Warp Knitted Structures Research Articles

PATTERN RELATED ISSUES IN THE MODELLING OF DEFORMED OVER SURFACE WARP KNITTED STRUCTURES WITH LONGER UNDERLAPS

The yarn level modelling of warp knitted structures is a complex process. For structures placed on the plane, it is well investigated and there are a few software solutions and papers reported. This paper considers the simulation of warp knitted structure, deformed in the 3D space. Especially the modelling of the areas of high curvature are detailed observed. Underlaps with longer lengths makes an unreal visualization for simulation results. Different pattern with different length of the underlaps are modelled with original algorithm developed by the authors. Modelling and visualization problems in the areas with long underlaps are discussed and possible solutions are proposed.

Investigation of flexible warp-knitted metal meshes for electromagnetic interference shielding

This paper develops four novel warp-knitted structures with two bars of the one-in one-out threading mode and describes the investigation of electromagnetic interference (EMI) shielding metal meshes composed of contiguous ultra-fine metal wires fabricated using warp-knitting technology. The mechanical properties, electrical conductivity, and microwave shielding effectiveness (SE) of the flexible meshes are assessed in terms of the mesh structure, mesh material, and warp run-in parameter. The results reveal that the metal mesh has a flexible and stable mechanical property, good electrical conductivity, and optimal EMI performance when the mesh structure is open lap atlas. The gold-coated molybdenum mesh exhibits the lowest electrical resistivity (5.18 × 10−5 ohm/sq) and optimal average EMI SE (–21.04 dB), making it a candidate for EMI shields for intelligent wearable electronic devices. The warp run-in can adjust the EMI SE, and the optimum average EMI SE (–25.52 dB) can be obtained when the warp run-in is set at the 2580 rack. Moreover, the warp-knitted metal meshes show desirable EMI shielding efficiency persisting at >70% even at large strain deformations and under cyclic bending and twisting deformations. Conductive warp-knitted meshes with a millimeter period and micron-scale wire diameter exhibit an adequately large scale, satisfactory mechanical stability, excellent flexibility, permanent conductivity, and strong microwave reflection, simultaneously. Moreover, the EMI SE of warp-knitted metal mesh reaches the protection level (–20 dB) prescribed by standard electronic devices. Therefore, the warp-knitted metal meshes show great potential in flexible and wearable EMI shielding devices.

Analysis of the stress relaxation of synthetic surgical meshes in view of loading direction

Surgical mesh is a porous fabric, which is used for healing hernia, pelvic organ prolapse, and urinary incontinence. In the present study, synthetic surgical meshes with five different warp knitted structures, including Tricot, Pin hole-net, Sandfly, quasi-Sandfly, and quasi-Marqussite, have been fabricated using polypropylene monofilament, and the effect of loading direction on the stress relaxation of the meshes in orthogonal directions have been considered in comparison with human fascia. The results revealed that loading direction has a significant effect on the stress relaxation of the mesh. Different stress relaxation of various mesh structures in orthogonal directions can be attributed to the pore shape (pore angle) and underlap angle in the structure of the mesh. Except for Pin-hole-net mesh, all mesh structures displayed a constant and independent level of orthotropy to the relaxation time. Moreover, Tricot mesh and quasi-Sandfly mesh, with the level of orthotropy near to1, revealed almost isotropic viscoelastic properties. Overall, the quasi-Marqussite mesh was proposed as the most desirable surgical mesh due to its lighter weight, higher porosity, lower stress relaxation, and closest level of othotropy to the human fascia.

Experimental and CFD Analysis of Air Permeability of Warp-knitted Structures

Functionality of technical textiles and apparels in a wide spectrum of end-uses is intensely dominated by their air permeability, which in turn depends on fabric porosity and structure. This work aims to investigate air permeability of warp-knitted fabrics using experimental and numerical simulation methods. Samples of locknit, three-needle satin and four-needle satin two-bar warp-knitted fabrics at tight, medium and loose densities together with single-bar 1×1 and 2×1 warp-knitted fabrics were knitted. The effect of loop density, underlap length and fabric structure on air permeability was investigated. The 3D geometry of locknit and single-bar 1×1 and 2×1 warp-knitted fabrics was modeled using CATIA. The structural geometry of a knitted loop as the unit cell of warp-knitted fabrics was simulated based on Vassiliadis model. The geometry models then were coupled with a computational fluid dynamics (CFD) model for flow simulation. Fluid flow through the fabric structure was simulated by numerically solving incompressible creeping Newtonian flow through the pore space of generated knitted structures using Fluent. The results were then compared with experimental data. It was found that 1×1 single-bar fabrics have higher air permeability than 2×1 single-bar fabrics. It was also found that single-bar fabrics show significantly higher air permeability compared to two-bar warp-knitted fabrics. For the two-bar fabrics, the results point to higher air permeability of locknit fabrics, followed by three-needle satin and four-needle satin. The results also indicated that increase in underlap length and loop density leads to reduction of both pore size and fabric porosity and therefore decreasing air permeability. It was found that the experimental and numerical results are acceptability compatible with error of less than 16 %. It was concluded that the proposed model is potentially capable of prediction of air permeability of single-bar and two-bar warp-knitted fabrics.

Three-dimensional simulation of warp knitted structures based on geometric unit cell of loop yarns

Warp knitted fabrics are typically three-dimensional (3D) structures, and their design is strongly dependent on the structural simulation. Most of existing simulation methods are only capable of two-dimensional (2D) modeling, which lacks perceptual realism and cannot show design defects, making it hard for manufacturers to produce the required fabrics. The few existing methods capable of 3D structural simulation are computationally demanding and therefore can only run on powerful computers, which makes it hard to utilize online platforms (e.g. clouds, mobile devices, etc.) for simulation and design communication. To fill the gap, a novel, lightweight and agile geometric representation of warp knitting loops is proposed to establish a new framework of 3D simulation of complex warp knitted structures. Further, the new representation has great simplicity, flexibility and versatility and is used to build high-level models in representing the 3D structures of warp knitted fabrics with complex topologies. Simulations of a variety of warp knitted fabrics are presented to demonstrate the capacity and generalizability of this newly proposed methodology. It has also been used in virtual design of warp knitted fabrics in wireless mobile devices for digital manufacture and provides a functional reference model based on this simplified unit cell of warp knitted loops to simulate more realistic 3D warp knitted fabrics.

Development of auxetic warp knitted fabrics based on reentrant geometry

This paper reports a novel method to fabricate auxetic warp knitted fabrics on a tricot warp knitting machine based on a special structure design and knitting process. Three auxetic warp knitted structures were successfully developed and all of them were designed to form a reentrant hexagonal geometry to acquire an auxetic effect. To provide the fabrics with reentrant frames, elastomeric yarns and stiff yarns were used to form the elastic underlaps and stiff underlaps, respectively. While the elastic underlaps can keep the reentrant structures stable, the stiff underlaps can support the reentrant structures to keep their reentrant frames. To alleviate the transfer of yarns in the loops under tension, an additional front yarn guide bar was used to feed binding yarns to basic reentrant structures. After knitting, all the fabrics were subjected to a heat setting process to keep their shape and a tensile test to assess their auxetic behavior. The experimental results showed that all the fabrics exhibited an obvious auxetic behavior within a wide range of strain. In particular, the fabrics retained auxetic behavior until breaking when stretched in the wale direction, and the Poisson’s ratio of the fabrics could be as low as –0.5. On the other hand, the fabrics exhibited a large tensile elongation when stretched in the course direction and their Poisson’s ratios changed from negative to positive after the tensile strain exceeded a certain value.

Fibrous structures in augmentation for rotator cuff repair: an experimental comparison

Background and purpose. Rotator cuff tendon (RCT) injuries are difficult to manage as current strategies do not restore tissue functionality. Driven by this market demand, several devices have been developed during the last years, however, to develop a structure that properly mimics the mechanical performance of the native tissue remains a challenge. This way, in this work we study a set of design criteria for a synthetic graft development for RCT repair and reconstruction, in order to achieve a structure with an adequate mechanical performance to substitute the damaged tissue, and that can also act as a scaffold, allowing and guiding tissue ingrowth. Materials and Methods. Two three-dimensional (3D) and two planar (2D) warp-knitted structures were developed using polyethylene terephthalate (PET) yarn, and are characterized in terms of their morphology, absorption and wicking behavior, and mechanical properties. Results. The present warp-knitted structures mimic the non-linear mechanical behavior of the native tissue. A less porous planar structure was concluded to be the best to provide mechanical augmentation, as it demonstrated mechanical properties suitable for RCT repair. Pre-tensioning techniques were adequate to increase this type of textile’s mechanical properties. Furthermore, the physical properties of this structure can allow and guide tissue ingrowth, demonstrating its potential to act also as a scaffold. Conclusions. As this is a relatively new field of application of knitting technologies, this work provides important information on how a warp-knitted structure should be tailored and treated post-production, in order to be used for RCT repair.

Parametric modeling of three-dimensional geometry of warp-knitted loop based on variation of process parameters

The purpose of this study is to develop a parametric 3-D loop model for warp-knitted structures which can predict loop geometry with variation of process parameters. Firstly, a process matrix of warp-knitted structure and a matrix including 3-D coordinates of date points were, respectively, developed. Then, based on internal stress analysis, an algorithm were suggested which can calculate the process parameter into 3-D coordinates of date points in geometric model of warp-knitted loop. Moreover, loop model were improved by space curve fitting based on piecewise cubic Hermite interpolating polynomial (CHIP). Furthermore, the developed parametric loop model was programmed with software of Matlab2014, and four test samples with different process parameters were simulated. The accuracy of the proposed model was verified by between simulations and micrographs, as well as agreement between predicted run-in values and the measured ones. It is confirmed that 3-D geometry of warp-knitted loop can be well predicted by the proposed parametric loop model.

About the 3D Modelling of Jacquard Warp Knitted Structures

This paper presents a method for extending the algorithms for 3D modelling of warp knitted structures, which allows 3D simulation of jacquard warp knitted structures. It starts with short overview of the state of the art and presents some modelling issues and two simulated examples of jacquard structures.

Design and fabrication of auxetic warp-knitted structures with a rotational hexagonal loop

In this paper, the design, manufacturing and characterization of two-dimensional warp-knitted textiles with auxetic performance is reported. Four warp-knitted structures based on a rotational hexagonal structure are produced, and these structures can lead to a negative Poisson’s ratio mathematically. The testing results have confirmed that the knitting structure of the front bar, as well as let-off values of the front bar’s chain parts, has a great effect, and auxetic properties of the warp-knitted textiles have a complicated relationship with the rotation angle. These novel structures can expand the applied area of auxetic structures.

About the automated pattern creation of 3D jacquard double needle bed warp knitted structures

Three dimensional structures can be produced on jacquard warp knitting machines with double needle bed. This work presents theoretical considerations about the modelling and simulation of these structures. After that a method is described, how to obtain production parameters from the simulation data. The analysis demonstrates, that the automated pattern creation of 3D structures is not always possible and not all mathematical solutions of the problem can be knittable.

The influence of two bar warp-knitted structure on the fabric tensile stress relaxation Part II: (locknit, satin, loop raised)

When strain is applied constantly, there is a decreased stress with time in viscoelastic materials, which is called stress relaxation. During the manufacture and application of clothing and footwear, materials experience various long-lasting deformations, and relaxation process in materials arises. Thus, with theoretical and experimental study of the factors affecting stress relaxation, the ability to design and produce appropriate clothes will be increased. In the first part of this research, we studied the stress relaxation behavior of warp-knitted structures which have longer underlaps in back bar (reverse locknit, three- and four-needle sharkskin, and queens’ cord). Following the previous research, the aim of this study was to investigate the effect of fabric structure, strain percentage, and course density on the stress and stress relaxation of the warp-knitted structures which have longer underlaps in front bar (locknit, three- and four-needle satin, and loop raised). The results reveal that the fabric structure, strain value, and fabric density are important factors affecting the stress and stress relaxation percent of the fabrics. By increasing the strain and the length of underlap in the front guide bar, stress and stress relaxation percent will be increased. Also, fabrics with higher course density show higher stress and stress relaxation percent. Among the mechanical models used to describe the stress relaxation behavior of the fabrics, the three-component Maxwell’s model with parallel-connect nonlinear spring showed the best agreement with the experimental stress relaxation curves of the analyzed fabrics.

3D auxetic warp-knitted spacer fabrics

Auxetic fabrics have gained an increasing attention in the auxetic material area in recent years. Especially, auxetic fabrics fabricated based on both weft-knitted and warp-knitted structures have made some important progress. However, all the knitted auxetic fabrics reported so far are fabricated based on 2D knitted structures, although some of the weft-knitted auxetic fabrics are finally formed into 3D forms. This paper reports a novel kind of 3D auxetic fabric fabricated based on a warp-knitted spacer structure with adoption of a special geometrical configuration which is formed with parallelograms. The auxetic behavior of these fabrics is discussed in terms of tensile direction, geometrical parameter, and repeated extension. The shape fitting ability on a spherical surface is also demonstrated. The results shows that the 3D auxetic fabrics have auxetic behavior in all the fabric plane directions and the highest auxetic effect is obtained when stretched in the weft direction of the fabric. The 3D auxetic fabrics also display an excellent shape fitting ability, which makes them very attractive for various applications where fitting to the human body's shape is highly required, such as bra cups, protectors for knees and elbows, shoes covers, etc.

Analysis of warp-knitted fabric structure. Part II: theoretical study on initial modulus of warp-knitted fabrics (tricot, locknit, and satin)

In the first part of this series, a 3D straight line model was presented for the geometry of two-guide-bar warp-knitted fabrics. In this part, first, the variation of unit cell geometry under uniaxial tension in the range of elastic deformation is investigated. In the present study, the energy method and Castiglianos's theorem were used to derive the fabric's initial modulus. Energy terms which are considered in this study are extension, bending, and compression energies. Also, the friction phenomenon is taken into account by using frictional energy term. For this purpose, the loop of each bar is divided into eight elements. Free-body diagrams of elements are drawn to show the relative magnitude and direction of all forces acting upon an object in a given situation. After obtaining the force details, strain energy of each element is calculated. Finally, the initial modulus of warp-knitted fabrics is obtained. To verify the model, samples of tricot fabrics were produced and tested. Results showed that the generated model can predict precisely the initial modulus of aforementioned warp-knitted structures.

A relationship between dry-relaxed warp-knitted fabric structure and curling distance

This study provides us information about relationship between curling and knitted structure and density of two-guide bar warp-knitted fabrics. Five standard warp-knitted fabrics are produced with three densities (Tricot, Locknit, three-needle Satin, Reveres Locknit and three-needle Sharkskin). Certain cut lengths of sample fabrics in both wale and course direction are analyzed through image processing to define curling value. A mechanical model, a function of fabric structure and yarn parameters, is also suggested to predict the curling force. The results of experimental and theoretical results show a greater curling value for course-wise cut in comparison to that of wale-wise in all warp-knitted structures and densities. The lowest curling value is found for three-needle Sharkskin structure in both directions and all densities.

Analysis of warp knitted fabric structure. Part I: a 3D straight line model for warp knitted fabrics

It is clear that the geometry of fabrics has an important role in their properties. Hence, to predict the behavior of fabrics, it is essential to define a suitable geometrical model. The structure of warp knitted fabrics is more complex than other fabrics. Therefore, to investigate the behavior of warp knitted structures, we need to define a simple model. The aim of this paper is to present a 3D straight line model for two‐guide‐bar warp knitted fabrics. To verify the suggested model, geometrical parameters, i.e. run‐in and weight per unit area of different samples were used. The investigation showed that in comparison to previous loop models, results of the straight line model correspond more to the experimental values. It is concluded that a straight line model can predict satisfactory geometrical properties of two‐guide‐bar warp knitted fabrics.

Geometry modelling of warp knitted fabrics with 3D form

Warp knitted fabrics are widely used in medical and technical applications, as well as for clothing textiles. To predict their properties, it is useful to have knowledge about the yarn geometry and the yarn interactions within these fabrics. The geometrical modelling of warp knitted structures from single bed knitting machines is documented in the literature. This text concentrates on a computation of the geometry of warp knitted structures, which have complex 3D forms or which are draped over similar ones. The topological information about the structure produced on the machine, is used to create the basic geometry of this structure. After creating the basic geometry, it is transferred to a 3D deformed state of the relaxed fabric. The paper starts with a short overview of the research papers about simulation of warp knitted structures. General theoretical remarks about the transformation from basic structure to 3D structure are given. Examples of spacer fabric with constant or non-constant thickness and tubular fabrics are included. All the results concentrate on the topological representation of the geometry and at this stage do not take into account the influence of internal and external forces exerted on the fabric.

Knitting of basalt filament yarn

Basalt filament yarn is a kind of inorganic fiber material made from basalt rocks and has widely been used to produce textile preforms for composite reinforcement. However, most of basalt textile preforms found on the market are made with woven or directionally oriented warp knitted structures. Weft-knitted preforms made of basalt filament yarns are still very limited due to the difficulty of loop formation. This paper presents an experimental investigation of knitting basalt filament yarn using a computerized flat knitting machine. The knitting effects were discussed based on the analysis of the variations of the loop length and yarn damage with different important knitting parameters such as yarn input tension, stitch cam setting and fabric take-down tension. The comparisons with glass filament and wool/acrylic blended yarn under the same knitting conditions were also presented. The study has shown that the basalt filament yarns can be more easily knitted on a flat knitting machine than glass filament due to lower yarn damage effect and larger adjustment scope of stitch cam setting.

TEXTILE COMPOSITE MATERIALS FOR SMALL INTESTINE REPLACEMENT

Abstract Down to the present day there are no sufficient techniques for a small intestine replacement, mostly because of the high standards for such implants. An indication for the need of novel operation techniques is the small patient survival rate of just 80 % for isolated small intestine transplantation and 62 % for combined liver-small intestine transplantation. The five year survival rates of the patients are merely 42 %. In order to overcome these limitations the authors are developing a partly resorbable textile-foam-composite for small intestine replacement The novel implant consists of a non resorbable textile PVDF mesh which is foamed with a micro porous, resorbable, and drug loaded polymer. The resorbable polymer serves on the one hand as initial sealing, therefore no intestine substance and bacteria can leak out into surrounding tissue, on the other hand it needs to be micro porous in order to ensure cell ingrowth. For the macro porous textile mesh warp knitting technology is used. The warp knitted tubular structure remains inside the body as a long term implant and provides mechanical support to ingrowing cells. In order to evaluate biomechanical properties of the warp knitted tubular PVDF meshes to compare them to the mechanical characteristics of small intestine tissue, tensile tests were conducted. Results of tensile tests on warp knitted structures with three different loop densities of 8, 12, and 16 loops per cm were compared to tensile tests on native small intestine tissue probes. The recorded curves of small intestine and warp knitted structures showed similar characteristics. The two characteristic Young Modules as well as the curve progression of the warp knitted structure with 12 loops per cm showed good accordance to the values of the native small intestine. Morphological analysis of the textile structures by digital image processing showed adequate pore size and porosity of the textile mesh.

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.