1.1 Rationale and Scope of the Book

The issue of how metals undergo plastic deformation (including creep – see the end of §1.3 below), and how this behavior can be measured and characterized, is a very old (and important!) one. Attempts to measure the “strength” of metals (and other materials) date back to antiquity. Understanding of the deformation mechanisms involved, and implications for systematic control over mechanical properties, are more recent, but they still extend back several decades. A number of texts were produced during the period in which most of the key advances were made, which was mainly between the 1950s and 1980s, and in many cases these have been followed up with a number of updated editions. Also, several new books have been published more recently. Many are strongly oriented towards engineering, with materials treated as (isotropic) continua and their properties described by analytical equations (constitutive laws). There are also several books containing a mixture of physical metallurgy and associated mechanical properties. Mechanical testing procedures are often covered in some way, although relatively few books have been dedicated to this area.

This book is partly aimed at comprehensive description of mechanical testing procedures, over a wide range of conditions (notably temperature and strain rate). Even the standard, “simple” procedures, such as uniaxial tensile testing, do in fact incorporate complexities that are not always well understood, but a large part of the motivation for the book relates to the fact that other testing geometries, particularly indentation, have become widespread recently. Also, the technique of finite element method (FEM) modeling, which is a powerful tool for investigation of mechanical deformation, is now mature and ubiquitous. Its use figures strongly in the book.

The theme of the book is not strongly oriented towards “nanoindentation” as such, although it is included in the coverage. It is mainly related to the obtaining of bulk mechanical properties (in the context of microstructure and deformation mechanisms, and their relationship to such properties). What would commonly be regarded as nanoindentation is not well suited to this objective, since the volume being mechanically interrogated is often too small for its response to be representative of the bulk. However, the key recent advances concerning indentation do not really relate to it being carried out on a fine scale, but rather to the process being instrumented so as to obtain detailed information about the (bulk) mechanical characteristics of the sample. This represents a fundamental advance, relative to its origins in hardness testing.

1.2 Structure and Readership of the Book

The structure of the book effectively comprises three sections. After this general introduction, there is a chapter dedicated to the handling of stresses and strains (as second rank tensors) and to the relationships between them during elastic deformation. This is followed by a chapter focussed on how such relationships are modified after the onset of plastic deformation (with the material treated as a homogeneous continuum) and then one on the mechanisms of plastic deformation and how they are affected by the microstructure of the metal. All three of these chapters can certainly be regarded as providing background information, although it is clearly relevant to a full understanding of plasticity. Similar material is available from a wide range of sources, but it is considered to be potentially helpful to include it in the book, for ease of reference. There is then a set of three chapters that cover the “standard” procedures of tensile, compressive and hardness testing. These are undoubtedly familiar to people involved with mechanical (plasticity) testing, but they do involve some subtleties that are not universally understood. These chapters contain guidelines for obtaining and interpreting experimental data most effectively with these procedures.

The final section of three chapters concerns testing techniques that have been developed more recently, with the first of these, outlining the current state of the art in indentation plastometry, given particularly close attention in view of its scope for obtaining detailed plasticity characteristics in a convenient and flexible way. The following one covers “nanoindentation,” which has had a high profile over recent years, but actually suffers from some severe limitations in terms of obtaining information about bulk plasticity characteristics. The last chapter covers a range of fairly specialized test procedures, some in developmental form. There is thus a developing emphasis throughout the book on more recent (and research-oriented) topics, which is reflected in progressively increasing levels of citation of other published work. These references are collected at the end of each chapter. There is also a nomenclature listing at the beginning, with an attempt made to standardize the symbolism in use throughout the book.

It should be noted that the topic of fracture has a relatively low profile in the coverage. While indentation can sometimes be used to stimulate fracture, or at least crack propagation, it is not really an effect that can readily be investigated via indentation. Of course, some kind of fracture event does normally take place at the end of a tensile test, although local conditions during that event are often rather poorly defined. Testing aimed at obtaining fundamental fracture mechanics properties, such as the fracture energy or the fracture toughness, are carried out rather differently (and those tests are not described in the book). In fact, at least for most metals, it is largely the plastic deformation (yield stress and work hardening characteristics) that dictates their “strength” (as measured by the “ultimate tensile stress,” UTS), with their fracture toughness as such (important as that is for other purposes) being of limited significance during such testing. Moreover, by interpreting the outcome of a tensile test via its simulation in an FEM model, it is possible to obtain an estimate of the critical strain for fracture, which is a widely used “property.” Furthermore, coverage is included (in the final chapter) of certain types of cyclic loading test that may involve crack propagation and some of the basics of fracture mechanics are presented there. Such tests are closely related to real industrial service environments and, indeed, there is reference throughout the book to the relevance of various tests to industrial usage of metallic components.

A natural part of the readership is those already involved in mechanical testing (of metals), who need an update and a book for future reference. However, it is potentially of much broader appeal, since many people involved in the development of novel or improved materials and components have a need for understanding of the issues associated with mechanical deformation. Indentation plastometry has many attractions, and its usage is likely to increase over coming years. This book is aimed at providing a source of information for potential users of the technology. Furthermore, there is certainly potential for usage in university level teaching. The prerequisites are simply a very basic background in materials science and mechanics. Finally, there are many people involved in various manufacturing and processing sectors with an interest in improved understanding of the links between processing conditions and component performance in service. Metal plasticity and its characterization are central to this.

1.3 Basic Elastic and Plastic Property Ranges

There is, of course, a fundamental difference between elastic and plastic properties (of metals). Elastic deformation essentially arises because the distances between atoms are being changed (by an applied load). These distances increase in the direction of an applied tensile force, and decrease in directions transverse to this. Since the transverse reductions do not compensate fully for the axial extension, there is normally an associated volume change (unless the Poisson ratio has a value of 0.5, which is not the case for any metal). The “building blocks” (unit cells of the structure for crystalline materials, i.e. for the vast majority of metals) have all been distorted. Of course, the deformation is fully reversible on removing the applied load. This is the basic meaning of “elastic.”

Plastic deformation, on the other hand, involves no volume change. It effectively occurs by moving some unit cells with respect to others (without distorting them). This most commonly occurs via the motion of dislocations (line defects within the structure). Other important differences, compared with the case of elastic deformation, are that the strains involved are commonly much greater and that, in general, the deformation is not readily reversible. (The word “plastic” effectively means “permanent.”)

Details concerning plastic deformation – and there are many – are covered elsewhere in the book. However, it may be useful here to summarize the range likely to be encountered for the main parameter values (of metals). These are the Young’s modulus (E), defined as the applied stress over the resultant (elastic) strain, the yield stress (applied stress level at which plastic deformation starts) and the ultimate tensile stress, which is the peak (nominal) stress, given by the maximum load attained during the test divided by the original sectional area of the sample. Typical values for various metals are shown in Table 1.1, although it should be emphasized that the yielding and strength values supplied there are simply indicative and are subject to large variations for the same material.

It may be noted that the Young’s modulus (stiffness) depends only on the types of atoms present, which determines the interatomic forces. For example, all steels have a similar value of E, since they are all composed predominantly of Fe atoms. In contrast to this, the onset of plasticity (i.e. the yield stress) depends in a complex manner on the “microstructure” of the metal, a term that encompasses many features that can change with purity level, (relatively small) alloying additions, processing conditions, heat treatments etc. This arises mainly because these features are likely to have an effect on the ease of dislocation motion. Details of these effects are provided in Chapter 4. The yield stresses of different steels thus cover a wide range. This is also true of the ultimate tensile stress. (In fact, for much of the data in Table 1.1, a strong caveat should be appended to the yield stress and tensile strength values, to the effect that they could change significantly if the component concerned were to be heated or subjected to some other kind of treatment.) It also follows that the strain at the onset of yielding varies between different metals, although, as can be seen, the value is in all cases quite small (<~1%, i.e. 10 millistrain). In contrast, plastic strains are commonly large (typically several tens of %).

At this juncture, only a few broad points of this type should be noted concerning the data in Table 1.1. Reliable and meaningful information concerning the plasticity of metals is far from simple in terms of both experimental procedures and interpretation of the data. Of course, the values in the table do refer to material response at ambient temperature. It will tend to be different at both lower and (particularly) higher temperatures than this. It should also be noted that the behavior is assumed to be time-independent, so the rate at which the load is applied is taken to have no effect. In practice, this is often fairly reliable over the range of loading rates that is likely to be used, but both very high rates and very low rates can lead to a significantly different response. In particular, what is usually termed creep – i.e. progressive plastic deformation at a constant applied load – may become noticeable at low loading rates. However, such behavior should be characterized using laws that incorporate time, rather than making some sort of adjustment to the apparent plasticity parameters. All of these issues are covered in detail elsewhere in the book.

A final point can be made that concerns composite materials. Of course, most composites are based on polymers, which are outside the scope of the book, but metal matrix composites (MMC) are in industrial use and are certainly worthy of study. In most cases, they contain ceramic reinforcement, in the form of either particles or fibers. These raise the stiffness and also usually lead to a higher yield stress and UTS, although often at a cost in reduced ductility and toughness. Table 1.1 does contain data for an MMC, although such materials have not really reached the levels of development and stability typical of many alloys, so this material, and these values, are not very well defined. It is perhaps also worth recognizing that metals containing relatively high levels of porosity can be treated as a special type of composite. Metal components, particularly castings, do sometimes contain porosity, although in many cases the levels are too low (<~1%) to have much effect on elastic or plastic properties. On the other hand, metals containing high levels of porosity (>~30%), produced deliberately in some way, have found some applications: they would commonly be described as metallic foams. Their properties, both elastic and plastic, are naturally very different from the corresponding pore-free metal. Again, however, such materials are not well defined or mature, so it’s difficult to justify the inclusion of a specific example in Table 1.1. Nevertheless, it is worth noting that the presence of pores in a metal, at or above a level of a few %, can affect the mechanical response.