Multiaxial Warp-knitted Fabrics Research Articles

Mechanical properties of flexible composites reinforced with high-performance glass fiber multi-axial warp-knitted fabrics

This paper investigates the quasi-static mechanical properties and damage behavior of flexible composites prepared from high-performance glass fiber-based multi-axial warp knitted fabrics as reinforcement and high tenacity environmentally friendly thermoplastic polyurethane as a matrix. The composites were produced by attaching thermoplastic polyurethane (TPU) to the top and bottom of the fabric using a vulcanization machine, followed by hot-pressing for different temperatures and time settings. Moreover, the effects of different preparation processes on the tensile strength, static puncture resistance, and tear strength of flexible composites were investigated in this paper. The results indicated that both hot-pressing temperature and vulcanization time had a significant impact on the mechanical properties of the composites. With an optimum vulcanization temperature of 185°C and a vulcanization time of 10 min, the composite provides optimum tensile strength and puncture resistance. Tear strength is the worst and is related to the reinforcement’s organisational structure and the interfacial bond’s strength. The results of this study are of theoretical and practical significance for applying high-performance multi-axial warp-knitted flexible composites as raw materials in the construction field.

Uniaxial tensile strain correction of multi-axial warp-knitted fabric

This paper analyzes the error between the theoretical calculated and actual values of single inserting yarn strain of multi-axial warp-knitted fabric under the uniaxial stretching process, and the “open M-shaped curved surface” of error was obtained theoretically. The analysis results show that the error increases with the increase of fabric elongation. The error is smaller in the small deformation range (fabric maximum elongation is 5%) and larger in the large deformation range. In addition, the error is also related to the angle between the single inserting yarn axis and the uniaxial tensile direction. The error varies periodically with the angle ([Formula: see text]), reaching its maximum when it is close to [Formula: see text]. In order to realize the correction of the existing theoretical calculated formula of single inserting yarn strain, the error formula was fitted by combining the laws of errors. The correction of the single inserting yarn strain was successfully achieved by the fitted error formula. The correction equation can correct the inserting yarn at any angle in the fabric strain range of 0–50%. The correction equation has a larger range of use and more accurate results than the theoretical calculated formula for inserting yarn. Finally, the validation was carried out with bias inserting yarn at [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] angles and the results were very good. Therefore, the corrected equation can replace the actual value of bias inserting yarn strain.

Strength characteristic and failure criterion of flexible multi-axial warp-knitted fabrics coated with polyurethane on single side

Coated fabrics are widely used in tension structures. In order to investigate the tensile behaviors of the coated fabrics, one group of specimens was cut out from multi-axial warp-knitted polyester fabrics coated with polyurethane on one side. These failure strengths of polyurethane-coated multi-axial warp-knitted fabrics were obtained by the method of uniaxial tensile test and a failure stress criterion is presented for the kind of materials. The results show that the coated fabric is anisotropic material, and the coating has great influence on the mechanical properties of the material. Furthermore, the failure stress criterion can be used for references in material design and application.

Short-term creep behavior of multi-axial warp knitted fabric-reinforced polyethylene matrix composites

Short-term creep experiments of composites of polyethylene reinforced with polyester fiber multi-axial warp knitted (MWK) fabrics were carried out for tensile and shear modes in three directions. This work primarily focuses on using the Burgers and Findley models to obtain parameters associated with stress level, temperature, and structure of MWK fabrics. Specifically, the material parameter A of the Findley model has higher values at elevated temperatures, which is contrary to the impact of fiber content along the loading direction. The material parameter n rises with an increasing temperature and is not affected by the fiber content. In terms of the Burger parameters, material parameters E1, E2, and η1 have a downside as temperature increases. The relationship of these parameters with fiber content is contrary to their relationship with temperature. There is no significant effect of temperature and fiber content on relaxation time, whereas there is an upward trend of relaxation time with increases in stress.

Tensile characteristics of flexible PU-coated multi-axial warp-knitted fabrics

In practice, the coated fabric is available in many shapes and sizes, not just rectangle and plane. In order to increase the shape-shifting abilities of coated fabrics, polyurethane-coated multi-axial warp-knitted fabric (PU-CMWKF) is developed. In this paper, the tensile characteristics of PU-CMWKF were investigated. Five groups of tensile experiments, with off-axial angles of 0°, 22°, 45°, 67.5°, and 90°, were conducted under constant velocity. The failure mechanism was explored by analyzing the damaged specimens. Additionally, the deformation behavior of PU-CMWKF in three regions was investigated by utilizing the digital image correlation (DIC) system, and an orthogonal anisotropic model was used to predict the modulus and Poisson’s ratio of 22.5° and 67.5°. Research results showed that the apparent modulus of PU-CMWKF strongly depended on the cut of directions. And the failure mechanism under in-plane direction loading suggests that tensile and shear failure act together. The analytical model is validated along five directions in the representation of elastic constant under corresponding small strain.

Evaluation of adhesive binders for the development of yarn bonding for new stitch-free non-crimp fabrics

Non-crimp fabrics (NCFs), especially multi-axial warp-knitted fabrics, are used as reinforcement materials for fiber-reinforced composites. The manufacturing of multi-axial warp-knitted fabrics by a conventional stitch bonding process to produce NCF has several disadvantages, such as filament damage, low production speed, yarn disorientation, etc. In order to overcome the existing limitations, the idea of using an adhesive binder to attach the fabric layers is a promising approach, so that the use of stitching yarns can be eliminated. The fundamental investigations presented in this paper show that the selection of the binder material has a major influence on the parameters of the textile products. Whereas the tested hotmelt adhesives offer a short curing time and a small but nevertheless sufficient bonding strength between bonded yarns, the tested reactive adhesives show a bonding strength up to 10 times higher, but at a considerably longer curing time. The reason for the different bonding strength is identified in the different penetration into the yarns. The experiments also show a significant influence of the fiber type and sizing, which needs to be taken into account when selecting fabric binders.

Tear properties and meso-scale mechanism of multi-axial warp-knitted fabric from various architectures: Studied by photography

Tear properties tests of three kinds of multi-axial warp-knitted (MWK) fabric: triaxial fabrics (−45 °/90 °/+45 ° and −45 °/0 °/+45 °) and biaxial fabric (−45 °/+45 °) were respectively implemented along the orientations of 0 °, 45 ° and 90 °. Load-displacement curves were well obtained. The high-speed camera was employed to observe the whole tear damage process and series of still images were picked up to analyze the meso-scale mechanism of the extension and displacement of tows on oriented layers. The results indicated the tear properties of MWK fabric are closely related to the orientations of the layers and the deformation mechanism of the layers. Stitching yarns incarnated significant effects in restricting the deformation procedures of tows.

Mechanical analysis of geocomposites consisting of multi-axial warp knitted fabric and nonwoven mat

An eco-friendly shore protection system for preventing the erosion of shore surfaces has been developed using water permeable geocomposites that allow grass to take root in them. The geocomposites require enough stiffness to resist the flow of the water in the stream and good permeability of the flowing water. In this study, a geocomposite is designed using multi-axial warp knitted fabric (MAWKF) and nonwoven mats to ensure both the mechanical stiffness and water absorption. Firstly, a stress analysis is performed for a shore protection system to calculate stresses which are imposed onto geocomposites. To test geocomposites’ capacity to withstand the stress, the geometrical modeling of a MAWKF is carried out and the mechanical behavior of its unit-cell is analyzed using finite element method. Comparing the predicted results with experiments, the validity of the current modeling is demonstrated. Finally the adaptability of the geocomposite is evaluated using the calculated deformation.

Experimentally Study on Mechanical Behavior of CF and GF Hybrid Fiber-Reinforced Plastics

The multiaxial warp-knit fabrics of glass fiber or carbon fiber as the structure materials have widely applied to many industries. In this study, in order to combined the advantages of these two fibers, glass fiber and carbon fiber were employed as reinforcement materials in RIMR 135 epoxy resins, and hybrid composites were formed. The tensile behavior of hybrid fiber-reinforced plastics (HFRP) were compared with CFRP and GFRP on the longitudinal orientation . The results suggested that HFRP was high tensile strength and modulus of elasticity. Scanning electron microscopy was used to characterize the morphology of damaged surfaces. The micrographs revealed that CF and GF maintained their own tension behavior in the hybrid composite materials as well in the neat FRP materials. The hybrid effect of HFRP was investigated by the hybrid theory, which was compared to the experimental results. It was confirmed that the tensile behavior of the hybrid composite materials matched the plus hybrid effect.

Damage Tolerance Behaviour of Carbon Fibre Multi-axial Warp Knit Fabric Composites

The stitch formation in Multi-axial Warp Knit Fabric (MWKF) is different to that found in traditional overstitching. The effect of such stitching is investigated and reported in this paper. Three different variants of stitch architectures in carbon MWKF, used in an epoxy matrix, were considered to investigate the effect of stitch density. Low energy impact, through penetration impact and compression strength testing were carried out. During low energy impact, a higher stitch density offered more damage resistance, being directly influenced by the through-thickness stitches, which have been shown to stop or deviate crack propagation. Similar results were also found during through penetration impacts. Compression after Impact properties were found to vary directly with the damage area. Whilst a high stitch density resulted in better residual strength properties, the percentage retention of the compression after impact strength was superior for low stitch density MWKF.

ELASTIC PROPERTY PREDICTION OF THE STITCHED MULTI-AXIAL WARP KNIT FABRIC COMPOSITES

In order to improve the resistances to delamination, damage tolerance, some in-plane and out-of-plane properties of composite materials, a through-thickness reinforcement must be provided. This through-thickness reinforcement is achieved by stitching multi-axial warp knit (MWK) fabrics used as preforms for the fabrication of composite materials. The MWK fabrics are constructed with layers of insertion fiber bundles in the warp, weft and bias directions. In order to correlate the microstructure of a preform with the elastic properties of stitched MWK composite, the analytical model for stitched MKW composite is developed. The overall geometry and geometric parameters of a representative volume are determined from the photomicrographs of cross sections of the fabricated composite specimens. The various elastic properties of MWK fabric composites are predicted as functions of various geometric parameters using an averaging method. The experimental results are compared with the predicted results in order to validate the suggested model. It is found that the predicted elastic properties are in reasonably good agreement with the experimental values.

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.

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.

Reinforcement of CFRP Structures by Tailored Fibre Placement

By Tailored Fibre Placement (TFP), a new process based on embroidery technology, it becomes possible to reinforce composite parts very efficiently by stitching additional fibres with defined alignments to a given textile preform locally at highly loaded areas. The attachment of large CFRP aircraft structures is often achieved by shear pin couplings, applying bolt bearing pressure to the composite around the loaded hole. In this study, the potential of TFP to improve material strength of CFRP couplings was investigated by conducting pin loaded bearing tests with samples made of multi-axial warp knitted fabric (MWF) impregnated with Hexcel-RTM6 resin. Samples stitched with different fibre patterns to reinforce the loaded hole were compared to samples with a simple planar thickening in this critical area by additional layers of fabric. TFP was efficient (comparing the additional strength to additional structural mass) regarding bearing strength. However, the best results for the improvement of ultimate bearing strength (failure load) were achieved by increasing the part thickness locally with fabric patches.

Reinforcement of CFRP Structures by Tailored Fibre Placement

By Tailored Fibre Placement (TFP), a new process based on embroidery technology, it becomes possible to reinforce composite parts very efficiently by stitching additional fibres with defined alignments to a given textile preform locally at highly loaded areas. The attachment of large CFRP aircraft structures is often achieved by shear pin couplings, applying bolt bearing pressure to the composite around the loaded hole. In this study, the potential of TFP to improve material strength of CFRP couplings was investigated by conducting pin loaded bearing tests with samples made of multi-axial warp knitted fabric (MWF) impregnated with Hexcel-RTM6 resin. Samples stitched with different fibre patterns to reinforce the loaded hole were compared to samples with a simple planar thickening in this critical area by additional layers of fabric. TFP was efficient (comparing the additional strength to additional structural mass) regarding bearing strength. However, the best results for the improvement of ultimate bearing strength (failure load) were achieved by increasing the part thickness locally with fabric patches.

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.