Based on the results, it was shown that although the addition of a harder secondary phase improves strength, cracks that initiate and propagate within the harder phase and ultimately across the target plate drastically reduce the ballistic performance of the two-phase material. Rigorous characterization in the form of bulk mechanical testing, scanning electron microscopy, and spatially resolved nano-indentation on initial and ballistically impacted plates was used to connect microstructural details to aspects of ballistic behavior governing performance. By changing alloy composition in the Al-Co-Cr-Fe-Ni multi-principal system, the variation in microstructure included single-phase equiaxed grains, single-phase with bimodal grain-size distribution, and eutectic two-phase lamellar microstructure. The performance of complex concentrated alloys (or high entropy alloys) with widely varying microstructure is evaluated by ballistically impacting targets with spheres fired at normal incidence. In this article, we review the current state of knowledge in high strain rate engineering design and systematically assess the potential and emerging opportunities that can be realised via the AM approach. However, to date, research on AM of materials and structures for dynamic impact applications has been relatively limited. In these scenarios, the ability to optimise the three-dimensional (3D) geometry of the protection system can facilitate significant improvements in structural performance under high strain rate conditions, signifying the great potential of additive manufacturing (AM) methods in this field.
Prior studies have highlighted beneficial design features to enhance integrity under dynamic loading conditions, such as the ability to absorb the impact energy through cellular structural design or to deflect the impact. Three broad categories of such applications, according to the impact type and configuration requirements, including material and design characteristics, can be identified: (i) ballistic protection against projectiles or fragments (ii) protection against other types of high energy events (HEEs), such as a crash or a blast and (iii) micrometeoroid and orbital debris (MMODs) mitigation.
Applications involving strain rates in excess of 10² - 10³s⁻¹ are unique in both materials selection and design approaches and are of particular importance for protection of infrastructure and personnel in dynamic impact scenarios.
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