Numerical simulation of the ballistic protection performance of a laminated armor system with pre-existing debonding/delamination

Abstract

The ballistic protection performance of laminated body armor may be significantly reduced through defects such as debonding/delamination. To understand the effect of a pre-existing defect on body armor performance, a two-dimensional finite element model was developed. The numerical study shows that the laminated armor system with a pre-existing debonding/delamination has poorer ballistic protection performance than that without any imperfection. Also, it was noted that the ballistic protection performance of the laminated armor system with a pre-existing debonding/delamination is sensitive to the changes in the parameters considered such as size, location and pattern of the pre-existing debonding/delamination.

Introduction

Over the past decades, there has been an increase in the development and application of laminated body armor systems, such as composite armor systems made of a ceramic strike face and a fiber-reinforced composite backing layer. The hardened ceramic facing material initially erodes or fragments the projectile, followed by the composite backing material reacting against the blunted projectile. This second stage normally involves considerable membrane stretching, bulge formation, and delamination, and thus the backing material play a role in absorbing part of the projectile kinetic energy and catching the fragments of both ceramic and projectile to prevent the wearer from the fragment assaults. This design exploits the fact that ceramic composites are more weight-efficient than traditional monolithic armor materials. [1], [2], [3], [4], [5], [6], [7], [8], [9], [10].

Defence departments of many countries such as those of Australia, US, UK and Canada require that the ballistic protection performance of a body armor system must meet a certain design standard such as the National Institute of Justice (NIJ) standard [11]. According to the NIJ standard, the performance of a body armor system is judged by the ballistic limit V_bl and maximum back face deformation (MAXBFD) [11], [12].

In one of the initial attempts at simulating the ballistic protection performance of ceramic composite armor system, Woodward [13] developed a simple one-dimensional (1D) approach for analyzing and describing the penetration mechanism involved in the perforation of ceramic composite armor, which was made of a ceramic front layer and a metallic backing layer. For simulating the projectile impact on ceramic/composite armors, Chocron-Benloulo and Sánchez-Gálvez [2] proposed a very simple 1D analytical model, which was divided into three phases of penetration (intact ceramic, fractured ceramic and initial response of the composite substrate) and fabric response and failure. The predicted residual velocity and residual length of the projectile compared reasonably well with the numerical results evaluated using the commercial finite element (FE) software Autodyn.

Fawaz et al. [3] developed three-dimensional (3D) FE models using the non-linear FE code LS-DYNA3D for simulating the performance of ceramic composite armor systems subjected to normal and oblique projectile impacts. The numerical results correlated reasonably well with the experimental data available in the literature. Shokrieh and Javadpour [5] developed FE model using Ansys/Ls-dyna software for the penetration analysis of a two-layer armor system subjected to a projectile impact, where the armor system was composed of a boron carbide ceramic front layer and a Kevlar 49 fiber composite backing layer. There was good agreement of projectile residual velocity between their FE model and Chocron’s analytical model, especially at high velocities. Krishnan et al. [6] developed an FE model for designing a body armor system made of ceramic and high-performance polyethylene materials. Laboratory testing of the prototype package showed that the FE predictions of damage were excellent even though the back face deformations were under-predicted. Burger et al. [10] developed a numerical model using the ABAQUS/explicit FE code for simulating the response of hybrid ceramic/fiber reinforced composite armors. There was good agreement of the ballistic limit between the numerical and experimental results. Feli and Asgari [8] conducted a simulation using LS-Dyna code for the ballistic perforation of ceramic/composite armor, which consisted of an alumina 99.5% ceramic front layer and a Twaron fiber backing layer. In this simulation, the Johnson–Holmquist model was used for modeling the ceramic front layer and the composite-damage model was used for modeling the composite backing layer. Since distortions caused by larger deformation of elements may result in stopping computations or making them extremely slow, the material erosion model was used to delete the failed elements from the analysis. For the initial velocity ranging from 1000 m/s to 1600 m/s, the FE simulation results compared well with the numerical results from Chocron-Benloulo and Sánchez-Gálvez [2] and the analytical results from Feli et al. [7].

Owing to the fact that debonding or delamination defects can occur during the manufacturing process and in service, these pre-existing defects may significantly degrade the ballistic protection performance of laminated body armor systems. In order to improve the defect/damage-survivability and defect/damage-tolerance of a laminated body armor system, it is essential to understand the effect of a pre-existing debonding/delamination within a laminated body armor system on its ballistic protection performance. During a bullet or projectile impact event, the dynamic response of a laminated armor system is a combination of the local and global reactions. The relative contributions from these two reactions are generally determined by many parameters such as the impact velocity of the bullet or projectile, the geometries and material properties of the projectile and laminated armor system components, boundary conditions, damage location, size and pattern [14], [15]. As limited work has been done in this area, this paper focuses on modeling simulation for the ballistic protection performance of a laminated ceramic/Kevlar composite armor system with a pre-existing debonding/delamination. This investigation also formed part of the Defence Material Technology Centre (DMTC) Personnel Survivability Program on Life of Type of armor systems.

In this paper, an FE model was developed using the commercial FE software Autodyn [16] to study the effects of major parameters such as pre-existing debonding/delamination size, location and pattern on the predicted values of ballistic limit V_bl and MAXBFD for the laminated body armor system subjected to a flat-faced cylindrical projectile impact. This FE model can also be applied to investigate the ballistic protection performance of an armor system with multiple debondings/delaminations. It can be achieved by either simplifying the multiple debondings/delaminations as a single debonding/delamination or replacing the single debonding/delamination with multiple debondings/delaminations. The present FE modeling procedure was used previously for investigating the blast wave and fragment/projectile protection performance of laminated armor systems backed with or without an air spacer [17]. In order to investigate the effect of projectile geometry on the predicted values of V_bl and MAXBFD, three flat-faced cylindrical projectiles having the same mass but different geometries were considered.