Session:  Predictive Models for Fabric and Fabric-Reinforced Composites.

Session Description: Novel contributions are sought on models to predict physical properties (mechanical and other) of dry fabrics, coated fabrics, and fabric-reinforced composites. Fabrics include plain weave, twill, satin and others made of any type of fiber with practical application. Physical/mechanical modeling of filament, multifilament, strand, tow, roving, and other multifilament arrangements used in practice are welcomed. Physical/mechanical modeling of weaving, brading, stitching and other technologies used in practice are welcomed. Contributions about modeling of other related aspects will be considered.  Other sessions/topics can be found in the conference website.

Abstracts:

  1. Habib   Rai       University of Balamand             Deflection of Multilayered Composites Containing Wavy Fibers            Fiber waviness is a manufacturing defect found in composite materials. Wavy fibers have been observed to occur in, or through the thickness of, the plane of lamination. It is represented mathematically by sine wave function and quantified by the ration of amplitude to wavelength of fibers (A/L). The sinusoidal fiber orientation induces a change in the lamina elastic properties, which influence the laminate elastic extensional, coupling and bending stiffness.  Composite parts containing fiber waviness were suspected to initiate failure in the part.  It has been proven experimentally that composite containing wavy fibers have a reduced fatigue resistance, compressive strength, and stiffness.  The present research involves the development of mathematical model and numerical solutions to investigate the effect of the in-plane fiber waviness on the deflections of composite plates.  The theoretical frame work of research consists of coupling the model developed by Rai et al. which evaluate the lamina elastic properties as function of fiber waviness, with Ritz technique as well as finite difference method to evaluate the deflection of multilayered composite plates.  Percent reduction of maximum deflection versus fiber waviness is obtained by using both Ritz technique and finite difference method. An example is illustrated to substantiate the model predictions.  A clamped laminate containing fiber waviness subjected to transverse loading is analyzed and results for percent reduction of deflection versus fiber waviness are obtained.

  2. Cagri    Ayranci            University of Alberta     (780) 488-8841           Elastic Properties of Large-Open-Mesh Diamond-Braided Tubular Composites: Comparison of Predictive and Experimental Findings     Title: Elastic properties of large-open-mesh diamond-braided tubular composites: comparison of predictive and experimental findings, Abstract: Two-dimensional braiding is conventionally used in aerospace, automobile, reinforcement of concrete structures, and medical applications. Most of these applications require closed-mesh (tight mesh) braided structures, but a number of stiffness critical applications, such as braided medical tubular structures such as catheters, require large-open-mesh (loose mesh) braided structures. Properties of closed-mesh structures have been intensely studied; however, the published data on the open-mesh structures are very limited. A number of models that predict the elastic properties of large-open-mesh braids have been published, but verification of these models using experimental data is missing in the literature. Braided catheters are one specific example where open-mesh braided tubular composite materials are required. Open-mesh geometry of the braided catheters provides a means to tailor the elastic properties of catheters to specific requirements of the application. Reported work is an attempt to close the literature-gap in this area. Two-dimensional large-open-mesh diamond-braided tubular composites have been manufactured and tested to determine their elastic properties. The results have been compared to that of the predictive models developed and published over the years, and the findings have been discussed. 

  3. Chensong         Dong    Curtin University of Technology                        Statistical Modeling of Dimensional Variations of Composites          With the increasing requirements of energy efficiency and environment protection, composite materials have become an attractive alternative to traditional materials because of the advantages of low density, high strength, high stiffness to weight ratio, excellent durability, and design flexibility.  Despite all these advantages, composite materials have not been as widely used as expected because of the complexity and cost associated with the manufacturing process.  One of the main problems is poor dimensional control.  Dimensional variations lead to difficult assembly, costly rework, high waste rate, and structural failure, which limit the use of composites.  The dimensional variations are caused by a number of factors including CTE mismatch, curing shrinkage, and processing defects.  Previous research on dimensional variations was mainly focused on the deterministic factors.  Due to the existence of processing defects such as resin-rich zones, dimensional variations are not constant from part to part.  Hence, the influence of these part-to-part variations needs to be considered in the design of composites and assembly, i.e. tolerance analysis.  Currently, there is no known method for the tolerance analysis of composites.  This research aims to provide some fundamental understanding to the tolerance issues of composites.  The effective material properties of composite laminates were derived.  The resin-rich zones were identified and characterized statistically and the dimensional variations were modeled by finite element analysis incorporating these resin-rich zones.  The dimensional variations from FEA were analyzed statistically.  The significance of this research is that it provides the fundamentals of the tolerance analysis of composite components and assemblies, which is extremely useful for the realization of affordable tight tolerance composites.  It also provides the foundation of Integrated Product/Process Development (IPPD) and Design for Manufacturing/Assembly (DFM/DFA) for composites.

  4. Brice    Gnahore           West Virginia University                        Predictive Models of Thermal Conductivity for Flexible Aerogel Blankets         In this paper, simple theoretical and numerical models have been developed to predict the effective thermal conductivity of flexible aerogel blankets which consist of aerogel particles and microscale air-pockets dispersed in an isotropic matrix. We develop a simple conductivity model as a function of aerogel, air-pocket and matrix volume fractions.  The numerical study, the effective thermal conductivity of these composites is computed using the finite element software ANSYS. This numerical analysis of thermal conductivity was studied by generating 3D simulation models of the microstructure of the aerogel blanket materials at different aerogel to matrix volume ratios and various aerogel to matrix thermal conductivity ratios. These models show very interesting predictions concerning the effects of air-pocket and aerogel particles volume fraction. For low aerogel particles content (less than 10%), the results of the analytical and the numerical models show good agreement. At high aerogel particles content, there is a discrepancy between the theoretical and numerical models.

  5. Damien Durville CNRS / Ecole Centrale Paris                Finite Element Simulation of Textile Materials at Fiber Scale            In order to simulate the mechanical behaviour of fabrics or textile-reinforced materials at the scale of fibers, an approach based on the consideration of contact-friction interactions between individual fibers is presented. A finite element code, Multifil, is developped to implement this approach in a nonlinear framework. Considering few hundreds of fibers splitted into yarns, it allows to simulate first the weaving process in order to compute the initial configuration of various types of woven samples. An elastic matrix can then be added to the woven samples. The simulation of different loading cases is carried out to identify their nonlinear mechanical behaviour. Available results at the scale of fibers and their interactions are very useful to understand the global behaviour of dry fabrics and composite textile materials.

  6. Paul      Fazzino University of South Carolina                  Impedance Spectroscopy of Fabric Reinforced Composites            Current applications require an understanding of the relationships that exist between the functional characteristics of composite material systems and the long-term behavior of those materials under mechanical loading.  In the present work, Electrochemical Impedance Spectroscopy was used to characterize damage initiation and progression in woven glass/epoxy composites that were fatigued loaded out of plane.  A systematic approach was taken to establish a proof of concept that electrochemical impedance at different frequencies of excitation in woven glass/epoxy composites is related to the development of damage in that material under cyclic end-loaded bending.  Impedance spectroscopy measurements were found to show large, consistent, and clear distinctions as damage developed, and also proved capable of detecting non-visual localized damage in thin specimens.

  7. Ben      Bridgens           Newcastle University                A Predictive Model for Flexible Woven Composites     A predictive model has been developed to determine the biaxial stress-strain response of flexible woven composites, without the need for biaxial testing. Sawtooth and sinusoid models of the fabric unit cell have been formulated, with spring elements between crossovers used to represent the coating. In both models a constant yarn cross-sectional area has been maintained, resulting in a relationship between unit cell length and yarn thickness which eliminates the need to determine the yarn crushing stiffness. All model parameters are determined from standard tests: yarn and matrix tensile moduli are inferred from uniaxial strip tests, weave geometry is measured from digital photographs of the material cross-section. A state-of-the-art biaxial test rig and new test protocol have been developed to fully establish the stress-strain behaviour of flexible woven composites used for tensile roof structures. This enables meaningful comparison to be made between the model output and actual fabric response. The model is truly predictive: parameters are not optimised to fit the model output to a particular data-set. The model provides a more accurate representation of fabric behaviour than current industry best practice (i.e. use of elastic constants based on biaxial test data), but without the need for specialist testing, equipment or software. The model is currently being extended to include shear response, again using purpose built, innovative test equipment and test protocols. The model has been developed for architectural fabrics, but is equally applicable to other woven composites and could be applied to problems of feasible drape during lay-up. A key application is for reverse engineering of woven composites: appropriate fibre/yarn and matrix properties and weave geometry can be calculated to provide a material with optimal properties for a given application.

  8. Prasun  Majumdar        University of South Carolina      803-777-0183 Behavior of Woven Fabric Composites in Off-axis End-loaded Bending    Out of plane deformation behavior of fiber reinforced polymer composites is of interest in many applications requiring a thin structure for cost-effective design. However, bending of off-axis laminates can be highly nonlinear associated with a large strain and often difficult to fully characterize. Previous studies represented in-plane response (such as tensile loading) of off-axis laminates using elastic-plastic analysis and discrete damage mechanics. However, there is no existing model in the  literature  that  can  represent  the  whole  progressive  failure in off-axis bending  process  and the  nonlinear behavior up to specimen failure at a large strain value. The major objective of this paper is to develop a constitutive model for off-axis bending of woven fabric composites under end-loaded bending configuration. To collect necessary experimental data for model input as well as to validate the model, a plain weave glass fiber reinforced epoxy composite material has been chosen. The proposed model utilizes a single master curve to describe the out of plane deformation behavior over the full elastic-plastic range and therefore eliminates ambiguity associated with multiple curve fitting. This model can be used for determining residual strength and life prediction purposes. Detail about experimental results and modeling approaches are discussed in the paper.

  9. Seyed   Alavi-Soltani     Wichita State University            (316) 978-5613           Modeling the Gelation and Vitrification Points for Polymer Composites Using Shear Rheometry      This paper presents modeling of the gelation and vitrification points for two commercial carbon fiber prepregs (977-2 UD and E7K8 PW). First different definitions for the gelation and vitrification points were studied and based on each definition the rheological properties of the prepregs including the gelation and vitrification points were obtained using an Encapsulated Sample Rheometer (ESR). To find out the definition that best fits the gelation point for each prepreg, the frequency test was performed. Finally the gelation and vitrification points were modeled. The gelation and vitrification data and models can be used in the Time Temperature Transformation (TTT) diagram.  The TTT diagram and the associated models can be used as a cure map to provide a direct estimation of the material state during cure in order to resolve the issues resulting from uncertainties in composite processing.

  10. Seyed   Alavi-Soltani     Wichita State University            (316) 978-5613           Material State Modeling in Combined Ramp- Isothermal Cure Cycles of Polymer Composites Using Shear Rheometry and Thermal Analysis             This paper presents viscosity and degree of cure modeling of a commercial carbon fiber prepreg (977-2 UD) in combined ramp-isothermal cure cycles. Since the degree of cure (DOC) is an important variable in polymer composite viscosity modeling, first DOC for combined ramp-isothermal cure cycles was modeled. The experimental data for DOC modeling was obtained using a Differential Scanning Calorimeter (DSC) and the baseline issue of the DSC heat flow curves was addressed. Once the DOC was modeled and rheological properties of the prepreg were obtained using an Encapsulated Sample Rheometer (ESR), different viscosity models were introduced to the viscosity data. Finally the data and the models for both DOC and viscosity were compared. The results showed that the models closely followed the experimental data. The viscosity and degree of cure models can be used as a tool to provide a direct estimation of the material state during cure in order to resolve the issues resulting from uncertainties in composite processing.

  11. Seyed   Alavi-Soltani     Wichita State University            (316) 978-5613           Development of Time-Temperature-Sperposition and Phase Transition Diagram for Polymer Composites Using Shear Rheometer     This paper presents development of the Time-Temperature-Superposition (TTS) and Phase Transition diagram for a commercial carbon fiber prepreg (977-2 UD). The rheological properties including storage modulus, loss modulus and tanδ were obtained using an Encapsulated Sample Rheometer (ESR). These properties were combined using the well known Williams-Landel-Ferry (WLF) equation to develop the TTS Master curve and Phase Transition diagram. The TTS and Phase Transition diagram can be used as a cure map to provide a direct estimation of the material state during cure in order to resolve the issues resulting from uncertainties in composite processing.

  12. Adi       Adumitroaie      West Virginia University            304-293-3111 Modeling of Twill and Satin Woven Fabric Reinforced Composites A model is presented to predict the hygro-thermo-mechanical behavior, including strength, of planar woven fabric reinforced composites, of which twill, satin, and plain weave are particular cases. A novel nomenclature is proposed to allow unambiguous definition of weave patterns in a form that is amenable for computational representation. Novel mathematical formulae are proposed to describe univocally the geometry of the tows for general weave patterns such as twill and satin, in such a way that computation of various quantities of interest is possible and efficient. A comprehensive methodology is proposed, which is capable of predicting laminate properties such as moduli and strength from constituent (fiber and matrix) properties. Predictions are compared with experimental data.

  13. Hermann          Alcazar WVU   (304) 293 3111 x 2327            Plastic Damage Modeling for Unidrectional Composites      A new analytical model and associated software have been developed for predicting the mechanical behavior of a unidirectional composite in the presence of internal damage. The model uses an anisotropic associative continuum damage mechanics approach to account for the development of plastic strain in the material, as well as to consider changes in stiffness as a measure of mechanical damage resulting from possible failure mechanisms such as fiber failure, inter fiber failure, and delamination. The Tsai-Wu failure criterion is used as the main basis for the definition of plastic strain and damage surfaces. Experimental data obtained from an inter fiber load-unload cycle are used to define the isotropic evolution of plastic and mechanical damage, where the plastic and damage thresholds are obtained by using nonlinear extrapolation. The analytical model is implemented as a user defined material in the ASYS finite element software, where the return mapping algorithm is implemented by using the Newton’s method.

  14. Xavier  Martinez           West Virginia University            304 293 3111 x2306   Numerical Simulation of Welded Fabrics for Tensile Applications            With the increase in the number of stronger fabrics, obtained form the development of new fiber materials such as aramids or liquid crystal polymers, it has also increased the number of applications in which this strength is required. Coated fabric materials are found in large tensile structures such as stadium roofs and covers, inflatable buildings, sails, etc. In the development and design of all these applications, special attention has to be paid to the seams required to put together the different fabric parts. Seams in coated fabrics are usually welded by processes like radio frequency or hot air. Welded seams are usually made overlapping fabric parts, which increase the structure stiffness in the welded region and can modify the structural performance; besides, local effects can lead to local stresses that will reduce the overall strength of the structure. All these factors show the necessity to include an appropriate simulation of the welded regions in the numerical simulation of tensile structures. The present work presents an effective, and computationally efficient, approach to include different welded seam configurations in the finite element simulation of a tensile structure. The numerical results obtained are compared with experimental tests in order to validate the methodology proposed.

  15. Sam     Tucker University of Southern Mississippi         601-266-4016 Curing Kinetics of Aerospace Epoxy Resins by In Situ Variable Temperature NMR  Preliminary molecular dynamics simulation studies indicate von Mises strain parameters of composites are significantly influenced at the molecular level of the matrix network through a combined contribution of torsional and rotational segmental motions, bulky pendant side-group moieties, and cross-link density / network formation mechanisms.  These contributing factors may be controlled through chemical modifications or curing techniques that affect the vitrification process.  Molecular dynamics modeling has shown that isomer selection in aromatic amines influences critical strain parameters in composite matrices.  Relationships between geometry of the matrix and increased mobility of the polymer chains to explore conformational space to freely rotate, kink, and undergo torsional movements have been studied as energy dissipation mechanisms for distortional deformation.   Networks from a series of aromatic amine curatives have shown noteworthy changes in von Mises distortional deformation capability within glassy-networks and provide necessary fundamental data to validate and improve molecular dynamics models.  This work focuses specifically on the synthesis of 13C and 15N isotope-labeled amines, and the use of solid state variable temperature NMR to study cure in situ.

  16. Matthew           Jackson            Univeristy of Southern Mississippi                     Molecular Dynamics Simulation and Synthetic Experimentation Results for a Novel Triphenylamino Triazine Composite Matrix Curative        The incorporation of triazine rings into thermosetting polymers provides an opportunity to improve thermomechanical properties for polymer matrix composite resins.  The triazine ring is traditionally integrated into glassy network matrices through cycloaddition curing reactions of cyanate ester trimerization into triazine rings which become crosslink hubs of high-temperature glassy networks.  We used MD modeling techniques to simulate epoxy network properties cured with a novel triphenylamino triazine curative and, based upon favorable network thermomechanical property simulations for these networks we synthesized the novel monomer curative and evaluated DGEBA based networks for thermal and physical property enhancements.