Features of measurement of the strength properties of sewing thread in designing lap-sewn fabrics

Abstract

The conditions of testing sewing thread do not correspond to the conditions of testing lap-sewn fabrics, which makes it difficult to predict their properties. For designing lap-sewn fabrics with given strength properties, the breaking load of sewing thread should be measured at a stretching rate of 100 mm/min.In measuring the breaking load of lap-sewn fabric, the strength indexes of the fabric as a whole are determined by testing samples. To successfully design lap-sewn fabrics with given strength properties, it is important to perform tests with standard methods. Respecting the extension rate provided by the standard is one condition for obtaining reliable and comparable results in tests. How important this parameter is can be judged by the results of testing the three kinds of lap-sewn fabrics represented in Fig. 1. The characteristics of these fabrics are reported in Table 1.As Fig. 1 shows, increasing the extension rate of the samples from 100 mm/min (standard rate [1]) to 200 mm/min significantly increases the breaking load both in the longitudinal and in the transverse direction. Nonrespect of the extension rate provided by the standard makes it impossible to objectively compare different lap-sewn fabrics.Lap-sewn fabric consists of lap fibres and the sewing thread securing it. The strength of the fabric is determined by the strength and interaction of the structural elements it contains. To effectively control the strength properties of lap-sewn cloth, it is necessary to have information on the role of the individual structural elements. This primarily relates to the sewing thread, since it absorbs the basic load when lap-sewn fabrics are stretched.Since the fabric consists of two structural elements, its overall breaking load can be expressed as the sum of two constituents:where P f is the breaking load of the lap-sewn fabric, daN; P t , P l are the breaking load provided by the sewing thread and lap fibres, daN.The indexes of the breaking load of the thread are given in the corresponding standards and technical documentation. However, it is necessary to note the testing conditions in which the results were obtained. There are no unified testing conditions for the different types of thread. GOST 6611.0-73 [2] recommends establishing the thread extension rate calculated so that the duration of the test before the thread breaks would be 10±1 sec. The extension rate for cotton yarn with approximately 9% elongation at break is 300 mm/min. These testing conditions are undoubtedly reflected in the results obtained.To confirm this conclusion, we tested the basic types of sewing thread used in production of nonwovens at a different stretching rate. Table 2 shows that an increase in the extension rate is accompanied by a significant change in the breaking load of the sewing thread. It should be noted that the extension rate affects the breaking load of sewing thread of different types to a different degree: for cotton yarn and polyester thread, this index increases by 10 and 4%, it decreases by 7% for polypropylen...