Atomistic Studies of Deformation and Fracture in Materials with Mixed Metallic and Covalent Bonding

Materials with high melting temperatures (over 2000°C) tend to be brittle at ambient and even relatively high temperatures. High melting temperatures originate in strong interatomic bonding arising from formation of dd or dp bonds that also affect and/or control crystal structures and properties of extended defects, such as dislocations, grain boundaries. These, in turn, govern plastic deformation and fracture. General goal: Establish relationship between electronic structure and mechanical behavio

Frangibility Quantification of Frangible Bullet upon Impact on a Hard Target

The article presents a new method of quantitative evaluation of the degree of disintegration, called frangibility of frangible bullets, based on shooting experiments. On the basis of the dependence of the Frangibility Factor of the bullet on its impact velocity, the frangibility index is determined as a quantifier of the projectile’s ability to disintegrate into fragments applied for a certain range of interest. The method can be used to assess the ability to disintegrate into fragments of various types of bullets, preferably frangible bullets produced by powder metallurgy technologies. The proposed method was used to quantify the frangibility of five experimental frangible bullets

Bond-order potential for simulations of extended defects in tungsten

We present a bond-order potential (BOP) for the bcc transition metal tungsten. The bond-order potentials are a real-space semiempirical scheme for the description of interatomic interactions based on the tight-binding approximation. In the hierarchy of atomic-scale-modeling methods the BOPs thus provide a direct bridge between electronic-structure and atomistic techniques. Two variants of the BOP were constructed and extensively tested against accurate first-principles methods in order to assess the potentials\u27 reliability and applicability. A comparison of the BOP with a central-force potential is used to demonstrate that a correct description of directional mixed covalent and metallic bonds is crucial for a successful and fully transferable model. The potentials are applied in studies of low-index surfaces, symmetrical tilt grain boundaries, and dislocations

Study of relationship between motion of mechanisms in gas operated weapon and its shock absorber

The article deals with the motion of the breech block carrier and the weapon casing of an automatic weapon mounted on a flexible carriage and the base of the weapon. Earlier works, which did not consider the dynamic properties of the base of the weapon, did not allow to reconcile the calculated and experimental results of the weapon casing displacement when shooting from firing rests. For the analysis of the motion of individual parts, the methods of mathematical modelling and firing experiments using a high-speed camera were chosen. Calculations show the best accord with experiment when modelling the system with 4 degrees of freedom. The oscillation of the system regarding the movement of the breech block carrier and the weapon casing was investigated under changed conditions of rate of fire, the use of a muzzle brake and different types of shock absorbers. The velocities and displacements of the weapon casing and the breech block carrier at different values of the impulse of the gases to the breech block carrier were determined

Possibilities of Using the Duplex System Plasma Nitriding + CrN Coating for Special Components

The article deals with the replacement of hard chrome plating by applying the duplex system plasma nitriding + CrN coating (hereinafter referred to as PN + CrN). The goal of the research was to find a suitable alternative for steel surface treatment that would replace hard chrome plating and ensure similar mechanical and tribological properties. An exposed part of a small-bore weapon was selected for evaluation, namely the gas piston of the 42CrMo4 steel breech mechanism drive. The PN + CrN duplex system was compared with a hard chrome coating as well as a self-deposited CrN coating. The mentioned surface treatments were evaluated in terms of metallography, mechanical and tribological properties and surface texture. From the mechanical properties, the hardness of the surface was analyzed, an indentation test was performed (Mercedes test) and adhesive-cohesive behavior was evaluated (Scratch test). Furthermore, an instrumented penetration test was performed (an evaluation of plastic and elastic deformation work and indentation hardness). As part of the assessment of tribological properties, the Ball-on-Flat test, the measurement of the coefficient of friction and the measurement of traces of wear were performed. The surface texture was evaluated in terms of morphology and surface roughness measurement by selected 2D and 3D parameters. The PN + CrN duplex system showed higher hardness than hard chrome, better tribological properties (lower friction coefficient), but worse surface texture. The PN + CrN duplex system has proven to be a suitable alternative to the hard chrome coating for exposed parts of small-caliber weapons, which can be applied in arms production