1. Introduction
Aramid poly (p-phenylene terephthalamide) or PPTA consists of relatively rigid polymer chains with linked benzene rings and amide bonds. This structure affords aramid fibers high tenacity, high modulus, and toughness [1-3]. Based on these merits, aramid fibers are used in ballistic armor materials. Figure 1 shows the chemical structure of aramid fibers.
Generally, ballistic materials can be divided into hard and soft armors [4-6]. Unlike traditional structural composites, hard armors, also known as armor-grade composites, contain only 20% by weight matrix, and are made to readily delaminate. Conversely, soft armors consist of multilayered, woven textiles, and are used to protect against various types of bullets.
The ability of a woven fabric to protect against bullets depends primarily on the mechanical properties of the yarn, such as the tenacity, tensile modulus, and toughness. However, Laible [7] demonstrated that “the relationship between the mechanical properties of a yarn and the ballistic resistance of a plied fabric from such yarn has never been established”; i.e., other factors may exist that influence ballistic performance. Generally, the energy absorption mechanism of soft armor depends on several additional factors, such as the weave pattern, the number of fabric plies, and the weave density. Weave patterns used in ballistic applications are usually plain and basket weaves. Fabrics with unbalanced weaves typically yield inferior ballistic performance [8]. Lim et al. [9] investigated ballistic impacts on multiple systems to characterize the reinforcement effect of multiple layers. They concluded that the inter-ply friction inhibited the sideways motion of the yarns, resulting in an increased resistance to ballistic penetration. Weave density, which refers to the number of yarns per unit dimension along the principal yarn directions, affects the areal density of the fabric and the crimp. Shockey et al. [10] concluded that the energy absorbed by a fabric was proportional to the fabric’s areal density.
Our research group previously compared the effect of the weaving density of aramid fabrics on their resistance to ballistic impacts. It was demonstrated that the existence of an optimal weave minimizes damage to both the yarn and fabric. Establishing these optimal conditions can be crucial in implementing better ballistic properties into fabrics [11]. Yarn crimp refers to the degree of yarn undulation, and is a property of the weave. Tan et al. [12] compared two methods of modeling crimp using empirical results. They concluded that accounting for crimp by modeling the linear elements in a zigzag manner yielded more accurate results than trying to account for crimp as a constitutive property.
However, little has been reported regarding correlations between ballistic properties and multifilament aramid woven fabrics. Furthermore, a comparative study of ballistic performance has not been carried out that accounts for both fabric properties and individual yarn properties. In the current study, two types of aramid woven fabrics, each with a different number of multifilaments, were prepared, and the influences of the number of multifilaments on the fabric properties and ballistic behavior were ascertained. This study provides fundamental information on how the number of multifilaments regulates fabric properties and the ballistic behavior of aramid woven fabrics.
2. Experimental Section
2.1. Materials
Aramid fibers (trademark Heracron®) were produced by Kolon Inc. (Kwach’on, Korea) with a fiber fineness of 840 denier. Table 1 shows some of the basic properties of two Heracron® filament fibers, each composed of 666 and 1000 monofilament fibers, respectively. As shown in Figure 1, the two Heracron® yarns had comparable tenacity, elongation, and Young’s modulus. Two types of fabrics, HT840-1 and HT840-2, were woven from the fibers for ballistic tests. The two fabrics had the same weave structure, fiber fineness, and weaving density. The detailed fabric properties are described below. To eliminate finishing oil and surface contamination, the fabrics were washed using an industrial scouring process.
2.2. Analysis
Filament fiber tests were performed in accordance with ASTM Standard D2256-97 (ASTM 2000). Each fiber specimen had an initial length of 50 cm. At the start of the test, the middle 25 cm of the fiber spanned between the instrument grips. The crosshead separation rate was maintained at 2 mm∙s−1, and the specimen was elongated until rupture. All of the specimens were twisted at a rate of 1.2 turns cm−1. The data from 20 independent measurements were expressed as an average with a single standard deviation. Fabric tests were performed in accordance with ISO 13934-1. All sample fabrics were created from the HT840 fabrics using the ravel strip method. Each sample had an initial length of 1.2 m and a width of 50 cm. A 50-mm length at each end of each sample was clamped into place with a fabric grip. The
Table 1. Basic properties of two Heracron® yarns.
sample was then wrapped twice around each grip. During each test, the load frame crosshead moved at constant rate of 2 mm∙s−1. Samples were pulled until rupture. Twelve samples in total were tested. Six were elongated along the warp, with the weft running along the width. The other six were elongated along the weft, with the warp running along the fabric width. Ballistic shooting tests were performed on 32-ply samples of each of the two fabrics (HT840-1 and HT840-2) in accordance with NIJ Standard-0101.06, “Ballistic Resistance of Body Armor, Level 3A Methods”. Tests were performed with 44-Magnum semi-jacketed hollow-point (SJHP) bullets, with a mass of 15.6 g (240 g) impacting the fabrics at a velocity of 436 ± 9 m∙s−1 (1430 ± 30 ft∙s−1). Six bullets were shot into each sample. After shooting, the depth of the puncture and the back deformation signature (BFS) formed on the backing material were measured. All tests were conducted at the H.P. White Laboratory, Inc. (Street, MD, USA).
3. Results and Discussion
3.1. Fiber and Fabric Properties
Aramid fibers consist of highly oriented and ordered crystalline polymer chains, resulting in a rigid structure that does not endure bending. Therefore, aramid yarn is usually composed of one multifilament fiber, in conjunction with a large number of monofilament fibers. Generally, an aramid monofiber may be divided by its skin— core fibril structure. It is known that the skin, in which the fibrils are uniformly axially oriented to the fiber axis, strongly affects the fiber’s physical properties [1]. In contrast, fibril in the fiber core are imperfectly packed and ordered. Both the skin and core properties contribute to the strength and resilience of the fiber. Therefore, it is important to comprehend the various properties of monofilament fibers, as well as multifilament yarns. Table 2 lists the properties of Heracron® monofilament fibers. HT840-1, with a thicker monofilament fiber density, displays superior physical properties, compared with HT840-2. This suggests that the HT840-1 fiber is better suited for skin structure fabrication than that of HT840-2 during the dry-wet jet spinning process. Well oriented skin fibril may prohibit the fiber rupture and improve its physical property.
Table 3 lists the physical properties of the extracted