Weft-knitted Composites Research Articles

Experimental investigation of pseudo-ductility in plain weft-knitted epoxy composite laminates under tension

The inherent brittleness of unidirectional fiber-reinforced composites represents a major complication in their broad practical applications. However, exploiting different reinforcement architectures can offer an opportunity for highly desirable ductile fracture nature. This article is aimed at investigating experimentally the tensile behavior of an epoxy laminate reinforced by different layers of plain weft-knitted fabrics. The fabricated laminates were subjected to uniaxial tension in different loading directions, and the corresponding tensile responses were recorded. Basic mechanical properties were characterized and digital image correlation (DIC) was employed to capture surface deformations. Tensile test results demonstrated strong nonlinearity and pseudo-ductility as well as loading-direction dependence in this type of composite laminates. Fracture tests were conducted showing high fracture toughness ranging from 10 to 20 MPa·m1/2. Typical damage modes were characterized using microscopy, which were coupled to a few distinct tensile deformation stages. This work demonstrates that weft-knitted composites are promising in permitting great amounts of pseudo-ductility with clear warning before complete failure. It is also expected to contribute to the understanding of the specific role of weft knit fabrics in composite laminates.

Multi-scale finite element modeling of bending behavior of 3D multi-cell spacer weft-knitted composites with different structures

In the present study, a multi-scale finite element model is proposed to predict the linear and nonlinear behavior of the 3D multi-cell spacer weft-knitted composite under bending load. In this study, a unit-cell of the composite which includes plain and biaxial weft-knitted structures was modeled at the meso scale. Periodic boundary conditions were applied to the meso model to calculate the elastic constants of each composite structure. In order to obtain failure parameters of the composites, the Puck failure criterion model was utilized by a VUMAT code for the meso model. Afterward, the elastic constants of the composites based on a Python code were extracted from the meso model. Moreover, failure parameters that include tensile and compressive strength through the fiber and transverse directions were obtained from the meso model. All elastic and failure parameters were used for the macro model which is created with different profiles under bending load. The numerical results at the meso scale showed that the presence of the weft and warp yarns inside the biaxial weft-knitted composite increases the strength of the composite through the course and wale directions. Moreover, the stiffness of the composite would be improved. So, the samples that contained a biaxial composite had more stiffness and bending strength in comparison with plain composite samples because the top and bottom layers were manufactured by the biaxial weft-knitted structure. Besides, the comparison between numerical and experimental force–deflection curves showed that the proposed model could predict the linear and nonlinear behavior of the composites with high accuracy. So, this model can be used for other textile composites with complex shapes to predict the mechanical behavior of them.

Effect of profile and reinforcing the surface layers on bending behavior of 3D multi-cell spacer weft-knitted composites

3D multi-cell spacer-knitted reinforced composites are composed of knitted layers which are joined together by multiple fabric connecting layers. Despite numerous capabilities, they demonstrate inferior bending performance due to their highly looped construction as well as low fiber volume fraction. Bending characteristics could be enhanced by inserting the warp and weft yarns through the fabric’s surface layers in wale and course directions. This research aims to investigate the role of reinforcing warp and weft yarns and structure profile on bending behavior of the composites produced from 3D multi-cell spacer-knitted fabrics. 3D multi-cell spacer-knitted fabrics were produced with rectangular cross-sectional shape in single- and double-decker profiles on a computerized flat-knitting machine. The surface layers of both 3D multi-cell spacer-knitted fabrics were reinforced by glass straight yarns in course and wale directions. The produced fabrics were used to fabricate thermoset 3D composites using epoxy resin via vacuum-assisted resin-transfer method. Composite samples were subjected to three-point bending load. Statistical analysis revealed that the composite profile and presence of the reinforcing warp and weft yarns affect significantly the flexural properties of 3D-knitted composites. Accordingly, introduction of the reinforcing straight yarns causes an increase in maximum bending force by 75 and 62.5% for single and double-decker structures, respectively. In order to understand how the tensile and compressive stresses are distributed on the 3D-knitted composite structure as well as to predict their mechanical behavior, a macro-mechanical model was created in the ABAQUS software.

The effect of loop density on the tensile behavior of biaxial weft knitted composites using both experimental tests and numerical method

In this study, a high-tenacity polyester was used to produce biaxial weft knitted fabric in three different loop densities. All of the composite samples were manufactured using the vacuum injection process. Epoxy resin was used as the matrix in the composite samples. Tensile tests in the course and wale directions were carried out on all samples. The results showed that the tensile strength and the elastic modulus of the composites were improved by increasing the loop density. On the other hand, multi-scale finite element modeling was employed to predict the elastic constants and the tensile strength of the composites. In this method, unit-cells of biaxial weft knitted fabrics with a script were modeled by ABAQUS finite element software in the meso scale. Periodic boundary conditions were applied to the unit-cells. Stiffness matrices of composites were calculated by a python code. In the macro model, a shell geometry was created and the elastic constants calculated from the meso scale were assigned to the macro model. The tensile strength of composites in the course and wale directions was predicted by the Tsai-Wu failure criterion equations. The numerical results had a good agreement with the experimental ones. According to the numerical results, the difference in the loop densities as the inputs data could be used and elastic constants and strength of composites in the course and wale directions could be obtained. So, this model is a useful method to predict the tensile behavior of biaxial weft knitted composites with different geometries.

Investigation of tensile and flexural behavior of biaxial and rib 1 × 1 weft-knitted composite using experimental tests and multi-scale finite element modeling

In this study, tensile and flexural behavior of biaxial and rib weft-knitted composite is obtained numerically and experimentally. Multi-scale finite element modeling is employed to simulate the tensile and flexural behavior of composite samples. In the finite element modeling, the geometry of a unit cell of each fabric is initially modeled in ABAQUS software, and then periodic boundary conditions were applied to a unit cell. The stiffness matrix for each structure was obtained by a python code via meso scale modeling and used as input data for the macro modeling. To validate the numerical model, two types of weft-knitted fabrics (rib 1 × 1 and biaxial fabrics) are produced by a flat weft knitting machine. Epoxy resin is used to construct composite by the vacuum injection process (VIP). After that, the tensile and three-point bending tests were applied to composite samples. The experimental results showed that tensile strength and tensile modulus of biaxial composites are greater than rib composites, in both wale and course directions. Moreover, in three-point bending test, biaxial composite showed more strength and more stiffness in comparison to rib composite. Finite element results were compared to experimental results in tensile and bending tests. The results showed that good agreement with experimental results in the linear section of tensile and flexural behavior of composites. Consequently, the current multi-scale modeling can be used to predict the stiffness matrix and mechanical behavior of complex composite structures such as knitted composites.

Prediction of elastic behavior of plain weft-knitted composites

Design and analysis of parts constructed from weft-knitted textile composites need the elastic and fracture behavior of the composite. To avoid time-consuming and expensive experimental procedures, micromechanical models and finite element simulations can be used to estimate stiffness matrix of these composites. In the present study, at first, a 3D model of a plain weft-knitted fabric is presented. Then a micromechanical approach is proposed to derive the mechanical properties of the weft-knitted composite using this geometrical model. A finite element simulation is carried out to extract the elastic properties of the composite as an alternative procedure. Finally, obtained moduli from both methods are validated by comparing them with existing experimental values. Results show a good agreement between the calculated and measured data. It can be concluded that the proposed micromechanical approach can predict weft-knitted composite behavior well without any great effort; however, the finite element analysis gives acceptable results too. The effects of composite variables on the stiffness are investigated and discussed.

Processing and characterization of hemp fiber textile composites with micro-braiding technique

From the actual usage of natural fiber reinforced composites, textile composites are desired. In the present study, hemp fiber reinforced poly(lactic acid) composites were molded with micro-braiding technique. At first micro-braided yarn (MBY) was fabricated. Then, plain woven fabrics were weaved on a loom. In order to optimize molding condition, three molding conditions were selected based on the previous study on unidirectional composites. Tensile and shear tests were conducted on the specimens. Tensile and shear strength is highest for molding temperature 190°C, molding pressure 1MPa and molding time 4min. Higher molding pressure induce pouring out of matrix from the mold die, which result in lack of matrix. At higher temperature, slight decomposition of hemp fiber also occurred. Cross-ply laminates and weft knitted composites were also molded for comparison. Tensile properties of textile composites were much lower than cross-ply laminates, because of undulation structures.

Effect of stitch and biaxial yarn types on tensile, bending, and impact properties of biaxial weft-knitted composites

Within the scope of experiments, the effect of stitch yarn type (aramid, glass and nylon), and biaxial, warp and weft, and yarn type (aramid and glass) was investigated in the biaxial weft-knitted (BWK) composites. Five different types of composite panel, which include fiber contents, such as glass-glass-glass, glass-glass nylon, glass glass aramid, aramid glass aramid and aramid aramid aramid were fabricated by hand lay-up method. After production of composite panels, tensile, three-point bending and three-point, bending impact tests were conducted on specimens. Microstructural characterization of tested materials was performed using a scanning electron microscope. This study showed that composites with BWK preforms consisting of fiber combinations, such as aramid-aramid-aramid, had higher tensile strength (in course direction), of aramid-glass-aramid had higher bending strength (in course direction), and of glass glass aramid had higher three-point bending impact properties (in course and wale directions) than other types of tested composite structures.

Dynamic Behavior of 3D Biaxial Spacer Weft-Knitted Composite T-Beam Under Transverse Impact

The dynamic responses of T-beam structures made of 3D biaxial weft-knitted composite were tested under different impact velocities to characterize strength and energy absorptions of the T-beam with different rib heights. The transverse impact tests were carried out at a modified split Hopkinson pressure bar (SHPB) apparatus and also compared with the quasi-static test results. A user defined material subroutine named MAT/UMAT, which was based on a unit-cell model of the 3D knitted composite was developed to calculate the impact responses with a commercial FEM code ABAQUS. The load-displacement curves of the composite T-beam with different rib height under impact were calculated and compared with those from experiment. Mechanisms of the load-displacement curve fluctuations obtained from experiments are also discussed. The effect of the rib height on the load-displacement cures and energy absorptions have also been analyzed. Such an approach could also be extended to the design of other 3D knitted composite structures with different shapes under impulsive loading.

Manufacturing of Weft-Knitted Fabric Reinforced Composite Materials: A Review

Textile composites are composed of textile reinforcements combined with a binding matrix. The available textile reinforcements are manufactured by weaving, braiding, knitting and stitching. Since the studies about weft-knitted composites began then these materials have attracted increasing attention because of their potential uses and advantages. As a basis for understanding the steps in manufacturing of weft-knitted reinforced materials, this research will be helpful as it provides information on the characteristics, specifications, manufacturing process and performance of weft-knitted fabric reinforced composites.

Impact and compression-after-impact performance of weft-knitted glass textile composites

Abstract This research dealt with the impact resistance and tolerance of composite materials manufactured from weft-knitted E-glass fabrics. Three different styles of the weft-knit structures, Milano, 1×1 rib and plain knit, were investigated together with two further variations of Milano and rib structure and one variation of plain knit. The effect of weft-knitted fabric style and knit- structural parameters upon the impact and compression-after-impact properties of the weft-knitted composites was examined. Damage morphology and failure mechanisms of the impacted materials were characterised. It has been found that both changes to the knit style and structural parameters affected the damage resistance and tolerance of composites. A knit structure having a higher value of total loop density has effectively incorporated better knitted-mesh generation within the structure, thereby enhancing the structural integrity of composites and offered better impact damage resistance and tolerance.

Multi-scale study of tensile properties and large deformation mechanisms of polyethylene terephthalate/polypropylene knitted composites

This paper is concerned with large deformation tensile properties and energy absorption mechanisms of thermoplastic weft-knitted composites, fabricated by compression molding co-knitted preforms consisted of polyester (polyethylene terephthalate) and polypropylene continuous filament yarns. The large deformation mechanisms have been studied by observing morphological changes during quasi-static tensile loading in meso- and micro-scales. In-situ SEM observations have identified critical points including brittle matrix crack initiation from imperfect composite surfaces, fibre/matrix debonding, serious loop distortion along the loading directions and fibre straining during the large deformation until composite fracture. The energy absorption mechanisms have been identified as localized matrix compression and extension, matrix crack propagation, localized loop distortion, and fibre straining during the deformation process. Effects of fabric structure, fibre volume fraction and plasma treatment on fibre surface have been also studied.