The development of lightweight and highly protective armor materials remains a critical challenge in the fields of body armor and vehicle armor systems. The need for materials with exceptional ballistic resistance has led to extensive research in high-performance fiber-reinforced polymer composites. Aramid fibers, such as Kevlar, and ultra-high molecular weight polyethylene (UHMWPE) fibers, such as Dyneema, have emerged as promising candidates due to their impressive specific strength and stiffness.
Selecting the appropriate materials for advanced armor systems involves considering factors like projected deformation characteristics and failure mechanisms, derived from the specific structural design and fundamental mechanical properties of the constituents. For Kevlar-reinforced composites, previous studies have measured their static and dynamic properties and used them in numerical simulations. Ballistic impact loads can induce various deformation and damage mechanisms in composite laminates, depending on factors such as projectile morphology, target architecture, and specimen size. Similarly, UHMWPE fibers have garnered attention for ballistic protection applications due to their exceptional strength and lightweight properties. Research has compared the ballistic performance of UHMWPE-based composites with different architectures and examined the effects of target thickness and projectile morphology on their behavior. Additionally, non-perforated impacts in composite plates have revealed key damage mechanisms, including fiber breaking, delamination, and permanent back face deflection. Many ballistic protection applications involve integrating fiber-reinforced composites with other materials, such as ceramic tiles, to optimize impact resistance. One notable example is the ceramic-faced composite armor system, where a ceramic tile is positioned in front of a fiber-reinforced composite panel to create a cohesive protective structure. In this system, the ceramic tile’s role is to disrupt and shatter the high-speed projectile, while the fiber-reinforced composite’s role is to preserve the armor’s integrity and absorb the remaining kinetic energy. This integration introduces new complexities in terms of deformation and failure mechanisms when subjected to ballistic impact loads.
In a new study by researchers at the Department of Mechanical and Industrial Engineering, Texas A&M University-Kingsville, including Dr. Guodong Guo, Associate Professor Shah Alam, and Professor Larry D. Peel, have conducted a comprehensive investigation into the ballistic resistance and deformation mechanisms of fiber-reinforced composites when integrated into ceramic-faced armor systems. Their work is now published in the Journal Composite Structures. The researchers’ primary objectives are twofold. Firstly, to directly compare the ballistic resistance of ceramic-faced armor backed by two distinct types of fiber-reinforced composites: Kevlar-29/epoxy composite and UHMWPE fiber-reinforced composite. Secondly, to identify the deformation characteristics and failure mechanisms of these two composite materials when integrated into ceramic armor systems subjected to ballistic impact loading. They utilized 7.62 mm armor-piercing projectiles (APM2) at varying velocities to assess ballistic resistance. Post-mortem visual and scanning electron microscopy analysis are performed to discern failure mechanisms. Additionally, finite element (FE) simulations are conducted to further investigate the behavior of layered armor backed by hybrid composite panels with different mix ratios, offering insights for optimal armor design.
In any ballistic event, understanding the damage mechanisms at play is paramount. The authors meticulously examined how Kevlar and UHMWPE composites respond to ballistic impacts, shedding light on their deformation characteristics and failure modes. In the case of Kevlar-epoxy composite panels, the study revealed a multifaceted response:
884 m/s impact: The panel exhibited minimal visible damage on the top and bottom surfaces but revealed interface delamination between sub-layers upon closer inspection.
1070 m/s impact: A pyramid-shaped bulge formed on the backside due to transverse wave propagation. The front surface exhibited a shallow crater with fractured fabric layers, while one layer detached from the bottom surface.
1164 m/s impact: Complete perforation occurred at the impact center, accompanied by fractured fibers and extended interface delamination.
These findings demonstrated that the damage mechanism in Kevlar panels is a complex interplay of shear plugging in the front layers and fiber breakage in the rear layers. Moreover, the authors highlighted the significance of projectile erosion and shattering, which contribute substantially to the bulging of the backing plate.
On the other hand for UHMWPE composite panels, the observed damage mechanisms were equally intriguing:
877 m/s impact: Similar to Kevlar panels, a pyramid-shaped bulge formed on the backside, albeit with a higher residual bulge.
1163 m/s impact: Interface delamination became more pronounced, with damage primarily limited to the top layers in the thickness direction.
1212 m/s impact: Partial penetration occurred at the impact center, with no systematic fiber breakage observed on the back face. This revealed the remarkable tensile resistance of UHMWPE fibers.
The authors conducted further examination using scanning electron microscopy (SEM) which unveiled distinct fracture morphologies. The rear layer of the UHMWPE panel exhibited severe axial splitting, attributed to the highly crystalline and ordered fibrils in aramid fibers. In contrast, the front layer displayed a relatively flat fractured surface, potentially resulting from shear forces perpendicular to the fiber direction. The research team also conducted FE simulation and found that it closely mirrored the experimental results, capturing the trends in deflection history accurately.
The study’s findings carry significant implications for the design and optimization of armor systems. For example, the choice between Kevlar and UHMWPE composites hinges on specific requirements. Kevlar panels excel in shear resistance, while UHMWPE panels offer exceptional tensile resistance. Designers must strategically select materials based on the anticipated threat and the desired balance between protection and weight. Moreover, the study underscores the importance of placing high shear-resistant materials in the front layers and high tensile-resistant materials in the rear layers of armor systems. This strategic placement maximizes the individual merits of each material and enhances overall protection. Furthermore, numerical simulations conducted in the study demonstrated the potential for hybrid panel designs. These designs, which combine Kevlar and UHMWPE in varying ratios, can significantly improve ballistic resistance while mitigating back face deflection. By carefully adjusting the material ratios, designers can achieve a balance that optimizes protection without compromising on weight constraints. Additionally, the shape and characteristics of the projectile can significantly influence the damage mechanisms and overall performance of armor systems. Projectile erosion and shattering, caused by interactions with ceramic tiles, play a crucial role in determining damage patterns. Indeed, the study’s results highlight the need for tailoring armor solutions to specific structural designs and loading conditions. What works for standalone composite panels may differ substantially from what is effective in layered ceramic armor systems.
In a nutshell, the study by Dr. Guodong Guo and colleagues has provided a wealth of insights into the behavior of Kevlar and UHMWPE composites when integrated into layered ceramic armor systems. These findings empower armor designers and engineers to make informed decisions regarding material selection, placement, and hybridization.
Guodong Guo, Shah Alam, Larry D. Peel, An investigation of deformation and failure mechanisms of fiber-reinforced composites in layered composite armor, Composite Structures, Volume 281, 2022, 115125,