Ballistic performance of nanocrystalline and nanotwinned ultrafine crystal steel

Abstract

Because of their remarkable mechanical properties, nanocrystalline metals have been the focus of much research in recent years. Refining their grain size to the nanometer range (<100 nm) effectively reduces their dislocation mobility, thus achieving very high yield strength and surface hardness-as predicted by the Hall-Petch relation-as well as higher strain-rate sensitivity. Recent works have additionally suggested that nanocrystalline metals exhibit an even higher compressive strength under shock loading. However, the increase in strength of these materials is generally accompanied by an important reduction in ductility. As an alternative, efforts have been focused on ultrafine crystals, i.e. polycrystals with a grain size in the range of 500 nm to 1 \im, in which "growth twins" (twins introduced inside the grain before deformation) act as barriers against dislocation movement, thus increasing the strength in a similar way as nanocrystals but without significant loss of ductility. Due to their outstanding mechanical properties, both nanocrystalline and nanotwinned ultrafine crystalline steels appear to be relevant candidates for ballistic protection. The aim of the present work is to compare their ballistic performance against coarse-grained steel, as well as to identify the effect of the hybridization with a carbon fiber-epoxy composite layer. Hybridization is proposed as a way to improve the nanocrystalline brittle properties in a similar way as is done with ceramics in other protection systems. The experimental campaign is finally complemented by numerical simulations to help identify some of the intrinsic deformation mechanisms not observable experimentally. As a conclusion, nanocrystalline and nanotwinned ultrafine crystals show a lower energy absorption than coarse-grained steel under ballistic loading, but under equal impact conditions with no penetration, deformation in the impact direction is smaller by nearly 40%. This a priori surprising difference in the energy absorption is rationalized by the more important local contribution of the deviatoric stress vs. volumetric stress under impact than under uniaxial deformation. Ultimately, the deformation advantage could be exploited in the future for personal protection systems where a small deformation under impact is of key importance.

Local Monte Carlo Method for Fatigue Analysis of Coarse-Grained Metals with a Nanograined Surface Layer

“…Here, the cohesive strength is modeled by Weibull A bilinear cohesive law has two important parameters, namely the cohesive strength T 0 n and fracture energy G IC . The fracture energy of the CG 304 SS is obtained from [46]. The flow stress of the CG specimen is from the experiments [46], while that of the NGL is estimated from experiments of the SMATed 316L SS [47].…”

“…Young's modulus E and Poisson's ratio v of both the CG and the NGL are taken as 200 GPa and 0.33, respectively [46]. The flow stress of the CG specimen is from the experiments [46], while that of the NGL is estimated from experiments of the SMATed 316L SS [47]. The kinematic hardening model is used to describe the cyclic plastic behavior of the metals.…”

Twin boundary spacing effects on shock response and spall behaviors of hierarchically nanotwinned fcc metals

“…Recent experiments and theoretical models have shown that the HTS with the secondary twins at the nanoscale can be a novel nanostructured design to achieve higher strength and toughness both under conventional deformation conditions and under extreme deformation conditions. 16,[36][37][38][39] So, it is highly desirable to examine the microstructure evolutions and underlying physics of deformation mechanisms for hierarchically NT fcc metals with low SFEs subjected to shock loading. In this regard, the focus of this paper is to understand the effect of spacing of secondary twins on the strength behind the shock front, spall strength, and related atomic-level deformation mechanisms in hierarchically NT Ag (a model material with low SFE) subjected to shock loading using MD simulations.…”

Explicit finite element analysis of high speed piston impacts

“…The fitting parameters for 316L, obtained by Olleak, A. et al (2015), was also included in the table below so that a comparison between the material properties of these two different grades of stainless steel could be made. The fitting parameters obtained by Frontán, J. et al (2012) matched those obtained by Krasauskas, P. et al (2015). Thus, 304 Stainless Steel III could be ignored from the analysis since it was already represented by 304 Stainless Steel I.…”

“…In order to evaluate the influence that the fitting parameters from the Johnson-Cook material model had on the stress-strain curve for 304 stainless steel, the fitting parameters obtained by Frontán, J. et al (2012) was compared to that obtained by Mori, L. et al (2007) and Krasauskas, P. et al (2015). These fitting parameters can be found in the table below.…”