Abstract: UHMWPE fibers represent the third generation of high-performance fibers, following carbon fibers and aramid fibers. They boast multiple advantages such as high modulus, high strength, and excellent energy absorption characteristics. UHMWPE fibers are notably lightweight, being approximately 20% lighter than conventional aramid fibers and 30% lighter than carbon fiber-reinforced composites. The molecular chains of UHMWPE fibers possess an extremely high linearity, with their mechanical properties further enhanced by high orientation and crystallization during the spinning process. The molecular structure with high orientation and crystallization   provides UHMWPE fibers with exceptional strength and modulus. Additionally, it demonstrates excellent energy absorption and acoustic emission transmission capabilities under dynamic loading conditions. Overall, due to its unique molecular structure and outstanding mechanical properties, UHMWPE fibers hold significant application potential in the field of lightweight and high-strength composites, especially in the area of personal protection. Therefore, conducting experimental research on the anti-fragmentation penetration of soft bulletproof layers made of unidirectional UHMWPE fabric is crucial for optimizing body armor design to enhance their safety. 
Soft bulletproof layers are a crucial component of bulletproof vests, and ultra-high molecular weight polyethylene (UHMWPE) fibers are widely utilized within these soft ballistic layers. This study focused on soft bulletproof layers made of unidirectional UHMWPE fabric to conduct fragmentation penetration tests using two different shapes of targets: flat-head and wedge-head ones. A 1.1g cylindrical simulated fragment was employed for these tests to determine the ballistic ultimate velocity, known as V50. Image J software was utilized to measure the damage areas of bullet holes of each layer, in order to analyze the damage morphology caused by the fragments and study the ballistic penetration mechanism under different target-hitting morphologies. The results revealed that when fragments penetrate the soft bulletproof layers in the two different morphologies at similar impact velocities, the total damage area to a single layer of unidirectional fabric is comparable. The ballistic limit (V50) for fragments impacting with a flat head is, on average, 4.01% higher than that for fragments impacting with a wedge head. Additionally, the ultimate energy absorption of the soft bulletproof layer when fragments impact with a flat head is, on average, 8.19% higher than that when fragments impact with a wedge head. Furthermore, as the areal density increases, the energy absorbed per unit areal density by the unidirectional UHMWPE fabric soft ballistic layer decreases.
The unidirectional UHMWPE fabric soft ballistic layer is a key component of soft body armor and is commonly used in military, police, and civilian personal protective equipment. Research on the anti-penetration mechanisms and the penetration process of the soft bulletproof layers made of unidirectional UHMWPE fabric has been enhanced to provide a theoretical foundation and technical support for the effective design of soft body armor.  

Key words: soft bulletproof layer, bullet hole damage, ballistic limit V50, ultimate energy absorption, limit specific energy absorption

摘要： 软质防弹层是防弹衣的核心组件，超高分子量聚乙烯（UHMWPE）纤维因其优异的力学性能在此类防弹层中的应用十分普遍。以UHMWPE无纬布制成的软质防弹层为研究对象，开展防弹层样品抗11 g柱状模拟破片的弹道侵彻和弹道极限V50试验研究，利用Image J软件对防弹层的损伤特征和穿透面积进行量化表征，探讨了破片平头和楔头着靶对穿透层数、穿透面积、弹道极限V50以及极限比吸能的影响。结果表明：平头着靶时软质防弹层弹道极限V50平均比楔头着靶时高401%，UHMWPE无纬布软质防弹层在破片平头着靶时有更好的防护效能。弹速相近的情况下，两种着靶形态的穿透面积基本一致；随着面密度增加，弹道极限V50和极限吸能均有所提高，但极限比吸能（单位面密度的能量吸收）呈下降趋势。研究结果可为破片侵彻软质防弹层时的破坏机制和软质防弹层的防护机理提供试验参考。

关键词: 软质防弹层, 弹孔损伤, 弹道极限V50, 极限吸能, 极限比吸能