Extensional properties of plain knitting fabrics are discussed in relation to the change of bending rigidity of yarn under the extension of loop form of yarn. Increasing of bending rigidity of yarn in the fabrics as extending of the fabric in wale direction is due to the in creasing of compactness between -fibers. The following two results are obtained.
(1) Bending rigidity of yarn in knit goods increases in logarithmic by the extension of knittings.
(2) Bending rigidity of yarn in knit goods under no extension not only depends on the stitch density of the fabrics, but on the yarn states, crimps of fibers and the yarn twist.

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To discuss the extension behavior of warp knit fabrics, we must investigate the extension behavior of their loops. We made experiments both on our model loop and on knit fabrics according to Grosberg's, theory on the extension behavior of loops with the following results.
(1) Grosberg's theoretical expression is fully reliable so far as the model loop is concerned
(2) However, his expression is not always reliable as to the extension of the knit fabrics, because their extension is much influenced by their structure. The experimental . value of half the structure agrees well with their theoretical value if the extension is low. The experimental value of denbigh structure, however, differes considerably from their theoretical value. Therefore, Grosberg's theoretical expression is inapplicable just as it is. The experimental value of an individual loop agrees comparatively well with its theoretical value if the extension is low.
From the above observations we are satisfied that Grosberg's theory is quite applicable to practical use if the fabric tension is low.

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Conventional stretch method for elongation test of knitted fabrics does not give accurate data at some points. We have developed new method for it, namely the method by inflation.
The tester for new method consists principally of cylinder, thin rubber on upper end of cylinder, pressure gauge, and elongation measure divice. Knitted fabrics are cramped on said rubber and compressed air is sent from compressor to cylinder. Fabrics then are inflated together with rubber by air pressure.This gives similar effect to that of free inflation of knitted fabrics. Therefore, this is proper method for the study of structure of knitted fabrics.
The results obtained.
1. Elongation of warp knitted fabrics by inflation is given by βk = Ewx Ec, where βk, Ew, Ec are constant, elongation of wale and elongation of course respectively.
2. The shape of inflated warp knit fabrics is that of ellipsoid of rotation just as rotated around major axis of wale.
3. The shape of inflated weft knit fabrics is that of convex body made by rotating convex curve around longitudinal axis : which curve is drawn when one ellipsoid is put on the other ellipsoid symmetrically : each ellipsoids takes wale and course in major axis.

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