ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation

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31. P.F. Thomason, Ductile Fracture of Metals, Pergamon Press, Oxford, 1990

32. J.R. Rice and M.A. Johnson, Inelastic Behavior of Solids, M.F. Kanninen, Ed., McGraw-Hill, New

York, 1970, p 641

33. J. R. Rice, Fracture—An Advanced Treatise, H. Liebowitz, Ed., Academic Press, New York, 1968, p

191

Introduction to the Mechanical Behavior of Metals

Todd M. Osman, U.S. Steel Research; Joseph D. Rigney, General Electric Aircraft Engines

Selected References

Structure of Metals

• D.A. Porter and K.E. Easterling, Phase Transformations in Metals and Alloys, Van Nostrand Reinhold,

Birkshire, UK, 1987

• C.R. Barrett, W.D. Nix, and A.S. Tetelman, The Principles of Engineering Materials, Prentice-Hall,

Inc., Englewood Cliffs, New Jersey, 1973

• W. Hume-Rothery and G.V. Raynor, The Structure of Metals and Alloys, The Institute of Metals,

London, 1956

• D. Hull and D.J. Bacon, Introduction to Dislocations, Pergamon Press, London, 1984

• P.B. Hirsch, Ed., Defects, Vol 2, The Physics of Metals, Cambridge University Press, Cambridge, 1975

• U.F. Kocks, C.N. Tome, and H.-R. Wenk, Texture and Anisotropy, Cambridge University Press,

Cambridge, 1998

• D. Hull, An Introduction to Composite Materials, Cambridge University Press, 1975

Deformation of Metals and Strength of Metals

• M.A. Meyers and K.K. Chawla, Mechanical Metallurgy: Principles and Applications, Prentice Hall,

Inc., 1984

• J.M. Gere and S.P. Timoshenko, Mechanics of Materials, 2nd ed., PWS Publishers, 1984

• T.H. Courtney, Mechanical Behavior of Materials, McGraw-Hill, New York, 1990

• J.W. Martin, Micromechanisms in Particle Hardened Alloys, Cambridge University Press, 1980

• P.F. Thomason, Ductile Fracture of Metals, Pergamon Press, Oxford, 1990

• R.W.K. Honeycombe, The Plastic Deformation of Metals, 2nd ed., Edward Arnold, London, 1984

Special Conditions in Flow and Fracture

• J.F. Knott, Fundamentals of Fracture Mechanics, Butterworths, 1981

• B.R. Lawn and T.R. Wilshaw, Fracture of Brittle Solids, Cambridge University Press, 1975

• H.L. Ewalds and R.J.H. Wanhill, Fracture Mechanics, Edward Arnold, London, 1985

• D. Broek, Elementary Engineering Fracture Mechanics, Martinus Nishoff Publishers, 4th ed.,

Dordrecht, Netherlands, 1987

Introduction to the Mechanical Behavior of

Nonmetallic Materials

M.L. Weaver and M.E. Stevenson, The University of Alabama, Tuscaloosa

Introduction

MANY DIFFERENT types of materials are used in applications where a resistance to mechanical loading is

necessary. The type of material used depends strongly upon a number of factors including the type of loading

that the material will experience and the environment in which the materials will be loaded. Collectively known

as engineering materials (Ref 1), they can be pure elements, or they can be combinations of different elements

(alloys and compounds), molecules (polymers), or phases and materials (composites). All solid materials are

typified by the presence of definite bonds between component atoms or molecules. Ultimately, it is the type of

bonding present that imparts each class of materials with distinct microstructural features and with unique

mechanical and physical properties.

Crystalline solids exhibit atomic or molecular structures that repeat over large atomic distances (i.e., they

exhibit long-range-ordered, LRO, structures) whereas noncrystalline solids exhibit no long-range periodicity.

The atomic and molecular components of both crystalline and noncrystalline solids are held together by a series

of strong primary (i.e., ionic, covalent, and metallic) and/or weak secondary (i.e., hydrogen and Van der Waals)

bonds. Primary bonds are usually more than an order of magnitude stronger than secondary bonds. As a result,

ceramics and glasses, which have strong ionic-covalent chemical bonds, are very strong and stiff (i.e., they

have large elastic moduli). They are also resistant to high temperatures and corrosion, but are brittle and prone

to failure at ambient temperatures. In contrast, thermoplastic polymers such as polyethylene, which have weak

secondary bonds between long chain molecules, exhibit low strength, low stiffness, and a susceptibility to creep

at ambient temperatures. These polymers, however, tend to be extremely ductile at ambient temperatures.

In this article, some of the fundamental relationships between microstructure and mechanical properties are

reviewed for the major classes of nonmetallic engineering materials. The individual topics include chemical

bonding, crystal structures, and their relative influences on mechanical properties. The present article has been

derived in structure and content from the article “Fundamental Structure-Property Relationships in Engineering

Materials,” in Materials Selection and Design, Volume 20 of ASM Handbook (Ref 2). In light of the

bewildering number of different engineering materials within each class, discussions were limited to a number

of general examples typifying the general features of the major classes of nonmetallic materials.

References cited in this section

1. N.E. Dowling, Mechanical Behavior of Materials: Engineering Methods for Deformation, Fracture,

and Fatigue, 2nd ed., Prentice Hall, 1999, p 23

2. T.H. Courtney, Materials Selection and Design, Vol 20, ASM Handbook, ASM International, 1997, p

336–356

Introduction to the Mechanical Behavior of Nonmetallic Materials

M.L. Weaver and M.E. Stevenson, The University of Alabama, Tuscaloosa

General Characteristics of Solid Materials

Engineering materials can be conveniently grouped into five broad classes: metals, ceramics and glasses,

intermetallic compounds, polymers, and composite materials. Metals, ceramics and glasses, polymers, and

composites represent the most widely utilized classes of engineering materials, whereas intermetallic

compounds (i.e., intermetallics), which are actually subcategories of metals and ceramics, are an emerging class

of monolithic materials. The general features of five major classes of materials are summarized in Fig. 1 and are

described in the following sections. Though this article deals with the properties of nonmetallic materials, a

brief discussion of the general characteristics of metallic materials is included where pertinent.

Fig. 1 General characteristics of major classes of engineering materials. Adapted from Ref 3

Metals

Metals represent the majority of the pure elements and form the basis for the majority of the structural

materials. The mechanical behavior of metals depends on a combination of microstructural and macrostructural

features, which ultimately depend upon bonding, chemical composition, and mode of manufacture. Metals are

held together by metallic bonds. Metallic bonds arise because on an atomic scale, the outer electron shells in

metals are less than half full. As a result, each atom donates its available outer shell (i.e., valence) electrons to

an electron cloud that is collectively shared by all of the atoms in the solid. This is referred to as metallic

bonding and is responsible for the high elastic moduli and the high thermal and electrical conductivity exhibited

by metals. Many metals also exhibit a limited solid solubility for other atoms (i.e., one metal can dissolve into

another). Consequently, engineers can often vary their properties by varying composition. In terms of atomic

arrangements, metals also have large coordination numbers (CNs), typically 8 to 12, which account for their

relatively high densities. Metals, by their nature, tend to be ductile in comparison to other engineering materials

and exhibit a high tolerance for stress concentrations. As such, many metals can deform locally to redistribute

load. Structurally, metals are generally crystalline, though amorphous structures (i.e., metallic glasses) are

possible using special processing techniques. Further information concerning structure-property relationships in

metals is provided in the article “Introduction to the Mechanical Behavior of Metals” in this volume.

Ceramics and Glasses

Ceramics and glasses include a broad range of inorganic materials containing nonmetallic and metallic

elements. Like metals, these materials can be formed directly from the melt or via powder processing

techniques (e.g., sintering or hot isostatic pressing) and their mechanical properties depend on structural (i.e.,

microstructural and macrostructural) features and chemical composition. They differ from metals in that strong

ionic, covalent, or intermediate bonds, which often result in higher hardness, stiffness, and melting

temperatures compared with metals, hold them together.

Ionic bonding occurs in compounds containing electropositive (i.e., metals, atoms on the left side of the

periodic table) and electronegative (i.e., nonmetals, atoms on the right side of the periodic table) elements. This

type of bonding involves the transfer of electrons whereby electropositive elements readily donate their valence

electrons to the electronegative elements, allowing the establishment of stable outer shell configurations in each

element. Figure 2 depicts ordinary table salt, NaCl, which is a perfect example of an ionically bonded solid. In

ionic solids, the coordination number (CN), which is defined as the number of cation/anion (i.e., positive

ion/negative ion) nearest neighbors, are typically lower than those in metals, which accounts for their slightly

lower densities compared with metals. Ionic solids are typically hard and brittle, and electrically and thermally

insulative (i.e., with lower electrical and thermal conductivity than metals). The insulative properties are a

direct result of the electron configurations within the ionic bond. Ionic solids usually form only in

stoichiometric proportions (e.g., NaCl and Al

), which cause them to have little tolerance for alloying.

Fig. 2 Schematic representation of ionically bonded NaCl. Note that this structure consists of Na

and Cl

ions sitting on interpenetrating fcc Bravais lattices.

Covalent bonding occurs in compounds containing electronegative elements. Covalent bonding involves the

sharing of valence electrons with specific neighboring atoms. This is schematically illustrated for methane

(CH

) in Fig. 3. For a covalent bond to occur between C and H, for example, each atom must contribute at least

one electron to the bond. These electrons are shared by both atoms, resulting in a strong directional bond

between atoms. The number of covalent bonds that form depends on the number of valence atoms that are

available in each atom. In methane, carbon has four valence electrons, while each hydrogen atom has only one.

Thus, each hydrogen atom can acquire one valence electron to fill its outer orbital shell. Similarly, each carbon

atom can accommodate four valence electrons to fill its outer shell. This type of bonding makes covalent solids

strong, brittle, and highly insulative because electrons are incapable of detaching themselves from their parent

and moving freely through the solid. Covalent solids also have lower CNs due to this localized electron sharing

resulting in lower densities. For example, diamond, which is an elemental covalent compound, has a CN of 4

(Fig. 4). Like ionic solids, covalent solids tend to exhibit very narrow composition ranges and exhibit little

tolerance for alloying additions. Examples of covalent molecules, elements, and compounds include H

HNO

, H

, diamond, silicon, GaAs, and SiC.

Fig. 3 Schematic representation of covalent bonding in methane (CH

)

Fig. 4 An example of perfect covalent bonding in diamond

In general, most ceramic compounds exhibit a mixture of ionic and covalent bonding, the degree of which

depends on the positions of the constituent elements on the periodic table. For elements exhibiting a greater

difference in electronegativity, bonding tends to be more ionic, while for elements with smaller differences,

bonding tends to be more covalent. Solids that exhibit mixed bonding, which are termed polar covalent solids,

often exhibit high melting points and elastic moduli. Silica (SiO

) is a good example of a polar covalent solid.

Ceramics and glasses have higher elastic moduli than most metals and exhibit extremely high strengths when

deformed in compression. The presence of strong ionic and covalent bonds allows these materials to retain their

strength to high temperature and makes them extremely resistant to corrosion. However, these same bonds

render ceramics and glasses brittle at ambient temperatures, resulting in little tolerance for stress concentrations

(e.g., holes, cracks, and flaws) and usually in catastrophic failure during tensile or shear loading.

Intermetallic Compounds

In some cases, intermetallic compounds can form within alloys. These materials are composed of two or more

metallic or metalloid constituents and exhibit crystal structures that are distinctly different from its constituents.

Unlike solid solution alloys, these mixtures form stoichiometric compounds (e.g., NiAl, Ni

Al, TiAl, and

Al), and their bonding is typically a combination of metallic, ionic, and/or covalent types. In terms of

mechanical and physical properties, intermetallics occupy a position between metals and ceramics. As in the

case of ionic and covalent solids, extremely strong bonds exist between unlike constituents, which imparts

intermetallics with lower CNs and densities than metals, highly directional properties, higher stiffness and

strength, and good resistance to temperatures or chemical attack. These materials, which have intrinsically high

strengths and elastic moduli, are often used in precipitate form to strengthen commercial alloys (e.g., Ni

Al in

Ni-base superalloys), and their low densities and high microstructural stability makes them attractive for use in

high-temperature structural applications such as turbine blades, exhaust nozzles, and automotive valves.

Examples of some typical intermetallic compounds are illustrated in Fig. 5. Of the more than 25,000 known

intermetallic compounds, recent emphasis has focused on the development of NiAl, FeAl, Ni

Al, TiAl, and

MoSi

base alloys for use as monolithic alloys in structural applications (Ref 4). As in the cases of ceramics and

glasses, the same bonds that impart intermetallics with high strengths render most of them with low ductility

and fracture toughness at ambient temperatures.

Fig. 5 Some simple intermetallic crystal structures

Polymers

Polymers are long chain molecules (macromolecules) consisting of a series of small repeating molecular units

(monomers). Most common polymers have carbon (organic material) backbones, though polymers with

inorganic backbones (e.g., silicates and silicones) are possible. Polymeric materials exhibit strong covalent

bonds within each chain; however, individual chains are frequently linked via secondary bonds (i.e., van der

Waals, hydrogen, and so on) though cross-linking via primary bonds is possible. The polymer polyethylene, for

example, forms when the double bond between carbon atoms in the ethylene molecule (C

) is replaced by a

single bond to each of the adjacent carbon atoms, resulting in a long chain molecule (Fig. 6). In polymeric

materials, secondary bonds arise from atomic or molecular dipoles that form when positively charged and

negatively charged regions of an atom or molecule separate. Bonding results from coulombic attraction

between the positive and negative regions of adjacent dipoles as illustrated schematically in Fig. 7. These types

of interactions occur between induced dipoles, between induced dipoles and polar molecules, and between polar

molecules. The secondary bonds holding adjacent macromolecules together in Fig. 8 are a direct result of the

formation of molecular dipoles along the length of the polymer chain. Secondary bonds are much weaker than

primary bonds as indicated in Table 1, which accounts for the low melting temperatures, low stiffness, and low

strength exhibited by many polymers. Table 1 Bond energies for various materials

Bond energy

Bond type Material

kJ/mol

kcal/mol

NaCl 640

153

Ionic

MgO 1000

239

Si 450

108

Covalent

C (diamond)

713

170

Hg 68

Al 324

Fe 406

Metallic

W 849

203

Ar 7.8

1.8

van der Waals

7.4

8.4

Hydrogen

O 51 12.2

Source: Ref 5

Fig. 6 Schematic representation of ethylene and polyethylene

Fig. 7 Schematic representation of secondary bonding between two molecular dipoles

Fig. 8 Schematic representation of secondary bonding between two polymer chains

In comparison to metals, intermetallics, and ceramics and glasses, polymers have very low CNs, which is part

of the cause for their low densities; however, they also consist primarily of light atoms such as C and H, which

tends to result in lower density. The localized nature of electrons in polymers renders them good electrical

insulators and poor thermal conductors.

There are three categories of polymers: thermoplastics, thermosetting plastics, and elastomers. Thermoplastics

have linear chain configurations where chains are joined by weak secondary bonds as described above. These

materials often melt upon heating but return to their original solid condition when cooled. In thermosetting

polymers, covalent cross-links or strong hydrogen bonds occur between polymer chains resulting in three-

dimensional networks of cross-linked molecular chains. Phenol formaldehyde, or Bakelite, is a good example.

Thermosettings change chemically during processing and will not melt upon reheating. Instead, they will

remain strong until they break down chemically via charring or burning. Elastomers differ from thermoplastics

and thermosetting polymers in that they are capable of rubbery behavior and are capable of very large amounts

of recoverable deformation (often in excess of 200%). Structurally, these materials consist of networks of

heavily coiled and heavily cross-linked polymer chains, which impacts higher strengths than thermoplastics by

inhibiting the sliding of polymer chains past each other. Elastomers are typified by natural rubber and by a

series of synthetic polymers exhibiting similar mechanical behavior (e.g., polyisoprene).

Composites

Composites are relatively macroscopic arrangements of phases or materials designed to take advantage of the

most desirable aspects of each. As a result, the strengths and/or physical properties of composites are usually an

average of the strengths and/or properties of the individual constituents/phases. Most composites are composed

of a compliant, damage-tolerant matrix and a strong reinforcing phase/constituent, usually filaments, fibers, or

whiskers, that are too brittle for use in a monolithic form. In composites with brittle matrices (e.g., ceramic

matrix composites), the reinforcing constituent may toughen the material more than strengthen it.

References cited in this section

3. N.E. Dowling, Mechanical Behavior of Materials: Engineering Methods for Deformation, Fracture,

and Fatigue, 2nd ed., Prentice Hall, 1999, p 24

4. G. Sauthoff, Intermetallics, VCH Publishers, New York, 1995

5. W.D. Callister, Materials Science and Engineering—An Introduction, 4th ed., John Wiley & Sons,

1997, p 21

Introduction to the Mechanical Behavior of Nonmetallic Materials

M.L. Weaver and M.E. Stevenson, The University of Alabama, Tuscaloosa

Structures of Materials

Depending upon the application, engineering materials can be pure elements such as silicon, compounds such

as aluminum oxide (Al

) or gamma titanium aluminide (γ-TiAl), or combinations of different molecules

(polymers) or materials (composites). All materials are composed of a three-dimensional arrangement of atoms,

the general details of which are described subsequently.

Inorganic Crystalline Solids

The basic building blocks of crystalline solids are unit cells, which represent the smallest repeating unit within a

crystal. When stacked together, these repeating unit cells form a space lattice, which is a repeating three-

dimensional array of atoms. Due to geometrical considerations, atoms can only have one of 14 possible

arrangements, known as Bravais lattices (Fig. 9). Most metals and metallic alloys crystallize with face-centered

cubic (fcc), hexagonal close-packed (hcp), or body-centered cubic (bcc) crystal structures. However, the

structures in nonmetallic solids tend to be more complicated.

Fig. 9 The 14 Bravais lattices illustrated by a unit cell of each: 1, triclinic, primitive; 2, monoclinic,

primitive; 3, monoclinic, base centered; 4, orthorhombic, primitive; 5, orthorhombic, base centered; 6,

orthorhombic, body centered; 7, orthorhombic, face centered; 8, tetragonal, primitive; 9, tetragonal,

body centered; 10, hexagonal, primitive; 11, rhombohedral, primitive; 12, cubic, primitive; 13, cubic,

body centered; 14, cubic, face centered. Source: Ref 6

Crystalline ceramics and intermetallics have crystal structures consisting of multiple interpenetrating Bravais

lattices, each of which is occupied by a specific atomic constituent. For example, common table salt (NaCl)

consists of two fcc (cubic F) Bravais lattices, slightly offset and overlaid on top of each other. One Bravais

lattice contains Cl

ions and is centered at origin, 0 0 0, while the second Bravais lattice contains Na

ions and is

centered at 0 ½0 (Fig. 10). The structure of the intermetallic compound β-NiAl, which is often called an ordered

bcc structure, actually consists of two simple cubic (primitive) Bravais lattices, one containing Ni atoms

centered at 0 0 0 and the second containing Al atoms centered at .

Fig. 10 Schematic illustration of the construction of NaCl from two interpenetrating fcc Bravais lattices

Inorganic Noncrystalline Solids

Not all solids are crystalline. Unlike their crystalline counterparts, noncrystalline materials do not display long-

range order. Instead, these solids exhibit some local order (i.e., short-range order) where atomic or molecular

subunits repeat over short distances. This group of materials, often collectively referred to as glasses, includes

many high molecular weight polymers, some polar-covalent ceramics, and some metallic alloys. Amorphous

structures arise because the mobility of atoms within these materials is restricted such that low energy

configurations (crystalline) cannot be reached.

To fully describe the nature of glassy materials, it is useful to consider the structure and properties of

commercial inorganic glasses. When cooling from the liquid state, materials may solidify in two different ways.

If the cooling rate is sufficiently slow, the liquid may freeze in the form of a crystalline solid. If the cooling rate

is extremely high, the liquid may pass through the freezing range without crystallizing so that it becomes a

supercooled liquid, which transforms to glass at lower temperatures. The critical cooling rate required for glass

formation in common inorganic glass is very low (≤10

-1

K/s), which means that it is very easy to form inorganic

glasses with these compositions. In metallic alloys, glass formation is more difficult and requires cooling rates

in excess of 10

K/s (Ref 7).

The supercooled liquid transforms to glass at the glass transition point, T

in Fig. 11(a). At this point, the

temperature dependence of the specific volume of the liquid changes. In glassy materials, there is little or no

change in volume upon cooling below the melting point, T

. In contrast, crystallization is accompanied by a

sharp decrease in volume below T

. At temperatures below T

, the slopes of both the glass and crystallized

solid curves are the same; however, the volume of glass is greater than the crystalline solid at all temperatures

where both forms can exist. The volume difference and the glass transition temperature depend on the cooling

rate, as illustrated in Fig. 11(b). The volume difference is related to the more open structure in the glass. The

difference is usually very small, in the range of a few percent for silicate glasses. In metallic glasses, it is

usually less than one percent (Ref 7).