Vishal Mehra

High velocity impact of metal sphere on thin metallic plates: A comparative smooth particle hydrodynamics study

Four different shock-capturing schemes used in smooth particle hydrodynamics are compared as applied to moderately high-velocity impacts (at 3km/s) and hypervelocity impacts (at ~6km/s) of metallic projectiles on thin metal plates. The target and projectile may be of same metal or different. The simulations are compared with previously published experimental data and simulations. The schemes differ in how artificial viscosity (AV) is treated and include (i) standard SPH AV, (ii) Balsaras AV (BAL), (iii) Morris and Monaghans AV (MON) and (iv) Riemann-based contact algorithm (CON) of Parshikov et al. At moderately high impact velocity, CON performed best overall, in particular, in being free from numerical fracture and formation of clumps, problems that plague SAV and its modifications, BAL and MON. For the hypervelocity impact, CON does not produce correct debris cloud. The BAL and MON while reproducing the debris cloud more closely, overestimate the crater diameter significantly. An excessive AV, by enhancing transverse momentum flow, may be the source of the overestimation of crater diameter. Experimental agreement is generally worse when the target and projectile are formed of distinct metals.

Tensile Instability and Artificial Stresses in Impact Problems in SPH

The smooth particle hydrodynamics (SPH) is a meshless computational technique that is popular in the modeling of impact and penetration problems. However, SPH is liable to a tensile instability that manifests itself as a bunching of nodes and formation of artificial voids and no generally accepted formulation exists to counter this instability. We examine the performance of two methods that have been proposed to deal with the tensile instability— the Monaghan artificial stresses and the Godunov-type SPH. The impact and penetration of 0.5 cm radii steel spheres on 2 mm thick aluminium plate at 3.1 km/s is chosen for comparison. We show that the artificial void formation in St-Al impact is suppressed but not eliminated by using Monaghan stresses while the void formation is entirely eliminated by using Godunov-type formulation of SPH that was proposed by Parshikov and Medin.

Buckling and longitudinal cracks in electromagnetically accelerated hollow cylinders

Hollow cylindrical Al-6061 T6 projectiles, driven in a coilgun system, suffer radial compression and buckle into quadrilateral or pentagonal cross sections. In some cases, the projectiles fail by developing longitudinal cracks in the compressed region. Simulations of the coilgun-projectile system, using Johnson–Cook and Bao–Wierzbicki failure models, reproduce buckling and formation of longitudinal cracks via localization of plastic strain and high temperatures around the bends of the buckled geometry. Failed specimens were micro-graphically investigated and the cause of failure attributed to synergistic effect of buckled geometry and localized high temperature zones.

A VOF based multi-material method to study impact and penetration problems

A 2D axisymmetric Eulerian method is developed for studying multi-material elastic-plastic problems involving large material deformations. It consists of a Lagrangian plus remap strategy using a volume-of-fluid (VOF) based material interface tracking. The multi-material formulation used in this method allows one to update the energy and stress components of each material in a mixed cell independently. This assumes common strain-rates to all materials present in a mixed cell. The equivalent pressure and stress in a mixed cell are determined using volume weighted average. The present scheme, therefore, eliminates the commonly used pressure relaxation method, mixed EOS evaluation and energy partition schemes. The stress components of each material are transported using a second-order monotonic upwind scheme (MUSCL) due to van Leer. The capability of the proposed method is demonstrated by applying to various impact and penetration problems involving large material deformations. Reasonable agreement with experimental results are observed.

Comparisons between Fast Shock Tube Simulations and Tests

The experiments of Menikoff et al on a projectile hypervelocity launcher using a fast shock tube (FST) are modelled using smooth particle hydrodynamics (SPH) technique. In a FST, the progressive detonation of a co-axial HE cylinder induces a cumulative shock in the liquid-filled core. This shock hits a thin flyer and accelerates it to hypervelocity. The comparisons are made on flyer velocity profile, peak pressure and shock speed in liquid core. The SPH reproduces the qualitative and quantitative aspects of the FST and is well-suited to the high strain-rate feature of this experiment.

Equation of state and electrical conductivity of expanded Al

Ab-initio molecular dynamics (AIMD) simulations have been performed for generating equation of state (EOS) data as well as atomic configurations of expanded states of Al. The generated atomic configurations have been used for the calculation of electrical conductivity. AIMD simulations have been performed using the ABINIT code and electrical conductivity has been calculated using the Kubo-Greenwood formula as implemented in ABINIT. The generated equation of state data have been fitted to a three-term EOS model using adjustable parameters of ionic and electronic Gruneisen parameters. This three-term EOS model has been used to estimate critical density, temperature and pressure in the liquid-vapour region. The calculated values of critical density, temperature and pressure show good agreement with results available in the literature.

Penetration of Long Steel Rods into Thick Steel and Sandstone Targets

The impact of a ogive‐nosed steel projectile into thick sandstone and steel targets has been simulated over impact velocities from 200 m/s to 750 m/s, and the projectile deceleration histories compared. Both targets have an entrance phase in which deceleration increases sharply. This phase is followed in a steel target by a steady deceleration phase. In sandstone, however, a double‐peak structure is seen in the 300–550 m/s range. This is similar to the double peak that was observed in a reported rock penetration experiment at 520 m/s. These studies are aimed at determination of material‐specific parameters related to dynamic strength and fracture properties.