Explosion Blast Response of Composites

Adrian P. Mouritz, Yapa D.S. Rajapakse

• Publisher: Woodhead Publishing
• Year: 2017
• Print ISBN: 9780081020920
• eBook ISBN: 9780081020937
• Topic: Technologie et ingénierie
• Subtopic: Science des matériaux

Explosion Blast Response of Composites contains key information on the effects of explosions, shock waves, and detonation products (e.g. fragments, shrapnel) on the deformation and damage to composites. The book considers the blast response of laminates and sandwich composites, along with blast mitigation of composites (including coating systems and energy absorbing materials).Broken down under the following key themes: Introduction to explosive blast response of composites, Air explosion blast response of composites, Underwater explosion blast response of composites, and High strain rate and dynamic properties of composites, the book deals with an important and contemporary topic due to the extensive use of composites in applications where explosive blasts are an ever-present threat, such as military aircraft, armoured vehicles, naval ships and submarines, body armour, and other defense applications.In addition, the growing use of IEDs and other types of bombs used by terrorists to attack civilian and military targets highlights the need for this book. Many terrorist attacks occur in subways, trains, buses, aircraft, buildings, and other civil infrastructure made of composite materials. Designers, engineers and terrorist experts need the essential information to protect civilians, military personnel, and assets from explosive blasts.- Focuses on key aspects, including both modeling, analysis, and experimental work- Written by leading international experts from academia, defense agencies, and other organizations- Timely book due to the extensive use of composites in areas where explosive blasts are an ever-present threat in military applications

Chapter 1

Physics of Explosive Loading of Structures

Stephen J. Cimpoeru¹, David V. Ritzel² and John M. Brett¹, ¹Defence Science and Technology Group, Fishermans Bend, VIC, Australia, ²Dyn-FX Consulting Ltd, Amherstburg, ON, Canada

Abstract

The detonation of an explosive charge in air or underwater creates a complex and dynamic physical environment in the surrounding medium which includes shock waves and high velocity detonation-product or fluid flows. Any structure in the immediate vicinity will be subject to loading from these phenomena and the ensuing blast-wave or fluid-flow structure interaction can be significant, and in the case of underwater explosions greatly modify this loading. Knowledge of the explosive loading process is a prerequisite for a thorough understanding of the resulting structural damage and blast-protective design. At a practical level this understanding is required for scientific trial design as well as interpretation of experimental data and computational studies of blast damage. In this chapter we provide a concise summary of the basic loading physics associated with air and underwater explosions.

Keywords

Air blast; blast physics; cavitation; explosive loading; gas bubbles; shock waves

Introduction

Blast and shock protection is complex, and the blast-resistant design of structures has often been compromised because the loading condition is ill-defined and dynamically changing. Furthermore, there is often misunderstanding of critical factors such as the underlying physics of the blast output and how loading is imparted. This chapter explains the nature of explosive blast and the primary means by which loading is imparted in both air and underwater explosions. The purpose is not to provide a review of the extensive literature on this topic; rather our objective is to provide a concise summary of the complex and dynamic loading conditions that a structure will be exposed to when subject to air or underwater explosions. This is illustrated with examples taken from the authors’ previous research.

Air Blast

Simple Spherical Air Blast

The destructive power of explosives has been known since at least 220 BC, the first documented explosive accident injured early alchemists in China and led to the development of black powder. However, the fundamental scientific understanding of the underlying blast physics only really developed in earnest in the 1940s due to the need to understand the blasts generated by the first nuclear weapons. Blast physics concerns the processes by which the energy of an explosion source propagates into its surrounding environment then interacts, loads and damages materials, structures, and systems. Understanding the complex nature of these blast processes is a key to devising the most effective blast protection technologies. This section describes the basic phenomena of the blast flow-field loading conditions important for the air-blast loading of structures.

The simplest case of an idealized blast from a bare spherical high explosive charge in air is first discussed. In reality, actual blast events are mostly nonideal and are strongly affected by explosive type and many other factors including the charge shape and casing. Fig. 1.1 depicts the very early development of the blast-wave flow for an idealized centrally-detonated bare spherical charge of high explosive. Following initiation, a detonation wave sweeps through the unreacted explosive material at speeds typically about 6–8 km/s, effectively converting the solid explosive to hot and extremely high-pressure gases at about 3000 K and 40 GPa (400,000 atm). Due to their extreme pressure, the gaseous detonation products expand rapidly to about 4000-fold the original charge volume and are visible as a radiant fireball; it is the hydrodynamics of this expansion process which generates the blast-wave flow.

Figure 1.1 Schematic depiction of the very early development of the blast wave flow for a centrally-initiated bare spherical charge of high explosive, (A) and (B), with spatial profiles of the change in static pressure, ΔP, with respect to charge radius, R, in lower images, (C) and (D), corresponding to the physical depiction in the upper images. (A) Detonation front $\overrightarrow{D}$

prior to reaching the charge surface and (B) latter time generation of the air-shock wave $\overrightarrow{S}$

being driven by the expansion of the fireball contact surface $\overrightarrow{C}$

. (D) Flow within the expanding detonation products is further partitioned by an “embedded” rearward-facing shock, $\overleftarrow{S_2}$

. The various materials are shown with green representing unreacted explosive, yellow/orange representing gaseous detonation products (orange being the center of detonation) and blue designating air; R₀ designates the original charge radius. Images (A) and (B) courtesy of DRDC Suffield.

The rapid expansion of this fireball of detonation-products drives a shock wave into the surrounding air ahead of it much like the action of a spherical piston. The edge of the expanding fireball is effectively a material front designated as a contact surface across which there is theoretically no mass or heat transfer. In reality, there is always some degree of turbulent mixing at this interface and consequent momentum, material, and heat transfer. The most distinctive feature of the propagated air-blast wave is the shock front through which there is a nearly instantaneous step change in all gas dynamic conditions of the air including static pressure¹, density, flow velocity and temperature.

A potentially significant and generally unrecognized aspect of the early blast flow development is that the air immediately surrounding the charge is “shock heated” by the passage of the intense shock front to extreme temperatures of the order of 8000 K. This process is not related to heat transfer from the fireball as might have been expected; ironically, heat would only be transferred from the shock-heated air to the fireball. The presence of superheated air just beyond the periphery of the fireball after passage of the shock is the cause for secondary combustion of afterburning material from the fireball that may have been mixed in by turbulence or projected into this zone of shock-heated air. Although the fireball is typically highly radiant and luminous, in fact at its full expansion it is much colder (~500 K) than the thick annular shell of air immediately beyond its perimeter which will persist at temperatures up to about 3000 K until dissipated some seconds after the passage of the shock wave.

The combined violent expansion of detonation-product gases and the resultant propagated air-shock wave constitutes the “blast flow-field” loading condition, and it is important to recognize the dual nature of these blast flow conditions. That is, close to the charge within the region of the fireball expansion not only is the amplitude of the blast forces more extreme as would be expected, but the flow field consists primarily of the expanding detonation-product gases as distinct from air. Compared to blast at greater distance, there is proportionately much higher kinetic energy in near-field blast [1] as well as dramatically variant spatial and temporal energy partitions.

Near-field blast conditions from explosive detonations have significant implications regarding consequent loading and damage processes which should be distinguished from classical understandings of static overpressure loading and damage in the far field [2]. The very strong near-field flow forces due to dyn...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of Contributors
• Preface
• Chapter 1. Physics of Explosive Loading of Structures
• Chapter 2. Experimental Techniques and Testing of Lightweight Naval Structures Against Weapons Effects
• Chapter 3. The Dynamic Behavior of Composite Panels Subjected to Air Blast Loading: Experiment and Theory
• Chapter 4. Computational Methods to Predict the Nonlinear Dynamic Response of Blast Loaded Laminated Composite Plates
• Chapter 5. Explosive Blast Resistance of Naval Composites: Effects of Fiber, Matrix, and Interfacial Bonding
• Chapter 6. Influence of Curvature and Load Direction on the Air-Blast Response of Singly Curved Glass Fiber Reinforced Epoxy Laminate and Sandwich Panels
• Chapter 7. Full-Scale Air and Underwater-Blast Loading of Composite Sandwich Panels
• Chapter 8. Design and Modeling of Bio-inspired Lightweight Composite Panels for Blast Resistance
• Chapter 9. Observations and Numerical Modeling of the Response of Composite Plates to Underwater Blast
• Chapter 10. Instabilities in Underwater Composite Structures: Hydrostatic and Shock Loading
• Chapter 11. Underwater Explosive Blast Response of Fiberglass Laminates
• Chapter 12. Low-Speed Impact on Composite Box Containing Water
• Chapter 13. Physical Mechanisms for Near-Field Blast Mitigation With Fluid Containers
• Chapter 14. Progress Toward Explosive Blast-Resistant Naval Composites
• Index