Ekmekci, Dursun; Cora, Ömer Necati

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.

Ekmekci, Dursun; Uşun, Altuğ; Cora, Ömer Necati

The mechanical properties of two advanced high-strength steels (AHSS), DP 1000 and DP 1200 dual-phase (DP) grades, under deformed and undeformed conditions, were investigated using nanoindentation, and the results were compared with those obtained from the conventional methods. To this goal, 3-point bending tests were applied to induce deformation in the samples. Before and after these tests, nanoindentations were performed at different forces and indentation depths. In addition to the hardness and modulus of elasticity values of the steels, the residual stresses on the samples after deformation were obtained by using the equations suggested in the literature, also with x-ray diffraction (XRD). Finite element (FE) modeling of 3-point bending and nanoindentation were performed to obtain stress–strain curves of the materials numerically. The stress–strain curves obtained by numerical analysis agree well with those reported in the literature. The variations of the hardness and modulus of elasticity values are narrower for deeper indentation (50 nm versus 200 nm), and the effect of deformation is more pronounced on the modulus of elasticity values (10–30% increase with the deformation) while hardness values increased with the effect of deformation, 10% at most.

Ekmekci, Dursun; Cora, Ömer Necati

The goal of this study was to acquire mechanical properties of a dual-phase grade advanced high strength steel (AHSS) by means of different microindentation loading conditions. Conventional, cyclic, and multi-step indentations were performed on DP 800 sample; and Young’s modulus, hardness values were obtained by the depth-sensing indentation technique. Effects of different load levels (50–300 mN) and type of indentation on the results were also analyzed through ANOVA. The load levels experimented yielded overall material response rather than its constituents (e.g., martensite and ferrite). Both hardness and Young’s modulus tend to decrease with increasing maximum indentation load level, especially from 50 to 100 mN, which is regarded as indentation size effect.