Warp-knitted Fabrics Research Articles

Numerical investigation on central tearing behaviors and propagation mechanisms of coated warp-knitted fabrics

To reveal the central tearing behaviors and propagation mechanisms of the architectural non-crimp fabric (NCF) composites with an initial crack, a microscopic finite element (FE) model was proposed and developed to simulate the tearing propagation process of materials in this paper. Firstly, the experimental program including the central crack tearing test of NCF composites and uniaxial tensile test of yarns was performed for gaining the material parameters. Secondly, a finite element (FE) model was developed and validated from the good agreement between numerical and experimental results. Furthermore, tearing damage mechanisms and failure performances of the NCF composites with an initial central crack were simulated and analyzed. Finally, the effects of initial crack length, yarn orientation, and arch curvature of weft yarn on mechanical properties were investigated in depth. The analysis and comparison results indicate that the NCF composite shows the unneglectable orthotropic characteristics, in which the initial crack length, yarn orientation, and arch curvature of weft yarn have the significant effects on the mechanical properties. This paper thus provides an adequately feasible and accurate FE model for the numerical simulation on the central tearing propagation behaviors of the NCF composites with an initial crack.

Time-Dependence of Stop Marks in Warp-Knitted Fabrics

Stop marks are one of the most frequently occurring errors in warp-knitted fabrics. They become visible in a fabric each time a warp-knitting machine stops and restarts. Nevertheless, investigations of such stop marks are rarely found in scientific literature. Here, we report on time-dependent investigations of stop marks in warp-knitted fabrics. Microscopic examination of stop marks after stopping times ranging between 1 s and 7 weeks revealed a superposition of the common stop mark due to imperfectly matching rotational speeds of the warp beam and main shaft, and an additional effect due to relaxation in the machine.

Analysis of the tensile creep performance of warp-knitted fabrics in technical applications in view of fabric structure

Creep is one of the time-dependent mechanical behavior of textile materials, which can effect on the appearance and performance of the fabrics in various applications from garment to technical applications such as tensile structures and geotextiles. In the present study, the effect of fabric structure and loading direction on the creep behavior of two bar warp-knitted fabrics (Tricot, Reverse Locknit, 3 needle Sharkskin, 4 needle Sharkskin and Queen’s cord) have been investigated. According to the results, the fabric structure and loading direction have outstanding influence on the creep of the fabrics. The creep of the fabric in the course direction decreases by increasing the length of underlaps in the back-guide bar, meanwhile increases in the wale direction. Moreover, Tricot and Revers Locknit structures demonstrated more creep in the course direction, whilst sharkskin 3, sharkskin 4 and Queen’s cord structures exhibited higher creep values in the wale direction. Moreover, among considered fabric structures, Reverse Locknit with level of orthotropy near to1, revealed almost isotropic viscoelastic properties.

A Calculation Method for the Deformation Behavior of Warp-Knitted Fabric

Abstract In this paper, a new method to simulate the structure and loop deformation behavior of double-bar reflex-lapping warp-knitted fabrics based on the structural characteristics is proposed. A simplified mass-spring model was built in which loops knitted by filaments were considered as particles with the uniform mass distribution connected by structure springs for overlaps and shear springs for underlaps. Deformation forces and direction on particles were analyzed to describe the displacement and deformation behavior of particles. A loop model with eight control points was established, and the relationship between control points and particles was studied combining the quadratic Bezier curves. The deformation simulation was implemented by a simulator program with C# and JavaScript via web technology on Visual Studio 2015. The stereoscopic sense of filaments was realized by changing the direction and intensity of the light. The results show that the fabric deformation and the loop shape can be accurately achieve using the simplified mass-spring model compared with the real sample.

Detailed Deformation Behaviors and Tensile Parameters for Coated Warp-Knitted Fabrics in 2D Stress Space

AbstractUnderstanding the mechanical properties of membrane materials in two-dimensional (2D) stress space is critical for structural design and mechanical analysis of membrane structures. In this ...

An intelligent defect detection system for warp-knitted fabric

In textile factories, the most typical warp-knitted fabric defects include point defects, holes, and color differences. Traditional manual inspection methods are inefficient for detecting these defects. Existing intelligent inspection systems often have a single function. Factories require a real-time inspection system that can detect common defects and color difference. The YOLO (you only look once) neural network is faster than the two-stage neural network and has lower hardware requirements. The system’s color difference detection algorithm compares the color difference between the standard image and the image to be measured and records where the color difference value is exceeded. Finally, the comparison of the factory application proves that the designed system has good real-time performance and accuracy and can meet the fabric inspection requirements of warp-knitted fabric factories.

Detailed characteristics of shear stiffness for coated biaxial warp-knitted fabrics subjected to coupled shear-tension loads

In order to reveal shear deformation mechanisms of architectural non-crimp fabric (NCF) composites, detailed shear stiffness-nonlinearities for a typical coated warp-knitted NCF composite under shear-tension coupling loading were studied from experimental investigation and FEA models. Shear-tension coupling effects on shear stiffness of the NCF composite were investigated for different stress states. Its shear stiffness shows significant nonlinear characteristics which are highly tensile stress level (TSL) related, and therefore a critical TSL value was obtained for the studied material. Moreover, a new method for determination of shear stiffness was proposed, and its feasibility was verified by FEA simulation using UMAT. In addition, differences in shear stiffness and micro-deformation mechanisms between NCF and plain-woven fabric (PWF) composites were analyzed specifically. Apparently, the yarn-to-yarn compression on the yarns could contribute to the different stiffness variations between these two composites.

Design and manufacture of three-dimensional auxetic warp-knitted spacer fabrics based on re-entrant and rotating geometries

Warp knitting technology is a fabric-forming technologies that is very suitable to fabricate three-dimensional (3D) auxetic fabrics due to its high efficiency and powerful pattern designing possibilities. In this study, two typical auxetic geometries, namely the re-entrant hexagonal network and rotating square solids, were selected as the design prototypes for the design and manufacture of 3D warp-knitted spacer fabrics. While two 3D warp-knitted spacer fabric structures with representative units of different sizes designed based on the re-entrant hexagonal geometry were manufactured by using a RD7 double needle bar Raschel machine with seven yarn guide bars, two 3D jacquard warp-knitted spacer fabrics with different base fabric structures designed based on the rotating squares geometry were fabricated by using a RDPJ4/2 double needle bar jacquard machine with two ground yarn guide bars and four jacquard guide bars. The Poisson’s ratios of these 3D warp-knitted fabrics in the course direction and wale direction were evaluated respectively through constant-rate tensile tests. The results revealed that the re-entrant hexagonal fabric structure with double chain stitches has auxetic behavior across a wide range of tensile strains along the course direction, while the rotating square fabric structure with elastic chain stitches as the base is auxetic within a narrow range of tensile strains along the wale direction. The study provides an alternative method to directly produce auxetic warp-knitted spacer fabrics through a single knitting process instead of using an additional post-compression and heat-setting process.

Role of Fabric Structure on the Second Tensile Elastic Modulus of Net Warp-Knitted Fabrics

This work aimed to study the second elastic modulus of net warp-knitted fabrics (NWKF) experimentally and theoretically. Net knitted fabrics were selected to study owing to their wide practical application and also lack of enough attention. Knitted fabrics have two regions of tensile elastic behavior which the initial one refers to the displacement of the elements of fabrics before jamming of the yarns. As soon as jamming takes place completely, the constituent yarns in the structure of fabrics go through tension. In other words, yarns in their spatial configuration resist the applied loads. This resistance is important in the ultimate strength of the fabrics and also in the composite materials reinforced with knitted fabrics. NWKFs were produced using a Raschel knitting machine. Uniaxial tensile tests were conducted in the course and wale directions. The second elastic modulus was measured using a statistical approach. In the theoretical part, a mechanical model based on the configuration of elements of fabrics after jamming status and using Energy method and Castigliano’s Theorem was proposed. The proposed model was used to calculate the second elastic modulus. To validate the model, the authors’ experimental data along with the collected data from the other researches were compared with the calculated values, and the results showed that the proposed model can predict the modulus reasonably. The results showed that the fabric structure including wale spacing, course spacing, and the number of lapping movements creating the holes affects the second elastic modulus despite the accepted concept, but the mechanical properties of yarns only influence the second elastic modulus of the fabrics.

Characterization of tensile creep behavior of fabric-reinforced PVC flexible composites

This paper presents an experimental study on the tensile creep behaviors of PVC flexible composites reinforced with both polyester warp-knitted biaxial fabrics and woven fabrics, and their viscoelastic properties under different temperatures (20 °C, 40 °C, 60 °C) were explored after 24 h creep with 10% stress. The results showed that all the samples presented the typical viscoelastic behavior, and their creep rates could reach to 90-97% after 7 h creep, and then being steady up to 24 h. Moreover, the strength retention enhancement of PVC flexible composites by warp-knitted fabrics was higher than that of PVC flexible composites enhanced by woven fabric. The Findley model was utilized to describe the creep behavior of PVC flexible composites enhanced by two kinds of fabrics. Through fitting of experimental data with the model, the anti-creep behavior of PVC flexible composites enhanced by biaxial warp-knitted fabric was superior than that of one improved by woven fabrics. The characterization of tensile creep behavior has great application potential in predicting the long-term performance of fabric-reinforced PVC flexible composites.

Comprehensive study on mechanical properties of coated biaxial warp-knitted fabrics

In recent years, coated fabrics have become the major material used in membrane structures. Due to the special structure of base layer and mechanical properties, coated biaxial warp-knitted fabrics are increasingly applied in pneumatic structures. In this article, the mechanical properties of coated biaxial warp-knitted fabrics are investigated comprehensively. First, off-axial tensile tests are carried out in seven in-plane directions: 0°, 15°, 30°, 45°, 60°, 75°, and 90°. Based on the stress–strain relationship, tensile strengths are obtained and failure modes are studied. The adaptability of Tsai–Hill criterion is analyzed. Then, the uniaxial tensile creep test is performed under 24-h sustained load and the creep elongation is calculated. Besides, tearing strengths in warp and weft directions are obtained by tearing tests. Finally, the biaxial tensile tests under five different load ratios of 1:1, 2:1, 1:2, 1:0, and 0:1 are carried out, and the elastic constants and Poisson’s ratio are calculated using the least squares method based on linear orthotropic assumption. Moreover, biaxial specimens under four load ratios of 3:1, 1:3, 5:1, and 1:5 are further tensile tested to verify the adaptability of linear orthotropic model. These experimental data offer a deeper and comprehensive understanding of mechanical properties of coated biaxial warp-knitted fabrics and could be conveniently adopted in structural design.

Response and failure modes of biaxial warp-knitted flexible composite subject to low-velocity impact

In this paper, biaxial warp-knitted fabrics were produced with different high tenacity polyester linear density and inserted yarns density. The low-velocity impact property of flexible composites made of polyurethane as matrix and biaxial warp-knitted fabric as reinforcement has been investigated. The effect of impactor shape and initial impact energy on the impact response of flexible composite is tested. The results show that the initial impact energy have minor effect on the impact response of the biaxial warp-knitted flexible composites. The impact resistance of flexible composite specimen increases with the increase of high tenacity polyester linear density and inserted yarns density. The damage morphology of flexible composite materials is completely different under different impactor shapes. The findings have theoretical and practical significance for the applications of biaxial warp-knitted flexible composite.

Long-term viability and extensibility of an in situ regenerated canine aortic wall using hybrid warp-knitted fabric

Many surgical materials promoting tissue regeneration have been explored for use in paediatric cardiac surgery. The aim of this study is to evaluate the long-term viability and extensibility of the canine aortic wall regenerated using a novel synthetic hybrid fabric. The sheet is a warp-knitted fabric of biodegradable (poly-l-lactic acid) and non-biodegradable (polyethylene terephthalate) yarns coated with cross-linked gelatine. This material was implanted as a patch to fill an oval-shaped defect created in the canine descending aorta. The tissue samples were explanted after 12, 24 or 36 months (N = 3, 2, 2, respectively) for histological examination and biomechanical testing. There was no shrinkage, rupture or aneurysmal change after 24 months. The regenerated wall showed prototypical vascular healing without material degeneration, chronic inflammation, calcification or abnormal intimal overgrowth. Bridging tissue across the patch was well-formed and had expanded over time. The biodegradable yarns had completely degraded at 24 months after implantation, as scheduled, but the regenerated aortic wall demonstrated satisfactory levels of mechanical strength and extensibility in tensile strength tests. The sheet achieved good long-term viability and extensibility in the regenerated aortic wall. These findings suggest that it is a promising surgical material for repairing congenital heart defects. Further developments of the sheet are required, including clinical studies.

Force attenuation capacity of weft-knitted spacer fabric in low-velocity impact

PurposeThis study aimed to determine the peak impact force and force attenuation capacity of weft-knitted spacer fabrics intended for padding that can be used for human body protection against impact.Design/methodology/approachA total of five weft-knitted spacer fabrics were fabricated with four different diameters of nylon monofilament yarns and one doubled monofilament yarns, respectively. The impact performances of the weft-knitted spacer fabrics were tested using a drop test method with a customized test rig to simulate falling. Impact tests were conducted on single- and multilayered experimental spacer fabrics to investigate the peak impact force and force attenuation capacity.FindingsIt was found that weft-knitted spacer fabric with a coarser or larger diameter of monofilament spacer yarn generated lower impact force and higher force attenuation capacity, thus resulting in better impact performance. Greater force attenuation can be achieved by utilizing a higher number of spacer fabric layers. However, the increase in thickness must be considered with the spacer fabric end use.Originality/valueThis study employed relatively coarse nylon monofilament yarn as spacer yarns to gain knowledge on the impact performance of weft-knitted spacer fabrics compared to warp-knitted spacer fabrics which are more common. The results showed that the diameter of spacer yarn significantly influenced the impact performance of the experimental weft-knitted spacer fabrics. These results could be useful for designing and engineering textile-based impact protectors.

Low-Velocity Impact Properties of Polyurethane Composites Reinforced with Warp-Knit Spacer Fabric

Eight kinds of polyurethane (PU)-matrix composite samples were prepared by impregnating the warp-knit spacer fabrics with flexible PU foam. An impact test was carried out to investigate the influence of fabric structure parameters on the compression behaviors and energy-absorption efficiency of the composites. By using finite element (FE) analysis and ANSYS software, the unit cell models of the composites were built to simulate the impact responses and get strain-stress curves. Comparison of FE simulation and experimental curves showed good consistency between them.

Auxetic behavior of warp knitted fabric under repeating tension

In our previous study, a novel class of auxetic warp knitted fabrics were developed and their auxetic behaviors were studied under a single tensile test. However, during daily use, the fabrics are usually subjected to repeating tension rather than single tension. Therefore, the durability of the fabrics’ auxetic performance is of great importance. So far, the auxetic behavior of fabrics under repeating tension has not systematically been investigated. In this paper, we report a study on the auxetic behavior of warp knitted fabrics under repeating tension. All the fabric samples were subjected to a repeating tensile test within a tensile strain of 25% until 100 tensile cycles. The results show that the fabrics can keep their auxetic effect in both course and wale testing directions after 100 tensile cycles, and the auxetic effect in the wale direction is retained longer under higher tensile strains than that under lower tensile strains with the increase of tensile cycles. The results also indicate that auxetic stability in the course direction is much better than that in the wale direction. We hope that this study can offer useful information to improve the auxetic stability of auxetic fabrics for practical use.

Mesh modeling and simulation for three-dimensional warp-knitted tubular fabrics

This article concentrates on a geometric-based simulation method for warp-knitted tubular fabrics. The warp-knitted tubular fabric is unfolded as a two-dimensional flat fabric after analyzing the structural characteristics of this kind of fabric knitted on the double needle bar warp knitting machine. Based on the principle of spatial transformation, a coordinate transformation algorithm using matrix operations is proposed, which allows a two-dimensional flat fabric to convert into a three-dimensional shaped tubular fabric. Spatial positions for data points of every single stitch are confirmed by coordinate mapping to fit the curved surface. Stitches undergo the deformation with the principle of the mass-spring model so as to make the mesh structure appear. Integrated with spatial transformation matrix operations, the validity of the proposed method is experimentally confirmed by simulating the warp-knitted tubular fabrics with the tube diameter changing of different segments, which realize the three-dimensional simulation for vivid shaped warp-knitted tubular products.

A generalized method of measuring the Poisson's ratio of warp knitted fabrics under uniaxial loading based on image processing technique

In this work, a generalized method for measuring the Poisson's ratio of fabrics is presented. The basis of this method is the two-dimensional nature of the fabric deformation. Through this method, the measurement error is very low compared to conventional methods. In the following, the error of measurement in the conventional methods has been proved theoretically then an experiment with five different knitted fabrics and an elastic sheet is designed. In addition, an algorithm based on the image processing technique is presented to measure longitudinal, transverse and area strain. The results of fabrics showed that the theoretical prediction about the error of conventional methods was predicted correctly. In the case of the elastic sheet, the same result has been found, but it was not significant statistically. It is suggested that the proposed method is capable of replacing all available methods of measuring the Poisson's ratio of fabrics and structures like them.

An algorithm for online detection of colour differences in warp knitted fabrics

The current color difference online detection algorithm has poor real-time performance. The noise affects color difference monitoring and sampling detection results are different from online detection. Therefore, a color difference online detection algorithm is studied. Firstly, the Flat Field and Perfect Reflector Method (PRM) are used to correct the color illumination uniformity of warp knitted fabric images. The image pyramid principle is used to down-sample the images to improve the online detection speed. Secondly, the principle of the histogram intersection method is used to pre-judge the color difference of warp knitted fabric image, and judge whether there is a color difference. The improved K-means algorithm is used to analyze the color types of the fabric, and the calculation formula is used to quantitatively calculate the color difference value. Finally, the algorithm has been verified in the factory. The speed of fabric inspection using the algorithm can reach 0.75–0.8m/s, and the color difference detection algorithm is compared with manual detection. The results show that the method can meet the real-time requirements of the system and it is consistent with the sampling test, and the detection speed increased from 0.2 to 0.8

Preparation and moisture management performance of double guide bar warp-knitted fabric based on special covering structure

This article provides a knitting method for warp-knitted fabrics with good moisture management performance. By adjusting of the warp knitting machine (mainly the bar), the design of lay-up method of the yarn, the selection of the yarn, and the warp let-off tension, the yarn covering relationship has been adjusted. By using two kinds of fineness polyester DTY, cotton yarn and spandex, designing two kinds of warp-knitting structures, six different warp-knitted fabrics have been knitted. The moisture management performance of these knitted samples was compared and evaluated. The study found that the greater the difference between the inner yarn and the outer yarn, the better the moisture management ability of the fabric. Polyester DTY with 50D/14F on the inner side and polyester DTY with 75D/288F on the outer side have the best moisture management performance. The polyester DTY with 50D/14F on the inside and 9.83Tex cotton yarn on the outside have higher wetting time and absorption rate.