The second edition of Mechanical Behavior of Materials has revised and updated material in every chapter to reflect the changes occurring in the field. In view of the increasing importance of bioengineering, a special emphasis is given to the mechanical behavior of biologi-cal materials and biomaterials throughout this second edition. A new chapter on environmental effects has been added. Professors Fine and Voorhees make a cogent case for integrating biological materials into materials science and engineering curricula. This trend is already in progress at many US and European universities. Our second edition takes due recognition of this important trend. We have resisted the temptation to make a separate chapter on biological and biomaterials. Instead, we treat these materials together with traditional materials, viz., metals, ceramics, polymers, etc. In addition, taking due cognizance of the importance of electronic materials, we have emphasized the distinctive features of these materials from a mechanical behavior point of view.

The underlying theme in the second edition is the same as in the first edition. The text connects the fundamental mechanisms to the wide range of mechanical properties of different materials under a variety of environments. This book is unique in that it presents, in a unified manner, important principles involved in the mechanical behavior of different materials: metals, polymers, ceramics, composites, electronic materials, and biomaterials. The unifying thread running throughout is that the nano/microstructure of a material controls its mechanical behavior. A wealth of micrographs and line diagrams are provided to clarify the concepts.

Introduction

In this chapter, we discuss one important means of altering the mechanical response of metals and ceramics: martensitic transformation. Martensitic transformation is a highly effective means of increasing the strength of steel. An annealed medium-carbon steel (such as AISI 1040) has a strength of approximately 100 MPa. By quenching (and producing martensite), the strength may be made to reach about 1 GPa, a tenfold increase. The ductility of the steel is, alas, decreased.

A quite different effect is observed in ceramics. Martensitic transformation can be exploited to enhance the toughness of some ceramics. If a ceramic undergoes a martensitic transformation during the application of a mechanical load, the propagation of cracks is inhibited. For example, partially stabilized zirconia has a fracture toughness of approximately 7 MPa m1/2. An equivalent ceramic not undergoing martensitic transformation would have a toughness less than or equal to 3 MPa m1/2.

An additional, and very important, effect associated with martensitic transformations is the “shape-memory effect.” Alloys undergoing this effect “remember” their shape prior to deformation. The three effects just described have important technological applications.

Structures and Morphologies of Martensite

Quenching has been known for over 3,000 years and is, up to this day, the single most effective mechanism known for strengthening steel. However, it is only fairly recently that the underlying mechanism has been studied in a scientific manner and understood.

Introduction

There is some confusion in the literature about the terminology pertaining to fatigue. We define fatigue as a degradation of mechanical properties leading to failure of a material or a component under cyclic loading. This definition excludes the so-called phenomenon of static fatigue, which is sometimes used to describe stress corrosion cracking in glasses and ceramics in the presence of moisture. Brittle solids (glasses and crystalline ceramics) undergo subcritical crack growth in an aggressive environment under static loads. Silica-based glasses are especially susceptible to this kind of crack growth in the presence of moisture. If a glassy phase exists at grain boundaries and interfaces, it will be susceptible to such an attack. Thus, static fatigue is more appropriately a stress corrosion phenomenon, rather than a cyclic stress-related phenomenon.

In general, fatigue is a problem that affects any structural component or part that moves. Automobiles on roads, aircraft (principally the wings) in the air, ships on the high sea constantly battered by waves, nuclear reactors and turbines under cyclic temperature conditions (i.e., cyclic thermal stresses), and many other components in motion are examples in which the fatigue behavior of a material assumes a singular importance. It is estimated that 90% of service failures of metallic components that undergo movement of one form or another can be attributed to fatigue. Often, a fatigue fracture surface will show some easily identifiable macroscopic features, such as beach markings.

THE SOPHISTS, PROTAGORAS AND THE PROTAGORAS

These days, the term ‘sophist’ is used solely as a term of disdain, for those who hope to get away with shoddy reasoning. It was not always thus. Our term ‘sophist’ derives from a Greek term σοφιστής; and in the fifth century bc, when that term was first used, σοφισταί were men to be reckoned with.

The first σοφισταί were so called because of some expertise or σοφία. In principle, any expert might be given the name σοφιστής. We hear, for example, of those who were given the name because they were experts in poetry, statecraft or ritual (311e4n.). In practice, the main bearers of the name were men like three of the characters in the Protagoras: Protagoras of Abdera himself, Hippias of Elis and Prodicus of Ceos. Among the better documented of the others like them were Gorgias of Leontini, Thrasymachus of Chalcedon and Antiphon of Athens. These men did not all make claim to exactly the same expertise (312d9–e1n.): for example, Prodicus had a special flair for distinguishing between words of very similar meaning (337a1–c4); Hippias cultivated a special mnemonic technique that enabled him to repeat a list of fifty names after hearing it just once (Hp. Ma. 285e; cf. 318e3n.); and Protagoras won so special a reputation for his understanding of how institutions can be managed (318e4–319a6) that he was commissioned to devise the constitution for a new Panhellenic settlement at Thurii (DK 80 A 1.50).

SOCRATES MEETS SOME FRIENDS

The Introduction to this book contains general remarks that could not conveniently be digested into the piecemeal format of the commentary. In spite of its name ‘Introduction’, and its position before the text, there is no need to have read the Introduction before starting to read the rest of the book. If some preliminary orientation to the Protagoras is required, it will be found in the italicised paragraphs of summary that are scattered throughout the commentary.

I have incurred many intellectual debts in writing this book: to the Editors of this series; to the unfailingly efficient and helpful staff of that marvellous resource, the Thesaurus Linguae Graecae; to those who took part in the Mayweek 2004 seminars on the Protagoras; to Bernard Dod, an exact and scrupulous copy-editor; and to Adam Beresford, Lynne Broughton, Myles Burnyeat, Andrea Capra, Giovanni di Pasquale, David Konstan, Geoffrey Lloyd, Catherine Osborne, Philomen Probert, Christopher Rowe, Catherine Steel, Liba Taub, Christopher Taylor, James Warren, Roslyn Weiss, and Jo Willmott.

More important than any intellectual debt is my debt to my father, Ronald Denyer. He died while I was writing this book. I dedicate it to his memory.

Recall the major steps in inverted index construction: