Experimental work

The experimental work is based on the direct study of the yarn after knitting, in order to identify the damage degree of the glass fibres inflicted by the knitting process. All models for the mechanical behaviour of the glass knitted fabric are based on the Young’s modulus for the glass yarns. During the knitting process the filaments are damaged in a significant proportion, therefore altering the initial value of Young’s modulus and altering the fabric properties. No previous study indicated the relation between technological parameters and the final value of the Young’s modulus.

Two types of glass fibre were considered for the experiment: EC 11 408 Z28 T6 – Vetrotex and EC 13 136 Z30 P 100. The yarns are knitted using single jersey, as being the simplest possible structure.

The fabrics were produced on a CMS 320 TC (Stoll) flat machine with the following characteristics: gauge 10 E, negative feeding – IRO NOVA (Fig. 3) and holding down sinkers and presser foot (Fig. 4).

Fig. 3. IRO NOVA negative feeding

Fig. 4. Holding down sinkers and presser foot

In order to adapt to the yarn count, the fabrics are knitted with different values for the stitch quality cam, presented in Table 3. Each sample had 100 wales and 100 courses. The fabrics were relaxed until they presented no dimensional variation. The structural parameters (horizontal and vertical stitch density and stitch length) are illustrated in Table 4.

After relaxation 10 yarn lengths were drawn from the fabrics in order to determine their tensile properties, avoiding the edges, visibly more damaged then the rest. The tensile strength was tested on a HOUSENFIELD H10K-S (Tinius Olsen), according to ASTM 2256. According to previous studies, the glass yarns break in less than the minimum 20 seconds indicated by the standard. Therefore, the testing speed selected was the minimum value of 50 mm/minute. The data confirmed the breaking of the glass yarns less than 7 seconds.