Shock enhancement effect of lightweight composite structures and materials

Abstract

Polymeric foams, textile materials and metallic foams are commonly used lightweight composite structures and materials for personal protective equipment (PPE) and protective structures in the battle field, because of their capabilities in reducing the risk of damages against ballistic impact. Under blast loading, however, a “shock enhancement” phenomenon has been observed in these materials, that is, the transmitted pressure is amplified, rather than attenuated as one would expect. Experimental evidences also demonstrated that covering animals with a layer of foam or textile could markedly increase the severity of lung injury. A number of studies have been published documenting such counter-intuitive effect. This review attempts to compile the state-of-arts and latest advances in this important subject. Experimental investigations on the pressure amplification of the aforementioned materials are summarized. Analytical/computational models that describe this phenomenon, particularly emphasizing on the mechanism and some key parameters affecting shock enhancement behavior, are also included. Finally, limitations of studies reviewed herein are discussed and issues which need to be addressed in further research are highlighted.

Introduction

Recent terrorist blast attacks have caused catastrophic consequences, in terms of fatalities and psyche of fear among the population, as well as enormous financial losses. When an explosive charge is detonated in air, the rapidly expanding gaseous reaction products compress and move the surrounding air outwards at supersonic speeds. The rapid expansion of the detonation products creates a shock wave with discontinuities in the pressure intensity, temperature, and velocity. Upon entering a body, the sudden pressure changes and complex stress/shear wave interaction can cause large compression of air-filled organs (e.g. the lungs) and tearing of muscle tissue and blood vessel [1]. In the last decade, blast induced injury has become a major medical concern in both academia and clinical research [2].

To reduce the injury or damage caused by explosion, it is conceptually straightforward to employ soft media (e.g. polymeric or metallic foams) and ballistic-resistant textile materials (e.g. Kevlar) as a passive shock mitigation device. Soft or porous media are generally excellent energy absorbers which attenuate impact loads by cell collapse mechanism at almost constant stress. Textile body armor materials consist of a large number of high-strength fibers to provide an effective protection against ballistic loads. Currently, these lightweight composite structures/materials have been used widely in personal protective equipment (PPE). However, when they are applied for blast wave mitigation, their protective performance is highly questionable. Numerous experimental evidences have demonstrated that the transmitted overpressures are significantly enhanced through a foam or fabric barrier, and animals covered with a layer of foam markedly increased the severity of lung injury [3], [4]. At occupational overpressure levels for heavy weapon operators, tests on the volunteers showed that the ballistic vest caused an increase in intra-thoracic pressure over that was observed when the vest was not worn [5]. Such counter-intuitive phenomenon was termed as shock enhancement. The shock enhancement phenomenon is an amplification of shock amplitude or impulse/energy transfer that takes place instead of expected mitigation. It is clear that a better understanding on this effect would be essentially critical to the development of more effective protective structures/materials for PPE against blast loads.

To provide a deeper insight into the shock enhancement effect, this paper presents a review of publications appeared in the open literature. The review focuses on three types of lightweight composite structures/materials commonly used in the protective applications, namely low-density polymeric foams, textile materials, and high-density metallic foams. Studies pertaining to fundamental investigations (mainly experimental and theoretical studies) are considered with the emphasis placed on the shock enhancement mechanism, and the key parameters affecting such behavior. Finally, limitations of the studies reviewed herein are discussed and the issues which need to be addressed in further research are highlighted.