Triaxial Braid Research Articles

Axial crushing of triaxially braided composite tubes at quasi-static and dynamic rates

This paper presents the results of an experimental investigation into the behaviour of braided CFRP tubes subjected to quasi-static and dynamic axial crushing. Tubes of six different geometries, with round, rectangular or square cross-sections, with 2–4 plies of triaxial braids, were tested. Two types of square tubes were also tested with a plug initiator. Comparisons were made by examining the failure modes, load-displacement curves, specific energy absorption (SEA), and the ratio of the peak load to the average progressive crush load. When tested without a plug initiator, the crush characteristics of these tubes differed significantly under the two loading rates. Under quasi-static crush, tubes showed a greater tendency to fail in an unstable manner, characterized by global buckling, folding of the walls and crushing at both ends of the tube. Therefore, quasi-static crush testing should be used with caution when evaluating dynamic response of a structure. In dynamic testing, seven out of the eight configurations failed in a stable manner, with damage initiating at the bottom and propagating up the length of the tube in a splaying type failure mode. The smaller square and the round tubes performed the best, displaying higher SEA and lower load ratio values.

The tensile behaviour of biaxial and triaxial braided fabrics

The tensile behaviour of braid reinforcement is classically described by the behaviour of composite elaborated from these reinforcements. Few studies concern the tensile behaviour of braided fabrics. In this paper biaxial and triaxial braids are manufactured on a braiding loom. The evolution of key parameters as linear mass and braiding angle in function of process parameters is presented. Braid reinforcements are characterized in uniaxial tensile. The mechanical behaviour is analysed and compared in function of the braiding angle, but also different kinds of braid are considered. A specific behaviour called “double-peak” is identified for triaxial braids which have a higher braiding angle. The evolution of the braiding angle measured during tensile tests gives a comprehension on the mechanical behaviour of dry braids. Associated with this experimental study, an analytical model is also proposed, to predict mechanical properties of braid reinforcements.

Tensile behaviour of structurally gradient braided prostheses for anterior cruciate ligaments

Anterior cruciate ligament (ACL) is a key fibrous connective tissue that maintains the stability of a knee joint and it is the most commonly injured ligament of the knee. A synthetic prosthesis in the form of a braided structure can be an attractive alternative to biological grafts provided that the mechanical properties can be tailored to mimic the natural ACL. In the present work, the polypropylene based structurally gradient braided prostheses have been designed and developed by understanding their tensile properties. Circular braiding process was employed to fabricate structurally gradient braided prostheses by systematically placing different types of braids in defined set of layers. An analytical model for predicting the tensile properties of structurally gradient braided prostheses has been presented by modifying and combining the existing models available in the literature. Specifically, the full set of stress–strain behaviour of structurally gradient braided prostheses has been computed based upon braid structural characteristics, constituent strand properties and braid kinematics. A triaxial braid in the outer layer of braided prostheses was found to withstand higher tensile stresses in comparison to a biaxial braid having same structural characteristics. A comparison has been made between the theoretical and experimental results of tensile properties of structurally gradient braided prostheses. The tensile properties of structurally gradient braided prostheses predicted through analytical route matched reasonably well with the experimental results.

Tensile behaviour of regular triaxial braided structures

Triaxial braided structures have proliferated in many applications and specifically, they are extensively used as main constituent materials in various biomedical and composite systems. The topological nature of triaxial braids provides significant enhancement in mechanical properties which can be further tuned by elucidating the most important geometrical and material parameters. In this study, the geometrical models of various braid parameters of triaxial braided structures have been developed for predefined strand geometries. These braid parameters were then used for computing various tensile properties of triaxial braids. Particularly, the elastic properties of triaxial braided structures and their composites have been analysed by including the effect of axial strand crimp. Furthermore, the full set of stress–strain characteristics of triaxial braided structures has been predicted based upon braid topological characteristics, constituent strand properties and braid kinematics. The research findings of stress–strain characteristics of triaxial braids have been confirmed by making a comparison between the theoretical and experimental results.

Geometrical modeling of near-net shape braided preforms

Circular braiding has been successfully adapted for producing near-net shape structures for advanced fiber-reinforced composites. Net-shape manufacturing is significantly important for fabricating complex three-dimensional (3D) preforms. Geometrical modeling of braid patterns on 3D preforms plays a key role in determining their mechanical behavior. In this research work, the geometrical models of strand trajectory on the surface of cylindrical and conical mandrels with diamond, regular and triaxial braid patterns have been developed. These geometrical models of strand profiles were then simulated using Virtual Reality Modeling Language. Subsequently, the strands on complex-shaped mandrels, including ‘bottle’ and ‘funnel’, were simulated and accordingly, the braid angles have been predicted and compared with the experimental results. A virtual experiment was also conducted to compare the trajectory of the strands having constant and varying braid angles on the surface of conical mandrels.

Impact of polyurethane yarns on the mechanical properties of braided artificial ligaments

Anterior cruciate ligaments (ACL) have to meet a number of physiological and biomechanical requirements. Most of the permanent prostheses that have been developed suffer from fatigue, creep, and mechanical failure. This study aims to develop a new braided synthetic ligament which offers a high elasticity ensuring a long-term mechanical performance. Two braided structures were designed: a biaxial quadruple braid and a triaxial quadruple braid incorporating polyurethane filaments. The mechanical properties of these structures were measured and compared to those of the natural ligaments. The elastic recovery under a traumatic force was also studied in order to compare the elasticity of the manufactured samples. The obtained results showed that the elastic recuperation was improved, thanks to the incorporation of polyurethane yarns. These yarns provide prostheses with mechanical properties that closely match those of the native ACL.

Circular braiding take-up speed generation using inverse kinematics

Circular overbraiding of composite preforms on complex mandrels currently lacks automatic generation of machine control data. To solve this limitation, an inverse kinematics-based procedure was designed and implemented for circular braiding machines with optional guide rings, resulting in a take-up speed profile for a given braid angle distribution on mandrels with complex 3D shapes including non-axisymmetric, optionally eccentric cross-sections that can vary in shape and size along an optionally curved mandrel centerline, allowing a curved machine movement. This procedure reduces the problem size, resulting in a short computation time, fit for CAE process chain integration. Numerical control data was generated for a complex mandrel with a specified braid angle and a triaxial braid. A simulation using this control data yields a braid angle that deviates a few degrees from the specified braid angle. The simulation was validated experimentally, using the generated instructions to control the braiding machine. This showed a deviation from the simulated braid angle of 3 degrees in the centered, non-tapered mandrel regions, up to 10 degrees in tapered regions and an experimental scatter of 7 degrees. The deviation is mainly attributed to the neglect of yarn interaction and guide ring contact friction in the model, leading to an incorrectly modeled convergence zone length.

New Directions in Braiding

It is the intent of this manuscript to provide a general treatment of braiding: past, present, and future. A history and evolution of braiding, braiding machinery, and related engineering developments is provided with emphasis on the design, manufacture, and analysis of braided fabrics and composites. Some recent developments are briefly described, including: 1. a composite braider with axial yarns which interlace with the helicals, and in which the helical yarns do not interlace with each other – a machine now under commercial development, 2. a new braided structure, called the true triaxial braid, produced by the new machine or by proper carrier loading on a conventional Maypole braider; and 3. a computer controlled take-up system using image analysis to monitor and control braid formation. Original work ongoing at Auburn University is described and involves Jacquard lace braids with open structures for use in composites, computer aided design (CAD), computer aided manufacturing (CAM), and analysis of ordinary and lace braids for composite applications. This paper is an expanded version of an invited presentation under the title “New Directions in Braiding” at a Fiber Society presentation in Bursa, Turkey, in the spring of 2010 [1].

Macromechanical Approach to Modeling Barely Visible Damage in Braided Composites

AbstractA method has been developed to analyze the impact response of triaxially braided carbon fiber composites, including barely visible damage. In the analytical model, the triaxial braid architecture is simulated using four parallel shell elements, each of which is modeled as a laminated composite. Because of the construction of the composite material, the resin regions between layers are weak areas in the material structure and can initiate damage by delaminating. To simulate these regions, each shell element is modeled as a homogeneous material with a zero-thickness cohesive zone (simulated with LS-DYNA’s tie-break contact) between the layers of braid. To accurately define the tie breaks used to model delamination, the resin materials’ cohesive zone must be defined. The length of the cohesive zone is calculated internally from parameters such as convergence scale factor, elastic modulus, peak traction, and strain energy release rate. To reliably model a cohesive zone, the zone should encompass three...

Design and manufacturing of an L-shaped thermoplastic composite beam by braid-trusion

Braid-trusion is a manufacturing process for composite materials in which a braiding machine is coupled with a pultrusion die to continuously produce beams with constant cross-section and off-axis fiber orientation. This study presents a geometrical model of the tri-axial braid which allows the design of braided preforms that achieve correct filling of the pultrusion die at the same time as it limits fiber friction on die walls. The typic design parameters are listed and used for the manufacturing of a braid-truded thermoplastic composite beam where fibers are aligned at ±69° as well as 0° with respect to the beam axis. Tensile mechanical characterization and cross-section observations are also presented.

Meso-scale and multicontinuum modeling of a triaxial braided textile composite

Accurately predicting failure in woven composites requires knowledge of the stress states within the meso-scale structure of the fabric reinforcement. Multicontinuum technology provides a computationally efficient way of extracting constituent stresses and strains from a structural-level finite element analysis. This study investigates the value in extending the capabilities of multicontinuum technology to materials with complex heterogeneity that could benefit from the definition of many constituents. To determine the feasibility of this extension, a meso-scale finite element model of a triaxial braid was developed and used as a test case. The model’s predictions of initial matrix failure were in good agreement with the limited experimental data. Also, trends in initial failure predictions for multi-axial load cases are in agreement with physically intuitive expectations. These results show promise for the success of future research in extending multicontinuum technology for application to composites with complex multiscale heterogeneity.

Prediction of material properties of biaxial and triaxial braided textile composites

Biaxial and triaxial braided composites are modeled using finite element methods to predict effective material properties: for biaxial braids, diamond pattern (1/1), regular pattern (2/2), and Hercules pattern (3/3) are modeled, while for triaxial braids, a regular pattern (2/2) is modeled. A micromechanical approach is adopted to calculate material properties of the tows, which are critical compositions of braided composites. By applying periodical boundary conditions (PBC), four representative unit cells (RUC) in correspondence to the four types of braids are analyzed and compared. Subsequently, effective material properties are obtained with the braiding angle varying from 15° to 75° with an increment of 5°, from which the variation of the engineering constants with the braiding angle is studied. The prediction results are compared with the experimental values for two material systems, and good agreement is achieved.

A notched coupon approach for tensile testing of braided composites

A notched coupon geometry was evaluated as a method for tensile testing of 2D triaxial braid composites. Edge initiated shear failure has been observed in transverse tension tests using straight-sided coupons based on ASTM D3039. The notched coupon was designed to reduce the effects of edge initiated failure and produce the desired tensile failure. A limited set of tests were performed with partial pressurization of tubes to determine the transverse tensile strength in the absence of edge initiated failure. The transverse strength measured with the notched coupons was considerably higher than the straight-sided coupons, comparable to the tube results, and closer to the maximum possible strength based on maximum fiber strain. Further investigations of the effects of the observed biaxial stress state and stress concentrations in the notched geometry are needed.

3D geometrical modelling of tubular braids

Geometrical modelling for tubular braids of different structures is studied and a simple versatile three-dimensional model is proposed after considering the crimp of the braiding yarn together with the tubular curvature of the tubular braid structure. The proposed model is versatile and suitable not only for different braid structures, but also, with the changes in the structural parameters such as braid angle, number of yarns in a set, yarn and mandrel diameter the model is still applicable. Application and 3D drawings of the model for diamond, regular and triaxial braids are given with the aid of Visual Basic and 3DSMax Studio.

Modification of a Macromechanical Finite Element–Based Model for Impact Analysis of Triaxially Braided Composites

Abstract A macro level finite element-based model has been developed to simulate the mechanical and impact response of triaxially-braided polymer matrix composites. In the analytical model, the triaxial braid architecture is simulated by using four parallel shell elements, each of which is modeled as a laminated composite. For the current analytical approach, each shell element is considered to be a smeared homogeneous material. The commercial transient dynamic finite element code LS-DYNA is used to conduct the simulations, and a continuum damage mechanics model internal to LS-DYNA is used as the material constitutive model. The constitutive model requires stiffness and strength properties of an equivalent unidirectional composite. Simplified micromechanics methods are used to determine the equivalent stiffness properties, and results from coupon level tests on the braided composite are utilized to back out the required strength properties. Simulations of quasi-static coupon tests of several representative braided composites are conducted to demonstrate the correlation of the model. Impact simulations of a represented braided composites are conducted to demonstrate the capability of the model to predict the penetration velocity and damage patterns obtained experimentally.

Characterization of Biaxial and Triaxial Braids: Fiber Architecture and Mechanical Properties

Biaxial and triaxial carbon fiber braids with off-axis braiding angles of 30°, 45° and 55° are characterized with respect to their fiber architecture. All braids are produced on a round mandrel with constant cross section. Detailed geometric information on the different braids, like roving dimensions, roving shapes and the degree of nesting is given. The findings from measurements in photomicrographs are used to construct meso-model yarn architectures at the unit cell level which are then analyzed with the WiseTex software (Lomov et al. Compos. Sci. Technol. 60:2083–2095, 2000). The results of the models’ analysis with TexComp and comparison of mechanical properties with tests are consistent and essential for further steps in predictive modeling. Predictive modeling was also performed for biaxial braids based on the packing density in the material and parameters of the braiding process. The good conformance of the predictive models gives a validated starting point for development of braided structures concerning stiffness behavior. In addition, the information about the fiber architecture can be used for failure analysis on unit cell level.

Investigation of a Macromechanical Approach to Analyzing Triaxially-Braided Polymer Composites

A macro level finite element-based model has been developed to simulate the mechanical and impact response of triaxially-braided polymer matrix composites. In the analytical model, the triaxial braid architecture is simulated by using four parallel shell elements, each of which is modeled as a laminated composite. The commercial transient dynamic finite element code LS-DYNA is used to conduct the simulations, and a continuum damage mechanics model internal to LS-DYNA is used as the material constitutive model. The material stiffness and strength values required for the constitutive model are determined based on coupon level tests on the braided composite. Simulations of quasi-static coupon tests of a representative braided composite are conducted. Varying the strength values that are input to the material model is found to have a significant influence on the effective material response predicted by the finite element analysis, sometimes in ways that at first glance appear non-intuitive. A parametric study involving the input strength parameters provides guidance on how the analysis model can be improved.

A model for the in-plane permeability of triaxially braided reinforcements

For multilayer preforms from three different triaxial carbon fibre braids with bias angles 45°, 60° and 70° at fibre volume fractions of 0.54 and 0.59, the in-plane permeability was characterised experimentally. A finding of high practical relevance is that, at a bias angle of approximately 55°, the principal flow direction switches from the preform 0° direction to the preform 90° direction. For different zones of characteristic yarn arrangement in the braid unit cell, the local permeability was modelled as a function of bias angle, global fibre volume fraction, and geometrical yarn parameters. The global braid permeability was derived from numerical flow simulation based on simplified 2D models of the textile architecture and local permeabilities, taking into account effects of nesting between adjacent layers. Calculated trends for the permeability as a function of the braid angle reproduce the experimental results qualitatively well. Quantitatively, agreement depends on the value of a geometry parameter, which decreases with increasing fibre volume fraction.

Effect of microscopic damage events on static and ballistic impact strength of triaxial braid composites

The reliability of impact simulations for aircraft components made with triaxial braided carbon fiber composites is currently limited by inadequate material property data and lack of validated material models for analysis. Methods to characterize the material properties used in the analytical models from a systematically obtained set of test data are also lacking. A macroscopic finite element based analytical model to analyze the impact response of these materials has been developed. The stiffness and strength properties utilized in the material model are obtained from a set of quasi-static in-plane tension, compression and shear coupon level tests. Full-field optical strain measurement techniques are applied in the testing, and the results are used to help in characterizing the model. The unit cell of the braided composite is modeled as a series of shell elements, where each element is modeled as a laminated composite. The braided architecture can thus be approximated within the analytical model. The transient dynamic finite element code LS-DYNA is utilized to conduct the finite element simulations, and an internal LS-DYNA constitutive model is utilized in the analysis. Methods to obtain the stiffness and strength properties required by the constitutive model from the available test data are developed. Simulations of quasi-static coupon tests and impact tests of a represented braided composite are conducted. Overall, the developed method shows promise, but improvements that are needed in test and analysis methods for better predictive capability are examined.

Characterization of Damage in Triaxial Braided Composites under Tensile Loading

Carbon fiber composites utilizing flattened, large tow yarns in woven or braided forms are being used in many aerospace applications. Their complex fiber architecture and large unit cell size present challenges in both understanding deformation processes and measuring reliable material properties. This report examines composites made using flattened 12k and 24k standard modulus carbon fiber yarns in a 0 /+60 /-60 triaxial braid architecture. Standard straight-sided tensile coupons are tested with the 0 axial braid fibers either parallel with or perpendicular to the applied tensile load (axial or transverse tensile test, respectively). Nonuniform surface strain resulting from the triaxial braid architecture is examined using photogrammetry. Local regions of high strain concentration are examined to identify where failure initiates and to determine the local strain at the time of initiation. Splitting within fiber bundles is the first failure mode observed at low to intermediate strains. For axial tensile tests splitting is primarily in the 60 bias fibers, which were oriented 60 to the applied load. At higher strains, out-of-plane deformation associated with localized delamination between fiber bundles or damage within fiber bundles is observed. For transverse tensile tests, the splitting is primarily in the 0 axial fibers, which were oriented transverse to the applied load. The initiation and accumulation of local damage causes the global transverse stress-strain curves to become nonlinear and causes failure to occur at a reduced ultimate strain. Extensive delamination at the specimen edges is also observed.