M S & M Engineering Inc

ABSTRACT

This white paper discusses the fracture toughness based criteria for the initiation and propagation of cracks due to ballistic impact in multi-layered composite panels and development of effective protective shield. The light weight composite proposed is a sandwich, consisting of a thin layer of ceramic on an aluminum panel on one side, and Kevlar laminate on the other side of an EPP foam core. The study shows that the inter-laminar bonding plays very important role in energy absorption during delamination and debonding and the use of higher strength adhesive results in significantly reducing the projectile velocity and penetration.

INTRODUCTION

Multilayered metal/polymer composites are evolving as protective shields against various levels of ballistic threats in armored vehicles. The added mass of the some of the protective shields involving steel, hinder the mobility of the vehicles and lead to the premature fatigue failure of the suspension components.

An alternative aluminum/foam/Kevlar sandwich composite is studied for its effectiveness as a ballistic shield.

The ballistic impact energy is absorbed mainly by the initiation and propagation of cracks from the point of impact. The magnitude of energy absorption depends on the fracture toughness of the material.

When combination of multilayered materials are used, the impact load causes very high inter laminar normal and shear stresses leading to delamination and debonding of layered composites. While some energy is dissipated during delamination mode fracture, it weakens the shield against penetration of the projectile.

This white paper discusses how using higher strength adhesives and bonding techniques can improve the effectiveness of the sandwich composite in mitigating ballistic impact energy

SIMULATION

It was proposed to develop lightweight alternative to quarter inch thick steel shields currently used to meet the Level IV threat. The level IV threat ballistic chart is shown in Table 1. Table 2 shows the projectiles and threat classification

Table 1. Armor's Ballistic Chart for threat level IV

Table 2 Projectiles and threat classification

The simulation was carried out using LS-DYNA with user defined fracture toughness crack initiation and propagation criteria developed for composite materials [1,2].

Figure 1.  3D-Finite Element Model of the proposed Ceramic /Aluminum /Foam /Kevlar composite.

For the parametric variations an axisymmetric solid model was used as shown in Figure 2 .

Figure 2.  3D Axisymmetric Solid Model of the proposed Ceramic /Aluminum /Foam /Kevlar composite.

Interfaces are modeled using adhesive properties.

The elements reaching the failure criteria are eliminated as the projectile penetrates the composite panel.

Figure 3. Initiation of delamination

The impact load causes the composite to delaminate even at an early stage of the impact when the laminate also undergoes high frequency oscillations.

Figure 4 shows the projectile penetration through the laminate.

Figure 4. Projectile penetration

Figure 5. Energy Absorbed During penetration

Figure 6. Projectile travel and arrest

CONCLUSIONS

A parametric study was performed and the thicknesses of various layers were optimized to arrest the projectile before penetration into the Kevlar composite layer.

The properties of high temperature and pressure cure adhesive seem to significantly improve the bond strength of the interface and thus enabling to achieve the desired shield effectiveness.

References

 [1] Mahishi J. M., "An Integrated Micromechanical and Macromechanical Approach to Fracture Behaviour of Fiber-Reinforced Composites”, Engineering Fracture Mechanics, Vol. 25, No. 2, pp. 197-228, 1986.

[2] Mahishi JM, Adams DF “Energy Release Rate During Delamination Crack Growth in Notched Composite Laminates”

Delamination and Debonding of Materials

Committee: D30 Paper ID: STP876-EB DOI: 10.1520/STP876-EB. ASTM by W. Steven Johnson, ASTM Committee E-24 on Fracture Testing - 1985