Experimental and numerical study on fragmentation of steel projectiles

K.G Råkvaag,O.S Hopperstad,T Børvik,I Westermann

doi:10.1051/epjconf/20122604020

K.G Råkvaag, O.S Hopperstad + Show 2 more

Open Access

https://doi.org/10.1051/epjconf/20122604020

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Abstract

A previous experimental study on penetration and perforation of circular Weldox 460E target plates with varying thicknesses struck by blunt-nose projectiles revealed that fragmentation of the projectile occurred if the target thickness or impact velocity exceeded a certain value. Thus, numerical simulations that do not account for fragmentation during impact can underestimate the perforation resistance of protective structures. Previous numerical studies have focused primarily on the target plate behaviour. This study considers the behaviour of the projectile and its possible fragmentation during impact. Hardened steel projectiles were launched at varying velocities in a series of Taylor tests. The impact events were captured using a high-speed camera. Fractography of the fragmented projectiles showed that there are several fracture mechanisms present during the fragmentation process. Tensile tests of the projectile material revealed that the hardened material has considerable variations in yield stress and fracture stress and strain. In the finite element model, the stress-strain behaviour from tensile tests was used to model the projectile material with solid elements and the modified Johnson-Cook constitutive relation. Numerical simulations incorporating the variations in material properties are capable of reproducing the experimental fracture patterns, albeit the predicted fragmentation velocities are too low.

Highlights

IntroductionThe classical Taylor test is used to test and provoke fragmentation. The test was originally conceived in 1948 by Taylor [1] as a material test to determine the dynamic compressive yield stress of the material
In this study, the classical Taylor test is used to test and provoke fragmentation
The results from the Taylor analysis are at best approximations, and it has been shown through experiments that the analysis does not predict the deformation or derive the yield strength accurately [2]

Summary

Introduction

The classical Taylor test is used to test and provoke fragmentation. The test was originally conceived in 1948 by Taylor [1] as a material test to determine the dynamic compressive yield stress of the material. Chen et al [5] performed a study on soft steel projectiles impacting a harder, but not rigid, target plate with velocities ranging from 200 − 800 m/s They experimentally identified the sunflower-like petalling mode predicted by Teng et al They noted an important difference between the Taylor test and their own nonpenetrating experiments. The failure modes for the hard projectiles were mushrooming, shear cracking and fragmentation For the mushrooming they experienced the same inner and outer loop pattern as Chen et al [5] due to a small indentation in the target plate, even though the target plate was harder than the projectile. Meyer and Brannon [8] used a Weibull distribution of the fracture properties in simulations of a Behind Armor Debris experiment They found that the statistical failure model predicted the size distribution of fragments better than a homogenous failure model. It loses much of its penetrating capacity and this critical velocity is important for design of protective structures

Material tests

Taylor impact tests

Numerical study

Numerical results

Conclusion