting being ignored at this point in time. In this paper, the formulation of the proposed model is presented. Numerical results generated from the model are compared with experimental data and other analytical methods, in order to validate the model. A discussion on the findings from the analysis are also given, with particular focus on the initial failure mode and stress/strain distributions predicted by the model for a longitudinal tensile load case at various levels of compaction. The model was found to easily model fabrics of varying fiber architecture, making it a useful tool in providing insight and quantification on the effects on stiffness and strength caused by fiber architecture changes resulting from compaction.
Optimum In Situ Strength Design of Composite Laminates. Part I: In Situ Strength Parameters
Authors: Wang J. , Karihaloo B. L.
Published: April 2011

In Part I of this two-part paper, a cracked fibre-reinforced angle-ply composite laminate is analysed using fracture mechanics. Based upon this analysis, a set of in situ strength parameters for unidirectional laminae in a multidirectional laminate is proposed. The in situ strength parameters take into account the influence of adjacent laminae and thickness of a particular lamina upon its transverse tensile and in-plane shear strengths when it is used in a multidirectional laminate. These strength parameters are then employed to calculate a stress norm which determines how close the stress state in the lamina is to its failure state. In the companion paper (Part II), the stress norm is incorporated into the formalism of an optimization problem in order to enhance the load bearing capacity of multidirectional laminates.
Strength Prediction of Mechanically Fastened Quasi-Isotropic Carbon/Epoxy Joints
Authors: Hamada Hiroyuki , Maekawa Zen-Ichiro , Haruna Kazushi
Published: April 2011

A study on predicting the joint strength of mechanical fasteners has been performed. However bearing failure that is most important failure mode for designing joints has not been taken into consideration. So in this study, the prediction method in consideration with bearing failure was examined. Two joint geometries that exhibit net-tension failure and bearing failure were chosen. In the proposed method, the characteristic length is combined with the Yamada-Sun failure criterion, and the characteristic length for compression was determined from the "bearing failure test" that was newly conceived to take bearing failure into consideration. The proposed prediction method was applied to quasi-isotropic carbon/epoxy T300/#2500 joints showing net-tension and bearing failure experimentally. Good agreement was found between the predicted and experimental results for each joint geometry.
A Thermoviscoplastic Theory for Composite Materials by Using a Matrix—Partitioned Unmixing-Mixing Scheme
Authors: Kim Seung-Jo , Shin Eui-Sup
Published: April 2011

A thermo-elasto-viscoplastic constitutive theory for fiber-reinforced composite materials is suggested by extending the existing unmixing-mixing scheme which is based upon some micromechanical observations. A new technique, which is called the "matrix-partition method,' is introduced in the model development. By this method, deformation states in the matrix phase can be represented with a set of mechanical variables for the respective parts of the partitioned matrix. As material parameters, which explain the three-dimensional microstructural effects resulting from kinematic compatibilities in the boundaries of the fibers and the matrix, the stress variation factors and the strain contribution factors are defined. The strain component of thermal expansion due to temperature changes is also included for the mathematical formulation. To verify the derived constitutive equations, the representative volume element of a unidirectionally fiber-reinforced lamina is discretized and analyzed by the developed finite element code. The overall stress-strain curves are obtained through the finite element analyses subject to various loading conditions, and compared with the predicted curves by the constitutive equations based on the matrix-partioned unmixing-mixing model. The numerical results illustrate the validity and the usefulness of the proposed theory on composite thermoviscoplasticity.
An Alternate Unified Approach to the Micromechanical Analysis of Composite Materials
Authors: Bennett Joel G. , Haberman Keith S.
Published: April 2011

An alternate approach to Aboudi's Method of Cells [1], for a unified treatment of the micromechanical analysis of composite materials is presented. This approach retains the philosophy of Aboudi's Method of Cells but is indeed a finite element (FE) based approach. The equations of equilibrium are applied to a representative volume model (RVM) and a unified method of homogenization (UMH) of micromechanical effects is presented. The method as developed here can be applied to find the homogenized properties of any representative volume model that meets the requirements discussed. In this paper, we first apply it to the analysis of grinding wheels, a "three" constituent isotropic material, wherein the void is treated as a material having volume property only. The equations, which are derived for an orthotropic material, are then specialized to fiber reinforced composites and the results are compared to Aboudi's and others. Several interesting points arise regarding the FE "load vector" for specialists that typically work in developing FE methods and codes.
The Effects of a Single Mode III Loading Cycle on Mode I Crack Initiation and Growth Toughnesses in a Cross-Ply [90°/0°]2s Ti-6A1-4V/SiCf Composite
Authors: Kamat S. V. , Hirth J. P. , Zok F. W.
Published: April 2011

The effects of a single Mode III loading cycle on the subsequent Mode I crack initiation and growth toughnesses in a cross-ply [90°/0°]2 s, Ti-6A1-4V/SiC f composite have been investigated. A Mode III cycle with Δ J III = 8 kJ/m2 was found to have no measurable effect on either the initiation or growth toughnesses. In contrast, a Mode III cycle with Δ J III = 14 kJ/m2 resulted in a significant decrease in the subsequent Mode I crack initiation toughness. It also caused a reduction in the crack growth toughness until the crack had extended beyond the damage zone produced during the Mode III excursion. The results are rationalized on the basis of the fracture mechanisms as well as the deformation field produced by Mode III loading. The Mode III damage effect has implications for failure criteria for composites under multiaxial loading.
Analysis of Acoustic Emission Signals Generated during the Fatigue Testing of GFRP
Authors: Shiwa Mitsharu , Carpenter Steve , Kishi Teruo
Published: April 2011

The acoustic emission AE generated during the fatigue testing of GFRP have been investigated. Four separate stage of AE activity were observed during a standard test. Each stage of AE activity was characterized by the AE event and energy rate. Different AE sources have been suggested by the analysis of AE signal parameters. Three distinct fracture surfaces have been observed using a SEM. Result of this investigation strongly suggest that AE measurement can be used as an effective probe for the assessment of damage accumulation and to characterize mechanisms and types of crack growth in GFRP during fatigue loading.
Notched and Pin Bearing Strengths of GFRP Composite Laminates
Authors: Khashaba Usama A.
Published: April 2011

An experimental study was conducted to determine the notched and pin bearing strengths of randomly oriented GFRP composites having various values of fiber volume fractions ( V f). To achieve this, the effect of varying hole size ( D = 3 to 12 mm) on the tensile strength of GFRP specimens, with constant width ( W), has been investigated. A series of pin bearing tests has been carried out to examine the effect of W/D ratio on the bearing strength of the composite laminates. The failure sequence and the fractured surfaces of bearing specimens were examined using the scanning electron microscope.

The results show that, fiber volume fraction has a significant effect on load-pin bearing displacement behavior. The value of W/D must be greater than 5 for the development of full bearing strength of the composite laminates. The stress intensity factor ( K lc) was accurately calculated for GFRP composites with various values of V f, in the range of hole sizes investigated.
Self-Heating Effects in Thermoset Composites
Authors: Buckmaster J. , Vedarajan T. G.
Published: April 2011

We describe a simple mathematical strategy, familiar from combustion studies, which can be used to predict whether or not temperature spiking occurs when a thermoset composite specimen is cured in an autoclave. For large specimens, thermal spiking cannot be avoided, so that Kim et al. (1995) have proposed an alternative process in which lay-up occurs contemporaneously with curing and consolidation. This steady-state process and its one-dimensional stability have been analysed by Teng (1993) for simple kinetic schemes using an asymptotic approach, but we describe a non-asymptotic strategy called the δ-function model. This has the advantage that complex cure kinetics can be handled with as much ease as simple kinetics. We present, for the first time, two-dimensional stability results, and show that, for nth-order kinetics, one-dimensional results will suffice.
Life Prediction Methodology for Composite Structures. Part II—Spectrum Fatigue
Authors: Schaff Jeffery R. , Davidson Barry D.
Published: April 2011

A previously developed model for predicting the life of composite structures under constant amplitude and two-stress level fatigue loadings is extended and applied to structures subjected to randomly-ordered loading spectra. The model is phenomenological and a limited amount of experimental data is required for its characterization. For uniaxially loaded laminates, this consists of static tension and compression strength distributions, S-N curves based on constant amplitude fatigue life distributions for two-to-three stress ratios, and a limited amount of two-stress level fatigue test results. The model is verified by comparing predicted fatigue life distributions to experimentally observed fatigue life data for a variety of laminates and load spectrums. Good correlation between theory and experiment is obtained for all loadings and laminates studied.
Glass Fabric Vinyl-Ester Composites: Tailoring the Fiber Bundle/Matrix Interphase with Nylon Coatings to Modify Energy Absorption Behavior
Authors: Rydin R. W. , Varelidis P. C. , Papaspyrides C. D. , Karbhari V. M.
Published: April 2011

The influence of a polyamide coating on the energy absorption behavior of an E-glass fabric-Vinylester composite was investigated using low velocity impact events to assess composite response. The evaluation included three different plate thicknesses and two concentrations of polyamide to achieve a thin and thick coating of the fabric reinforcement. Inelastic Energy Curves (IEC) were used to describe and compare the dominant failure modes. Thin ductile coatings were seen to enhance energy absorption primarily through crack blunting and enhanced frictional sliding, while thicker coatings appear to facilitate delamination in addition to matrix-coating slip due to the weak chemical bond between the vinylester matrix and polyamide. These mechanisms combine to allow greater plate deflections which in turn induce transitions between the dominant plate reactions and ultimately translate into greater energy absorption.
An Experimental Investigation of the Mini-Sandwich Laminate as Used to Obtain Unidirectional Composite Compression Strengths
Authors: Welsh Jeffry S. , Adams Donald F.
Published: April 2011

A mini-sandwich laminate configuration was used to test unidirectional AS4/3501-6 carbon/epoxy and S-2/5216 glass/epoxy composite material systems in established shear- and end-loaded compression test fixtures. Four different analysis techniques were used to predict the unidirectional lamina strength and stiffness from the measured tabbed and untabbed mini-sandwich specimen strength. The unidirectional composite compressive strengths obtained from both co-cured and adhesively bonded mini-sandwich specimens tested in the IITRI fixture using a nonlinear laminate theory analysis technique produced the highest strength values. Although the unidirectional composite compressive strengths measured using these specimens were higher than strengths typically obtained using conventional unidirectional specimen configurations, the adhesive bond between the face sheets and the 3501-6 epoxy resin core of the co-cured mini-sandwich specimens and the Poisson's effect in the core material were identified as possible limitations to this test method.
Mechanical Properties and Solid Lubricant Wear Behavior of Al/Al2O3/C Hybrid Metal Matrix Composites Fabricated by Squeeze Casting Method
Authors: Song J. I. , Han K. S.
Published: April 2011

In this study, mechanical properties and wear behavior of Al/Al2O3/C hybrid metal matrix composites fabricated by squeeze casting method was characterized. The effects of volume fraction of carbon fiber on mechanical properties of hybrid composites was investigated. Tensile strength of Al/Al2O3/C composites was decreased by increasing the addition of carbon fiber. The carbon fiber additions were seen to be less effective in strength than when only alumina fibers were incorporated, because carbon fibers in hybrid systems are difficult to wet by molten aluminum alloys and react with a matrix alloy to form brittle aluminum carbide (Al4C3). Ductility was considerably reduced by increasing the incorporation of carbon fiber. Wear behavior of Al/Al2O3/C composites was characterized by the dry spindle wear test under various sliding speeds. The wear resistance of Al/Al2O3/C composites was remarkably improved over Al/Al2O3 composites by adding carbon fibers to Al/Al2O3/C composites. Specifically, at the intermediate sliding speed the wear resistance of Al/Al2O3/C composites containing 8 vol.% carbon fiber was found to be better than that of the rest of the carbon hybrid composites, but amounts of carbon volume fraction were of little effect on wear resistance at high sliding speed of 3.62 m/sec. From fractographic studies, damaged sections in wear surfaces of hybrid composites at intermediate sliding speed were seldom observed due to the formation of solid lubrication film. The solid lubrication film which was formed as a result of adding carbon fibers improved the wear resistance of carbon hybrid composites because this film reduced the high friction force between MMCs and counter material. Wear model of Al/Al2O3/C composites was also suggested by the examination of worn surfaces.
Experimental Procedure and Results for the Identification of Elastic Constants of Thick Orthotropic Plates
Authors: Frederiksen Per S.
Published: April 2011

Ability to determine six independent material parameters of thick orthotropic plates by a non-destructive numerical/experimental method is demonstrated. The parameters, which are the four in-plane elastic constants and the two transverse shear moduli, are deduced simultaneously from natural frequencies of the completely free plate by a nonlinear least squares approach. The main new contribution of the present paper is the consideration of higher mode natural frequencies of thick plates linked with an accurate numerical model based on a higher-order shear deformation theory for the theoretical predictions. This has enabled both transverse shear moduli to be estimated along with the in-plane parameters. Focus is here on the experimental technique and the application of the method in real tests. High quality frequency measurements are performed by the impulse technique employing a non-contacting microphone combined with advanced curve fitting of the frequency response function. The first 12 to 15 natural frequencies are typically considered. The different sources of errors associated with the method are discussed. The method is illustrated by evaluating the six elastic constants of an aluminium plate and of two different unidirectional composite plates. For the composites, close agreement is found between in-plane and out-of-plane shear moduli in the planes parallel to the fibres, confirming transverse isotropy.
Buckling Analysis of Thick-Walled Composite Pipe under External Pressure
Authors: Yang Chihdar , Pang Su-Seng , Zhao Yi
Published: April 2011

Composite pipe has been used in transporting corrosive fluid in many chemical processes in the petrochemical and pulp and paper industries. It is often subjected to external pressures due to process conditions (vacuum), water hammer, pump suction, and differences in elevation head. Most composite pipe designers are currently using traditional metallic pipe standards which do not take into account the anisotropic nature of composite materials. An analytical model for the buckling of orthotropic composite pipe under external pressure has been developed in this study. In this study, first-order laminated anisotropic plate theory was used to construct models of the kinematic and constitutive behavior of the composite cylinders. The Ritz Method was then applied to determine the buckling load under external pressure. Results obtained from the developed model were compared with the theory derived by Flügge for the case of metallic pipe. Due to the inclusion of shear deformation, the new model gave lower buckling loads than Flügge's results especially for the case of thick-walled pipe. Good agreement was found when comparing the developed model with experimental results provided by Fibercast Company following ASTM D 2924 for centrifugally cast composite pipe.
Thermo-Oxidative Stability and Fiber Surface Modification Effects on the Inplane Shear Properties of Graphite/PMR-15 Composites
Authors: Madhukar Madhu S. , Bowles Kenneth J. , Papadopoulos Demetrios S.
Published: April 2011

Experiments were conducted to study the effects of thermo-oxidative stability (weight loss) and fiber surface modification on the inplane shear properties of graphite/PMR-15 unidirectional composites. The isothermal aging was conducted at 316°C and up to 1000 hours of aging times. The role of fiber surface treatment on the composite degradation during the thermo-oxidative aging was investigated by using A-4 graphite fibers having three different surface modifications, namely untreated (AU-4), surface treated (AS-4), and surface treated and sized with epoxy-compatible sizing (AS-4G). Weight loss of matrix, fibers, and composites was determined during the aging. The effect of thermal aging was seen in all the fiber samples in terms of their weight loss and reduction in fiber diameter. Calculated values of weight loss fluxes for different surfaces of rectangular unidirectional composite plates showed that the largest weight loss occurs at those cut surfaces where fibers are perpendicular to the surface. Consequently, the largest amount of damage was also noted on these cut surfaces. Optical observation of neat matrix and composites plates subjected to the different aging times revealed that the degradation (such as matrix microcracking, void growth, etc.) occurred within a thin surface layer near specimen edges. The inplane shear modulus of the composites was unaffected by the fiber surface treatment and the thermal aging. The shear strength of the composites having the untreated fibers was the lowest and it decreased with aging. Fracture surface examination of the composites having untreated fibers suggests that the weak interface allows the oxidation reaction to proceed along the interface and thus expose the inner material to further oxidation. The results indicate that fiber-matrix interface affects the composite degradation process during its thermal aging and that the weak interface accelerates the composite degradation.
A Layer-Wise Analysis for Free Vibrations of Thick Composite Spherical Panels
Authors: Dasgupta A. , Huang K. H.
Published: April 2011

The authors have previously presented layer-wise models for modeling the vibrations of thick composite cylindrical shells. The layer-wise theory is needed to overcome the deficiencies of conventional shear-deformable plate theories because the gradients of the deformation field are not necessarily continuous through the thickness, due to the discontinuity of material properties at layer interfaces. Fully three-dimensional finite element models place prohibitive demands on computational resources, and are not economically feasible. In this paper a similar layer-wise laminated shell theory is developed for doubly curved thick composite panels subjected to different combinations of three-dimensional boundary conditions. Piece-wise continuous, quadratic interpolation functions through the thickness, are combined with beam function expansions in the two in-plane directions of the laminate, to model the dynamic behavior of laminated spherical panels. This captures the discontinuities in the transverse shear and other strain distributions, from one layer to another. Past development of dynamic analyses of such structures using layer-wise theories were limited to flat plates. The present study is applicable to spherical laminates of arbitrary lamination sequence, and allows a generalized choice of boundary conditions which can be varied arbitrarily through the thickness. Preliminary results for spherical shells are presented and compared with existing results from other analytical methods in the literature. The ability of this layer-wise model to capture in-plane modes is illustrated. Particular attention is paid to the stiffening due to the spherical curvature, and to the influence of three-dimensional layer-wise boundary conditions on the natural frequencies and mode shapes.
Moldability of Tackified Fiber Preforms in Liquid Composite Molding
Authors: Rohatgi Vivek , Lee L. James
Published: April 2011

In this work, the effectiveness of using a reactive (bismaleimide) and a non-reactive (polymethylmethacrylate) tackifier in controlling preform springback is investigated using a 6k, 4HS woven graphite fiber mat. Effects of tackifier on preform permeability and fiber wetting are also addressed. Preform springback occurs when the force exerted by the compressed fibers, upon release of an externally applied load, is greater than the "holding" force provided by the tackifier. The latter depends upon the tackifier modulus and the wetted surface area of the preform. The surface area of the pre-form covered by the tackifier can be either interor intralayer, depending upon the rheological properties of the tackifier and the preforming conditions. For the same tackifier concentration, better springback control is achieved when tackifier is inside the fiber tows. Moreover, since mold filling is governed primarily by resin flow between the fiber tows, permeability or flow resistance is not affected much by intra-layer coverage of the tackifier. Blocking of the interstitial gaps within the fiber tows may, however, affect fiber wetting by lowering the capillary pressure.
New Ideas on the Measurement of the In-Plane Shear Strength of Unidirectional Composites
Authors: Pierron Fabrice , Vautrin Alain
Published: April 2011

This paper addresses the issue of the measurement of the in-plane shear strength of unidirectional composites. It is shown from recently published material that for both Iosipescu and 100 off-axis tensile tests, failure under a homogeneous stress state is achieved. However, these stress states are not pure shear and it is shown that ignoring the presence of transverse tensile and compressive stresses, respectively in the 100 off-axis and the Iosipescu tests, would lead to important apparent differences in the in-plane shear strength measured from both tests.

An analysis using a quadratic failure criterion underlines the role of the transverse stresses and values of 95 and 98 MPa for the shear strength of unidirectional T300/914 are derived respectively from the 100 off-axis and the Iosipescu tests.
Acoustic Emission Applied to Pipes of GFRP that have been Exposed to Corrosive Mediums
Authors: Morrás M. L. , Rodríguez M. , León L. M. , Bilbao P. , Ereño F. J.
Published: April 2011

The aim of this study was to validate that the acoustic emission technique as a nondestructive test is able to check GFRP which are going to transport corrosive liquids. For this purpose, different tubes made by filament winding, were exposed to different corrosive liquids at 50°C and finally tested at compression and acoustic emission simultaneously. These pipes were also tested by microscopy to verify that the results achieved by Acoustic Emission and by this technique were comparable. Besides, in order to study several variables an experimental design was carried out. All the results were introduced in a statistical program. The purpose was to consider all these factors and assess which ones were the most important in the corrosion process. The main factors considered in the design were: the different corrosive mediums (HCl and NaClO), the postcured of the samples, concentration of the corrosive medium, and the exposure time. The results showed that the A. E. technique allows to determine the state of the pipes which have been exposed to corrosive mediums and even, it allows to distinguish between two different mediums. So, the utility of the technique is proved in this kind of degradation processes.
Transverse Ply Cracking in Cross-Ply CFRP Laminates—Initiation or Propagation Controlled?
Authors: Boniface L. , Smith P. A. , Bader M. G. , Rezaifard A. H.
Published: April 2011

The initiation and propagation stages of transverse ply cracking in crossply CFRP laminates under quasi-static loading have been studied using laminates with a range of transverse ply thicknesses. Specimens of two types have been tested-coupons with polished edges and coupons with defects (notches) introduced into the transverse ply prior to testing. Detailed observations of crack propagation across the transverse ply thickness and width have been made, along with measurements of crack density and residual properties. In laminates with thin transverse plies (less than 0.25 mm for the materials tested here), fully formed transverse cracks are observed at about the same strain in undamaged and notched laminates. In laminates with thicker transverse plies (greater than 0.25 mm), the notched laminates show fully formed transverse cracks at lower strains than the undamaged laminates. The experimental results have illustrated the situations (laminate properties, presence of flaws) in which fracture mechanics models for transverse cracking available in the literature can be applied. In particular, it is shown that the fracture mechanics models which assume implicitly the presence of initial defects spanning the thickness of the ply may underestimate the failure strain of laminates with thick transverse plies. The effect of composite intralaminar toughness is also discussed.
Determination of Non-Symmetric 3-D Fiber-Orientation Distribution and Average Fiber Length in Short-Fiber Composites
Authors: Zhu Y. T. , Blumenthal W. R. , Lowe T. C.
Published: April 2011

A mathematical procedure is proposed for recovering from image analysis the three-dimensional (3-D) non-symmetric density distribution of fiber orientation and the average fiber length in short-fiber composites. The determination of fiber-orientation distribution and average fiber length is essential for assessing the mechanical and physical properties of a short-fiber composite. The average fiber length can be obtained from one micrograph, while the determination of 3-D fiber orientation requires micrographs from two orthogonal planar faces of a composite sample. In addition, a simple procedure is proposed to obtain the single-angle fiber-orientation distribution from one micrograph. This distribution is often needed for predicting the mechanical and physical properties in the direction normal to the plane of the photographed sample cross section. In processing the fiber-orientation density data obtained from a micrograph, a cumulative fiber-orientation density curve is used to derive the fiber-orientation function, which is simpler and more accurate than the histogram that has been commonly used. Previously reported procedures all assume symmetries in fiber-orientation distribution, and are therefore highly idealized. Without assuming any symmetry, the present procedure improves upon previous works.
Composite Implant for Bone Replacement
Authors: Kelsey Deborah J. , Springer George S. , Goodman Stuart B.
Published: April 2011

A procedure is described for designing prosthetic implants made of composite materials used in central bone and combined joint and bone replacements. The primary objective was to arrive at an implant made of fiber reinforced composites which has a higher bone growth stimulus and a lower micromotion level than titanium alloy (Ti6Al4V) implants currently in use. A secondary objective was to provide a means by which the physician can select, rapidly and with ease, the proper implant for a specific patient. To achieve these objectives, a procedure was developed consisting of three major elements. The first is a performance parameter which indicates the relative merits of different implant designs and takes into account that bone growth and micromotion have different importance from a clinical point of view. This relative importance is taken into account with weighting factors, the values of which were estimated from a survey of orthopedic surgeons. The second element of the procedure is a finite element code for calculating the strains in the bone, which are the parameters of importance in evaluating the implant's performance. The third element is a strain library which stores strains precalculated for a large number of different patient characteristics and different implant types. The strains for the specific patient under treatment are obtained by interpolation of the data in this strain library. The performance of an implant is then evaluated based on these patient specific strains. An example is presented for implants made of graphite AS4/PEEK composite. The results indicate that, in most applications, composite implants have better performances than titanium alloy implants both from the point of the overall implant performance and from the points of view of bone growth and micromotion.
Three-Dimensional Modeling of Damage in Laminated Composites Containing a Central Hole
Authors: Nguyen Ba Nghiep
Published: April 2011

Renard et all's transverse matrix cracking model is extended to the three-dimensional analysis of laminated composite structures. In this model, a thermodynamic approach to transverse matrix cracking is used. In order to determine the material parameters which modulate the stiffness reduction following the exponential laws assumed by Renard et al., an identification method based on the self-consistent model is proposed. Fiber breakage is detected by the maximum longitudinal strain criterion. The three-dimensional formulation of the damage model taking into account transverse matrix cracking and fiber breakage is essential in order to implement it in the general finite element system, SAMCEF, by which a three-dimensional modeling permits to fully describe the progressive damage in laminated composites containing stress concentrations. This damage model is used to determine the damage accumulations in a laminated graphite/epoxy AS4-3502 plate containing a central hole and under axial tension. Our results are compared to Tan's.
Analysis of the Transverse Cracking of Cross-Ply Laminates: A Generalized Approach
Authors: Berthelot J. M.
Published: April 2011

A generalized approach has been developed to evaluate the two-dimensional stress distribution in cracked cross-ply laminates. Comparisons between finite element analysis and analytical models show that a complete parabolic analysis extended to 0° and 90° layers yields a fairly good approximation of the strain and stress distributions in the case of [0°/90°] s, with equal thicknesses of 0° and 90° layers. However a deviation is observed in the case of thick 0° layers. The analysis presented in this paper keeps a parabolic variation of the longitudinal displacement across the thickness of 90° layer, and generalizes the longitudinal displacement distribution across the thickness of 0° layers by taking an indeterminate distribution. The development of this analysis allows us to obtain the general form of the stress distributions in 0° and 90° layers. Finally, it is proposed a particular form of the variation of the longitudinal displacement across the thickness of 0° plies which yields a good agreement of the stress distributions with finite element analysis for the different stacking sequences investigated.
Strain Measurement with Microsensors
Authors: Itoh Toru , Springer George S.
Published: April 2011

A model is developed for determining the farfield strains in laminated composites from the strain values indicated by a microsensing device embedded inside the material. The microsensing device considered here consists of three elements: a sensor, a circuit chip, and an antenna. The sensor detects changes in the strains in the material surrounding the sensor. The circuit chip processes the sensor signals which are then transmitted, via the antenna, to a receiver located outside of the composite. The sensor and the circuit are powered by electromagnetic waves generated outside the material and transmitted to the antenna.

The model proposed provides a means by which the signal emitted by the microsensing device, and picked up by a receiver, is converted to the engineering value of the farfield strain, i.e., to the strain which would exist in the material at the location of the sensor if the sensor were not present. On the basis of the model, two computer codes were written. The first code calculates the influence coefficient matrix, which is the parameter required to convert the sensor strain to the farfield strain; the second gives the farfield strain directly from the measured sensor strain.

Tests were performed to verify the model and the computer codes. In these tests, a strain gauge, a rectangular steel plate, and a circular steel disk (simulating the components of a microsensing device) were embedded in a glass/epoxy beam. The sensor strains were measured by the embedded sensor; the farfield strains were measured by two surface mounted sensors. The measured farfield strains were compared to the farfield strains calculated by the present model, and good agreements were found between the measured and calculated farfield strains.
Mechanical Evaluation of Axially Loaded Composite Insulators with Crimped End-Fittings
Authors: Bansal A. , Kumosa M.
Published: April 2011

Composite insulators consist of a glass reinforced polymer (GRP) rod, with two metal end-fittings attached to the ends of the rod during assembly. This paper aims to evaluate the strength and design of mechanically crimped insulator end-fittings subjected to axial tensile or compressive loads during service. A non-destructive ultrasonic technique has been used to measure the extent of radial shortening applied to two composite substation insulators during crimping. The axial compressive strength of these insulators have been determined from destructive mechanical tests. In addition, detailed nonlinear axisymmetric and three-dimensional finite element analyses have been performed to evaluate the effects of several design variables on the mechanical performance of the crimped joints subjected to axial loads. Results obtained from this study demonstrate that the insulators are likely to encounter failure due to sliding of the GRP rod (within the end-fittings) under excessive axial loads. Based upon the finite element results, semi-empirical relationships have been presented to predict the axial loading capacity of the crimped insulator ends, and the maximum internal stresses on the GRP rod surface.
Flow Instabilities during the Squeeze Flow of Multiaxial Laminates
Authors: Shuler S. F. , Advani S. G.
Published: April 2011

This paper examines the effect that ply arrangement has on the processing characteristics of aligned long fiber reinforced composite laminates. More specifically, this study focuses on the difference between the consolidation squeeze flow behavior of unidirectional and multiaxial ply laminates. A commercially produced material tape composed of aligned carbon fibers in a thermoplastic PEEK matrix (APC-2) was used to construct the laminates. Using specially designed hot platens, constant platen closure rate squeeze flow-experiments were performed on 70 ply unidirectional [0°] laminates. Flow visualization techniques were then used to measure the velocity profile of the material during flow. Similar squeeze flow experiments were also carried out on macro-ply [014,9014,07] s and cross-ply [0°,90°]18 s laminates. Comparisons are made between the generated squeeze forces and macroscopic flow deformation of the three laminate configurations.
Development of an Autoclave Cure Cycle with Cooling and Reheating Steps for Thick Thermoset Composite Laminates
Authors: Kim Jin Soo , Lee Dai Gil
Published: April 2011

In this study, an autoclave cure cycle for thick thermosetting resin matrix composite materials was developed to reduce the temperature overshoot. To predict the temperature distribution of thick thermosetting composite laminates during cure, the heat transfer equation, including the heat generation term, was simulated by the finite difference method (FDM). Using the simulated results, the cure cycle was obtained by modifying the conventional cure cycle. The steps of cooling and reheating, which were determined by the cure rate and temperature at the midpoint of the laminate, were introduced into the conventional cure cycle. The developed cure cycle was used to cure 15 and 30 mm (100 and 200 ply) thick laminates and was found to be effective for the reduction of temperature over-shoot
Failure Behavior of Knitted Fabric Composites
Authors: Ruan Xiaoping , Chou Tsu-Wei
Published: April 2011

This paper presents experimental and theoretical studies of the failure behavior of knitted fabric composites. Test results of the tensile and shear strengths of plain-stitch and rib-stitch fabric composites are presented. It has been determined that fracture initiates due to yam/matrix debonding, and the interfacial crack propagates along the yam/matrix interface. An analytical model of the failure behavior based upon observation of the fracture surface is presented. A good agreement between experimental and predicted failure strengths is obtained for plain-stitch fabric composites under warp direction loading. Large discrepancies exist between predicted and experimental strengths for other cases.
A Study on the Room-Temperature Curvature Shapes of Unsymmetric Laminates Including Slippage Effects
Authors: Cho Maenghyo , Kim Min-Ho , Choi Heung Soap , Chung Chung Hwa , Ahn Kyu-Jong , Eom Yong Sung
Published: April 2011

The room-temperature shapes of cured unsymmetric composite laminates have out-of-plane warping after autoclave processing. In addition, they exhibit two stable room-temperature configurations due to snap-through phenomena when the side length of laminates exceed a critical value. The cured shape of unsymmetric laminates are influenced by many factors. Experiments show that the effect of tool-plate cannot be ignored and has significant influence on the cured shape.

This study examines slippage effects resulting from the interaction between the laminates and the tool-plate which are ignored in the previous researches. By introducing a dimensionless slippage coefficient and correlating the corresponding value with experimental results, the influence of processing parameters is investigated. Modeling is extended to predict curvatures of general lamination layup configurations.