High hardness steel used as ballistic armor: mechanical properties

ABSTRACT Ballistic armors are designed to resist specific threats, which depend on the application. These threats can be generated by firearm projectiles or fragments from explosions with different kinetic energies. The materials usually used for this purpose are metallic, nonmetallic and composite, being evaluated by dynamic tests, such as the Hopkinson Pressure Bar and the ballistic ones at the desired level of protection. However, the material can be previously evaluated by traditional mechanical tests, such as impact and surface hardness tests, classifying it as a material that can be used in ballistic protection. Also, during of serial production of military combat vehicles, the quality control of the applied material can be done by mechanical tests like the mentioned above to verify compliance with the established requirements. Due to its cost / ballistic resistance ratio, steel is still the most used material to produce of combat vehicles. There are international standards that show the minimum mechanical properties that a steel need to get specific ballistic protection. The objective of this work was to evaluate the compliance of a high hardness steel, produced by Brazilian steel company, proposed for the production of military vehicles with protection level up to ammunition 7.62 mm, through the impact, hardness and bending tests with the MIL-DTL-46100E. The analysis of the fracture surface of the samples tested by Charpy impact was carried out to correlate the mechanical properties of the material with the fracture mode. The results obtained and the characterization of the fracture surface suggest that the studied steel does not have the mechanical behavior suitable for the use in combat vehicles, been necessary to prove it through the ballistic test.摘要 防弹装甲的设计需针对特定应用场景匹配对应防护威胁，此类威胁可由不同动能的枪弹弹丸或爆炸破片产生。当前用于防弹装甲的材料主要包括金属、非金属与复合材料，其性能需通过动态试验评估，例如霍普金森压杆（Hopkinson Pressure Bar）试验与防弹试验，以达到预设防护等级。此外，材料也可先通过传统力学试验（如冲击试验与表面硬度试验）进行预评估，初步判定其是否可应用于防弹防护场景。在军用作战车辆的批量生产过程中，同样可通过前述力学试验对所用材料开展质量控制，验证其是否符合既定技术要求。鉴于钢材料具备优异的成本-防弹性能比，其仍是目前军用作战车辆最常用的制造材料。现有国际标准明确了钢材实现特定防弹防护等级所需满足的最低力学性能要求。本研究旨在通过符合MIL-DTL-46100E标准的冲击、硬度与弯曲试验，评估某巴西钢铁企业生产的高硬度钢是否适配7.62毫米弹药级别的军用车辆防护要求。本研究还对经夏比冲击试验的试样断口形貌进行了分析，以建立材料力学性能与断裂模式之间的关联。试验结果与断口形貌表征结果表明，本次研究的高硬度钢并不具备适配军用作战车辆的力学性能，需通过防弹试验进一步验证其性能。