Multiaxial Warp Knitted Research Articles

Flexural Strength, Performance, and Cost Analysis of Multiaxial Warp-Knitted Preforms for Composite Reinforcements

ABSTRACT In the scope of this study, performance analysis of industrial fabrics for composite reinforcement was evaluated from the cost point of view. For the production of composite structures, major considerations can be summarized as stiffness, strength, weight, and cost. In general, manufacturing costs of composites consist of material, labor, overhead, and development costs. The priorities and demands will determine characteristics of the material and process. Specific strength per cost relation is in favor of multiaxial warp-knitted (MWK) fabric. In order to evaluate fiber reinforcements within the group, MWK fabric laminate was compared with woven fabric laminate from the cost/performance point of view. To do so, laminate comparison was performed between MWK fabric and woven fabric reinforced composites. It was found that relative to woven reinforced composite, MWK fabric reinforced composite has lower resin weight, overall composite weight, and labor cost and higher flexural moment with slightly higher material cost.

Effects of through-the-thickness stitches on the elastic behavior of multi-axial warp knit fabric composites

In order to improve the resistance to delamination and some in-plane and out-of-plane properties of composite materials for structural integrity, through-the-thickness reinforcement must be provided. The reinforcement is achieved by using the stitched multi-axial warp knit (MWK) fabrics as preforms for the fabrication of composite structures. In this study, the influence of stitches on the elastic behavior of MWK fabric composites under tensile and shear loadings was investigated by utilizing a unified micromechanical model. In the analysis, the in situ constituent properties and fiber volume ratios of insertion and stitching fibers determined from the geometric parameters set by the representative volume were used. The crucial step in the analysis was to correlate the averaged stress states in the constituents by adopting the bridging matrix. The experimental results were compared with the predicted results. It was found that the predicted results are in reasonably good agreement with the experimental results.

An Experimental and Numerical Study on the Impact Energy Absorption Characteristics of the Multiaxial Warp Knitted (MWK) Reinforced Composites

In this paper, an experimental and numerical study on the energy absorption characteristics due to the low-velocity impact of the composites reinforced by the multiaxial warp knitted (MWK) structures with double loop pillar stitch and common tricot stitch are presented. The influences of the MWK structural parameters on the impact behavior and energy absorption properties are discussed. A good agreement between the experimental results and simulation is obtained.

Predictions of Elastic Properties of Multi-Axial Warp Knitted Fabric Composites

An analytical model was proposed to predict the elastic properties of multi-axial warp knitted (MWK) fabric composites for three-dimensional structures. The characteristics of MWK fabric composites are the assemblage of multilayers of rovings in the warp, weft and bias directions for in-plane reinforcement and out-of-plane stitches by knitting rovings to provide through-the- thickness reinforcement. For analysis, a representative volume of the MWK fabric composite was identified. The geometric limitations, effects of stitch fibers and design parameters of MWK composites are considered in the model. Then, the elastic properties of MWK fabric composites are predicted by using an averaging method. The experiments are also conducted on the MWK fabric composites to compare the predicted results with the experimental results for the verification of suggested model. The predicted elastic properties are in reasonably good agreement with the experimental values. Finally the effects of design parameters of the MWK fabric composites are discussed.

Low velocity impact responses of hollow core sandwich laminate and interply hybrid laminate

Two different fabrics, an integrated hollow pile glass fabric and a nonwoven mat, inlaid into multiaxial warp knit blankets were laminated to form a sandwich structure and a hybrid structure. The impact loads and the energies of the panels corresponding to different damage characteristics under drop-weight tests were calculated. Fracture patterns and damage characteristics were observed and photographed using a camera. The damage characteristics of the penetration process in both laminates are described in this study. Significant delamination was found in both the sandwich laminate and the hybrid laminate, which would be a reason to improve the impact energy absorption. Results showed that the inlaid materials played a very important role in the impact behavior and damage characteristics of the laminates.

Effects of fibre architecture on reinforcement fabric deformation

In this paper, the shear properties of a number of woven and non-crimp (multiaxial warp knit) fabrics are analysed. These are shown to be related to the fibre architecture and, for non-crimp fabrics, the pattern and orientation of the stitching thread. A detailed geometric model has been developed for the deformation of woven fabrics during in plane shear, and this is used as a basis for a mechanical analysis to predict shear compliance. This model incorporates intertow friction, tow compaction, and in plane (membrane) tension. Finally an iterative model for fabric forming is described, based on fabric shear energy obtained either from the mechanical model or from experimental measurements. This is validated for a hemisphere and an automotive transmission tunnel and is shown to offer greater accuracy than the traditional geometric mapping, while associated computation times are at least an order of magnitude lower than those associated with non-linear finite element analysis.

Modeling formability of multiaxial warp knitted fabrics on a hemisphere

Abstract The aim of this paper is to describe a model for the prediction of the formability of a multiaxial warp knitted (MWK) fabric to a 3D surface. For this purpose, we first characterized in detail the forming behavior of MWK fabrics containing two bias inserting yarns (TBMWK fabric). Through experimental observation, it was found that the two bias inserting yarns always tend to gather along the weft direction. The angle between the two bias yarns has a linear relationship with the perpendicular distance from the measured points to the longitudinal axis of the hemisphere during forming process. The slope of this linear relationship is also linear with the magnitude of radius of the pressing hemisphere provided that the radius is larger than 7 cm. Based on the above finding, a mathematical model is established for predicting the deformations of TBMWK fabrics during the hemisphere-forming process. The shape of flat TBMWK fabric that can yield the corresponding hemisphere during the forming process as well as local deformations can be calculated through this model. The hemisphere-forming experiments show that the present model is workable and accurate. The results from both the model and experiments suggest that the shape of flat TBMWK fabric that can yield the corresponding hemisphere which is close to a rectangular, not to a square as presented by woven fabric. The method developed in the paper has laid a foundation for further modeling of the forming behavior of MWK fabrics onto other 3D surfaces. More importantly, it is of great value to find that the two bias inserting yarns always tend to gather along the weft direction of the fabric which is a starting point for modeling of the forming behavior of MWK fabrics.

Characterizing and Modeling Bending Properties of Multiaxial Warp Knitted Fabrics

The bending behavior of multiaxial warp knitted (MWK) fabrics is quite different from that of plain weaves and other apparel materials. According to the bending hysteresis curves obtained from KES-FB-2 (Kawabata's evaluation system), MWK fabric bending involves not only the spreading of filaments in inserting yarns, but also the buckling of inserting yams, which accounts for the more remarkable irregularity and nonsymmetry of MWK fabric bending hysteresis curves compared with those of plain weaves. Furthermore, the inserting yarns not completely parallel to the applied bending moment direction will undergo both bending and torsional deformation during a bending cycle. Experimental observation and theoretical study has shown that the bending paths of those non- orthogonally bent yarn systems follow different cylindrical helices. In this paper, a predictive bending model proves to be applicable in assessing the bending behavior of MWK fabrics in different bending directions, and the bending rigidity corresponding to an arbitrary curvature can be calculated with this model as well. Frictional restraint within fabrics is also discussed.

Modeling Uniaxial Tensile Properties of Multiaxial Warp Knitted Fabrics

Multiaxial warp knitted (MWK) fabrics were formerly developed as substrates for flexible coated fabrics. They have an important role in industrial applications due to their outstanding mechanical properties. At the beginning of the 1990s, MWK fabrics entered the field of rigid structural composites to produce aerospace quality compo nents, marine parts, and automotive frames. Although many papers have been published on the technology, structure, and properties of MWK fabrics as well as their composites, so far there is no model dealing with mechanical properties, which greatly restricts the designs and applications of these fabrics. In this paper, a uniaxial tensile model is built, and a formula for calculating the tensile modulus of the fabric in any direction is presented. In addition, the theoretical model is justified by the tests from the Instron 4466, which show good agreement between theoretical and experimental results.

Mechanical and Impact Properties of Composite Laminates Reinforced with Kevlar Multiaxial Warp Knit Fabrics

The mechanical and impact properties of multiaxial warp knit fabric reinforced composite laminates have been studied. Kevlar® fibre was used to fabricate the multiaxial warp knit structure for unidirectional reinforcing as well as stitching yarns. Tensile, flexural, short beam shear, and impact properties have been compared with those of woven laminated composites. The results showed that increased interlaminar shear strength is the main characteristic feature of the multiaxial warp knit composite compared with plain woven laminates. The interlaminar shear strength of multiaxial warp knit composites showed up to 52% increase compared with that of woven laminates composites. Also the delamination was retarded due to the increase in interlaminar shear strength of the multiaxial warp knit composites. It is shown that the through-the-thickness reinforcing loops resist any delamination. The tensile strength of the multiaxial warp knit composites increased by about 5–12% compared with the theoretical values computed from classical lamination theory. It is generally accepted that impact induced delamination area increases with the increase in the total absorbed impact energy in collisions with foreign objects. But the multiaxial warp knit composites showed that the total absorbed impact energy decreased with the increase in through-the-thickness reinforcement fibres. The multiaxial warp knit composites showed reduced vertical deformations compared with woven laminates, as the reinforcing loop density increased.

Mechanical and Impact Properties of Composite Laminates Reinforced with Kevlar Multiaxial Warp Knit Fabrics

The mechanical and impact properties of multiaxial warp knit fabric reinforced composite laminates have been studied. Kevlar® fibre was used to fabricate the multiaxial warp knit structure for unidirectional reinforcing as well as stitching yarns. Tensile, flexural, short beam shear, and impact properties have been compared with those of woven laminated composites. The results showed that increased interlaminar shear strength is the main characteristic feature of the multiaxial warp knit composite compared with plain woven laminates. The interlaminar shear strength of multiaxial warp knit composites showed up to 52% increase compared with that of woven laminates composites. Also the delamination was retarded due to the increase in interlaminar shear strength of the multiaxial warp knit composites. It is shown that the through-the-thickness reinforcing loops resist any delamination. The tensile strength of the multiaxial warp knit composites increased by about 5–12% compared with the theoretical values computed from classical lamination theory. It is generally accepted that impact induced delamination area increases with the increase in the total absorbed impact energy in collisions with foreign objects. But the multiaxial warp knit composites showed that the total absorbed impact energy decreased with the increase in through-the-thickness reinforcement fibres. The multiaxial warp knit composites showed reduced vertical deformations compared with woven laminates, as the reinforcing loop density increased.

Analysis of multiaxial warp-knit preforms for composite reinforcement

Three-dimensional structures of multiaxial warp-knit (MWK) fabrics have been recently developed for multidirectional reinforcement of composites. Multilayers of linear yarns are assembled in warp (0 °), weft (90 °), and bias (±θ) directions to provide in-plane reinforcement in specific directions, and they are stitched together by knitting yarns to provide structural integrity and through-the-thickness reinforcement. Based on the experimental observations, the unit cell geometry of the MWK fabric is identified along with idealized cross-sectional shapes of insertion and stitch yarns. A geometric model is developed relating the geometric parameters and process variables. Expressing fiber volume fraction and fiber orientation in terms of structural and processing parameters, this geometric model provides a basis for the establishment of process windows for the MWK fabric preforms as well as for the prediction of the mechanical behavior of the MWK reinforced composites.