bluuebunny/crossref_metadata_2025 huggingface dataset:
Consolidation and Interlaminar Fracture Properties of Unidirectional Commingled CF/PEEK Composites
Authors: Beehag Andrew , Ye Lin
Published: June 2014

The main objective of this study was to investigate the relationship between consolidation quality and interlaminar fracture of CF/PEEK composites manufactured from commingled prepreg. A compression molding system using a hot press was applied to simulate the effect of processing conditions, such as holding time (10, 30 and 60 min) and processing temperature (400'C and 415'C). Consolidation quality was studied through void content and density measurement, while transverse flexural (TF) properties of consolidated CF/PEEK composites were measured and correlated with the processing conditions. Mode-I and mode-II interlaminar fracture behavior was studied using double cantilever beam (DCB) and end notch flexure (ENF) specimens. The results indicated that the consolidation quality of the composites is highly dependent on processing conditions, and it was clearly improved at 400'C when the holding time was increased from 10 to 60 min. Material degradation was observed for the composite produced at 415'C, especially when a long holding time was applied. Mode-I interlaminar fracture growth resistance had little correlation with consolidation quality, while apparent mode-II interlaminar fracture toughness decreased as the void content was reduced. Fracture mechanisms associated with material microstructure are discussed. The results indicate that consolidation quality is not the only aspect that controls interlaminar fracture properties.
Die-Less Forming of Large and Variable-Radii of Curvature in Continuous-Fiber Thermoplastic-Matrix Composite Materials
Authors: Gertner Yosef , Miller Alan K.
Published: June 2014

Die-less forming is a method of forming complex shapes in continuousfiber thermoplastic-matrix laminates under only software control, i.e., without fixed dies. The "continuous" die-less forming concept for thermoplastic composite laminates has been extended to encompass (a) constant-curvature parts having curvature larger than the forming rollers, and (b) variable-curvature parts. Shape accuracies of 0.016" have been attained. Prior work with the same process indicates that aircraft quality (< 6 dB ultrasonic loss) can also be maintained throughout the operations. The key to good shape accuracy and quality is maintenance of a substantial (e.g., 25"C) temperature gradient from the main part of the forming zone to the tangent point where the material exits this zone. Manipulation of advective heat transport by proper selection of the forming velocity was effective in maintaining this temperature gradient. Key internal behavior which governs the shape accuracy includes adequate interply sliding within the forming zone and avoidance of plastic bending outside of it. This work contributes to methods for forming continuous-fiber thermoplastic composites to final net shapes without dies.
Processing Characteristics in Different Semi-Crystalline Thermoplastic Composites Using Process Simulated Laminate (PSL) Methodology
Authors: Deshpande Abhijit P. , Seferis James C.
Published: June 2014

Issues related to process-induced internal stresses are very important in processing semi-crystalline thermoplastic composites. Internal stresses have potential impact on the processing outcomes as well as the performance of composite materials. The effect of processing parameters on the factors leading to internal stresses was considered in this study. Morphological gradients and residual stress profiles in semi-crystalline thermoplastic composites were evaluated using the Process Simulated Laminate technique that has been applied to poly-ether-ether-ketone/carbon fiber (PEEK/CF). The thermoplastic composite materials investigated in this study were polyethylene-terephthalate/carbon fiber (PET/CF) and poly-ether-ketone-ketone/long discontinuous carbon fiber (PEKKLDFTM), in comparison with other material systems. It is shown that the Process Simulated Laminate technique can be used to examine the processing characteristics of polymeric composites and can provide a rational methodology for comparison of different material systems.
Recycling Carbon-Fiber-Reinforced Thermoplastic Composites
Authors: Schinner G. , Brandt J. , Richter H.
Published: June 2014

Advanced composite materials are being used more and more in structural applications, which raises the question of the disposal of this kind of material. In this paper two different possibilities for recycling carbon-fiber-reinforced thermoplastic composites are presented. One approach focuses on the grinding of thermoplastic composites in order to use the various grinding fractions as a high-quality reinforcing material in injection molds or as press molding compounds. The manufacturing processes investigated are suitable to reuse fiber-reinforced thermoplastic waste. The properties evaluated are on the same level as comparable injection molding materials. Another approach investigated for reusing thermoplastic carbon-fiber-reinforced composite parts is a reforming process. The reforming experiments performed did not change the material properties evaluated.
Effects of Water on a Graphite/Epoxy Composite
Authors: Zhou Jiming , Lucas James P.
Published: June 2014

The effects of water-induced physical and chemical property modification on a graphite/epoxy (GrEp) laminate were investigated. The laminate was consolidated using unidirectional 1300/934 graphite/epoxy prepreg. Specimens were conditioned, hydrothermally, by immersion in distilled water at 45, 60, 75 and 90'C for more than 9000 hours. The composite properties were characterized by evaluating the specimen's weight change due to water absorption and desorption, by hygrothermally induced expansion, and by water-induced change of the glass transition temperature (T,). Optical microscopy and scanning electron microscopy (SEM) were conducted to assess surface modification associated with resin cracking and resin mass loss. Water sorption in the Gr/Ep material exhibited both Fickian and non-Fickian diffusion behavior. Diffusion data showed that the time for the onset of non-Fickian behavior was inversely related to the increase in exposure temperature. Anomalous (i.e., non-Fickian) behavior in the composite resulted from both chemical modification and physical damage to the epoxy resin. Moisture-induced cracks, voids, and surface resin peeling were revealed by SEM and optical microscopy. Surface cracking and resin mass loss phenomena were used to describe water sorption behavior in composites. Thermomechanical analysis (TMA), water desorption, and infrared (IR) spectral results suggest that water retention in the composite can be characterized: (1) by water trapped at defects such as crack tips, interfaces, and voids; (2) by water interaction associated with weak hydrogen bonding within the resin; and (3) by water bonded to hydrophilic functional groups of the polymer chains.
Effects of Impact Modification on the Mechanical Properties of Wood-Fiber Thermoplastic Composites with High Impact Polypropylene (HIPP)
Authors: Park Byung-Dae , Balatinecz John J.
Published: June 2014

This paper reports on the effects of impact modification on the mechanical properties of wood-fiber thermoplastic composites with high-impact polypropylene (HIPP). Homo polypropylene (PP) was modified through its melt blending with HIPP to obtain an inherently tough matrix resin for PP/wood-fiber composites. The resin modification improved the impact strength of the composites, while it reduced the stiffness and strength properties acquired by the addition of wood-fibers. The requirements of plane strain fracture toughness were not satisfied in this study. In order to compensate the nonplane strain fracture toughness, the specimen strength ratio was used as a comparative measure of fracture toughness. The strength ratio depended on impact modifier levels as well as on wood-fiber concentrations. The work of fracture measured as the area under the load-displacement curve increased with impact modifier levels except at 40% wood-fiber concentration. Microstructural probing showed a degree of wood-fiber alignment in the skin and core of injection-molded specimens and provided some insight into the slight increase in the impact strength of composites.
Calculating Strain Energy Release Rate in Cracked Orthotropic Beams
Authors: Pandey R. K. , Sun C. T.
Published: June 2014

The total strain energy release rate of a homogeneous orthotropic cracked beam-like structure is calculated using Timoshenko's beam theory. The total strain energy release rate is modified by including the energy spent in the deformation of the beam near the crack tip. The contribution due to the root rotation is obtained by solving an approximately equivalent 2-D elasticity problem. By including the contributions due to the root rotations of the beam segments during crack extension, the modified beam model prediction of strain energy release rate is shown to agree very well with the 2-D finite element results.
The Design and Manufacture of a Transverse Floor Beam from Thermoplastic Composite Material-A Case Study
Authors: Walls K. O. , Crawford R. J.
Published: June 2014

In recent years the use of thermoplastic materials for primary aircraft structures has increased due to the benefits they offer in terms of reduced structural weight and improved payload. This work is a detailed assessment of the potential application of thermoplastic composites for a transverse floor beam. A number of beam sections were designed and manufactured from APC-2 material and subjected to a variety of tests. In all the areas investigated, significant developments have been made and important lessons learned.
Stress-Strain Relations of a Viscoelastic Composite Reinforced with Elliptic Cylinders
Authors: Li J. , Weng G. J.
Published: June 2014

The stress-strain behaviors of a two-phase composite comprising a viscoelastic matrix and elastic elliptic cylinders are examined by means of a micromechanical theory. The reinforcing cylinders are taken to be aligned along direction 1 but randomly oriented in the 2-3 plane, with a common aspect ratio a (the thickness-to-width ratio). The strain-rate sensitivity of the stress-strain curves of the transversely isotropic composite is investigated as a function of the cross-sectional aspect ratio of the elliptic cylinders, and it is found that when the aspect ratio is one-corresponding to the traditional aligned fibers -the reinforcement is the poorest and that when the aspect ratio approaches zero corresponding to long, thin ribbons it is the most effective. Other types of crosssectional aspect ratios all give rise to a stress-strain curve lying between these two. Except for the thin ribbon with a and axial tension for all, other types of reinforcement all exhibit a saturation stress under a constant strain rate and result in strong strain-rate sensitivity for the overall response of the system.
An Analysis for Cracks Parallel to the Interface of Bimaterial Systems
Authors: Wang Wen-Xue , Takao Yoshihiro
Published: June 2014

This paper solves a plane strain problem for two bonded dissimilar planes containing two cracks parallel to the interface. The bimaterial system is loaded by tractions distributed along the crack surfaces. Based on the Fourier transform, the problem is reduced to a system of Cauchy-type singular integral equations that contain exact and explicit kernel functions. The solution of these equations are obtained easily by utilizing Gauss-Chebyshev formulae for various material combinations and geometrical parameters. Several numerical examples of stress intensity factors, energy release rates, and crack opening displacement are presented to exhibit the interactions among the interface and cracks.
Mechanical Deformation Mechanisms in Knitted Fabric Composites
Authors: Kelay M. S. , Bader D. L. , Reed P. E.
Published: June 2014

Knitted fabric composites have certain advantages over woven composites, particularly in their ability to conform to complicated contours. As a consequence, however, they demonstrate inferior mechanical characteristics compared to woven materials as a direct result of the presence of bent fibers. Such a knitted fabric composite made from, for example, glass fibers in a polyurethane matrix, can be used as an orthopaedic splinting bandage for immobilizing fractures of the upper and lower limbs.

Relatively little research has been reported on knitted fabric composites; therefore, a study on the mechanical deformation mechanisms of knitted fabric composites was initiated. It was observed that knit patterns, type of fiber, size of fibers used, size of loops, coatings and lay-up procedure were all variables that could affect the structure/property relationship of knitted fabric composites. Tensile testing with optical measurement of strain was performed on knitted substrate and coated bandages in both the course and wale directions. Results indicated that the knitted fabrics function as link mechanisms at the microscopic level, with knitted loops straightening and bending before the individual elements of the knitted yarn take up significant load and material deformation. Theoretical modeling of the glass knit structure, in both the course and wale directions, agrees well with experimental testing.
The Effect of Polyamic Acid Binder Concentration on the Processability and Properties of LaRC TPI Composites Made by Suspension Prepregging
Authors: Gonzalez-Ibarra A. , Davis R. M. , Heisey C. L. , Wightman J. P. , Lesko J. J.
Published: June 2014

Unidirectional carbon fiber/LaRC TPI composites were prepared by aqueous suspension prepregging. The matrix material was fully imidized LaRC TPI powder, and the binder was LaRC TPI polyamic acid. The effect of the concentration of the polyamic acid binder on composite processability and properties was studied. An optimum concentration range of 2.9-9.4% weight/weight binder/matrix was found that resulted in good prepreg handling characteristics and good consolidation. A binder concentration 2 13.1% led to poorer consolidation of the composites due to an increase in the melt viscosity of the matrix. This increase was probably due to a combination of crosslinking and chain extension of the polyamic acid during the imidization step. Formation of the crosslinked structure was supported by transverse flexure, meso-indentation, and solubility tests.
Influence of Interfacial Debonding and Free Edges upon Compressive Strength of Unidirectional FRP Composites
Authors: Zhang Guigen , Latour Robert A.
Published: June 2014

The uniaxial compressive behavior of fiber-reinforced polymer (FRP) composites is investigated in this paper. In the present study, the analytical model developed by the authors in previous studies is applied to investigate the effect of free edges and interfacial debonding upon the compressive strength of unidirectional composites. Results show that interfacial debonding has a great effect on the compressive strength, with fiber microbuckling strength falling to zero for the extreme case of complete fiber/matrix debonding with a frictionless interface. It is also found that free edges significantly affect the microbuckling behavior of unidirectional FRP composites by lowering the compressive strength of fibers located along a free edge as compared to that of inner fibers. Furthermore, the model indicates that the compressive strength of surface fibers can be strengthened by increasing the thickness of the outer matrix coating to the extent of theoretically being twice as high as the inner fibers. This finding may have significant practical implications for the strengthening of composite structures under compression by suppressing kink band initiation at free edges of the structure.
Effect of an Interlaminar Adhesive on the Yielding Behavior of Aluminum/Fiber-Reinforced Thermoplastic Composite Laminates
Authors: Sun W.
Published: June 2014

An adhesive stress-release model is proposed to predict the relief of thermal residual stress due to the presence of an adhesive layer in hybrid aluminum/fiberreinforced laminates. The model is developed based on the thermal curing temperature and composite constituent properties and is used to determine the laminate initial yielding and deformation behavior. The adhesive layer functions as a compliant layer to release the thermal residual stresses and to delay the yield occurrence, thus enhancing the laminate's deformation behavior. Compared with available experimental data for graphite/PPS and glass/PPS with 2024-T3 or 7075-T6 aluminum hybrid thermoplastic laminates, the proposed model has better predictive capabilities than some other commonly used models.
Experimental Analysis and Process Window Development for Continuous Filament-Wound APC-2
Authors: James D. L. , Black W. Z.
Published: June 2014

A small-scale filament winding apparatus was built so that surface temperature distributions could be determined during actual filament winding runs and the resulting temperatures could be compared to those predicted by a numerical model. Several experimental runs were performed using various combinations of infrared energy input and prepreg tape velocities. Infrared thermography was used to determine the surface temperature of the composite during each experiment. The field of view of the infrared camera included approximately 90 degrees of the mandrel in the angular direction and two-thirds of the axial length of the composite on the mandrel.

The numerical and experimental results in this paper suggest that varying the position of the infrared lamp, changing the prepreg tape velocity (winding speed), or the heat flux incident on the composite have a major impact on the temperature distribution throughout the composite; these factors may therefore be used to alter the physical properties and overall quality of a filament-wound part. Improved consolidation will likely occur if a low-power infrared lamp is used to preheat the prepreg tape prior to contact with the mandrel and another infrared preheater is focused on the mandrel at an angular location prior to the tape-mandrel interface. The majority of the composite heating should be achieved by a high-power lamp located such that the energy is focused on the mandrel in the vicinity of the prepreg tape-mandrel interface but displaced in the direction opposite the axial velocity of the tape. Therefore, the location of the lamp must move relative to the tape when the crosshead feed mechanism changes directions. By displacing the axial location of the lamp slightly downstream (in the direction of motion of the crosshead feed mechanism) of the tape-mandrel interface, preheating of the composite on the mandrel occurs prior to the tape being laid on the mandrel.

The numerical model is used to identify processing windows that lead to a wellconsolidated part. The processing window is defined by upper temperature limits that will damage the part and lower temperature limits that will leave the part unconsolidated. The temperature limits are selected from a knowledge of the autohesive characteristics of APC-2 (complete intimate contact and void-free prepreg tape are assumed). The model is then used to identify winding conditions such as infrared energy input, winding speed, and composite substrate thickness that result in a consolidated product.
The Wear of a Composite of High-Modulus HDPE Fibers in a Matrix of LDPE in Unidirectional Motion against 316L Steel
Authors: Tepupatump S. , Atkinson J. R.
Published: June 2014

Injection molding has been used to produce a composite of short, randomly dispersed, high-modulus HDPE fibers in an LDPE matrix. With a fiber loading of 10 wt% the wear coefficient of the composite rubbing against smooth 316L steel on a tri-pin-on-disc machine is lower by nearly an order of magnitude than that of LDPE. There is a reduction in the tensile creep rate of the composite compared with LDPE and modest improvements in strength and modulus.
Composites from Commingled Polyethylene Terephthalate, High-Density Polyethylene and Newspaper Fibers
Authors: Hon David N.-S. , Buhion Caroline J.
Published: June 2014

In this investigation it was demonstrated that composites made from commingled polyethylene terephthalate (PET), high-density polyethylene (HDPE), and newspaper fiber showed incompatibility among all components. Statistical analysis revealed a significant contribution of HDPE and fiber to energy consumption; HDPE, PET and fiber had disparate influences on tensile strength. HDPE was significantly low at the 99% confidence level, and fiber was significantly higher at the 5% confidence level. The three individual components had significant effects on Young's modulus at the 99% confidence level. However, the interaction of the three components had no significant effect on tensile strength or modulus. Scanning electron microscopic (SEM) analysis revealed the appropriate dispersion of PET or fiber in the HDPE matrix and poor interfacial adhesion. Methods of improving the interfacial interaction are discussed.
A Comparative Study on the Impact Resistance of Composite Laminates and Sandwich Panels
Authors: Ferri Rocco , Sankar Bhavani V.
Published: June 2014

An experimental study was conducted to compare the damage resistance and energy-absorbing capabilities of graphite/bismaleimide composite laminates and foam-core sandwich panels. Two types of tests were conducted: static indentation and drop weight impact. The load-displacement response and energy absorbed were recorded for quasi-static and impact tests. The amount of core compression in sandwich panels was also measured. Damage in the laminates and sandwich panels was assessed by using ultrasonic C-scan, X-radiography, and photo-micrography. The load-displacement responses and damage area obtained from quasi-static and impact testing matched very well for the laminates. The quasi-static and impact responses for the sandwich panels have the same overall trend but differ due to the viscoelastic properties of the foam core. Test results show that the sandwich panels can absorb more energy than the laminates and undergo less deflection to absorb the same maximum energy as the laminates. Impact energy above a statically determined threshold level has been shown to cause extensive foam core damage.
High-Performance Glass-Fiber-Reinforced Thermoplastic Composites Made of Textile Prepregs
Authors: Karhe J. , Scherieble G. , Ehrler P.
Published: June 2014

High-performance composites were made of knitted fabrics using weft insertion. The stitch was formed by means of thermoplastic yarns (PA6 or PETP); the weft yarns were E-or S2-glass yarns. These fabrics were very flexible and well-suited to produce doubly curved components. Then eight layers of these fabrics were pressed. At that point, the glass yarn reinforcement was arranged unidirectionally. At a proportion of 46 volume-% of E-or S2-glass-with PETP as the matrix-ILS-values from 60-70 MPa could be achieved. The corresponding values of breaking strength were about 900 MPa. They could be achieved by means of S2-glass yarn as the reinforcement component.
A Modified Layer Removal Analysis for the Determination of Internal Stresses in Polymer Composites
Authors: Eupe M. P. I. M. , Powell P. C.
Published: June 2014

The layer removal method is often used for measurement of internal stresses in homogeneous polymer materials. In order to extend the use of the method to polymer composites, certain refinements are needed. These include (1) use of varying mechanical properties (elastic moduli) through the thickness of the composite plate and (2) measurement of not only curvatures but also strains. These refinements are necessary because a nonsymnetric laminate is created when layers are removed. The modified layer removal method was applied to PEI-glass fiber composite cross-ply laminates. The average internal stresses measured within a layer of constant fiber orientation agree well with stress levels predicted by Classical Lamination Theory. The computed stress levels fulfill the condition of stress equilibrium, indicating that the procedure applied in the measurements is correct.
Thermoviscoelastic Analysis of the Thermoplastic Composite Tape Placement Process
Authors: Sonmez Fazil O. , Hahn H. Thomas
Published: June 2014

The thermomechanical behavior of a thermoplastic composite laminate during the tape placement process has been studied. Stresses induced by the compaction roller are predicted using a finite element method. These stresses control the compaction and bonding behavior of the laminate. In the analysis, the tape placement process is considered as a quasi-steady state rolling contact problem. The effect of temperature on creep compliance is included by treating the composite as a thermorheologicaliy simple material. Thermal expansion coefficients are allowed to be temperature dependent. The effect of friction at the contact surface is also taken into account.

The numerical results show how the process parameters (e.g., roller velocity, roller pressure, heat input) are related to temperature and stress distributions within the composite. These relationships can be used to predict the bonding behavior of the laminate.
Factors Affecting Adhesion at Copper-Polyamide 11 and Aluminum-Polyamide 11 Interfaces
Authors: Gundjian M. , Fisa B.
Published: June 2014

Adhesion at polyamide 11-copper and polyamide 11-aluminum interfaces was studied. Metal-polymer-metal laminates were prepared by compression molding using processing conditions similar to the normal melt processing parameters of polyamide 11. The results show that the time of contact at the molding temperature required to reach contact level of adhesion is significant (several minutes). The state of the metal surface affects adhesion. Mild oxidation (prior to molding) improves adhesion with aluminum; with copper, a monotonous slow decrease of adhesion with oxidation time is observed. The presence of the metal surface affects the crystallization behavior of polyamide. Upon a relatively slow cooling rate (40-50'C/min), a transcrystalline layer about 15 1Lm thick is formed having a degree of crystallinity almost 10% higher than the material away from the interface. In the absence of the transcrystalline layer, the level of adhesion is significantly lower.
Fiber Orientation Morphological Layers in Injection Molded Long Fiber Reinforced Thermoplastics
Authors: Skourlis T. P. , Chassapis C. , Manoochehri S.
Published: June 2014

Flow-induced fiber orientation can vary significantly across the thickness of injection molded parts and is usually present in the form of through-thickness layers. In this work, fiber orientation was examined both on the flow plane and through the thickness. The thickness of a shell layer with fiber orientation in the flow direction and a core layer with a fiber orientation mainly transverse to the flow were measured via microscopy. The effect of filling speed and mold thickness on the molded plaque morphology was identified. Mold-filling simulations were undertaken in order to provide the characteristic shell and core layer fiber orientations as well as the frozen layer thickness at the end of filling and the gap-wise shear rate gradients. The combined frozen and high shear rate regions were correlated with the experimental shell layer thicknesses.
The Effects of Temperature and Moisture Exposure on the Properties of Wood-Fiber Thermoplastic Composites
Authors: Balatinecz John J. , Park Byung-Dae
Published: June 2014

Wood-fiber thermoplastic composites were prepared with polypropylene (PP) and high-density polyethylene (HDPE) matrix resins employing high shear thermokinetic compounding for fiber dispersion. Wood fibers obtained from recycled newspapers were mixed with the matrix polymers at a level of 30% by weight. In one experiment, sets of six samples were immersed in boiling water for up to 48 hours and subsequently tested for mechanical properties and water absorption. In another experiment, samples were exposed to -40'C, -20'C, 00C, 230C, 40'C and 60'C for two hours and then tested at the same temperatures for tensile and flexural properties. Results showed that immersion in boiling water resulted in water absorption of between 3 and 5%, a decrease in tensile and flexural properties, and an increase in impact strength. When the composites were exposed to various temperatures, both strength and modulus decreased significantly with increasing temperatures above ambient (23°C) level for both PPand HDPE-based composites. The opposite trend was evidenced below freezing.
Dynamic Mechanical Behavior and Interphase Adhesion of Thermoplastic (PEEK, PES) Short Fiber Composites
Authors: Sarasua J. R. , Pouyet J.
Published: June 2014

The dynamic mechanical behavior of four thermoplastic composite systems is investigated between 50 and 2500C. Poly(etheretherketone) (PEEK) and poly(ethersul-fone) (PES) are reinforced with 0-30 wt.% short glass (GF) and carbon fibers (CF) and then microstructurally characterized. Fiber length distributions and orientation in the injection molded samples are determined by quantitative optical microscopy and scanning electron microscopy (SEM) respectively. The mechanical damping (tan b) and storage modulus (E') are studied for a given microstructure in the context of the properties of the various constituents of composites. The quantitative analysis of the tan 6 depression of composites at the glass transition temperature (Tg) is interpreted in terms of fiber-matrix interphase adhesion. This study reveals that, in contrast to carbon fibers, which are seen to adhere better to PEEK, glass fibers adhere better to PES. SEM observations of fractured surfaces of composites corroborate the estimations about fiber-matrix adhesion obtained by Dynamic Mechanical Thermal Analysis (DMTA).
Manufacturing of Thermoplastic Composites from Commingled Yarns-A Review
Authors: Svensson N. , Shishoo R. , Gilchrist M.
Published: June 2014

The use of commingled yarns is one of the more promising routes for producing structural thermoplastic composites. The textile processes available enable faster manufacturing and tailoring of the fiber architecture of preforms. Development of this technology is pushed by significant interest from, for example, the transportation industry. This paper reviews work done on commingled materials, including yarn manufacturing and preforming, modeling of impregnation and consolidation, and mechanical properties of the composites.
Delamination Crack Tip Behavior at Failure in Composite Laminates under Mode I Loading
Authors: Paris Isabelle , Poursartip Anoush
Published: June 2014

A technique is presented for measuring the local crack tip displacement fields and the global applied parameters during mode I loading of delaminations in composite laminates up to and including failure. To achieve this objective, an instrumented loading set-up has been designed to fit inside the chamber of a scanning electron microscope (SEM).

With this technique, the SEM images of loaded specimens are analyzed to obtain the crack opening displacements (COD) as a function of distance from the crack tip. For mode I tests conducted on AS4/3501-6 unidirectional material, good agreement is obtained between the Linear Elastic Fracture Mechanics (LEFM) calculations and the measurements, in terms of both general shape and magnitude. The results show that the COD increases linearly with the applied displacement up to failure.

Thus, for the case of a brittle material under mode I loading, there is good agreement between the measured and calculated crack tip displacements, and the material behaves in a linear elastic way up to failure. By extending the technique to mode II and mixed-mode loading and by testing other tougher materials in the future, we hope to increase our understanding of mixed-mode failure criteria, which play an important role in the detailed analysis of complex composite structures under realistic loading.
Interlaminar Shear Strength and Fracture Behavior in Aged Composite Laminates
Authors: Tao J. , Sun C. T. , Arendt C. , Brunner M.
Published: June 2014

An experimental procedure for measuring interlaminar shear strength and fracture toughness in composite laminates was proposed. The procedure was used to determine the interlaminar properties of a quasi-isotropic laminate of IM7/K3B thermoplastic composite that had undergone thermal or thermo-mechanical aging. A five-point bending device and the ENF fixture were used to determine the interlaminar shear strength and interlaminar toughness, respectively. The aging effects on these two properties of IM7/K3B under various aging conditions were evaluated. It was found that the aging effect on interlaminar toughness is similar to that on interlaminar shear strength.
Integral Fit Joint Technologies for Composites
Authors: Lee David E. , Hahn H. Thomas
Published: June 2014

Seeking to expand the varieties of joining approaches for polymer-matrix composites beyond discrete fasteners as well as adhesive and non-adhesive bonds, novel assembly joints that are integral to the design and fabrication of individual components are presented. Grouped under the title of integral fit joint technologies, prior approaches to eliminating fasteners, avoiding adhesives, or bonding components by ultrasonic or other energy-augmented methods are identified and illustrated. By building upon prior efforts to apply integral fit joints in areas such as snap fits, electronic assemblies, wooden joint structures, and plastic building blocks, potential uses of thesejoining technologies in composites can be identified and evaluated. Application of integral fit joint technologies in composites requires a fundamental understanding of the strengths and weaknesses of composite material systems, component fabrication processes, the forces acting on joints as well as methods for assembling composite components together. A composite snap joint technology developed for high voltage transmission towers serves as an example in demonstrating the potential for using integral fit joints in composite structures.
Damage Mechanics Modeling and Analysis for a Hydrothermally Aged Composite
Authors: Morii Tohru , Yokoyama Atsushi , Hamada Hiroyuki
Published: June 2014

The elastic modulus of a randomly oriented glass-fliber reinforced plastic (GFRP) after hydrothermal aging was estimated using the finite element method and a damage mechanics model. Fiber-matrix debonding of the hydrothermally aged GFRP was assumed as the internal damage, and then the damage mechanics was introduced to the finite element analysis. The damage angle around the fiber greatly affected the modulus reduction of the aged GFRP and was independent of the damage thickness. The internal damage of the aged GFRP was determined quantitatively by the weight loss due to water immersion, and the modulus of the aged GFRP was calculated by the finite element analysis introducing the damage parameter. The calculated results correspond well to the experimental results, and it was analytically shown that the modulus reduction of the aged GFRP was caused by fiber-matrix debonding.
Damage Progression in Bolted Composites
Authors: Minnetyan Levon , Chamis Christos C. , Gotsist Pascal K.
Published: June 2014

Structural durability, damage tolerance, and progressive fracture characteristics of bolted graphite/epoxy composite laminates are evaluated via computational simulation. Constituent material properties and stress and strain limits are scaled up to the structure level to evaluate the overall damage and fracture propagation for bolted composites. Single and double bolted composite specimens with various widths and bolt spacings are evaluated. The effect of bolt spacing is investigated with regard to the structural durability of a bolted joint. Damage initiation, growth, accumulation, and propagation to fracture are included in the simulations. Results show the damage progression sequence and structural fracture resistance during different degradation stages. A procedure is outlined for the use of computational simulation data in the assessment of damage tolerance, determination of sensitive parameters affecting fracture, and interpretation of experimental results with insight for design decisions.
Stresses Around a Biaxially Loaded Fastener Hole in a Laminate of Finite Geometry
Authors: Xiong Y. , Poon C.
Published: June 2014

The stress analysis of a biaxially loaded fastener hole in a laminate of finite width has been conducted using a complex variational approach developed at the Institute for Aerospace Research. The formulation of the complex variational approach developed is described in this paper. In this approach, an infinitely stiff fastener is assumed to be in contact with one half of the hole edge. The fastener load is in equilibrium with the applied loads along the exterior edges, and a cosine distribution over the contact region is assumed. The stresses around a single fastener hole under biaxial loading conditions were determined. Various anisotropic laminates were analyzed. The effects of the biaxial loading ratios on the joint performance were examined. Results from the analytical approach are presented and discussed in this paper.
An Analysis of Mechanisms Governing Fusion Bonding of Thermoplastic Composites
Authors: Butler Christine A. , Mccullough Roy L. , Pitchumani Ranga , Gillespie John W.
Published: June 2014

A number of mechanisms have been proposed in the literature as contributors to the strength development at the polymer-polymer interface during fusion bonding of thermoplastic composites. Of these, healing and intimate contact emerge as fundamental mechanisms governing bonding. Intimate contact refers to the development of the amount of surface area that is physically contacted at the interface at any time, and healing describes the migration of polymer chains across the interface in intimate contact.

This work provides a new theoretical development of a coupled bonding model that accounts for variability in initiation time for healing due to growth in the area in intimate contact. The generalized coupled bonding model is valid for any set of processing conditions and reduces to the proper controlling mechanism as dictated by the process. Analysis revealed a key dimensionless group, Q, that captures the coupled nature of the mechanisms governing fusion bonding. By evaluating Q, which is a function of material and process parameters, one can determine the relative contributions of each mechanism.

Experimental validation of the coupled model using two different processes, tow placement and resistance welding, is also presented. An evaluation of Q for the tow-placement process indicates that both mechanisms are controlling. For this case, the coupled model demonstrates better strength predictions than the conventional healing model alone. In contrast, the resistance welding process is shown to be intimate-contact controlled, in which case the coupled model reduces to a more simplified model. The ability to rigorously determine the controlling mechanisms is of critical importance to accurately model the strength development during fusion bonding processes.
Effects of Thermal Aging on the Mechanical Behavior of K3B Matrix Material
Authors: Sacks Serena , Johnson W. Steven
Published: June 2014

In this study, the timeand temperature-dependent behavior of unaged as well as aged samples of neat K3B (a thermoplastic polyimide made by Du Pont) has been characterized. The aging temperature was 1770C (350'F), and aging times are 5,000 hours and 10,000 hours. Testing temperatures were room temperature, 930C, 1350C, and 1770C (200'F, 2750F, and 350'F). A combination of monotonic tensile tests and stress relaxation provided the necessary information. Notch bend tests were conducted to determine fracture toughness. Neat K3B has been found to be very brittle and prone to defects. Aging for both 5,000 and 10,000 hours resulted in much lower failure strains, especially at elevated temperatures. The time dependence exhibited, especially after aging, may not be enough to affect correlations to composite behavior, and a time-dependent approach may not be necessary. FTIR data shows some slight changes in peak size, so a change in the structure of K3B cannot be ruled out. DSC data shows a very slight (negligible) change in the glass transition and no melting point (as expected). The polymer molecules are approaching equilibrium during aging (physical aging), but crosslinking has not been ruled out.

The main goal of this study was to gather more information on the timeand temperaturedependent behavior of K3B. This has been accomplished by first characterizing the viscoplastic behavior of neat resin samples after aging for long periods of time at elevated temperatures. The data gathered was used to determine the effects of thermal aging on the mechanical behavior of K3B. The work being discussed presently focuses primarily on the neat matrix material.
Meso-Mechanics and Meso-Structures: A Matter of Scale
Authors: Chamis Christos C. , Gotsis P. K. , Mital S. K.
Published: June 2014

Meso-mechanics and meso-structures are described in terms of the scales at which they are observed and formulated. Select composite examples are presented to illustrate that meso-mechanics and/or meso-structures are meaningful only when they refer to a specific scale in a hierarchical scale observation/simulation. These examples include different types of composite unit cells, woven fabric unit cells, and progressive fracture as a composite enhanced infrastructure made from reinforced concrete. The results from the select examples indicate that meso-mechanics and meso-structures are elusive terms and depend mainly on the investigators' knowledge and available information.
Recent Developments Encompassing Non-Isothermal/Isothermal Liquid Composite Molding Process Modeling/Analysis: Physically Accurate, Computationally Effective, and Affordable Simulations and Validations
Authors: Ngo N. D. , Mohan R. V. , Chung P. W. , Tamma K. K. , Shires D. R.
Published: June 2014

The mathematical and associated computational modeling and analysis of mold filling, heat transfer, and polymerization reaction kinetics in Resin Transfer Molding (RTM) are quite complex and not only require accurate computational approaches to capture the process physics during the simulations, but also must permit complex geometric configurations to be effectively analyzed. The process simulations at a macroscopic level require the representative macroscopic constitutive behavior which can be predicted from a microscopic analysis of the representative volume element (RVE) of the fiber preform configurations. This is first presented here for purposes of illustration in reference to determination of the preform flow permeabilities. Next, an effective integrated micro/macro approach and developments including a viable flow solution modeling and analysis methodology with emphasis on providing improved physical accuracy of solutions and computational advantages are described for the transient flow progression inside a mold cavity filled with a fiber preform under isothennal and non-isothermal flow conditions. The improved physical accuracy and the overall effectiveness of the new computational developments for realistic process modeling simulations are first demonstrated for isothermal conditions. Subsequently, the new integrated flow/ thermal methodology and developments are extended for non-isothermal conditions. The highly advective nature of the non-isothermal conditions involving thermal and polymerization reactions also require special numerical considerations and stabilization techniques and are also addressed here. Finally, validations and comparisons are presented with available analytical and experimental results whenever feasible. Emphasis is also placed upon demonstrations for practical engineering problems.
Intelligent Control of the Thermoplastic Composite Tow-Placement Process
Authors: Heider Dirk , Piovoso M. J. , Gillespie J. W.
Published: June 2014

Model-based control in the manufacturing of advanced composite materials is one approach to improve the overall quality and performance of the final part. Frequently, quantitative nondestructive methods are not available for feedback control, and therefore a predictive controller is required. This study investigates a neural-network-based control system for the automated thermoplastic composite tow-placement process. This non-autoclave process incorporates an advanced controller that predicts desired part quality at minimal cost. The control system combines a neural network (NN) based model. numerical and neural network optimization, and an infrared thermal imaging camera to sense the temperature profile on the part surface. The optimization utilizes the NN process model to calculate the optimum inputs for the desired quality, whereas the image camera is used for temperature feedback control.
Free Vibration of Thin-Walled Closed-Section Composite Beams with Optimum and Near-Optimum Coupling
Authors: Dancila D. Stefan , Armanios Erian A. , Lentz W. Karl
Published: June 2014

An investigation of the free vibration characteristics for a set of extensiontwist coupled thin-walled single-cell composite beams with optimal and near-optimal coupling is performed using an analytical model, a finite element simulation, and testing. It is found that, despite the relatively large beam slenderness ratio, the applicability of the analytical model is limited to the first bending modes, for which excellent agreement with the finite element and experimental results is obtained. For analytically predicted higher order modes, increasingly significant in-plane warping is shown by the corresponding finite element results, and an increasingly large variance is observed between the two sets of results. The occurrence of shell modes that cannot be captured by the analytical model is also shown by the finite element results. Test data confirm finite element calculations as well as low modes predicted by the analytical method. The sensitivity of natural frequencies to lay-up perturbation angle is established over a frequency range of interest, providing insight into the robustness of the modal characteristics for the global and the local optima, respectively.
Reduction of Fiber Pull-Out Stress Due to Hot Water Immersion: Carbon/PEEK
Authors: Chen P. , Piggott M. R.
Published: June 2014

Single fiber pull-out tests have been carried out to investigate the effect of hot water immersion on the carbon fiber/PEEK interface. Although this interface appears to be quite strong and very durable, both the interface and the polymer were slightly weakened and softened by water immersion at 90'C. The interface weakening effect was noticeable after I h at 90'C, and there was further slight weakening with prolonged immersion up to 3000 h, with a maximum loss in debonding force of about 33%. For immersion at 60'C the effect was significant only after 1000 h. Polymer softening was noticeable after 1000 h at both 60'C and 90'C. Post debonding friction was also reduced by about 50%. Drying out at 90'C for 360 h did not restore the interface strength or friction, but there was some restoration of the polymer yield stress.
Manufacturing and Property Analysis of a Novel Class of 3-D Woven Composites
Authors: Yushanov S. P. , Bogdanovich A. E. , Mohamed M. H.
Published: June 2014

Structural and elastic properties of a novel class of 3-D woven orthogonally reinforced composites are studied. Comparative analysis of various theoretical approaches for elastic properties prediction of spatially reinforced composites is carried out. Theoretical predictions are compared with available experimental data. The effect of local yarn waviness on the elastic response of 3-D woven composites is studied using developed novel probabilistic theory.