Comparison of Mechanical Properties Obtained for Armor Steels: Conventional Methods vs. Nanoindentation

The mechanical properties of three armor steels, namely Ramor 500, Ramor 550, and Armox Advance, were acquired by nanoindentation and conventional methods, and then the results were compared. To this goal, samples were extracted from different (e.g., undeformed and deformed) zones of the ballistically tested armor plates. Then, nanoindentations were performed at 10-mN maximum load and different maximum indentation depths (50-150 nm). In addition to the hardness and modulus of elasticity values of the examined materials, residual stress values of the samples were calculated through the equations suggested in the literature. These values were then compared with the residual stress measurements obtained with the conventional x-ray diffraction (XRD) - sin2ψ method. Moreover, Charpy V-notch impact tests were performed to calculate the fracture toughness of armor steel samples. Microstructural analysis (e.g., optical microscope, scanning electron microscope, atomic force microscope), surface roughness, and micro-hardness (micro-Vickers) measurements were also obtained. It was noted that hardness values for the Ramor 500 and Ramor 550 armor steels were in the range of 5.5-9 GPa, whereas the hardness values of Armox Advance samples were obtained in the 8.6-10.2 GPa range at a 50-nm indentation depth. The residual stress values obtained with nanoindentation were found to be in quite good agreement with the one obtained through XRD. It was proven that nanoindentation can be used to acquire several mechanical properties through considerably less sample preparation procedure, in an automated, and a more representative manner because of the measurements obtained at multiple points.