Session W6: ME.5 Ballistics I

Numerical Study of Liner Modification Methods Leading to Fin-stabilized Explosively Formed Penetrators

One of the methods of increasing aerodynamic properties of elongated explosively formed penetrators (EFP) is inducing fins in the rear part of the projectile. In this work a set of fins generation methods was examined. Analysis showed that small amplitude wave-shaped imperfections in circumferential area of the liner provide projectile with desired finned shape without significant changes in formation process, which can reduce development cycle compared to axisymmetric EFP. Three methods of fins generation based on imperfections induced in liner were ~simulated numerically using commercial software. Suitable simulation technique was chosen and modified in order to treat geometrical imperfections of small amplitudes without large computational efforts. The relation between imperfection amplitude and projectile shape was obtained. It was shown that periodic nonuniform thickness is effective method of fins generation. For example, imperfection amplitude of 2{\%} leads to fins with amplitude 10{\%}. Material damage properties were taken into account and yielded range of acceptable imperfection amplitude.   

Deformation quantification during impact testing of mild steel for velocities of 1 to 3 km/s

Understanding the high rate deformation of metals during hypervelocity impact is important in mitigating damage in shielding systems. Well characterized experiments are needed to improve, validate and provide reference for numerical design of these complex systems. To better understand the high rate failure of metals, this work examines the response of mild steel plates during hypervelocity impact. An electromagnetic railgun at the French-German Research Institute of Saint-Louis, France, was used as the launch platform for impact velocities of 1 to 3~km/s. The targets were 50~mm thick. Image analysis of the highly deformed pearlite grains indicate that strains upwards of 100\% occur prior to failure near the impact point. Strain values decrease and grain orientations gradually change from aligned in the shot direction to random away from the impact site. Electron back scatter diffraction is used to quantify micro-structural deformation, and localized thermal and fracture effects are characterized with secondary electron microscopy.   

Applied Impact Physics Research

Applied impact physics research is based on the capability to examine impact processes for a wide range of impact conditions with respect to velocity as well as mass and shape of the projectile. For this reason, Fraunhofer EMI operates a large variety of launchers that address velocities up to ordnance velocities as single stage powder gun but which can also be operated as two-stage light gas guns achieving the regime of low earth orbital velocity. Thereby for projectile masses of up to 100 g hypervelocity impact phenomena up to 7.8 km/s can be addressed. Advanced optical diagnostic techniques like microsecond video are used as commercial systems but - since impact phenomena are mostly related with debris or dust - specialized diagnostics are developed in-house like x-ray cinematography and x-ray tomography. Selected topics of the field of applied impact physics will be presented like the interesting behavior of long rods penetrating low-density materials or experimental findings at hypervelocity for this class of materials as well as new x-ray diagnositic techniques.   

Strength effects in an imploding cylinder with constant mass-to-explosive loading

High explosives were used to implode thin-walled metal cylinders of different strengths (6061-T0, 6061-T6, mild steel, and stainless steel) at a constant mass-to-explosive (M/C) ratio. The velocity history of the inner surface of the imploding cylinder was recorded via Photonic Doppler Velocimetry (PDV). The time histories and peak velocities were compared to imploding Gurney models and LS-DYNA hydrocode simulations. A model for the acceleration of the wall using a detonation pressure-based time constant gave good agreement with both the experiments and simulations. The deceleration caused by strength effects was modeled from high-strain rate theory and was used to predict the entire velocity history.   

Influence of the multilayer coating obtained by the HVOF method on behavior of the steel barrier at dynamic loading

The high velocity (supersonic) oxy-fuel (HVOF) thermal spray technology is a rather recent addition to family of thermal spray processes. This technique is considered most modern of technologies of spraying. The increase in velocity of the particles at lower temperatures allowed reducing level of oxidation of the particles and to increase the density of a powder coating. In HVOF dry dusting applicators of the first and second generations was used the cylindrical nozzle, whereas in the third generation expanding Laval nozzles are used. This method allows the velocity of a gas flow to exceed to 2000 m/sec, and the velocities of the powder particles 800 m/sec. Recently many results on elastic and strength properties of the multilayer coatings obtained by supersonic flame spraying method are received. But the main part of works on research of the coating obtained by the HVOF method is devoted to research of their stress-strain state at static loadings. In this work the behavior of the steel barrier with the multilayer coating applied by HVOF is researched, at dynamic loading of projectile structure at different velocities of interaction. The problem was solved numerically within Lagrangian approach, a finite element method with the use of the explicit finite difference scheme of G. Johnson.   

The strength of reinforced concrete constructions under dynamic loads

At projection of industrial and civil facilities it is necessary to consider probable dynamic loads: impact, seismic waves, etc. Results of the coordinated experimental and numerical investigations of destruction of various types of ferroconcrete elements of constructions at impact loads are given in the work.