The Study Of Formability Of Multi-Layered Biaxial Weft Knitted Fabrics

This paper mainly studied formability of the multi-layered biaxial weft knitted fabrics (MBWK fabrics). The geometric structure of MBWK fabrics was studied first owing to the close relationship between fabric properties and structure parameters. Based on the idealized cross-sectional shapes of inserting and stitch yarns, the geometric unit cell model of MBWK fabrics was built. The concept of stitch yarn tightness factor was raised for the first time according to the peculiar structure of MBWK fabrics, i.e. the ratio between real length and geometric length of the stitch yarn. It affected the formability of MBWK fabrics together with the insertion yarn tightness factor.The shear properties of fabrics directly determined whether this fabric could transform from its original plane state to 3D curved state without buckling during the forming process. So the critical shear angle (also called locking angle) of MBWK fabrics was studied in the second section of this paper, and the model of critical shear angle of MBWK fabrics was established for the first time under the condition of yarns' simplification owing to its unique construction. According to different restraint conditions considered, the MBWK fabrics had two critical shear angles: the critical angle θ₁ controlled by the inserting yarns and the critical angle θ₂ controlled by the stitch yarns, the larger of which should be the ultimate critical shear angle θ.Three kinds of geometric shapes, i.e., hemisphere, rectangular box, and cylinder, were selected for forming experiments to characterize the formability of MBWK fabrics. The forming characteristics of MBWK fabrics were concluded from the experiments data, and the corresponding models were established based on these conclusions. The forming experiment on hemisphere testified that the transforming pattern of MBWK fabrics was similar to woven fabrics under the condition of ignored yarns slippages. In the forming experiment on rectangular box, the obvious yarns slippages were observed during the forming process, so the pin-jointed net model used by woven fabrics didn't adapt to MBWK fabrics. Based on this finding, a new mathematical model using a macroscopic approach was established for predicting the unfolding shape of flat MBWK fabrics after deformation. In addition, the effect of position of constraint yarns on forming results was analyzed. The yarns slippages were mainly studied by theforming experiment on cylinder, and yams slippages characteristics were described.Finally, the forming experiments of MBWK fabrics on double-hemisphere were carried out for the first time. Based on the experimental results, the forming characteristics of MBWK fabrics on double-hemisphere were described. It was found that the angle changed between the inserting yams was related to the magnitude of diameter of the hemisphere, but had no correlations with the distance between the double-hemispheres. And the forming model of MBWK fabrics on double-hemisphere was established. This model not only showed the yams distribution and orientation after deformation but also predicted the unfolding shape of flat fabrics after deformation.