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1 Introduction
Nowadays, the role of garment industry in economic activities and human life is considerable ([7] Guo et al. , 2011). Among woven fabrics which are used in garment industry, the denim fabrics are widely used as a main part of the garment fashion. Denim apparel is promoted by many consumers around the world; in particular, the success of these fabrics usage is because of its compatibility with every society and culture ([3] Card et al. , 2006). The high quality of garments not only does depend on fabric quality, sewing threads and sewing machine parameters, but also depends on fabric sewability ([2] Behera and Chand, 1997). The sewability of fabrics and its importance has been considered in the process of garment manufacturing. The sewability is defined as the ability of the material to be sewn efficiently and to provide a suitable performance for its end use ([28] Stylios et al. , 1994). The study of sewability can help better comprehending of the interactions between one or more plies of fabric which are sewn with sewing thread ([31] Zeto et al. , 1996).
Needle penetration force (NPF) is an important factor influencing on the quality of seams and fabric sewability. The NPF is created by the friction between the martial and sewing needle ([30] Ujevic et al. , 2008).
The researchers measured NPF to investigate the sewing damage in fabrics during sewing process.
A high penetration force is one of the key reasons causing the sewing damage; on the other hand, a fabric with high penetration force is more susceptible to damage. Sewing damage has directly negative effects on quality of garment. Therefore, the quantitative value of NPF could be used to determine the damage of sewn fabrics during sewing process ([29] Stylios and Zhu, 1998; [31] Zeto et al. , 1996; [8] Gurarda and Meric, 2005).
The types of material, number of fabric layers, weave pattern, sewing needle size, and shape of needle point have a profound effect on the sewing NPF ([27] Stylios and Xu, 1995; [30] Ujevic et al. , 2008).
The researches on NPF can be classified into three categories including the development of an instrument to measure the NPF ([4] Carvalho et al. , 2009; [23] Rocha et al.