Introduction

In the next chapters, four groups of materials will be introduced and discussed. These will embrace metals, brittle solids, polymers and energetic materials. Some of these will be pure elements in various microstructures, others will be composites of several in different conformations. A lot of what follows has been described and studied by materials science and much terminology and commonplace understanding will be borrowed from there. Appendix A at the end of the book summarises some key concepts for those trained in other disciplines. At the most basic level, materials can be classified as metals or non-metals according to their ability to conduct electricity. The metals consist of cations in a delocalised electron cloud with structure determined by electrostatic bonds formed between the ions and the electron cloud. As pressure increases this bonding changes nature and above the finis extremis localisation of the electron density away from the nucleus occurs leading to new states.

Metals are the most common class of elements in the periodic table (Figure 5.1). Atomic stacking rules define a lattice of ions surrounded by a delocalised cloud of electrons, but from the point of view of the electronic states, one may equally consider them as materials where conduction and valence bands overlap. This definition opens the descriptor to metallic polymers and other organic metals and, considering the context within this book, one must consider the behaviour of materials that change their characteristics under high pressures and cause them to achieve metallic states (to conduct) at pressures below the finis extremis. A diagonal line drawn from aluminium (Al) to polonium (Po) separates the metals from the non-metals, and within that region the elements order themselves into subgroups defined by their electronic structures.

Introduction

This chapter will detail the response of a class of materials dubbed energetic to signify that they can break bonds and react under load. These substances contain a large amount of stored chemical energy that can be released if appropriate thermal thresholds are exceeded. Such materials combine a fuel and an oxidiser; fuels are typically carbon or hydrogen, oxidisers are oxygen or a halogen like chlorine, for example. Combining hydrogen and oxygen to form water liberates 13 260 J kg–1 and burning petrol with oxygen (air) 30000 J kg–1. Yet the high explosive TNT liberates only 4080 J kg–1, less than 15% of the amount liberated by petrol. The difference is that fuel alone burns only where oxygen is present; a spillage will burn for minutes with oxygen from air, for example. Yet a TNT molecule contains oxygen within it and can liberate energy in the microseconds the reaction front takes to transit the molecule and break bonds. Therefore the difference between these fuels lies in the power that the molecule supplies in the form in which the material exists on ignition. Energetic materials may be solids, liquids or gases, but condensed-phase materials will be followed here as earlier in the book. Further, they need not necessarily be organic. There is increasing need for higher performance, lighter weight and safer composites which use reacting metals as well as more conventional materials and using new material morphologies which have increased surface areas, such as mixtures of nano-materials or designed nano-composites. However, the principal energetics used at the present time include a range of elements that react with oxygen and these will be discussed in what follows.

Introduction

Matter in the universe exists in a series of states; three that are well known from standard experience, solid, liquid and gas, and the plasma state in which gas is ionised. Other more esoteric states are possible but the four above occupy the main thrust of this book. This volume has confined itself to pressures in the range up to a megabar and temperatures below 10000 K in which solids exhibit strength that is based upon the interaction of valence electrons. Beyond a critical energy density bonding is determined by further interactions of inner orbital electrons and concepts from the ambient cannot be extended.

Equally the forces acting on matter applicable to this work are electrostatic or gravitational. Electrostatic forces may act over great distance but as length scale increases there is sufficient matter that substances behave as neutral. The long-range attractions at the microscale are due to Van de Waals’ forces that might operate over distances of the order of 10 nm between polymer chains and are important in binding matter at the mesoscale. Components on a scale of centimetres are naturally held under gravity in stacks or compressed under lateral forces by some restraint. The strength of such an interface in tension is determined by that of the pin or joint that constrains the interface between the two components. At this scale, flow occurs by hinging around pivots under load or by slip along the fracture line with frictional heating at the interface. At the planetary scale forces are gravitational and slip occurs down faults that allow flow under shear.

I cannot explain my curiosity about extreme phenomena in nature; nevertheless I have been drawn to the science that surrounds them – from those occurring on the scale of solar systems to those at work at the smallest regimes within matter. Extreme forces surround us; they govern our weather, the cores of planets, components of engineering structures and the ordering of particles within atoms. At the scales of interest in this book they are either gravitational or electrostatic in origin. Forces drive mechanical routes to impose change and materials are forced to respond to these pressures in non-linear, counter-intuitive and utterly fascinating manners; frequently more quickly than not only the senses, but the recording media that exist today can track. Nothing that changes does so instantaneously; every mechanism takes some time, however small. This mean that the integrated response follows a delicate framework of competing pathways that reorder as the driver for the forces changes. As with many processes, one can only see patterns apparent in retrospect. Furthermore, the difficulties encountered achieving these states mean that there are many untracked routes that matter can take to respond about which we know little. Thus despite the years this book has taken to come to this point, it can only provide a snapshot of behaviour as I see it.

Nevertheless, matter allows the nature of its bonding to be probed by subjecting it to load and the reader will learn to appreciate the variety of materials behaviours and their causes that allow the design of structures or even new materials to withstand the environments considered. The behaviours observed are complex and seemingly counter-intuitive, and quantifying them has frequently filled books in the past with extended solid mechanics. This has made texts rich in analysis and specialised in application and required the reader to be expert in the mathematics of non-linear behaviour. However, it seemed that a reader with an appreciation of the physical sciences and elementary algebra required an open text to emphasise behaviours not analytical subtleties. Thus this book unites principles covering a broad canvas at a level accessible to graduate students.Further, it addresses the regime in which the strength of matter may be described with extensions of solid mechanics at the continuum rather than extrapolation of atomic theory and quantum mechanics at the atomic scale.

In the previous chapter, a series of examples was given to illustrate a range of material responses that stem from impact or explosion and result from a transient loading pulse within the material. These drivers propel waves travelling through solids, liquids and gases and place the material they have swept through into a state of compression, tension or shear. This chapter will describe these disturbances in more detail and attempt to give simple mathematical descriptions of the phenomena and the material's response. This basic approach is really a development of solid (a branch of continuum) mechanics to embrace additional features of loading at higher speeds and amplitudes; there are many, more complete texts available on the basics of solid mechanics that the reader may consult. The strategy here is to keep the derivations as simple as possible; again there are texts that derive relations with more generality than here but it is vital that the reader realises the assumptions, and more importantly their limits, in what follows. Particularly, it should be noted that solid mechanics assumes material behaviour based upon observations made in ambient states. Electronic bonding itself changes nature at around 300 GPa, so it is unrealistic to expect theory extended from the elastic state to apply in these regimes. Thus assumptions made and their limitations in the loading states the reader wishes to consider must be fully understood before using the formulae below. The basic laws of conservation of mass, momentum and energy, and classical mechanics will drive the descriptions of the thermodynamic states. To that will be added the concepts of elastic and inelastic (in metals, plastic) deformation bounded by a yield surface. To focus on material response, it is generally the simplest loading that is applied experimentally. Thus these states will be mentioned below to highlight particular relations to which the text will return.

Akrology

Extremes

The dynamic processes operating around us are often treated as transients that are not important when compared with the fixed states they precede. However, an ever-increasing knowledge base has illuminated this view of the operating physics and confirmed that extreme regimes can be accessed for engineering materials and structures. Matter is ever-changing, its form developing in a series of nested processes which complete on the timescales on which mechanisms operate; processes that occur on ever smaller timescales as length scales decrease. This book is concerned with the response that occurs when loads exceed the elastic limit. This affects behaviour in the regime beyond yield which encompasses a range of amplitudes and responses. However, it concerns condensed materials and loading, eventually taking them to a state where they bond in a different manner such that strength is not defined; this limit represents the highest amplitude of loading considered here. Nonetheless the driving forces are vast and awe-inspiring, while the different rates of change observed in operating processes are on scales that span many orders of magnitude. The following pages will highlight prime examples from the physical world and then provide a set of tools that classify mechanisms in order to analyse significant effects of these processes on the materials involved. The wide range of observations and applications create simple but powerful principles that are outlined in what follows.

Materials are central to the technologies required for future needs. Such platforms will place increasing demands on component performance in a range of extremes: stress, strain, temperature, pressure, chemical reactivity, photon or radiation flux, and electric or magnetic fields. For example, future vehicles will demand lighter-weight parts with increased strength and damage tolerance and next-generation fission reactors will require materials capable of withstanding higher temperatures and radiation fluxes. To counter security threats, defence agencies must protect their populations against terrorist attack and design critical facilities and buildings against atmospheric extremes.

The scientific method

The present chapter gives an overview of experimental platforms showing how they may be used to populate models for materials behaviour. The condensed phase defines the pressure and temperature range of interest, which may be approximately fixed at less than 1 TPa and below 10000 K. Indeed pressure has one of the largest ranges of all physical parameters in the universe (the pressure in a neutron star is c. 1033 Pa), so that most of the materials in nature are under conditions very different from those on Earth. The goal of shock experiments is to track response and mechanisms across the realms of stress and volume that are experienced by condensed-phase matter across the universe. At the highest pressures and temperatures, materials move from the solid to the liquid and then to plasma states as new correlations and bonding are formed. These high-density states have been termed warm dense matter (WDM) and lie beyond the finis extremis – outside the regime of extreme behaviour considered here. A summary of the phase space occupied by matter in these regions is shown in Figure 3.1.

The goal of experimental work is to provide adequate knowledge of the response of matter over the operating regimes of the relevant plasticity mechanisms. By this means, analytical descriptions can be constructed to try and capture the fundamental relationships between the independent variables – stress and stress state, strain and strain rate, and temperature – that determine the constitutive, damage and failure behaviour of materials. A shock impulse provides a pump to drive materials deformation and control of that impulse also allows a window into the operative mechanisms that lead to plasticity and damage evolution. This includes determining dynamic strength as a function of pressure as well as determining equation of state over the range of interest for particular applications.