Research on the Knittability of Metallic Yarns in the Warp-knitted Mesh of a Deployable Antenna

Specimen specifications of metallic yarns used in warp-knitting meshes are shown in Table 1.

The knittability of warp-knitted fabrics was able to be characterised by the damage degree of yarns after knitting. But warp-knitted fabrics could not be dislodged to obtain intact yarns against the knitting direction or with the knitting direction. Having been replaced, each knitted yarn’s strength utilization factor was calculated from single yarn’s tensile strength, which was obtained from the fabric’s tensile performance. The knitted yarn strength utilization factor could be used for knittability characterization. The higher the knitted yarn strength utilization factor, the better the knittability.

The number of yarns bearing loads during the tensile process was related to the stretching direction of the fabric. Mainly two yarns were loaded in longitudinal stretching, while only one in transverse stretching. Therefore, the fabric breakage strength was able to be calculated down to single yarn strength for a known number of stretched yarns [13].

In the longitudinal direction, 40 columns of LF-1and LF-2 specimens, which were actually 80 yarns, were stretched. 40 horizontal rows of LF-1 and LF-2, which were 40 yarns, were stretched along the transverse direction. While there were two overlapping loops in the HF-1 and HF-2 meshes, which amounted to 20 columns, with 80 yarns being stretched in the longitudinal direction, in the transverse direction, 20 horizontal rows including 40 yarns were stretched in HF-1 and HF-2 meshes.

From Table 7, the knitted yarn strength utilization factor of LF-1 yarn was higher than that of LF-2 yarn. While in HF antennas, the knitted yarn strength utilization factor of HF-1 was close to that of HF-2, that of HF-1 and HF-2 yarns was higher than that of both LF-1 and LF-2 yarns, even if their different structure and processing parameters were used. In general, the higher the gauge of the knitting machine used, the more difficult it was for fabrics to be knitted. Therefore, HF-1 and HF-2 yarns could be judged to have better knittability.

From Table 2 and Table 7, the relationship between the strength of metallic yarn and knittability is uncorrelated, but that of the elongation at break is positively correlated. The elongation at break of yarns in LF-1 and LF-2 samples is 23.8% and 1.8%, respectively (Table 2). From Table 7, the knitted yarn strength utilization factor of LF-1 yarn is 1.8 times that of LF-2 in both the longitudinal and transverse directions; that is, the knittability of LF-1 yarn is better than that of LF-2 yarn. The yarn elongation at break of HF-1 is close to that of HF-2, as seen from Table 2. And also, no significant difference was found in the knitted yarn strength utilization factor, which means the HF-1 and HF-2 meshes are very similar. Most high-performance yarns are of the long filament type, whose elongation at break is related to twist. To improve the knittability, the elongation at break of yarns should be increased by reducing the twist coefficient [14,15].

It can be seen from Tables 3 and Table 7 that the bending stiffness of metallic yarn is negatively correlated with knittability. The bending stiffness of LF-1yarn is close to that of LF-2 yarn, which is same for HF-1 and HF-2 yarns. The bending stiffness of LF-1 and LF-2 yarns is 3 times that of HF-1 and HF-2. From Table 7 it can be seen the knitted yarn strength utilization factor of HF-1 and HF-2 is slightly higher than that of LF-1; that is, HF-1 and HF-2 yarns possess better knittablity. The knitted yarn strength utilization factor of HF-1 and HF-2 is twice that of LF-2, leading to a much better knittability. Therefore, a reduction in yarn bending stiffness could be achieved by reducing fiber fineness [16]. The knittability was complexly affected by both the yarns’ stiffness and tensile strength. To some extent, bending stiffness may show a more significant effect.

It was shown that yarn elongation at break and yarn bending stiffness comprehensively affected the knittability of the yarns. To further investigate the quantitative relationship between yarn basic mechanical properties and yarn knittability, a multiple linear regression analysis was performed. The knitted yarn strength utilization factor was y1 and y2. The yarn elongation at break was x1, the yarn bending stiffness - x2, and the yarn breaking strength was x3. No reasonable fitting results were obtained. Based on the previous analysis, a reasonable fitting result should be obtained after removing the independent variable of the yarn breaking strength (x3). Results of the fitting of the knitted yarn strength utilization factor in the vertical and horizontal directions are shown in Table 8.

As a result, a very strong linear relationship was found between y1(y2), x1, y1(y2) and x2. The coefficient (R2) of the prediction models was 0.998. The correlation between the elongation at break of yarn and the knitted yarn strength utilization factor was noticeable over 10% in both the vertical and horizontal directions. The correlation between bending stiffness and the knitted yarn strength utilization factor was over 5%. The yarn knittability could be quantitatively characterized by knitted yarn strength utilization factor. Therefore, the model shown in Table 8 could be used to predict the yarn knittability of yarn based on the yarns’ mechanical properties.