bluuebunny/crossref_metadata_2025 huggingface dataset:
Prediction of Compression Strength of Unidirectional Polymer Matrix Composites
Authors: Barbero Ever J.
Published: April 2011

An analytical formulation is developed for the prediction of compression strength of unidirectional polymer matrix composites. The exact solution of the problem is simplified into an explicit equation for compression strength. Then, a nondimensional group is identified and used to propose a simple formula for the prediction of compression strength. The final formula can be used in design or to estimate the compression strength of production parts without performing compression testing. No empirical or semi-empirical parameters are used. The formula is validated for a broad class of materials including a variety of polymer matrices reinforced with carbon and glass fibers. Three parameters are required: the shear stiffness and strength of the composite and the standard deviation of fiber misalignment in the composite. All three parameters can be measured by well-established techniques. Data from the literature is used to validate the formula.
Progressive Damage of Unidirectional Graphite/Epoxy Composites Containing a Circular Hole
Authors: Yang Fan , Chow C. L.
Published: April 2011

In this paper, experimental and finite element results are presented to describe the anisotropic states of stress, strain and damage of unidirectional graphite/epoxy composite plates containing a central hole subjected to off-axis uniaxial tension. The nonlinear stress analysis of the laminates is based on the finite element procedure formulated with a newly developed anisotropic damage model, in which the inelastic behavior of composite material is attributed to the irreversible thermodynamics processes involving energy dissipation and stiffness change primarily caused by the presence of micro-structural changes of the material. Moiré interferometry, a high sensitivity whole-field experimental technique, with a computer image processing system is used to determine and to interrogate the two-dimensional deformation field in the composites. Results from the tests and analysis clearly reveal the anisotropic damage characteristics and the influence of principal material direction on the deformation response of the composites. Failure of the composite material near the hole area takes the form of a damage zone and the damage mechanism involves redistribution of stress and strain. From the analysis, it may be concluded that macrocrack initiates at the material point near the hole boundary with high damage value and propagates along the direction of damage zone extension. Good agreement between the predicted and experimental results confirms that the current approach can provide a convenient and accurate means to interrogate the complicated damage behavior of notched unidirectional composites with a hole and can be readily extended to examine the damage response of unidirectional composite structures with different notch configurations.
Material Damping in Composite Rotors
Authors: Wettergren Hakan L.
Published: April 2011

The paper is concerned with material damping in composite rotors. The material damping for a high-modulus carbon fiber in epoxy resin has been investigated for different angles of filament winding. When the separate contributions on the damping from the stresses in the directions of the fibers, transverse to the fibers and in shear are taken into account, the damping coefficient has a minimum when the filament winding angle is between 10° and 45°, which fortunately for most applications is also the optimal interval for the filament winding angle to get minimum weight.

To get the lowest possible dissipation energy for three different constraints, lateral eigenfrequency, maximum torque due to static stress and maximum torque due buckling, there exists an optimal filament winding angle interval. This interval is in the same region as the optimal filament winding for minimum weight.
Evaluation of Failure Criteria for Fiber Composites Using Finite Element Micromechanics
Authors: Zhu H. , Sankar B. V. , Marrey R. V.
Published: April 2011

A micromechanical analysis of the unit cell of a unidirectional composite is performed using the finite element method. The circular fibers are assumed to be packed in a periodic square array. Assuming that the failure criteria for the fiber and matrix materials and also for the fiber-matrix interface are known, the failure envelope of the composite is developed using the microstresses computed in the unit cell analysis. This method is referred to as the Direct Micromechanics Method (DMM). The micromechanical methods were also used to simulate different tests to determine the strength coefficients in phenomenological failure criteria such as maximum stress, maximum strain and Tsai-Wu theories. The failure envelopes from the phenomenological failure criteria are compared with those of the DMM for the cases of biaxial and off-axis loading of a model unidirectional composite material. It is found that none of the phenomenological criteria compare well with the DMM in the entire range. A conservative failure envelope obtained using a combination of maximum stress and Tsai-Wu criteria seems to be the best choice for predicting the failure of unidirectional fiber composites.
Compressive Behavior of Aramid Fiber-Reinforced Pultruded Rods
Authors: Andrews M. C. , Young R. J. , Mahy J. , Schaap A. A. , Grabandt O.
Published: April 2011

Raman spectroscopy has been used to follow the tensile and compressive deformation of aramid fibers in pultruded composite rods. The deformation behavior of the rods has been compared with that of individual aramid fibers. It has been found that the deformation of the fibers in the rods is similar to that of individual isolated fibers when both are deformed in tension. For deformation in compression, however, failure of the fibers in the rods is found to take place at a lower stress than for individual fibers due probably to the cooperative formation of kink bands in the rods. These findings give a unique insight into the compressive deformation of aramid/epoxy composites and have important implications for our understanding of the compressive failure of fiber-reinforced composites in general.
Long-Term Thermo-Oxidative Aging in Composite Materials: Experimental Methods
Authors: Tsotsis Thomas K.
Published: April 2011

The combination of this work, an earlier study [1] concerning the effects of thermo-oxidative aging on the thermal, fracture and compressive behavior of carbon-fiber reinforced epoxy composites, and a companion paper [2] discussing the failure mechanisms due to thermo-oxidative aging provide the first published comprehensive study of the effects of long-term thermo-oxidative aging on such a wide range of properties for the same materials systems. In this phase of the work, tensile tests (both unidirectional 0° and ±450 layups), edge delamination, and compression-after-impact (CAI) were examined for G30-500/R922-1 and G30-500/R6376 carbon/epoxy specimens aged in air at 177°C (350°F) up to 10,000 hours. Additionally, 10,000 hour data not included in the previous study are also included for comparison. The purpose of the present study is not to identify suitable candidate materials, but to evaluate the effect of test methods and failure modes in characterizing thermo-oxidative stability. The materials systems chosen were selected as model materials for the study of degradation phenomena and not for performance evaluation at 177°C which is clearly beyond the useful range of these materials. Nevertheless, unidirectional tensile properties were found to be relatively unaffected up to 5,000 hours of aging. By contrast ±45° tensile strengths and unidirectional compression strengths decreased between 10 and 35% after only 1,000 hours exposure. Mode I fracture toughness increased with aging due to increased fiber bridging and Mode II fracture toughness was, by contrast, relatively unaffected. Edge delamination fracture toughness was severely affected by thermo-oxidative aging decreasing to between 5 and 15% of the unaged value after 10,000 hours aging suggesting a strong effect of interfacial degradation. Because even small weight losses lead to correspondingly large reductions in certain mechanical properties, even small errors in the determination of weight loss lead to large errors in estimates of mechanical properties. Indeed, it is likely that weight loss, even with the presence of corresponding mechanical data is not a good indicator of property degradation. Weight loss is only useful as a negative indicator: it indicates an unacceptable material; it does not yield useful information as to useful lifetimes.
Inhibited Carbon-Carbon Composites: Isothermal and Fatigue Exposure
Authors: Ochoa Ozden O. , Elliott Charles H.
Published: April 2011

The oxidation process is examined under isothermal, cyclic thermal and thermomechanical fatigue conditions for inhibited carbon-carbon composites. Mass loss and material property degradation assessment is undertaken with subsequent exploratory nondestructive testing utilizing DMA and PUCOT techniques. An analytical diffusion model is developed to evaluate oxidation degradation for selected exposure conditions. The results are then compared with the experimental data generated. Nondestructive testing techniques introduced along with the diffusion model successfully trace the shear and axial modulus degradation of inhibited carbon-carbon composite as oxidation progresses. These results when compared with thermomechanical data establish the significance of the combined temperature and mechanical load on the overall material characterization of carbon-carbon laminates.
Solid Particle Erosion Behavior of FRP Degraded by Hot Water
Authors: Miyazaki N. , Funakura S.
Published: April 2011

Experiments were carried out to study solid particle erosion behavior of an FRP degraded by hot water. The FRP used was unsaturated polyester resin reinforced by chopped glass strand mats. The test specimens were soaked in hot water of 80°C and the degraded test specimens were used in the erosion tests. The Vickers hardness of a resin rich area in the FRP was measured and the interlaminar shear strength of the FRP was obtained from short beam three-point bending tests. According to these test data, the resin became soft and the interface between the matrix resin and fiber was weakened due to degradation by hot water. It is found from the erosion test data that the erosion behavior of the degraded FRP is affected both by decreasing hardness of the matrix resin which results in decreasing the erosion rate and by decreasing the interfacial strength between the matrix resin and fiber which results in increasing the erosion rate.
Coating Sequence Effect for Composites Containing Multiply Coated Long Fibers
Authors: Lu Shih-Yuan
Published: April 2011

The effect of coating sequence on the effective conductivity of composites containing multiply coated aligned long fibers, either periodically arranged or randomly distributed, is investigated. The effect is found to be dramatic. Reordering of the coating sequence may shift the coated long fiber from being enhancing to being impairing to the matrix. It is further found that outer coating layers play a more influential role than inner coating layers do in affecting the effective conductivity of the composite since outer coating layers are more exposed to the heat flow. The coating sequence effect is investigated for three different microstructures, namely square arrays, hexagonal arrays, and random arrays, and is most pronounced, at a fixed inclusion volume fraction, for the random array, less for the square array, and the least for the hexagonal array. This observation can be explained in terms of the involvement of relevant dimensionless multipole polarizabilities in the evaluation of the effective conductivity. The present development can also be used to compute the effective conductivity of composites containing either periodically arranged or randomly distributed aligned long fibers possessive of radially changing conductivities. Example calculations are offered to illustrate the converging behavior of this application.
Curing Behavior of Thick-Sectioned RTM Composites
Authors: Michaud D. J. , Beris A. N. , Dhurjati P. S.
Published: April 2011

The successful manufacture of thick-sectioned composites is challenging, since the highly exothermic nature of thermoset resins and limited temperature control make it difficult to avoid detrimental thermal and cure gradients within the composite. In order to make quality parts, it has been found experimentally that cure temperatures must be lowered as much as 50% from those suggested for thin parts. Differential Scanning Calorimetry (DSC) experiments of a vinyl-ester resin system at these lower temperatures revealed a significant dependence on temperature for the maximum extent of cure. If the resin is cured isothermally at 55°C, the final conversion of the resin was found to only reach 70%. When the maximum extent of cure parameter was incorporated into an empirical autocatalytic kinetic model, it was found to significantly improve the description of the cure kinetics. Inhibitors, added to the resin to improve shelf-life, disappear rapidly at higher cure temperatures but can double the time required to cure a thick composite processed at 55°C. A zeroth order kinetic relationship was developed to estimate the amount of inhibitor in the system during the resin's cure. The inhibitor relationship and the improved kinetic model were used in a finite difference cure simulation to successfully predict the thermal gradients during cure of a 2.54 cm thick composite manufactured by resin transfer molding (RTM).
Experimental Evaluation of Repair Efficiency of Composite Patching by ESPI
Authors: Wang Wei-Chung , Chiou Tsann-Bim
Published: April 2011

In this paper, the electronic speckle pattern interferometry (ESPI) technique was first used to investigate repair efficiency of patching by using composite materials on aluminum alloy plates containing a central crack. The effect of single-sided and double-sided patchings with same total thickness and four different fiber orientations of the patchings was discussed. A self-developed computer program for analyzing mode I stress intensity factor (SIF), K1 was performed to evaluate the repair efficiency. To verify the correctness of the experimental results, recontruction technique was employed. Well-matched condition between the experimentally obtained and reconstructed fringes indicates the correctness of the experimental results.
Calculation of the Stresses and Strains in Embedded Fiber Optic Sensors
Authors: Kollar Laszlo P. , Van Steenkiste Regis J.
Published: April 2011

Embedded fiber optic sensors operating on interferometric principles have recently been considered for measuring strains and temperature inside isotropic and orthotropic (composite) materials. Owing to the complex interactions between the sensor and the material surrounding it, the relationship between the sensor output and the strains and temperature inside the material cannot be determined by simple tests. In general, the relationships providing the bridge between the sensor output and the engineering values of strain and temperature must be established via analytical models. Once arrived at, the relationships between the sensor output and the engineering values of the strains and temperature can be inverted to provide the values of the strains and temperature in terms of the sensor output. The scope of this article is limited to the relationship between the strains and temperature in the material, far from the sensor, and the strains and temperature in the sensor. Closed form expressions are derived which relate the strains and temperature in the composite to the strains inside the optical sensor. The sensor may be either circular or elliptic. The sensor and the material surrounding it are considered to be orthotropic.
Effects of Braiding Parameters on Energy Absorption Capability of Triaxially Braided Composite Tubes
Authors: Chiu C. H. , Tsai K. H. , Huang W. J.
Published: April 2011

The effects of two different braiding parameters, the braiding angle and the axial yarn content, on the crush failure characteristics and specific energy absorption capability of two-dimensional (2D) triaxially braided composite circular tubes were studied. Quasi-static axial compression was employed for these crush tests. The crushing appearance and failure modes in the crushing zones were examined by optical microscopy. The results showed that the triaxially braided carbon composite tubes were crushed by a progressive splaying mode. The average width of the splaying fronds increased with increasing braiding angle, but decreased with increasing axial yarn content. The tube with 200 braiding angle showed the highest specific energy absorption among the tubes with various braiding angles. And the tubes using 6k-12k fiber tows as the axial yarn exhibited higher specific energy absorption among the tubes with various axial yarn content.
Analysis of a Stitched Double Cantilever Beam
Authors: Sankar Bhavani V. , Dharmapuri Sreerama M.
Published: April 2011

A simple analytical model is presented to describe crack propagation in stitched laminated Double Cantilever Beam (DCB) specimens. The stitches are smeared and modeled as continuous cohesive springs, which are assumed to be linear-elastic until final failure. Timoshenko beam theory is used to model the DCB specimen. As the specimen is loaded, the stitches are assumed to break when the strain in the stitches reaches the ultimate strain. The crack tip is assumed to advance as the energy release rate at the crack tip reaches the Mode I fracture toughness of the parent laminate. A closed form solution is obtained for the problem of beam on elastic foundation and it is used to simulate the DCB test and crack propagation. Results presented include, load-deflection diagrams, crack bridging length and apparent fracture toughness for various stitch parameters. A semi-empirical formula is derived for the bridging length as a function of the stitch ultimate strain and fracture toughness of the parent laminate. The results are compared with experimental results. It is found that, for the case with Kevlar stitches, the linear elastic model of the stitch yarn is not adequate. It is shown that inelastic behavior of the stitches plays a significant role in increasing the fracture toughness due to stitching.
Stress Rupture of Unidirectional High Performance Thermoplastic Composites in End-Loaded Bending at Elevated Temperatures. Part II: Analytical Modeling
Authors: Mahieux Céline A. , Russell Blair E. , Reifsnider Kenneth L.
Published: April 2011

The equations of shape for a specimen under end-loaded bending are derived for a different coordinate axis using the theory of large displacements of buckled bars (The Elastica). The strain and stress distributions along the length of a unidirectional fiber reinforced composite undergoing stress rupture in end-loaded bending are established. Two life prediction models are constructed and compared to experimental results for an AS4/PPS composite. The models are respectively based on the nucleation and propagation of the microbuckles. The latter model compares well with the experimental stress rupture data for AS4/PPS composites in end-loaded bending at 120°C.
On the Effective Coefficient of Thermal Expansion in Thermal Bending of Composites Undergoing Linear Heat Flow
Authors: Hasselman D. P. H.
Published: April 2011

It is suggested that the localized stresses and associated displacements resulting from the thermal conductivity mismatch of second-phase inclusions in dispersed phase-continuous matrix composites subjected to linear heat flow can contribute to the thermal bending (i.e., the curvature) of the composite at the continuum level. The original solutions of Tauchert for a single inclusion were used to derive an expression for the curvature and associated effective coefficient of thermal expansion of a matrix with spherical inclusions undergoing linear heat flow. The results are expressed in terms of the isothermal coefficient of thermal expansion, Young's modulus and thermal conductivity of the composite, which, in turn, depend on the relevant properties and volume fractions of the matrix and dispersed phase. Numerical examples are presented which indicate a significant effect. Thermal expansion mismatches between the matrix and dispersed phase are shown to play a particularly large role in either increasing or decreasing the degree of thermal bending, which may even become negative for matrices with near-zero expansion behavior. A numerical example based on a flat composite plate constrained from bending indicates that the above effect can have a large influence on the magnitude of maximum thermal stress.
Stress Transfer from the Matrix to the Fibre in a Fragmentation Test: Raman Experiments and Analytical Modeling
Authors: Paipetis Alkis , Galiotis Costas , Liu Yung Ching , Nairn John A.
Published: April 2011

The stress transfer properties of the fibre/matrix interface in the single fibre fragmentation test were investigated. Two carbon fibre-resin systems involving epoxysized and unsized fibres, were examined. Axial fibre stress data at resolutions of the order of one micron, were obtained with the technique of Remote Laser Raman Microscopy. Subsequent analytical modeling of the data was performed using a Bessel-Fourier Series stress analysis approach. The analysis provides a nearly exact solution for the stress field in the fragmentation test and simultaneously accounts for damaged or imperfect interfaces through the use of an interface parameter D, All data were fit using a two zone model in order to account for the propagation of interfacial damage as a function of applied strain. The fitting process was used to determine D(), the interface parameter in undamaged zones, Did), the interface parameter in the damage zones near fibre breaks, and Id, the length of the damage zones, all as a function of applied strain. The interface parameter in the undamaged zones, D('), was independent of applied strain. We claim D(')is a good property for comparing fibre/matrix interfaces and the most relevant property for predicting the role to the interface in real laminates.
Experimental Investigation of the Thermal Properties of Wood Fiber/Thermoplastic Composites
Authors: Oladipo A. B. , Wichman I. S. , Beck J. V.
Published: April 2011

The influences of fiber mass fraction and temperature on the effective thermal properties of aspen wood fiber/high density polyethylene composites are investigated. One major goal is to ascertain the composition at which the composite possesses thermal properties that are indicative of good fire safety characteristics. The paper also examines the extent to which analytical expressions presently available in the literature can be used for predicting the composite thermal properties from those of the individual components. The results show that temperature effects on the effective thermal properties of the composite diminish with increase of wood fiber mass fraction. Composites having 50 to 60% wood fiber by mass appear to possess the best processing and fire safety qualities.
Damage Development in Woven Fabric Composites during Tension-Tension Fatigue
Authors: Hansen Ulrich
Published: April 2011

Impacted woven fabric composites were tested in tension-tension fatigue. In contrast to results from static testing, the effects of low energy impact damage in a fatigue environment were found to be the critical element leading to failure of the specimen. This difference emphasizes the need to identify and understand the fatigue damage mechanism. A relatively new non-destructive inspection technique using infrared thermography was found to be a very useful tool in detecting damage initiation and growth. Furthermore, this technique supplies valuable information to the characterization of the operating fatigue damage mechanism(s).

Fatigue leads to a degradation of material properties. Consequently, in connection with impact induced local stress raisers, fatigue produces continuously changing non-uniform stress fields because of stress redistribution effects. Other models addressing evolution of fatigue damage in composite materials have not been able to simulate evolving non-uniform stress fields. Therefore. in the second part of this paper. an analytical/numerical approach capable of addressing these issues is also proposed.
The Strength of Bolted and Bonded Single-Lapped Composite Joints in Tension
Authors: Lin Wei-Hwang , Jen Ming-Hwa R.
Published: April 2011

This work is primarily concerned with a failure analysis of single-lapped bolted and bonded mixed-composite joints, representing practical situations, both in terms of experimental data and theoretical appreciation. Two kinds of stacking sequences in adherends are tested: quasi-isotropic and cross-ply. The adhesives include thermal-setting and thermal-plastic types. The bolts used to clamp the bonded joints are made of 304 stainless steel, whilst the rivets are of NAS 1919 C05 according to National Aerospace Standards. The important invented techniques in making riveted and bolted joints and measuring their respective clamping pressure are disclosed, and the main contribution of systematic experimental data in contrast with numerical prediction is highlighted as follows.

The bolting arrangements can be divided into one (1 x 1), three (1 x 3, 3 x 1), six (2 x 3) and nine (3 x 3) configurations. The empirical results indicate that the joint of six (2 x 3) bolts possess the highest strength, nine (3 x 3) bolts the second strongest, followed by (1 x 3), (3 x 1); while the one bolt geometry is the weakest. The mixed joints with more than three bolts will fail by adhesive delamination and tension failure in adherends transversely. However, the failure mechanism for the one bolt system is different, in that delamination occurs first, then bolt bending and shearing, and finally a tension-type failure. A typical tensile stress-strain diagram is linear before delamination. We also adopt the C-scan non-destructive testing (NDT) technique to examine damage initiation sites and to compare this information with the numerical prediction obtained using the finite element method. The comparison is satisfactory. Finally, the joint strength increases with increasing overlap length, until the strength approaches asymptotically a constant value for longer overlap lengths.
Effect of Particulate Reinforcing on Stress Corrosion Cracking Performance of a SiCp/2024 Aluminum Matrix Composite
Authors: Yao Hong-Yu
Published: April 2011

The stress corrosion cracking (SCC) performance of a SiCp/2024 aluminum matrix composite in NaCl aqueous solution is examined by means of bolt loaded double cantilever beam (DCB) experiment as well as slow strain rate tension (SSRT) experiment. Effect of the particulate reinforcing on composite's SCC performance is discussed. It is found that the composite suffers from SCC by mechanism of intergranularly anodic dis-solving, just as 2024 aluminum alloy. The experiment results lead to such new conclusion that although no essential change is introduced to the SCC performance by the presence of the particulate in the composite as compared to the monotonous aluminum alloy, the composite's SCC resistance is greatly enhanced with respect of the counter part 2024 aluminum alloy. The enhancement is attributed to: (1) the fine microstructure of the composite; (2) the strain concentration phenomenon around the reinforcing particles; (3) the hindering effect of the particles located at grain boundaries on intergranular cracking.
Prediction of Effective Three-Dimensional Elastic Constants of Translaminar Reinforced Composites
Authors: Dickinson L. C. , Farley G. L. , Hinders M. K.
Published: April 2011

A Trans-Laminar-Reinforced (TLR) composite is defined as composite laminate with up to five percent volume of fibrous reinforcement oriented in a trans-laminar fashion in the through-thickness direction. The objective of this work was to examine the effect of important parameters on elastic response. Detailed finite element models of unit cells were used to study the effects of adding TLR on the elastic constants. Parameters investigated included TLR material, TLR volume fraction, TLR diameter, TLR through-thickness angle, ply stacking sequence, and the microstructural features of pure resin regions and curved in-plane fibers. The work was limited to materials with at least one ply interface. Adding a few percent TLR had a small negative effect on the in-plane extensional and shear moduli, Ex, Ey and Gxy but a large positive effect (up to 60 percent) on the thickness direction extensional modulus, E,. The volume fraction and the axial modulus of the TLR were the controlling parameters affecting E,. The out-of-plane shear moduli, G, and Gy, were significantly affected only with the use of a TLR with a shear modulus an order of magnitude greater than that of the composite lamina. A simple stiffness averaging method for calculating the elastic constants was found to compare closely with the finite element results, with the greatest difference being found in the inter-laminar shear moduli, G., and Gy.
Effect of Isothermal Aging on Ultimate Strength of High-Temperature Composite Materials for SST Structures
Authors: Shimokawa Toshiyuki , Hamaguchi Yasumasa , Kakuta Yoshiaki , Katoh Hisaya , Sanda Tomio , Mizuno Hiroshi , Toi Yasuhiro
Published: April 2011

The objective of this experimental study was to evaluate the effect of iso-thermal aging on the ultimate strength of three kinds of carbon/high-temperature composite materials, i.e., G40-800/5260 and MR50K/MR2000N bismaleimide composites and T800H/PI-SP amorphous thermoplastic polyimide composite. These materials are current candidate structural materials for a supersonic transport of the next-generation. The hole-notched and unnotched panels, before being machined to specimens, were isothermally aged at 120°C and 180°C for up to 15,000 hours. Static tests at room and elevated temperatures before and after thermal aging provided the open-hole tensile, open-hole compressive, and short beam shear strengths. Moreover, the effects of five oxidation resistant treatments on open-hole compressive strength at 180°C were investigated after isothermal aging of 5,000 hours at 180°C. The test results clarified the effects of isothermal aging on ultimate strengths and oxidation resistant treatments on open-hole compressive strength. Moreover, the possibility of developing an accelerated aging test method is discussed using a modified Larson-Miller type equation.
Modelling and Experimental Analysis of Multi-Coating Effect on Thermal Expansion and Thermal Stresses of Polymer Fiber-Reinforced Composites
Authors: Agbossou Amen , Bergeret Anne
Published: April 2011

Due to Thermal Expansion Coefficient (TEC) mismatch between the fiber and matrix, thermal stresses can appear during cooling from the processing temperature to room temperature. In this paper, we (1) analyze experimentally the effect of coating on the TEC of fiber composite and (2) present a thermal self-consistent model to predict thermal stresses and TEC in multi-coated fiber. Over a large range of temperature (40°C to 200°C), the experimental results show several interesting features that result from the presence of (Silane and Polyvinylacetate) coating. These results are then modeled theoretically using the proposed thermal self-consistent model. The results show that variations in thermal stress may initiate damage in the vicinity of the fiber or promote interface contact during curing of the composite.
Damping Performance of Cocured Graphite/Epoxy Composite Laminates with Embedded Damping Materials
Authors: Biggerstaff J. M. , Kosmatka J. B.
Published: April 2011

Cocuring viscoelastic damping materials in composites has been shown to be successful in greatly increasing the damping of composite structures. The damping performance, however, is often not as high in cocured composites as in secondarily bonded composites, where the damping material does not undergo the laminate cure cycle. The reason for the discrepancy in damping between the cocured and secondarily bonded samples was found to be resin penetration into the damping material. Samples with a barrier layer between the damping material and the epoxy resin had a 15.7% to 92.3% higher effective loss factor (depending on the frequency) than cocured FasTape™ 1125 samples without the barrier and at least 168% higher effective loss factor than cocured ISD 112 samples without the barrier. These higher damping values are very close to the values achieved by secondarily bonding. Viscoelastic damping materials typically have maximum recommended temperatures below that of the composite cure cycles. The effect of cure temperature on viscoelastic damping materials was also studied and it was determined that most damping materials are marginally affected by cure cycle temperature.
Advanced Calculation of the Room-Temperature Shapes of Unsymmetric Laminates
Authors: Schlecht M. , Schulte K.
Published: April 2011

It is well known that the room-temperature shapes of unsymmetric laminates do not always conform to the predictions of classical lamination theory. Instead of being saddle shaped, as classical lamination theory predicts, the room-temperature shapes of unsymmetrically laminated composites are often cylindrical in nature. In addition, a second cylindrical shape can sometimes be obtained from the first by a simple snap-through action. Since 1981 several models, which are restricted to rectangular plates and sometimes only some special lay-ups, have been developed. The Finite Element Analysis (FEA) has just recently been used for the calculation of the room-temperature shapes of unsymmetric laminates, because more sophisticated finite element codes are now available and the calculations can be made in an acceptable time. Using the FEA there are no restrictions concerning the laminate geometry or the lay-up and the snap-through effect can be modeled. Additionally the edge-effects can also be monitored in the FEA results. Unsymmetric laminates can become important for aircraft applications especially for the aeroelastic tailoring.
Stress Transfer Model for Single Fibre and Platelet Composites
Authors: Mendels D. A. , Leterrier Y. , Manson J. A. E.
Published: April 2011

A model for elastic stress transfer across the interface in composites has been developed in particular to overcome the known problem of the matrix effective radius. Stresses were determined using a two dimensional method by introducing a novel hypothesis of structural response of the matrix. This is in contrast to previous approaches, which mainly consider that the axial derivative of axial stress does not depend upon the radius. This leads to a new form of the stress transfer equation, free of an adjustable parameter. This analysis was applied to linear elastic single fibre and single platelet composites. The comparison of the two reinforcements allowed the identification of a structural parameter for which the two geometries are shown to be equivalent. The model was validated using literature results obtained by Raman spectroscopy means or photo-elastic methods. The agreement between measured data and the model showed the relevance of the matrix structural response approach to stress transfer.
Composite Manufacturing: Simulation of 3-D Resin Transfer Molding
Authors: Tan Cheng P. , Springer George S.
Published: April 2011

A technique was developed for simulating the resin transfer molding (RTM) process. The major feature of the technique is a computational steering system that enables the user to make changes during the simulation. Specifically, at any instance, the user can inspect the progress of the resin front. On the basis of the observed resin front position, the user can, as needed, change the port and vent locations, open and close ports and vents, adjust the inlet and exit pressures or flow rates, and reorient the mold with respect to the gravitational field. Additionally, the user can "rewind" the simulator to any previous time in the mold filling process, make any of the above changes, and then continue the simulation.

The technique is augmented by a computer code which has three main components: the Simulator, the Gtaphics User Interface (GUI), and the Global Data Storage. The Simulator is a finite element code that calculates the resin flow inside the fiber preform. The GUI serves as the interface between the user and the Simulator; it provides the commands to the Simulator and displays the results. The Global Data Storage is the module that manages the exchange of data between the GUI and the Simulator.

The computer code (designated as SUPERTM_3D) is suitable for simulating the resin flow inside two-dimensional as well as three-dimensional fiber preforms of arbitrary shapes. The use of this computer code is illustrated through sample problems. These problems demonstrate how (with this code) the designer can establish the port and vent locations, opening and closing sequences of ports and vents such that the fiber preform is filled completely in the shortest time with the fewest number of vents.
Evaluation of Various Approximate Analyses for Plain Weave Composites
Authors: Whitcomb John , Noh Jae , Chapman Clinton
Published: April 2011

The literature contains a variety of analyses for analyzing plain weave composites. These differ in both geometric approximation and assumptions about stress and strain distribution. Herein the weave geometry for a wide range of waviness was described using three-dimensional finite element meshes. These meshes were used with various stress and strain assumptions to determine how these assumptions affect predicted in-plane extensional stiffness. Assumptions about stress and strain can be tied to whether an analysis is equivalent to a virtual work, complementary virtual work, or hybrid formulation. Since these variational principles have well understood convergence behavior, it is easy to explain the observed effects of the various assumptions. The results indicate that sometimes accurate predictions for particular cases result from cancellation of errors, rather than sound formulation.
The Influence of Aspect Ratio of Voids on the Effective Elastic Moduli of Foamed Metals
Authors: Chao Lu-Ping , Huang Jin H. , Huang Yung-Sung
Published: April 2011

An analytical methodology is developed to study the influence of void's aspect ratio on the overall elastic behavior of foamed materials. The method developed for the prediction of the effective elastic moduli is based upon Eshelby's equivalent inclusion method and Mori-Tanaka theory. Analysis results with foamed aluminum as demonstration of the effectiveness of the proposed methodology indicate that the effective longitudinal elastic modulus Ē11, the effective shear moduli Ḡl2 and Ḡ13 and effective Poisson's ratio v̄12 appear to increase with the increment of void's aspect ratio. Meanwhile, the effective transverse elastic moduli Ē22 and Ē33, the effective out-plane shear modulus Ḡ23, and the effective Poisson's ratios v̄21 as well as v̄23 decrease with increasing aspect ratio of the void. The numerical results correlate well with experimental data, demonstrating the accuracy of the proposed methodology with respect to predicting elastic moduli of foamed materials.
Failure Investigation of Graphite/Polyimide Fabric Composites at Room and Elevated Temperatures Using the Biaxial Iosipescu Test
Authors: Searles K. , Odegard G. , Kumosa M. , Castelli M.
Published: April 2011

The biaxial and modified biaxial Iosipescu shear test methods were applied to determine the shear dominated, biaxial mechanical response of graphite/PMR-15 and graphite/Avimid-R woven fabric composites at room and elevated temperatures. Three different composite architectures were examined: T650-35 warp-aligned, 8-harness satin (8HS) fabric in a PMR-15 matrix, T650-35 warp-aligned, 8HS fabric in an Avimid-R matrix and T650-35 0°/90°, 8HS fabric in an Avimid-R matrix. Several biaxial Iosipescu tests were performed at room temperature under shear, shear-tension, and shear-compression loading conditions to characterize damage and obtain biaxial, shear dominated failure properties. Shear tests were also conducted at elevated temperatures approaching 316 °C to determine the effects of temperature on the shear strengths of the composites investigated. A nonlinear finite element analysis is briefly introduced to evaluate the effects of specimen sliding and geometric nonlinearities on the stress and strain distributions in the biaxial Iosipescu specimens. In addition, the effects of different loading block geometries on the stress distribution in the Iosipescu specimens subjected to biaxial loads were also investigated. Within this investigation, it was found that graphite/Avimid-R was more resistant to biaxial, shear dominated failure at room temperature in comparison to graphite/PMR-15. However, the graphite/PMR-15 composite system exhibited better shear strength properties at elevated temperatures above 232 °C. It was also found that the effect of compression along the notch root axis generated by the loading blocks did not affect the loads at failure.
A Micromechanical Analysis of the Radial Elastic Response Associated with Slender Reinforcing Elements within a Matrix
Authors: Cox James V. , Yu Hailing
Published: April 2011

This paper addresses the elastic modulus associated with the radial interaction between a slender axisymmetric reinforcing element and a matrix. In particular, reinforcing elements with a significant surface structure are considered, and the elastic modulus of an interface model is defined to characterize the local elastic behavior resulting from the mechanical interaction that is not explicitly captured at a larger scale of modeling (i.e., a scale at which the surface structure is not explicitly modeled). An analytical justification for the elastic modulus is presented by determining the difference in the strain energy stored in a matrix that has a homogenized (or smoothed) interface traction distribution versus a more concentrated traction distribution that may occur with a complicated surface structure. Due to the importance of strain energy in driving cracks, it is postulated that the elastic modulus should be such that the composite with an idealized interface will store the same amount of strain energy as the actual composite having an interface with a surface structure. Analytical results show that the elastic modulus increases with the ratio of the contact area to the interface area and with a decrease in the period associated with a periodic traction distribution. A numerical example shows the effect of the elastic modulus on the prediction of longitudinal cracking in a quasibrittle matrix.
Prediction of Post-Impact Compressive Strength of Composite Laminates Using an Inhomogeneity Model
Authors: Wang J.
Published: April 2011

In this paper, the damaged area in fibre-reinforced quasi-isotropic laminates that have been subjected to low-velocity impact is treated as a circular inhomogeneity in a finite plane region. The modulus of the damaged area is estimated using a self-consistent model, together with the solution for the inhomogeneity problem and experimental data. The post-impact compressive strength of the laminates is then predicted using the inhomogeneity model. The predicted post-impact compressive strength is shown to be in good agreement with experimental results for the considered quasi-isotropic laminates of two thicknesses.
Effects of Adding Butyric Acid to PN on Gut‐Associated Lymphoid Tissue and Mucosal Immunoglobulin A Levels
Authors: Murakoshi Satoshi , Fukatsu Kazuhiko , Omata Jiro , Moriya Tomoyuki , Noguchi Midori , Saitoh Daizoh , Koyama Isamu
Published: April 2011

Background: Parenteral nutrition (PN) causes intestinal mucosal atrophy, gut‐associated lymphoid tissue (GALT) atrophy and dysfunction, leading to impaired mucosal immunity and increased susceptibility to infectious complications. Therefore, new PN formulations are needed to maintain mucosal immunity. Short‐chain fatty acids have been demonstrated to exert beneficial effects on the intestinal mucosa. We examined the effects of adding butyric acid to PN on GALT lymphocyte numbers, phenotypes, mucosal immunoglobulin A (IgA) levels, and intestinal morphology in mice. Methods: Male Institute of Cancer Research mice (n = 103) were randomized to receive either standard PN (S‐PN), butyric acid–supplemented PN (Bu‐PN), or ad libitum chow (control) groups. The mice were fed these respective diets for 5 days. In experiment 1, cells were isolated from Peyer's patches (PPs) to determine lymphocyte numbers and phenotypes (αβTCR+, γδTCR+, CD4+, CD8+, B220+ cells). IgA levels in small intestinal washings were also measured. In experiment 2, IgA levels in respiratory tract (bronchoalveolar and nasal) washings were measured. In experiment 3, small intestinal morphology was evaluated. Results: Lymphocyte yields from PPs and small intestinal, bronchoalveolar, and nasal washing IgA levels were all significantly lower in the S‐PN group than in the control group. Bu‐PN moderately, but significantly, restored PP lymphocyte numbers, as well as intestinal and bronchoalveolar IgA levels, as compared with S‐PN. Villous height and crypt depth in the small intestine were significantly decreased in the S‐PN group vs the control group, however Bu‐PN restored intestinal morphology. Conclusions: A new PN formula containing butyric acid is feasible and would ameliorate PN‐induced impairment of mucosal immunity.
Inhibition of Inflammatory Mediators
Authors: Wang Weihua , Le Wei , Ahuja Rahul , Cho Do‐Yeon , Hwang Peter H. , Upadhyay Daya
Published: April 2011

Objective

To determine if statins induce anti‐inflammatory effects in upper airway inflammation. Mediators of innate and adaptive immunity regulate airway inflammation. Release of these mediators involves enzymatic conversion of polyunsaturated fatty acids into biologically active mediators, which can be blocked by statins. Although upper airway inflammation and chronic sinusitis occur in millions of patients with asthma worldwide, the anti‐inflammatory effects of statins in upper airway inflammation have not been previously studied.

Study Design

Laboratory research.

Setting

Tertiary referral center.

Subjects and Methods

Analysis of sinus tissues collected from patients with chronic rhinosinusitis revealed suppression of highly expressed inflammatory mediators in patients who were found to be on statins, suggesting that statins may induce anti‐inflammatory effects. Therefore, the authors performed an in vitro study to determine if these anti‐inflammatory effects were induced by statins. Cultured primary human airway epithelial cells were exposed to ambient air pollution particulates (PM) to trigger the inflammation, with and without statins, and the expression of inflammatory mediators was analyzed.

Results

The authors found that expression of CCL5, CCL11, and IL13RA was suppressed in patients on statins. In vitro exposure to PM enhanced the expression of these mediators, while pretreatment with statins completely blocked these effects. Furthermore, the effects of statins were blocked by inhibition of the statin pathway using isopentenyl‐5‐pyrophosphate. Statins did not have any significant effect on the viability of normal cells.

Conclusion

Statins induce anti‐inflammatory effects in human airway epithelial inflammation. Statins may play a role in the treatment and prevention of chronic rhinosinusitis and pulmonary exacerbation of obstructive airway diseases.