Wen-Shyong Kuo

Compressive damage in 3-axis orthogonal fabric composites

This work is aimed at two goals: examining compression-induced damage in 3-axis orthogonal composites, and assessing the influence of surface loops on the damage behavior. Two types of 3-axis fabrics were made; one incorporated unidirectional carbon/epoxy rods along the axial direction, and the other employed carbon yarns in all the axes. After resin impregnation, one half of the specimens were ground to remove surface loops. Compressive tests were conducted by using an end-supported, end-loaded fixture designed for thick specimens. The damage configurations can be categorized into microscopic and miniscopic levels. The microscopic damage focuses on kinking of fibers and shear failure of fiber/matrix interface, whereas the miniscopic damage focuses on kinking of axial bundles and shear failure of transverse bundles. In many ways the miniscopic damage is a scale-up of the microscopic damage, although their underlying mechanisms of damage evolution are different. The loop-retained and loop-removed specimens manifest the role of loops in resisting compression. The usefulness of loops was found to lie in retaining the structural integrity after damage has developed. Theoretical predictions of the critical stress for the onset of fiber microbuckling have been examined and compared with the experimentally obtained results. Some keys for improving the compression behavior of the 3-axis composites are discussed.

Failure behavior of 3D woven composites under transverse shear

Abstract Much less research has been done on failure characteristics of composites under transverse shear, especially for 3D textile composites. This work is an attempt to this need. General characteristics of 3D composites related to the present study are first discussed. Three types of 3D woven carbon/epoxy composites were made with identical internal yarn structures but different external loop patterns. For comparison purposes, a unidirectional carbon/epoxy composite with the same numbers of axial fibers and a monolithic epoxy material were also made to reveal the role of transverse yarns in resisting the shear. To apply the transverse shear, a special fixture was used to clamp and cut the specimen using two cutters. With the fixture, no notch on the specimen is needed, and thus the interlacing loops on the surface remain intact before the test. The gap between the cutters was varied to examine its influence on the failure behavior. Damage in fibers is most intensive within the cutting zone. Microscopic observations on the induced damage were carried out. Two failure modes in axial yarns are prevailing: shear fracture and tensile rupture. Matrix cracking leading to the loss of the shear rigidity is responsible for the tensile rupture of the axial yarns. The transverse shear resulted in complex but intriguing damage modes. The loop pattern, gap length, and cutting position are the crucial influencing factors to the damage modes, maximum load, and the maximum shear displacement to failure.

Elastic moduli and damage evolution of three-axis woven fabric composites

Three-axis orthogonal woven fabric composites composed of carbon fibres and epoxy resin have been fabricated. Examined from micrographs, the fabric weaving yarns were found to be very slender with aspect ratios ranging from 11–13.6. Based upon the observed geometry, the composite has been modelled by a unit cell comprising wavy yarns. Both elliptical and lenticular cross-sections were adopted to simulate the slender weaving yarns. Taking into account one-dimensional stress concentration and yarn undulation, an iso-phase approach has been developed to analyse the composite elastic moduli. A higher weaving yarn aspect ratio was found to result in a lower modulus. Modulus reduction due to yarn undulation was more significant in weaving directions. Material characterization has been conducted based upon monotonic tensile and three-point flexural tests, and detailed damage mechanisms for both loadings have been examined. The onset of damage under tensile loading was found to be z-axis yarn debonding, followed by debonding and splitting in y-axis yarns. When subjected to flexural loading, yarn debonding, transverse cracking, and interyarn matrix cracking were the dominant damage mechanisms which appeared on specimen tensile sides. Stress transfer among yarns and how it relates to the composite damage have been discussed in detail.

Elastic moduli and damage mechanisms in 3D braided composites incorporating pultruded rods

This paper is aimed at elucidating the processing-property relationships of composites through experimental characterization. The fabrication processes involve three primary steps: the pultrusion of 1 mm diameter unidirectional rods, the formation of three-dimensional preforms incorporating the rods, and the impregnation of resin into the preforms. The use of the rods is intended to address crimp problems often seen in textile composites. A modified two-step set-up to incorporate the rods has been developed. A series of fabrics with varying braiding fibers, pitch lengths and braider sizes has been made to investigate their respective influence on processing and damage behavior. In comparison with conventional textile composites, the axial yarns are straight and are packed in a rather dense and orderly manner. The composite moduli have been analysed by using the fabric geometry model and compared with the experimental results. The material characterization has been carried out on the basis of flexure, short-beam and compression tests. Damage configurations and accumulation for each loading case have been examined. Unique features include pull-in and push-out of the rods, which are the dominant modes in the short-beam tests. Buckling of the rods is the major damage in the flexure and compression tests. The buckled rods form kink bands analogous to fiber microbuckling in compressed unidirectional composites. The kink bands were found to propagate along the interlacing loops on the surface. These unique damage characteristics associated with the use of the rods are discussed in detail.

Failure behavior of three-axis woven carbon/carbon composites under compressive and transverse shear loads

This work examines the responses of three-dimensional carbon/carbon composites under axial compression and transverse shear. By using a 3D weaving technique, two types of preforms with different bundle sizes of the weaving yarns were prepared for assessing its influence on the failure behavior. Carbon yarns were arranged in a three-axis, orthogonal form with interlacing loops on outer surfaces. The carbon matrix was added by using a phenolic resin as the matrix precursor. Resin transfer molding was used for resin impregnation, followed by the curing and carbonization of the resin. The matrix filling was repeated up to five cycles, and the efficiency of the matrix filling was examined. Special fixtures were designed for applying the loads to the 3D composites. Microscopic observations on the induced damage were carried out. The axial yarns were found to undergo bending fracture in the compressive tests. The yarn imperfection, rather than the fiber misalignment, was the major failure-determining factor. The bending of the yarns is analyzed, and the critical value of imperfection that leads to bending fracture is given. The transverse shear resulted in complex but intriguing damage modes, which are closely related to the surface loops and through-thickness yarns. Some keys for preform design to best withstanding the shear are discussed.

Processing and characterization of 3D woven and braided thermoplastic composites

Abstract This work is aimed at elucidating the processing–microstructure–property relationships of three-dimensional thermoplastic composites. The material processing involves fabric formation and hot-press molding. A powder-impregnated Nylon/carbon yarn was used to form three-dimensional fabrics. Two types of fabrics have been made in the present work, including 3-axis orthogonal woven and two-step braided. The woven fabric is characterized by more evenly distributed fibers along three orthogonal directions, whereas the braided fabric contains most fibers along the axial direction. Compression molding was employed for composite consolidation. Matched molds were designed for making the composites with predetermined thicknesses. The molding thickness and molding temperature were varied to examine their respective effects on the resulting properties. Yarn geometries in the molded composites were studied through microscope observations. The molding significantly distorted the through-thickness yarns of the woven fabric, and the mechanisms of yarn distortion were identified and related to the fabric structure. Material characterization was conducted by means of flexure tests. The loading curves show significant non-linearity with the development of damage. The molding thickness is a critical parameter governing the flexural modulus, flexural strength, and damage modes. A special mounting method was used to permit the examination of the fractured interiors of specimens. How surface loops affect damage modes and how damage grows within these non-uniform materials are discussed in detail.

The role of loops in 3D fabric composites

The role of interlacing loops on the performance of 3D composites has long been overlooked. One possibility is that loops occupy only a small fraction of the volume, yet experiments have shown that their influence can be disproportionate in determining damage behavior. The key lies in the fact that loops not only build the 3D network of reinforcements by connecting through-thickness yarns, they also cover and protect the composites from external attack. In the present work, the role is examined experimentally. Two types of three-axis woven composites were made, one combining solid carbon/epoxy rods along the axial direction, and the other employing carbon yarns in all axes. These composites are identical in loop patterns but distinct in yarn alignment. Composite geometry was first examined by introducing two unit cells that describe internal yarns and surface loops. Two terms describing the coverage of surface loops were defined and calculated for the fabrics under investigation. In order to elucidate the influence of loops, some specimens were ground to remove all loops on surfaces. Material characterizations based upon flexure and Izod impact tests were then carried out. The loop-retained and loop-removed specimens provide a sharp contrast in the configuration of damage. On the basis of the comparison, the role of loops has been examined. The results show that loops can provide at least two functions that enhance composite durability and damage tolerance.

Fabrication and microgeometry of two-step braided composites incorporating pultruded rods

Abstract The concept of using consolidated rods as axial reinforcements in two-step braided composites has been demonstrated in this paper. The composite fabrication consists of three major processes: the fabrication of unidirectional rods, 1 mm in diameter, by pultrusion, the formation of three-dimensional preforms by a modified two-step braiding set-up with the rods as axial reinforcements, and the impregnation of resin into the preforms by resin transfer molding, followed by a curing process to consolidate the composites. In comparison with conventional three-dimensional fabric composites, this approach (1) essentially eliminates both axial and braiding yarn crimp in the composite interior, (2) enhances fabric consistency, (3) provides fairly rigid fabrics, and (4) facilitates resin infiltration in resin transfer molding. The resulting composite geometries have been experimentally characterized. The mechanism of braider slip was found to dominate the resulting interlacing pattern. According to the micrographs, the rods were placed regularly within the fabrics with satisfactory compactness. As squeezed by the rods, the braiders are thin and close to rectangular. Two families of specimens with varying take-up distances and braider sizes have been made, and the corresponding pitch length, yarn content, rod gap, braider cross-section, and interlacing pattern have been correlated with these varying parameters. The processing characteristics and the inherent limitations of this method are discussed in detail.

The microstructural changes of carbon fiber pores in carbon–carbon composites during pyrolysis

Carbon/carbon (C/C) composites have been prepared from a phenolic resin and oxidized PAN felt with heat treatment to 1000, 1800, and 2400 °C. This paper explores the reasons for the formation of pores in the fibers and studies the microstructure of these pores. Most of the pores in the fiber with various sizes concentrate at a location 2 μm away from the fiber surface, and make a circle in the fiber. The structure of the carbon layer planes around the pores grows with increasing temperature, and has the feature of high orientation. The reasons for the formation of these pores might be: when the oxidized PAN fibers are heated, some low molecular weight species transforms into gases but do not leave the fibers due to the delicate fiber surface structure, so forms the small pores in the fibers. Through the temperature increase, the vibration of these low molecular weight molecules forms thermal stress, and thus results in the growth of the pores.

Processing and microstructures of 3D woven-fabric composites incorporating solid rods

This work has two objectives. The first is to examine the processability of incorporating pultruded rods in three-dimensional woven fabrics. A weaving set-up for this purpose has been developed. The second objective is to characterize the microstructures of the composites and to assess the merits and limitations associated with the rods. Three types of fabric have been made in this study. The first is a 3-axis orthogonal type combining the rods in the axial direction. The second type has a similar structure except that the rods are also used in the widthwise direction. The third is the addition of two off-axis weaving yarns to the first type, resulting in a 5-axis structure. Material processing involves rod pultrusion, fabric formation, and resin impregnation. Both Kevlar and carbon tows have been used in the rods and weaving yarns. A series of specimens with varying bundle size of the weaving yarns have been made. No pressing force was applied during resin transfer molding to preserve the as-formed fabric configuration. According to the micrographs, the rods are essentially free of distortion, while two kinds of local deformation have been identified in the yarns. Unit cells are selected to describe interior microstructures of the materials. Fiber distribution along each direction and matrix distribution in interbundle and intrabundle spaces have been examined on the basis of measured fabric dimensions. The influence of the pitch length on the distributions of fiber and matrix has been analyzed by assuming constant yarn areas. Unique features in processing/microstructure relationships resulting from the presence of the rods have been discussed.