Multi-axial Warp-knitted Composites Research Articles

Temperature effect on compression property of multiaxial warp-knitted composites

The wind turbine blades are exposed to various environments all the year round, especially temperature variations during the four seasons. To study the influences of temperatures on the mechanical properties of composite materials, the compression property in the 0°, 45°, and 90° directions of the multiaxial (triaxial and quadaxial) glass fiber warp-knitted composites at −30°C, 0°C, 20°C, and 40°C was analyzed, respectively. The stress–strain curves, strength, modulus, failure strain, and the relationship between strength and temperature were obtained. The results indicated that the compression performance decreased with the increase of temperature, and the effect of temperature on quadaxial composite was more significant than that of the triaxial composite. In addition, the failure mechanisms were also analyzed according to the fracture modes and scanning electron microscopic morphologies of composites.

Fatigue failure characterization of multi-axial warp-knitted composites under cyclic uniaxial tensile loading

In this work, the fatigue failure mechanisms of quadri- and tri-axial warp-knitted fabric composites (QWFC and TWFC) were investigated. A series of quasi-static state tensile tests and cyclic loading experiments were carried out, and the morphology of fatigue failure was visualized by optical and scanning electron microscopes (SEM). The results showed that while the failures of QWFC and TWFC were similar with X-shape morphology, QWFC had longer fatigue life than TWFC. The SEM images illustrated that the matrix of QWFC was not uniform, and contained cracks with scaly appearance, in addition to small amount of fiber curls. Moreover, the fracture surface was flat and layered. In contrast, the matrix of TWFC had propagation of cracks along ±45° direction; its fracture surface had sphere ‘budding yeast-like’ appearance. The wrap-knitted fabric contained increased numbers of layers in the composites, thereby had longer fatigue life.

Experimental investigation on the quasi-static penetration behavior of curved multi-axial warp-knitted composites

The quasi-static penetration tests of the curved glass/phenolic multi-axial warp-knitted (MWK) composites with two types of preform structures and in three different curvatures were implemented. Three phases — elastic deformation phase, progressive damage phase and tension-shear phase were identified to present the different damage behavior stages of curved glass/phenolic MWK composites under quasi-static penetration. Several types of damage modes including matrix cracking, tension-shear of the materials, fiber fracture and interlayer delamination were revealed based on the load-displacement curves and morphology analyses. The results showed that the reinforcement structure and the curvature of the specimens did provide obvious influence onto the quasi-static penetration behaviors of curved glass/phenolic MWK composites and they could be utilized in pointed purposes according to the different mechanical features.

Experimental study on the bending properties and failure mechanism of 3D MWK composites at elevated temperatures

This paper reports the effect of temperature on the bending properties and failure mechanism of 3D MWK (Multiaxial warp knitted) composites. Three-point bending experiments on composites with three fiber architectures were performed at four different temperatures. Then the effect of temperature on the load/deflection curves, bending strength and stiffness were analyzed. Macroscopic fracture morphology and SEM micrographs were also examined to understand the deformation and failure mechanism. The results show that the temperature has great impact on the bending properties of 3D MWK composites and the performance decreases significantly with the increase of the temperature. Meanwhile, the bending properties can be affected greatly by the fiber architecture and these decrease significantly with the increase of fiber orientation angle at room and elevated temperatures. Moreover, the damage and failure patterns of composites vary with the test temperature and fiber architecture. At room temperature, the composite exhibits brittle fracture feature and damages in the breakage and tearing of fibers. But at the higher temperature, the composite becomes more softened, cracked and plastic. It damages in fiber layer delaminating, matrix cracking and fiber/matrix interface debonding.

Charpy impact properties and failure mechanism of 3D MWK composites at room and cryogenic temperatures

The charpy impact experiments on the 3D MWK (Multi-axial warp knitted) composites with four different fiber architectures are performed at room (20°C) and liquid nitrogen temperatures (as low as −196°C). Macro-Fracture morphology and SEM micrographs are examined to understand the impact deformation and failure mechanism. The results show that the impact properties decrease significantly with the increase of the fiber ply angle at both room and liquid nitrogen temperatures. Meanwhile, the impact energy at liquid nitrogen temperature has been improved significantly than that at room temperature. Moreover, the fiber architecture has remarkable effect on the impact damage and failure patterns of composites at room and liquid nitrogen temperatures. At liquid nitrogen temperature, the matrix solidification and the interfacial adhesion capacity increase greatly, which effectively hinders the stress wave propagation. However, more micro-cracks appear and the brittle failure feature becomes more obvious.

A comparative study on the tensile properties and failure mechanism of 3D MWK composites at room and liquid nitrogen temperature

The tensile experiments on the three-dimensional (3D) multi-axial warp knitted (MWK) composites with four types of fiber architecture were performed at room and liquid nitrogen temperatures (as low as −196°C). Macroscopic fracture morphology and SEM micrographs both are examined to understand the deformation and failure mechanism. The results showed that the tensile properties can be affected greatly by the fiber architecture and these decrease significantly with the increase of the fiber orientation angle at room and liquid nitrogen temperatures. Meanwhile, the tensile properties at liquid nitrogen temperature have improved significantly than that of those at room temperature. Moreover, the damage and failure patterns of composites vary with the test temperature. At liquid nitrogen temperature, more microcracks appear and the brittle failure feature becomes more obvious; however, the interfacial adhesion capacity is enhanced significantly. In addition, the fiber architecture has remarkable effect on the failure mechanism at room and liquid nitrogen temperatures. POLYM. COMPOS., 35:1294–1305, 2014. © 2013 Society of Plastics Engineers

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.