## 1 Introduction  
Nonwoven fabrics are the most common single-fiber application for polypropylene usage in terms of sheet or web structures. These fabrics are defined as web structures made by bonding or interlocking fibers or filaments by mechanical, thermal, chemical, or solvent means. A thin layer of the nonwoven fabrics is continuously formed by the deposition process on a web belt. The uniformity and density of nonwoven fabrics can be controlled by adjusting both the velocity of the moving belt and the deposition rate [1]. The properties and structures of spunbond fabrics depend on the process parameters, such as the polymer throughput rate, the cooling and suction air speed, and also the bonding temperature [2]. Nowadays, in order to join nonwoven fabrics, various approaches such as thermal bonding, adhesive bonding, and ultrasonic welding are adopted [3]. In the thermal bonding process, the initial morphology of the nonwoven fibers significantly affects the structure and properties of the joints. For example, fibers with high molecular orientation and crystallinity tend to form a weak and brittle bond due to the lack of polymer flow. On the other hand, fibers with lower tenacity and higher elongation at break result in better bonding and higher fabric tensile properties [4]. In order to clarify the issues related to the bonding temperature process, [PERSON] et al. investigated thermal bonding conditions, involving the welding area, size, and temperature. The results showed that the strength of a thermal welding joint increased with the expansion of the welding area and size. However, the welding strength and the bending length varied according to the temperature distribution [5]. [PERSON] et al. studied the strength of thermal calendared joints using islands-in-the-sea (I/S) fibers. Nylon-6 (N6) and polyethylene (PE) of bicomponent structures were used as the islands and the sea in the I/S bicomponent fibers, respectively. The results indicated that the strength of the thermally bonded N6-PE fibers was higher than that of the original fibers [6]. However, the thermal bonding process is complex, and it is easy to cause a lack of the temperature for melting at the central region of the bond [7]. In order to improve the quality of the bonding structures, adhesive bonding technology has been applied instead of conventional thermal bonding processes. In adhesive bonding technology, ultraviolet curable adhesive is utilized to obtain better properties than when using structural adhesives [8]. However, the quality of the bonding structure (i.e., the mechanical properties) still has some issues, such as a low operating temperature and low shear stress [9]. To overcome these troubles, an innovative bonding technology using ultrasonic vibration has recently been researched and developed. In bonding by ultrasonic vibration, the mechanical vibrations combined with the high pressure can create enough temperature for softening nonwoven fabrics, causing molecular diffusions and cooling down to form welding joints [10]. In previous papers, the tensile strength and elongation at break values of ultrasonic welding joints were investigated according to the fabric's area density and row rollers. The results implied that an increase of the area density of the fabric and the row rollers can lead to an increase in the tensile strength. In addition, it was highlighted that the elongation at break values are proportional to the density and row rollers [11]. In another work, [PERSON] et al. estimated the possibility of ultrasonic welding on textile fibers. In this work, an ultrasonic welding system with a longitudinal horn moving in the vertical direction was utilized, after which, the mechanical force was applied on multilayers of the fabrics over the anvil. The results presented that an increase in the welding time and pressure can increase the joint strength initially, but afterward, the strength decreases further. Meanwhile, with an increase in the vibration amplitude, the strength of welding joints can be enhanced [12].