Yasser Gowayed

A Self-Consistent Fabric Geometry Model: Modification and Application of a Fabric Geometry Model to Predict the Elastic Properties of Textile Composites

A method for predicting the elastic properties of textile-reinforced composites is presented with applications. The method is a modification of a Fabric Geometry Model (FGM) [1–3] that relates fiber architecture and material properties of textile-reinforced composites to its global stiffness matrix through micromechanics and stiffness averaging technique. The FGM, although proven to be a quick and successful method [4], suffers two major drawbacks: 1. incompatibility of the basic transverse isotropy assumption with the theoretical mathematical derivation, (i.e., the mathematical derivation produces elastic constants that do not exhibit transverse isotropy) and 2. inconsistency of the transformation matrices associated with the stiffness calculations (i.e., the technique is not sufficiently robust to handle all cases). In this paper, these problems are discussed and solutions are presented. Comparison between stiffness and compliance averaging approaches is investigated. Moreover, predictions using the Self-Consistent FGM are compared with experimental data available in literature.

Mechanical properties of triaxially braided composites - Experimental and analytical results

This paper investigates the unnotched tensile properties of two-dimensional (2-D) triaxial braid-reinforced composites from both an experimental and an analytical viewpoint. The materials are graphite fibers in an epoxy matrix. Three different reinforcing fiber architectures were considered. Specimens were cut from resin transfer molded composite panels made from each briad. There were considerable differences in the observed elastic constants from different size strain gage and extensometer readings. Larger strain gages gave more consistent results and correlated better with the extensometer readings. Experimental strains correlated reasonably well with analytical predictions in the longitudinal, 0°, fiber direction but not in the transverse direction. Tensile strength results were not always predictable even in reinforcing directions. Minor changes in braid geometry led to disproportionate strength variations. The unit cell structure of the triaxial braid was discussed with the assistance of computer analysis of the microgeometry. Photomicrographs of braid geometry were used to improve upon the computer graphics representations of unit cells. These unit cells were used to predict the elastic moduli with various degrees of sophistication. The simple and the complex analyses were generally in agreement, but none adequately matched the experimental results for all the braids.

Detection and Classification of Defects in Knitted Fabric Structures

A new method for knitted fabric defect detection and classification using image analysis and neural networks is presented. Images of six different induced defects were obtained and used in the analysis. Statistical procedures and Fourier Transforms were utilized as two different approaches in the feature extraction effort and neural networks were used to detect and classify the defects. The results showed success in detection and classification of most defects especially when the Fourier transforms technique was utilized.

Mechanical Behavior of Circular Hybrid Braids Under Tensile Loads

In this work, the mechanical behavior of hybrid circular braids without a core under tensile loads is studied experimentally and analytically. A predictive model of the mechanical response of the braids based on the constituent yarn characteristics and machine parameters is developed. The structural geometry of the fabric is analyzed and used to build the mechanical model. Image analysis is employed to experimentally characterize the structural parameters of the braids, their deformation, and the braid and crimp angles of the yarps. Hybrid braids with two different yarn systems are manufactured at various braid angles. The structures are tested in tension and their strain response is recorded. Experimental results are compared to theoretical values, and the model predic tions have good accuracy. In the case of high braid angles, the theoretical model underestimates the strength of the fabric.

Thermal Conductivity of Textile Composites with Arbitrary Preform Structures

In this paper, a model is constructed to predict the thermal conductivity of textile composite materials under steady state heat transfer conditions. First, the composite under consideration is geometrically characterized to identify relative volume fraction and spatial orientation of each yarn along its path. This is followed by applying finite element analysis (FEA) and virtual work to a “unit cell” of the textile composite. Hybrid hexahedra brick elements with fibers and matrix around each integration point are employed in the finite element formulation by means of micro-level homogenization. Thermal conductivity predictions using this approach are found to agree with experimental results for polymer and ceramic composites. The same approach can be used to solve other field problems including electrical conductivity, electrostatics, and moisture diffusion.

Modification and application of a unit cell continuum model to predict the elastic properties of textile composites

Abstract The unit cell continuum model, presented by Foye, is based on the principles of finite element analysis using heterogeneous hexahedra brick elements to predict the elastic properties of textile composites. The use of heterogeneous elements, consisting of fibres, and matrix, often introduces mathematical instabilities in the solution due to the large difference in the stiffness of the fibres and matrix. This can reduce the accuracy of the results. In this paper a solution is presented to this problem. The solution is based on a micro-level homogenization approach utilizing a fabric geometry model. Moreover, this approach is integrated with a geometric model, based on processing science modelling, to ensure accurate representation of complex fabric preforms. Test results for in-plane elastic properties of a five-harness satin weave carbon/epoxy composite and a three-dimensional weave E-glass/poly(vinyl ester) composite were compared with mechanical predictions using this approach and gave matching results

Thermal conductivity of composite materials made from plain weaves and 3-D weaves

Abstract In this research activity, thermal conductivity of textile composites made from plain weaves and 3-D XYZ weaves is quantified. Plain weave composites are made from E-glass, Kevlar® and AS4 graphite fibers and epoxy resin. 3-D woven composites are manufactured from Toho graphite fibers and epoxy resin. The effect of fiber type and fiber volume fraction on the thermal conductivity of textile composites is investigated. The fabric Geometry Model is adapted to calculate the thermal conductivity of textile composites. Results using this tool are compared with experimental data and predictions using the Graphical Integrated Numerical Analysis. The analytical approach proved to be a good engineering tool to predict the thermal conductivity of textile composites.

Modeling of crack density in ceramic matrix composites

The feasibility of utilizing the shear lag theory to estimate crack density in fabric reinforced composites was investigated. A geometric model was constructed for the fabric and meshed using a hybrid finite element approach. The small segment of the yarn and the surrounding matrix enclosed within each element were treated as a unidirectional composite and the shear lag theory was used to estimate the crack density. Model results were compared to experimental data for a 5-harness satin melt-infiltrated SiC/SiC composite under tension and showed a pattern similar to experimental data with the model starting to accumulate cracks at a stress corresponding to the point of departure from linearity in the stress–strain curve while cracks were experimentally observed around 60 MPa higher. The model and experimental data had a similar value for the crack density at the saturation level. Sensitivity analysis showed that the crack density was highly sensitive to the fiber volume fraction in the load direction followed by the weave angle of the crimped segments of the yarns and the interfacial shear strength between the fibers and the matrix.

Theory and Practice of Cotton Fiber Selection Part II: Sources of Cotton Mix Variability and Critical Factors Affecting It

This part of the study demonstrates the complex nature of variability in multi component blends and the effectiveness of bale picking schemes in handling this com plexity, using analysis of variance. Such analysis provides useful guides in the selecting fibers for uniform blending. Critical factors affecting mix uniformity are examined, including population variability, location of category break points, number of cate gories, and laydown size. Methods for optimizing blend uniformity in view of the effects of these factors are recommended.

Theory and Practice of Cotton Fiber Selection Part I: Fiber Selection Techniques and Bale Picking Algorithms

The cotton mixture as a multi-component blend of inherently variable natural fibers imposes several challenges with regard to the proper method of selecting fibers for a uniform blend and consistent output characteristics. Part I introduces a number of fiber selection techniques and based on these techniques, proposes three different bale picking schemes. The first scheme is random picking, which resembles the tra ditional massive blending and serves as the basis for more advanced schemes. The second scheme is proportional weight category picking, in which the distribution of a fiber characteristic is divided into a number of classes, and bales are picked from each class in quantities proportional to class relative frequency. The third scheme is optimum category picking, in which bales are selected on the basis of optimizing factors that contribute to blend uniformity. These factors include category variance, picking cost, and category inventory.