bluuebunny/crossref_metadata_2025 huggingface dataset:
Energy Release Rate Determination for Edge Delamination under Combined in-Plane, Bending and Hygrothermal Loading. Part II-Two Symmetrically Located Delaminations
Authors: Davidson B. D.
Published: April 2011

Edge delamination in composite laminates subjected to combined in-plane, bending and hygrothermal loading is analyzed using a crack tip element approach. It is assumed that two delaminations exist which are symmetrically located with respect to the midplane of the laminate. It is shown that, under pure bending, a linear analysis indicates that one of the delaminations generally has a positive mode I stress intensity factor, whereas the other crack generally has a negative mode I stress intensity factor. This indicates that crack face compression exists and an analysis incorporating crack face contact constraints is required to accurately obtain energy release rates. However, for many cases of bending combined with in-plane loading or uniform temperature or moisture change, both cracks have a tendency to open. General expressions are derived for the total energy release rate and the individual mode I and mode II components under these conditions. Specific example problems and results are presented.
A Finite Element Analysis of Crack Bridging in Metal Matrix Composites
Authors: Kim K. S.
Published: April 2011

A finite element method is developed to model crack bridging in unidirectional metal matrix composites. The Marshall-Cox-Evans (MCE) model is used for the relationship between the stress of a bridging fiber and the crack opening displacement. Loading, unloading and reloading are modeled in detail for cyclic loads. Fiber failure is assumed to occur when the stress exceeds the tensile strength. The method is applied to Ti-matrix/SCS-6 composites. The critical interface shear stress (τ) predicted from the experimental crack opening displacements is found to be 10 MN or less. The maximum and minimum stress intensity factors (Kmax, Kmin) in a fatigue cycle are obtained as functions of the crack length. Kmax. decreases drastically due to bridging fibers as the crack propagates. Kmin at the unloaded state is not negligible and must be accounted for in computing ΔK. The da/dN-ΔK plots show that is significantly larger than 10 MPa yield better correlations. However, the quality of correlation is poor even at τ as large as 40 MPa.
Modeling Frequency-Dependent Longitudinal Dynamic Behavior of Linear Viscoelastic Long Fiber Composites
Authors: Lesieutre George A.
Published: April 2011

An approach to modeling the longitudinal dynamic behavior of unidirectional long fiber composites made from constituents having frequency-dependent material properties is described. In this approach, the total displacement field is considered to be comprised of two parts: an elastic part, and an anelastic part. Material dynamic behavior is described by constitutive equations and governing differential equations. These equations involve the coupled behavior of the total displacement field and the anelastic displacement fields. An "equation of motion" describes the time evolution of the total displacement field, while "relaxation equations" describe the time evolution of the anelastic displacement fields. Distinct anelastic displacement fields are initially associated with the constituent fiber and matrix materials. Effective composite anelastic displacement fields and material properties are then developed in terms of the corresponding constituent properties and their respective volume fractions. The determination of frequency-dependent composite modulus and damping properties from corresponding constituent properties is illustrated, using data from prior experiments. This approach to modeling composite longitudinal behavior suggests direction for the development of an approach to modeling more general behavior, as well as an approach to the analysis of thermorheologically complex materials.
Penetration into Glass/Epoxy Composite Laminates
Authors: Wu Enboa , Tsai Cheng-Zorn , Chen Yung-Chung
Published: April 2011

A new method using laser Doppler anemometry (LDA) was employed to measure the force history of (0°2/90°2). E-glass/epoxy laminates struck by hemispherically tipped projectiles. The striking velocity of the 35.5 g projectiles ranged from 18 m/s to 80 m/s. The ballistic limit for these 2.8 mm thick laminates was at ≈ 60 m/s. It was found that the highest impact force occurred at the ballistic limit, and that the peak impact force in each test was proportional to the initial velocity of the projectile when the specimen was not seriously damaged. Beyond the ballistic limit, the peak impact force was reduced slightly. Further, the magnitudes of the peak force and the absorbed energy of the laminates at the ballistic limit were more than twice those loaded quasi-statically; and the dynamic friction force was also found to be approximately 1.5 times larger. These results indicate a significant rate effect. Surprisingly, the effect of boundary sliding was found to not affect the peak impact force.
Indentation Failure in Cross Ply Laminates, Comparison between Observations and 3D Field Solutions
Authors: Jøorgensen Ole
Published: April 2011

A numerical as well as an experimental study of indentation damage in thick laminates is presented. Static indentation of a spherical steel ball into thick laminates, one of GFRP type and one of CFRP, is performed. The resulting fracture is in both material systems dominated by a bottom centre crack, inclined matrix cracks, accompanied by delaminations. The order of appearence is material dependent. The fracture initiates in the layer opposing the point of contact in the case of a GFRP laminate, whereas the first evidence of fracture is found in the layer nearest the loaded surface, in the case of a CFRP laminate. Full 3D finite element analyses of the elastic contact problem are performed, using a newly developed finite element specifically suited for the purpose. The numerical solutions indicate that the matrix cracking is governed by the failure criterion of maximum tensile stress perpendicular to fibres. Using this criterion the numerical solutions reveal a correlation between the ratio of moduli and the mode of failure. A low value of E, 1/E2 results in failure initiation in the lower layer, whereas a high value of El /b2 results in crack initiation and onset in the upper layers. The experimentally found divergence in terms of failure initiation between the GFRP laminate and the CFRP laminate supports this. The influence of cracks and delaminations on the overall stress distribution is studied qualitatively by inserting cavities of appropriate size and shape into the finite element model.
A Modified Procedure to Measure Bending Strength and Modulus of Advanced Composites by Means of Compression Bending
Authors: Fukuda H. , Katoh H. , Uesugi H.
Published: April 2011

This paper deals with a new bending test method for advanced composites. Although the conventional three and four-point bending tests are convenient to obtain bending modulus and strength, composites may fail at the loading nose due to the stress concentration, because composite materials have high anisotropy and they fail in a brittle manner.

To compensate for the above shortcoming, we developed a new bending test method which is based on a buckling of a column. Both strength and modulus could be measured. In the present paper, another refined methodology to evaluate the bending modulus is discussed in addition to the bending strength. 1300 and T800 carbon/epoxy unidirectional and quasi-isotropic laminates with various specimen lengths were tested. The superiority of the present method was demonstrated especially for evaluating the strength.
Fiber Fracture in Unidirectional Composites
Authors: Case S. W. , Carman G. P. , Lesko J. J. , Fajardo A. B. , Reifsnider K. L.
Published: April 2011

In this paper, an analysis is developed to predict the stress redistribution in the presence of single and multiple fractured fibers in a unidirectional composite material. This analysis includes the effects of constituent properties, fiber volume fraction, and crack size on the strain concentrations experienced by the adjacent fibers. These effects are not included in other predictions such as shear lag or those of Hedgepeth and Van Dyke. In addition, the predictions are compared with direct experimental measurements obtained from model composite tests.
Effect of Perturbation of Fibre Architecture on Permeability Inside Fibre Tows
Authors: Lundström T. Staffan , Gebart B. Rikard
Published: April 2011

The influence on the permeability inside fibre tows from fibre packing arrangement and non-uniform fibre diameter is investigated. The analysis mainly consider flow transverse to the fibres and is based on the fibre geometry. Perturbations from perfect quadratic and hexagonal arrangements of fibres at constant fibre volume fraction are considered. The importance of microgeometry on permeability is pointed out and it is indicated how to deal with this problem. The analysis shows that the fibre transverse permeability is strongly dependent on the fibre size distribution. When an arrangement of equally sized fibres is continuously changed from quadratic to hexagonal it is furthermore found that the transverse permeability is highest just before the packing gets hexagonal. The analysis is also applied to a simple FORTRAN code to simulate removal of fibers from a quadratic arrangement. The simulations shows that the transverse permeability decreases with increased number of removed fibres under constant fibre volume fraction.
Slip and Frictional Shear Stresses in Ceramic Matrix Composites
Authors: Peters P. W. M. , Martin E. , Quenisset J. M.
Published: April 2011

Ceramic materials reinforced by continuous ceramic fibres show enhanced toughness in the case of weak conditions of load transfer at the Fibre/Matrix interface. Load transfer in these materials mainly takes place through interfacial friction. The corresponding shear stresses are investigated in the present work with the aid of both an analytical and a finite element analysis of the thermomechanical stresses in a microcomposite consisting of one fibre surrounded by a cylindrical outer tube. The applied method is able to describe the non-homogeneous field of fictional shear stresses induced by the presence of a matrix crack. Stresses are analysed for two different material systems (SiC/Glass and SiC/SiC) and compared with experimental data for SiC/SiC reported elsewhere. Comparison between the analytical and experimental results has revealed the necessity to consider the effect of roughness of the created slip planes on the frictional shear stress.
Effects of Large Deflection on Mode II Fracture Test of Composite Materials
Authors: Wang H. , Vu-Khanh T. , Le V. N.
Published: April 2011

Large deflection response and its effect on measuring the Mode II fracture toughness of composites have been studied for the End-Loaded-Split (ELS) test. The response is analyzed by nonlinear beam theory and the correction to toughness calculation based on linear beam theory is obtained for any crack length. The effect of large deflection on accuracy of load measurement is also studied. In addition, the ELS test is used to measure the Mode II toughness of carbon fiber/polyetheretherketone (PEEK) composite. It is found that the data reduction procedure based on linear beam theory needs to be corrected if the fracture occurs at large deflections. Special care should also be taken towards a valid load measurement when using the ELS test in the large deflection range.
A Non-Isothermal Process Model for Consolidation and Void Reduction during In-Situ Tow Placement of Thermoplastic Composites
Authors: Ranganathan Sridhar , Advani Suresh G. , Lamontia Mark A.
Published: April 2011

In in-situ composites processing, heat and pressure are applied locally to composite tows to achieve consolidation. During the consolidation of thermoplastic composite tows or plies, a modest amount of flow takes place. Due to the high viscosity of thermoplastics, the fibers and the matrix move together. We propose a novel approach to model this process-approximate the medium as a compressible viscous continuum rather than use Darcy's law which has been successful in modeling thermoset composites processing. The void content of the composite changes during the consolidation process due to various mechanisms. A consolidation model for thermoplastic composite that incorporates—the relevant void growth and transport phenomena has been developed. The model is capable of predicting the final void fraction and the final thickness of a composite part as a function of the processing speed and the consolidation pressure under non-isothermal conditions.
Effects of Particle Orientation in Silicon Carbide Particulate Reinforced Aluminum Matrix Composite Extrusions on Ultrasonic Velocity Measurement
Authors: Lu Y. , Liaw P. K.
Published: April 2011

In silicon carbide particulate reinforced aluminum matrix composite extrusions, the particles are nonspherical in general and are randomly oriented due to the extrusion process, resulting in sometimes significant anisotropic elastic stiffness. Since the ultrasonic velocity depends upon the elastic stiffness, ultrasonic velocity measurement is employed to characterize the anisotropic elastic stiffness of the composite extrusions. To establish the relationship between the materials' macroscopic response (ultrasonic velocity) and the microstructure, the measured statistical particle orientation distribution has been taken into account in theoretical modeling to predict the velocity of ultrasound that propagates in the composite extrusions, an attempt that has never been made before. The numerically calculated velocity agreed well with the measured velocity. Both numerical calculations and ultrasonic measurements have shown that the randomly oriented particles have led to a higher velocity (stiffness) in the extrusion plane and a lower velocity (stiffness) in the direction perpendicular to the plane, which are just the manifestation of the nonuniform particle orientation. It has also been shown that satisfactory theoretical modeling requires the consideration of the particle orientation distribution in the composite extrusions.
Experimental and Numerical Studies of a Laminated Composite Single-Lap Adhesive Joint
Authors: Tsai M. Y. , Morton J. , Matthews F. L.
Published: April 2011

The problem of a single-lap bonded joint with laminated polymeric composite adherents and with a spew fillet, subjected to tensile loading, is investigated. Experimental and numerical analyses of this problem are presented to address the mechanics and deformation of such material and bonding configuration. Strain gages are employed to record the geometrically nonlinear deformation of the specimen. Full-field moiré interferometr is used to measure the surface deformation of the adherends and adhesive (including a spew fillet). A geometrically nonlinear, two-dimensional finite element analysis is performed to check the mechanics assumptions by comparing with the experimental measurement. A good correlation between experimental and numerical solutions is obtained. It is observed that the composite single-lap joint deforms nonlinearly when subjected to tensile loading. The resulting displacement and strain fields for the adherend and adhesive layer are also presented. The moire results show that the transverse normal and shear strains, but not the longitudinal stain, for composite adherents on the free surface suffer from the fire edge effect, anticlastic as well as bending-twisting coupling effects. For the adhesive strains, the transverse normal (peel) component on the free surface is compressive throughout all the bond line except for a small region near the tip of the spew fillet, wile this component in the interior becomes tensile near the end of the overlap. The adhesive longitudinal strain in the spew fillet is insensitive to the three-dimensional defamation effect, but not the adhesive shear strain in the spew fillet. The adhesive longitudinal strain is not small enough to be neglected in the stress analysis.
Response of SiC/Ti under Combined Loading Part II: Room Temperature Creep Effects
Authors: Lissenden C. J. , Herakovich C. T. , Pindera M. J.
Published: April 2011

Results from cyclic multiaxial tests on [±45], silicon carbide/titanium (SiC/Ti) tubular specimens at room temperature are presented. One test cycle consisted of applying axial stress at a constant rate, holding the axial stress constant at a predetermined level for a fixed period of time, applying shear stress at a constant rate, holding the shear stress constant at a predetermined level for a fixed period of time, unloading the shear stress, and finally, unloading the axial stress. Test results show that relatively large axial time dependent, and to a lesser extent, time independent strains are accumulated after the axial stress is held constant. The time dependent accumulated axial strains are termed creep, while the time independent strains are due to coupling between shear and axial strain. The results of short duration creep tests conducted on a bulk titanium alloy (Ti-15-3) are also presented and are consistent with the results from the multiaxial tests on SiC/Ti
Compressive Strengths of Commingled Fiber Composite Systems
Authors: Hwang Shun-Fa
Published: April 2011

The idea of fiber commingling is proposed to increase compressive strengths and the effect of fiber commingling is evaluated by considering two different failure models, buckling failure and shear failure. Micro-buckling failure theory is extended to account for the nonlinear effects of the fiber and matrix by using Ramberg-Osgood stress-strain relations and the tangential modulus concept. In the case of shear failure, a more appropriate method to estimate the shear strength of each component is proposed. The effect of fiber commingling can be estimated from the controlling mode.
Analytical Models for Low Cost Manufacturing of Composite Components by Filament Winding, Part I: Direct Kinematics
Authors: Mazumdar S. K. , Hoa S. V.
Published: April 2011

Filament winding technique is not used to its overall capacity because of insufficient understanding of the kinematics of filament winding. In order to effectively utilize the capability of filament winding technique, a Geometric Approach based on the algebraic and trigonometric relations is presented to determine the machine motions for generating desired fiber distribution on axisymmetric, non-axisymmetric, cylindrical, and non-cylindrical mandrel shapes.

Analytical models thus developed are found to be extremely useful tools for laying down the fiber on predetermined path in lieu of costly techniques such as teach-in-programming, simulation, CAD/CAM, etc. It is suggested that a sophisticated filament winding machine is not always necessary to fabricate a complex structure. A filament winding machine with two and three degrees of freedom can perform the same task on various types of mandrel surface with fewer driven parts, higher accuracy, and lower manufacturing cost. Present method determines the machine motion in a few seconds and provides exact solution in generating the desired fiber winding angle on certain types of mandrel surface which has polygonal cross-sections.

Based on the Geometric Approach, a user friendly computer program "DIRECTIKIN" is developed to study the direct kinematics of filament winding. Using the program "DIRECIXIN, effects of location of the delivery point, fiber winding angle distribution, and mandrel shape geometry on winding motions are discussed. Experimental verification of the model is performed on a McClean Anderson filament winding machine for cylindrical and non-cylindrical mandrels.
Evaluation of First Ply Failure in a Three-Dimensional Load Space
Authors: Zocher M. A. , Allen D. H. , Groves S. E. , Feng W. W.
Published: April 2011

Numerically generated failure envelopes for several three-dimensional failure criteria are presented and compared to experimental data. These envelopes are developed through finite element analysis and are based on first ply failure (FPF). The experimental data are based on ultimate failure of filament wound tubes constructed from Toray 1000/DER332-T403 and loaded in various combinations of axial traction (both tension and compression), internal pressure, and torsion. All tubes have a layup of [± 1.5, ± 45, ± 89] T. Five three-dimensional failure criteria are considered: max-stress, max-strain, one proposed by Tsai, and two recently proposed by Feng. In addition to the three-dimensional failure criteria evaluated, the finite element results are used in conjunction with classical lamination theory to test the predictive capability of some of the more common two-dimensional failure criteria (max-strain, Tsai-Hill, and Tsai-Wu). None of the predicted envelopes compares well with the experimental data; thereby illustrating the need for progressive failure analysis in structures subjected to complex stress states. It is shown that it is possible to improve the accuracy of at least one of the three-dimensional failure criteria through a very minor modification of the theory.
Modelling Mechanical Response of SiC/CAS-II Ceramic Composite under Quasi-Static Loads Using a Real-Time Acousto-Ultrasonic NDE Technique
Authors: Tiwari Anil , Reifsnider Kenneth L. , Henneke Edmund G.
Published: April 2011

The onset of matrix cracks and their progression have been successfully monitored by an acousto-ultrasonic (AU) NDE technique as shown by the authors in previous work. The nonlinear behavior observed in a stress-strain curve for a unidirectional ceramic composite has been predominantly attributed to the effect of matrix cracks. This effect of matrix cracks on the stress-strain curve has been previously modelled (AUSSR model) with the help of real-time AU data. The models presented in this paper (modified AUSSR, AUSSR-WZ, AUSSR-WZ-WB models) attempt to describe the damage mechanisms occurring by assessing the combined effect of matrix cracks and fiber breakage on the global strain response of the laminate to increasing loads. The strains at the outer surface predicted by the models described in this paper are compared to the experimentally observed strains from the extensometer attached to the outer surface. Previous models from the literature are introduced outlining the history of development of successive models. It is shown here that the modified acousto-ultrasonic stress-strain response (modified-AUSSR) model based on AU parameters is capable of predicting the strain response to increasing stress levels for unidirectional and cross-ply ceramic composite laminates. This work represents an integration of measured experimental damage with analytical modelling to produce a better physical understanding of the damage development process in ceramic composites.
Delamination-Free and High Efficiency Drilling of Carbon Fiber Reinforced Plastics
Authors: Park Kyu Yeol , Choi Jin Ho , Lee Dai Gil
Published: April 2011

The wear-resistant metal bonded diamond drill was applied to the drilling of carbon fiber epoxy composites in order to increase the drilling performance without fuzzing and delamination. For the first time, the helical-feed drilling method was applied to overcome the drilling depth limitation in the conventional drilling with metal bonded diamond drill. From the drilling experiments, it was found that the helical-feed method employed in this work was efficient and did not cause thermal damage in high speed drilling.
Stress Effects of Inhomogeneous Expansion-Controlled Matrices in Continuous Fiber Composites
Authors: Orso Joseph , Vizzini Anthony J.
Published: April 2011

The effect of adding an expanding monomer to counteract macroscopic shrinkage of the matrix on the residual stresses in a continuous fiber composite material is examined using a finite element analysis. The fiber volume fraction, expanding monomer fraction, and monomer location are varied. In a limited number of cases, the von Mises stress and the shear stress at the fiber-matrix interface is reduced. However, in most cases, especially as the particle nears the fiber, these stresses are increased dramatically.
Failure Analysis of Woven and Braided Fabric Reinforced Composites
Authors: Naik Rajiv A.
Published: April 2011

A general purpose micromechanics analysis that discretely models the yarn architecture within the textile repeating unit cell was developed to predict overall, three-dimensional, thermal and mechanical properties, damage initiation and progression, and strength. This analytical technique was implemented in a user-friendly, personal computer-based, menu-driven code called Textile Composite Analysis for Design (TEXCAD). TEXCAD was used to analyze plain weave and 2 × 2, 2-D triaxial braided composites. The calculated tension, compression, and shear strengths correlated well with available test data for both woven and braided composites. Parametric studies were performed on both woven and braided architectures to investigate the effects of parameters such as yarn size, yarn spacing, yarn crimp, braid angle, and overall fiber volume fraction on the strength properties of the textile composite.
Stitching vs. a Toughened Matrix: Compression Strength Effects
Authors: Reeder James R.
Published: April 2011

The compression strength of a stitched and a toughened matrix graphite/epoxy composite was determined and compared to a baseline unstitched untoughened composite. Two different layups with a variety of test lengths were tested under both ambient and hot/wet conditions. At longer gage lengths where failure was due to global buckling, no significant difference in strength was found between the different materials. For shorter specimens, a 30% reduction in strength due to stitching was found for both layups, presumably due to an increase in fiber misalignment. An observed increasing strength with decreasing gage length was found for all materials and was explained using statistics and a random distribution of flaws (misaligned fibers). The toughened material showed a small increase in strength over the baseline material, presumably due to the compensating effects of a more compliant matrix and straighter fibers. A hot/wet environment reduced the strength of the baseline and stitched material by 30% and the toughened material by 20%. Video images of the failing specimen supported the theory of a random distribution of flaws controlling composite failure.
An Evaluation of Chemical Aging/Oxidation in High Performance Composites Using the Vickers Micro-Indentation Technique
Authors: Parvatareddy H. , Wang J. Z. , Lesko J. J. , Dillard D. A. , Reifsnider K. L.
Published: April 2011

Vickers micro-indentation was utilized to follow the changes in micro-hardness and thereby evaluate the chemical aging/oaidation phenomena in two different high performance polymeric composite systems. The two systems under study were1M8/954-2, a carbon fiber-reinforced thermoplastic-toughened thermoset composite, and IM8/ITX, a semicrystalline thermoplastic composite. Oxidation profiles with aging, were obtained on [0°]8 s (unidirectional) composite specimens by indenn across the thickness of the samples in the fiber direction. Samples were aged up to 6 months in environmental chambers at 150°C in three different environments: an inert nitrogen environment, a reduced air pressure of 13.8 kPa (2 psi air), and ambient air pressure. The experimental results show a considerable drop in overall hardness of both IM8/954-2 and 1M8/ITX composites after a week of aging in all of the three environments, possibly attributable to a thermal stress-reieving effect. Very little additional change was observed after 6 weeks aging. Thereafter, the hardness profiles show drops in hardness starting at the specimen edges initially, and then gradually spread to the specimen interior with time. The above observations were indicative of diffusion controlled oxidation in the samples. Also, the hardness drops in the composites appeared to be sensitive to oxygen partial pressure in the aging environment. Samples aged in ambient air were most affected, those in nitrogen least affected, and specimens aged at the reduced pressure were intermediate. A similar diffusion controlled oxidative process was also seen from Vickers indentation results on aged 1M7/5260 thermoset (bismaleimide) composites. This study is a preliminary effort and one of the first of its kind to study the effect of chemical aging in composites using the micro-indentation technique. This work has left some questions unanswered, nevertheless it has shown potential for utilizing micro-indentation as a tool for studying composite behavior in the future. Some suitable modifications to this test technique could enhance its usefulness, thereby gaining greater insights in the area of composite durability.
Characterization of the Fiber-Matrix Interphase and its Influence on Mechanical Properties of Unidirectional Composites
Authors: Subramanian S. , Lesko J. J. , Reifsnider K. L. , Stinchcomb W. W.
Published: April 2011

The fiber-matrix interphase is commonly altered in graphite fiber rein-forced composites by changing the level of surface treatment imparted to the fibers and by using different fiber sizings. In this study, the influence of both surface treatment and fiber sizing on the mechanical properties of unidirectional composites was studied. Three material systems having the same high modulus Apollo carbon fibers (manufactured by Courtoulds Research) and HC 9106-3 toughened epoxy matrix, but with different fiber sizing and surface treatment, were used to study the effects of the interphase. For convenience, these are designated 810 A, 820 A and 8100 systems. The fibers used in the 810 A and 8100 systems received 100% industry standard surface treatment, while the fibers used in the 820 A system received 200% industry standard surface treatment. The "A" and "O" represent unreacted bisphenol-A epoxy and polyvinylpyrrolidone (PVP) sizing respectively. The formation of distinctly different interphase in these three material systems was confirmed using a pernanganic etching technique. The etching study also revealed a highly nonuniform distribution of PVP in the 8100 system. Unidirectional tensile test results indicated significant reduction in the longitudinal stiffness of the PVP sized 8100 laminates. However, the tensile strength and failure strain of this system were vastly greater than those of the epoxy sized 810 A laminates. The systems with 10% and 200% levels of fiber surface treatment had similar longitudinal tensile properties. Transverse tensile test results indicated that the interracial bond strength was greatest in the 820 A system and lowest in the PVP sized 8100 system. The transverse flexural strength of the 8100 and 820 A systems were however found to be significantly greater than that of the 810 A system. It is postulated that the differences in unidirectional mechanical properties in these material systems are essentially due to differences in the interfacial bond strength and efficiency of load transfer from the matrix to the fiber.
Normal Stress Distribution of Notched 2D Triaxial Braided Textile Composites and Tape Equivalents under Tension
Authors: Norman Timothy L. , Anglin Colin , Gaskin David , Patrick Michael
Published: April 2011

A theory developed to predict the normal stress distribution near an open hole for tape laminated composites is evaluated for its applicability to 2D triaxial braided textile composite materials. Four different textile architectures are considered; braid angle, yarn and braider size and percentage of longitudinal yarns varied. Tape laminates equivalent to these textile composites were also constructed for comparison. Compliance calculated using the measured normal stress distribution from notched coupons with three different open hole sizes were compared to compliance calculated from the normal stress distribution predicted using the extended isotropic solution [1]. Results indicated that correlations between measured and predicted compliances were stronger for the tape equivalents and at smaller braid angles, however no statistically significant differences between measured and predicted compliances for textile and tape equivalents were detected. The method developed to predict the normal stress distribution was equally effective for all materials, architectures, specimen widths and strain gage locations. Our results validate the use of approximate theory to predict near-notch stress distribution in 2D braided textiles tested in this study and suggest that the textiles behave as orthotropic material.
The Continuous Curing Process for Thermoset Polymer Composites. Part 2: Experimental Results for a Graphite/Epoxy Laminate
Authors: Kim Cheol , White Scott R.
Published: April 2011

A new composite manufacturing method called continuous curing has been developed to alleviate problems in the processing of thick composites such as long manufacturing cycle times and material degradation due to thermal spiking. A 10 cm-thick graphite/epoxy (AS4/3501-6) laminate was fabricated using the continuous curing method. The thermal distributions were measured during the process and the final cure distribution was obtained by DSC testing. The mechanical performance of the laminate was analyzed by destructive testing of 16-ply mechanical test specimens removed from the bulk laminate. These specimens were distributed at five locations across the thickness of the laminate. Results for longitudinal compressive strength, transverse tensile strength, and interlaminar shear strength show some degradation. An attempt to quantify the factors contributing to the lower measured strengths is presented. Optical microscopy and image analysis was performed to obtain void and fiber volume distributions across the laminate thickness. Longitudinal layer waviness was measured by optical microscopy. The degradation in strength can be attributed to the high degree of layer waviness, nonuniform consolidation, and excessive voids. These manufacturing defects are attributed to less than optimal processing parameters chosen for the demonstration experiment.
Quantification of Random Fiber Arrangements Using a Radial Distribution Function Approach
Authors: Everett R. K.
Published: April 2011

Radial distribution functions (RDFs) for random, hard-core model patterns and actual composite material microstructures have been determined. The environs of the typical fiber in the random pattern have a higher fiber areal density nearer to the fiber than farther away. These differences increase with volume fraction. The RDFs for random patterns show good agreement with theory. The RDFs for Nicalon/zirconia titanate composites appear similar to the random case, but also display an angular dependence on near-neighbor distribution. The proximity of near-neighbors has important ramifications for modeling the micromechanics of tow-based fiber composite mateials.
The Mechanical Structures of Bamboos in Viewpoint of Functionally Gradient and Composite Materials
Authors: Amada Shigeyasu , Munekata Tamotsu , Nagase Yukito , Ichikawa Yoshinobu , Kirigai Atsushi , Zhifei Yang
Published: April 2011

The structures of bamboos have been adapted to our natural environment over a long period of time. They are a composite material reinforced axially by fibers called bundle sheath. Furthermore, they have a hierarchical gradient structure, that is, a macroscopic gradient structure in culm (corresponding to trunk of wood) diameter, culm thickness and node length, and a microscopic one in the bundle sheath distribution. The macroscopic gradient structure leads to a constant surface stress at every height and the microscopic gradient structure provides a strength distribution in radial direction adapting to bending stress due to wind loads.
Dynamic and Static Torsional Characterization of Pultruded Hybrid Cylindrical Composite Rods
Authors: Kumar Sujit S. , Mantena P. Raju
Published: April 2011

The objective of this paper is to demonstrate the effects of hybridization on the static and dynamic behavior of pultruded composite rods subjected to a torsional mode of deformation. Candidate materials that were tested included cylindrical composites made from unidirectional glass and graphite fibers, and hybrids of glass-graphite/epoxy in various shell-core type configurations, fabricated by the pultrusion manufacturing process. The dynamic torsional stiffness and damping were evaluated by the FFT based impulse-frequency response vibration technique. The static shear modulus was obtained using a conventional low torque tester and the initial tangent modulus method. This study has shown that for a given fiber volume fraction glass/epoxy exhibits higher shear modulus than graphite/epoxy; and hybridization of these two material systems results in better dynamic performance by placing glass fibers in the outer shell region.
Three-Dimensional Failure Analysis of Plain Weave Textile Composites Using a Global/Local Finite Element Method
Authors: Woo Kyeongsik , Whitcomb John D.
Published: April 2011

Textile composites are known to have improved out-of-plane properties and impact resistance. However, detailed analysis of textile composites is very difficult to perform due to the geometric complexity. In the present study, a practical computational procedure based on a global/local finite element method was developed for detailed analysis of textile composites. This procedure utilizes two problem levels: global and local levels. At the global level, an initial global solution was obtained using a coarse global mesh. At the local level, a small portion of the textile composite was modeled with a refined local mesh. For the global analysis, macroelements were used since the use of effective engineering properties are not in general accurate for the larger microstructural scale found in textile composites.

Results indicated that the failure behavior of plain weave textile composites was sensitive to the fiber bundle waviness. The initial failure mode was the fiber bundle separation when the waviness ratio was large while it was the fiber bundle fracture when the waviness ratio was small. It was also found that the stress distributions near the free surface were different from those inside. The stress level was much higher inside than near the free surface. However, the boundary region was shallow and was limited to the outer-most layer.
Measurement of the Degree of Cure of Carbon Fiber Epoxy Composite Materials
Authors: Kim Jin Soo , Lee Dai Gil
Published: April 2011

In this study, a dielectric cure monitoring system which consists of an electric circuit, a sensor and a personal computer was developed to on-line monitor the dielectric properties of carbon fiber epoxy composite materials. Also, the kinetic model of carbon fiber epoxy composite materials was developed by curve fitting of the differential scanning calorimetry (DSC) data.

The start and end points of cure and the relationship between the dissipation factor and the degree of cure were obtained by comparing the dissipation factor from the dielectric properties with the degree of cure from the DSC data. The relationship between the dissipation factor and the degree of cure was tested under various temperature profiles.
Dynamic Punch Shear Behavior of AS4/3501-6
Authors: Riendeau S. , Nemes J. A.
Published: April 2011

Punch shear experiments are conducted on a 24 ply graphite/epoxy AS4/3501-6 quasi-isotropic layup over a wide range of loading rates. Low and medium rate experiments are performed using a hydraulic testing machine with a specially designed punch shear fixture. High rate experiments are performed using a punch shear version of the split Hopkinson bar apparatus. Optical microscopy of sectioned samples of partially punched specimens is used to construct a sequence of the damage process. Cracking initiates in the shear zone of the specimen side contacted by the punch bar. This is followed by delamination in the plug on the lower third of the specimen and finally after significant rotation of the material, tensile fiber failure results. The peak punch load and displacement at the peak are found to be relatively insensitive to the punching speed. It is postulated that the values of the peak are determined principally by the tensile fiber failure, which is known to be rate-independent for graphite fibers. The lower portion of the curve, however, which is thought to reflect the matrix cracking response, is seen to exhibit substantial rate-dependence. Since matrix and delamination cracking are extremely important in assessing damage in composite structures subjected to low velocity impact loading, this rate-dependence is considered to be quite significant.
The Mechanical Properties and Deformation of Shear-Induced Polymer Liquid Crystalline Fibers in an Engineering Thermoplastic
Authors: Shi Frank F.
Published: April 2011

In this paper, through the morphological study of surface-treated samples and by using the well-known Halpin-Tsai equations, the aspect ratio and the elastic modulus of the shear-induced fibers from a thermotropic liquid crystalline polymer (LCP, Vectra A950) in a polycarbonate (PC) matrix were determined. Based on Cox's classic theories about the development of disperse liquid drops in suspending medium, the deformation mechanism of the LCP phase in the thermoplastic matrix was quantitatively or semi-quantitatively studied. The relationships among the viscosity ratio of LCP and PC, Weber number and the fiber development are given.
Modelling of Translaminar Cracks in Cross-Ply Composite Laminates
Authors: Adolfsson Erik
Published: April 2011

An approximate model is developed for the reduced thermoelastic constants of cross-ply laminates containing an array of parallel equally spaced translaminar cracks. The crack lengths under consideration range from short tunnel cracks parallel to the fibres in the 90° plies to long cracks not only covering the full width of the 90° plies, but also cutting into the neighbouring 0° plies. No crack bridging is assumed in the 0° plies. Approximate closed-form expressions for the thermoelastic quantities of the cracked laminate are derived from the application of solutions for the crack opening displacements for an array of cracks in a homogenous orthotropic body. Asymptotic values of the discussed properties valid for infinite crack densities are also computed. The displayed expressions solely contain ply material properties and information about crack spacing and crack length. The presented method yields conservative estimates of the thermoelastic properties of cross-ply laminates containing bridged cracks. Comparisons between the present model and finite element results are presented for two types of material systems exhibiting different degrees of orthotropy. It is seen that the expressions obtained for the E-modulus and the shear moduli are in good agreement with the finite element results for a wide range of crack densities and crack lengths. The expressions for the Poisson's ratios and thermal expansion coefficients are less accurate.
Failure Behavior of Plain Weave Fabric Laminates under On-Axis Uniaxial Tensile Loading: I—Laminate Geometry
Authors: Ganesh V. K. , Naik N. K.
Published: April 2011

Mathematical expressions are presented for defining the geometry of the two-dimensional orthogonal plain weave fabric laminates. The geometry was defined with the view of developing the analytical models for the prediction of the thermo-mechanical behavior. The actual strand cross-sectional geometry. possible gap between adjacent strands, strand undulation and continuity along both the warp and fill directions are considered in the mathematical expressions. Simulated geometry was compared with the actual photomicrographs of the laminate cross sections. A good match was observed.
Impact Testing on Composites Laminates and Sandwich Panels
Authors: Scarponi Claudio , Briotti Giuseppe , Barboni Renato , Marcone Antonio , Iannone Michele
Published: April 2011

The purpose of this work was the evaluation of the behavior of fiber-reinforced composites and sandwich panels for aeronautical applications under impact.

Experimental tests were performed on several specimen configurations, based on different quasi-isotropic lay-up and materials such CFRP, CFRTP and a syntactic foam as the core of sandwich panels. Considering the high specific mechanics characteristics of such a foam and its cocurability with the facesheet material, symmetric and non-symmetric sandwich configurations have been tested. The study has pointed out the advantages for aeronautic constructions of different materials and the influence of the core position along the panel thickness.

The impact tests were performed with a falling weight machine, which allowed the most important dynamic and kinetic parameters, such as the contact force, impactor velocity and displacement and perforation energy to be measured. Delamination areas of each specimen were also measured using N.D.I. reflection ultrasonic techniques. In order to determine the residual strength, Compression After Impact tests were performed on specimens that have an indentation of approximately 0.3-0.4 mm.

A numerical simulation was performed using a transient dynamic Finite Element Analysis. The main goal of this analysis was to define the controlling factors for high velocity impact simulation, such as the increase of geometric non-linearity and the delamination effects. Moreover, the delamination areas and positions were determined taking in account the different boundary conditions.

The study have pointed out the behavior of thermoset, thermoplastic and unsymmetric sandwich panels.
Single Fiber Microbuckling in a Model Composite
Authors: Mueller C. , Gorius A. , Nazarenko S. , Hiltner A. , Baer E.
Published: April 2011

Fiber microbuckling, a compressive failure mechanism in unidirectional fiber-reinforced composites, was studied in a model system composed of a single polyamide fiber in a silicone matrix. The transparent matrix permitted direct observation of fiber microbuckling during compression. In all instances the shape of the microbuckling was planar and sinusoidal. The effects of fiber diameter, fiber modulus, and matrix modulus on the critical wavelength were observed. Excellent agreement with predictions from the stability analyses of Sadowsky et al. and Darby and Kanellopoulos was obtained.
Performance Limits for Stiffness-Critical Graphitic Foam Structures. Part I: Comparisons with High-Modulus Foams, Refractory Alloys and Graphite-Epoxy Composites
Authors: Hall Richard B. , Hager Joseph W.
Published: April 2011

Graphitic foams offer exciting potential for mass savings in structures. The present investigation seeks to identify the potential mass savings and corresponding dimensional tradeoffs for application of such materials to simple, stiffness-critical structural elements, in support of an in-house materials development program. A hypothetical family of graphitic foams having ligament stiffness properties identical to those of P120 graphite fiber is considered; the stiffness properties of the hypothetical foams are viewed as realistic upper bounds for the performance of this class of materials. A representative refractory alloy, AS4/3501-6 unidirectional and [0/90], composites, and commercially available SiC and amorphous carbon foams are compared to the graphitic foams for application to plates in bending and buckling, beams in bending and buckling, and tension bars, in analogy to Gordon [1] and Ashby [2,3]. The mass and thickness ratios of the foams to the comparison materials for the chosen applications depend primarily on the Young's modulus of the foam, which is modeled according to a well-established relation due to Gibson and Ashby [4]. The hypothetical foams are found to offer stiffness properties leading to structural masses ranging from competitive to dramatically improved versus the described comparison materials. Increased structural dimensions are often associated with the savings in mass. Strength properties have not been considered in the present modeling, as processing issues still need to be resolved and the resulting defect distributions are unknown; foams of substantial promise are, however, being fabricated in small quantities.