ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation
Подождите немного. Документ загружается.
To overcome the presence of particles, dislocations will either cut through (i.e., shear) them or will bow around
them. If the particles are small and coherent, they are more likely to be shearable. Whether or not shearing
occurs depends on a number of factors including particle misfit, particle modulus, and particle structure. In
shearable particles, the presence of coherent particles with lattice parameters that are either larger or smaller
than the matrix leads to internal stress fields that interact with dislocations. Similarly, particles exhibiting
different shear moduli than the matrix will also alter or inhibit dislocation motion. Additional strengthening can
also be achieved if the particles have low stacking fault energies or exhibit ordered crystal structures. An
additional form of strengthening, chemical strengthening, is related to the energy required to create an
additional area of particle-matrix interface when the particle is sheared. If the particles are large or incoherent,
dislocations will bow around the particles until the bowing segments join. The dislocation can then proceed,
leaving a dislocation loop around the particles. This is know as Orowan looping. As particle size increases, the
bowing stress decreases. In precipitation-hardened systems, maximum strengthening (at a fixed volume) is
achieved when the shearing stresses and the bowing stresses are equal.
The above descriptions refer to materials with a relatively low volume-fraction of particles. However, in metal-
matrix composites, higher concentrations of particles have been used to strengthen materials. Usually in these
dispersion-strengthened systems, high strength, high-modulus particles or fibers are artificially added to a low
modulus substrate. The strengthening is believed to result in part from the modulus difference between the
matrix and the dispersed particles. The high-modulus particles or fibers carry most of the stress, while the
relatively ductile matrix accommodates most of the strain. There is also usually enhanced work hardening
within the matrix near the particles, which leads to strengthening.
Deformation and Strengthening of Polymers (Ref 16, 17)
Mechanical properties of polymers are influenced by a variety of the structural parameters introduced
previously. The strength of many polymers is directly related to the molecular weight and to the degree of
crystallinity. Properties related to tension are often expressed in the form:
(Eq 6)
where
n
is the number average molecular weight and a and b are experimental constants. The number
average molecular weight is defined as:
(Eq 7)
where x
i
is the number of molecules in each size fraction and M
i
is the molecular weight in each size fraction.
Increased crystallinity can lead to increased tensile strength, stiffness, and yield strength. When polymers
exhibit some crystallinity, stiffness and elastic modulus can change by more than an order of magnitude.
Copolymerization of multiple monomers also can modify mechanical properties by decreasing crystallinity and,
thus, lowering stiffness and yield point.
Many polymers exhibit viscoelastic behavior. When viscoelastic polymers are stressed, there is an immediate
elastic response as indicated in Fig. 21 (Ref 17), followed by viscous flow, which decreases with increasing
time until a steady state is achieved. If the material is then unloaded, the elastic strain is recovered followed by
time dependent (delayed) recovery. Some permanent strain, denoted as permanent recovery in Fig. 21, remains.
Fig. 21 Schematic representation of viscoelastic behavior of a polymer. Loading produces an immediate
elastic strain followed by viscous flow. Unloading produces an immediate elastic recovery followed by
additional recovery over a period of time. Source: Ref 17
In thermoplastic polymers, permanent deformation occurs via interchain sliding. Strengthening is accomplished
by impeding chain sliding. One way to accomplish this is by adding “bulky” side groups to each polymer chain.
This results in a geometric (steric) hindrance to chain sliding. For example, for a polymeric series based on the
monomer C
2
H
3
R, where R represents a specific atom or side group, strength increases proportionally with the
size of the side group. For example, polypropylene R = CH
3
) is stronger than polyethylene R = H).
In general, crystalline polymers are much stronger than noncrystalline ones. Higher density in crystalline
polymers can be correlated with the fact that the polymer chains are aligned closer together, which makes chain
displacement more difficult. One good example of this is polyethylene where strengthening can be achieved by
drawing, which effectively increases the amount of crystalline material. Most thermoplastic polymers are
difficult to crystallize, however, due to their bulky side groups and greater viscosity (Ref 18).
Drawing is a technique used to strengthen thermoplastics. During drawing, the long axes of the polymer chains
align along the drawing direction. In filament form, the strength of drawn polymers is related directly to the
strength of the covalent bonds in the polymer backbone rather than the resistance to interchain sliding. This
type of strengthening, which is truly realized only in filaments, forms the backbone of the textile industry.
Though different polymers can exhibit limited solubility between each other, polymers cannot really be solid-
solution strengthened as metals, ceramics, and intermetallics can be. Exceptions occur when rigid rod
molecules with very stiff backbones are added. These molecules reinforce the polymer when dispersed in it in
the same way that fibers reinforce composite materials.
Deformation and Strengthening of Composite Materials
Composites are classified either by the type of matrix or the type of reinforcement. Common matrix
classifications include polymer-matrix composite (PMC), metal-matrix composite (MMC), ceramic-matrix
composite (CMC) and intermetallic-matrix composite (IMC). Reinforcement classification schemes include
particulate-reinforced composite, fiber-reinforced composite, and laminate composite. Composites are designed
to take advantage of the best characteristics of each component.
Microstructurally, composites consist of a matrix and a reinforcing phase. In fiber-reinforced composites, the
matrix holds the fibers together, protects them from damage, and transmits load to the fibers. The reinforcing
phases, which can be fibers, filaments, laminates, and so on, usually involves high strength, high modulus
materials that are too fragile for use in a monolithic form. Ductile reinforcements can also be used to toughen
brittle matrix materials.
Polymer-matrix composites are the commercially important composites. The polymer matrix is typically a
thermoset, as opposed to a thermoplastic. Unlike thermoplastics, thermosets are relatively stable over a wide
range of temperatures and are resistant to plastic deformation. Many common PMCs contain glass fibers
embedded in epoxy resins. Stronger fiber, such as graphite fiber, is used as well.
Thermoset composites are made by adding a reinforcement to the thermoset during or prior to setting. The
thermoset bonds the reinforcement together. The polymer matrix also protects glass-fiber interfaces from
degradation. Glass is prone to react with water vapor in air, which induces corrosive surface flaws (crack
initiation sites).
Basic composite mechanics can be addressed by considering Fig. 22. Consider a two-phase material consisting
of N lamellae of α and β phases with thickness of l
α
and l
β
, respectively. The volume fractions of each phase
are:
(Eq 8)
The application of a force along the y-axis and the stresses experienced by each lamellae are equal (i.e., σ
α
= σ
β
= σ
c
= F/L
2
); however, the strains differ from lamellae to lamellae. The composite strain is, thus, the weighted
average of the individual strains or:
ε
c
= V
a
ε
a
+ V
β
ε
β
(Eq 9)
From Hooke's Law, the composite modulus thus becomes:
(Eq 10)
Fig. 22 Schematic representation of lamellar arrangement in a two-phase composite material. Source:
Ref 19
When a force is applied along the z-direction, each lamellae experiences equal strains (i.e., ε
α
= ε = ε
β
= ε
c
);
however, the stresses experienced differ. For example, the force carried by each α lamella can be approximated
by assuming that the sum of the forces borne by the individual phases equals the total external force:
F = (F
α
+ F
β
)
(Eq 11)
Because each lamellar carries a stress, the forces experienced by the respective α and β lamellae are:
F
α
= σ
α
Nl
α
L and F
β
= σ
β
Nl
β
L
(Eq 12)
Substitution of these relationships into Eq 11 allows the respective volume fraction rules for composite stress
and modulus to be calculated as:
σ
c
= V
a
σ
a
+ V
β
σ
β
and E
c
= V
a
E
a
+ V
β
E
β
(Eq 13)
This volume fraction rule is the one most commonly followed by composites since the phase distributions in
most composites are close to that for which the equal strain approximation is valid.
Using these relationships, it is useful to plot the influence of volume fraction upon the load carried by the
reinforcement in the elastic regime. Such a plot is shown in Fig. 23 assuming that the α-phase is stronger than
the β-phase. This plot effectively shows that the strengthening phase is much more effective in carrying load
when forces are applied parallel to the axis of the reinforcing phase. Reinforcements can be dispersed in a
variety of ways in composite materials.
Fig. 23 Ratio of the force carried by the strong phase to that carried by the weak phase for deformation
parallel and perpendicular to the lamellae. Source: Ref 19
Most composites are not as ideally arranged as those just described. Thus, variants of Eq 10 and 13 are used to
describe the properties of materials possessing different types of reinforcements. For particle-reinforced
materials, the composite stress and modulus can be expressed by:
σ
c
= V
m
σ
m
+ K
s
V
p
σ
p
and
E
c
= V
m
E
m
+ K
c
V
p
E
p
(Eq 14)
where E
p
and E
m
represent the particle and matrix moduli, V
p
and V
m
represent the particle and matrix volume
fractions, σ
m
and σ
p
represent the matrix and particle tensile strengths, and K
s
and K
c
represent constants.
One of the most common ways is to use reinforcements in the form of continuous fibers. A similar expression is
valid for continuous fiber-reinforced composites; the volume fraction rule is similarly expressed as:
σ
c
= V
f
σ
f
+ V
m
σ
m
= V
f
σ
f
+ (1 - V
f
)σ
m
(Eq 15)
where σ
c
, σ
f
, and σ
m
represent the tensile strengths of the composite, fibers, and matrix, respectively, and V
f
and
V
m
represent the respective fiber and matrix volume fractions in the composite. This equation applies when the
long axis of the fibers is aligned parallel to the stress axis. Table 2 lists the properties of a number of continuous
fibers. Nonmetallic fibers demonstrate high strengths and higher strength-to-density ratios, which make them
attractive in applications such as aerospace where low weight is a premium. The volume fraction rule applies
provided the long axis of the fibers remains aligned with the stress axis. Random distributions of fibers result in
lower strengthening. In addition, strengthening is lower for particulates or when the fibers are not parallel to the
stress axis. Expressions similar to the volume fraction rule derived here are used to estimate other composite
properties like elastic modulus. Not all properties can be calculated using such a simple rule.
Table 2 Properties of selected fibers
Tensile strength
(TS)
Materials class Material Density (ρ),
mg/m
3
GPa ksi
TS/ρ,
MNm/kg
Pearlitic steel (piano wire) 7.9 4.2 610
0.53
Be 1.8 1.3 190
0.72
Mo 10.3 2.1 310
0.2
Metals
W 19.3 3.9 570
0.20
Al
2
O
3
3.96 2.0 290
0.51
Graphite (Kevlar) 1.5 2.8 410
1.87
S glass 2.5 6.0 870
2.4
Ceramics and
glasses
SiC 2.7 2.8 210
1.04
Nylon 66 1.1 1.05 150
0.95
Polyamide-hydrazide 1.47 2.4 350
1.63
Copolyhydrazide 1.47 2.7 390
1.84
Polymers
Poly(P-phenylene)
terephthalamide
1.44 2.8 410 1.94
Source: Ref 19
The use of metals to reinforce ceramics (or intermetallics) usually increases toughness but decreases strength.
Increased toughness is the result of the blunting and/or arrest of cracks by the ductile reinforcement.
Reinforcement geometry is also important. Fibers and laminates tend to improve toughness more than equiaxed
particles. When cracks approach fibers or lamellae (plates), they are often deflected along the reinforcement-
matrix interface. This increase in crack tortuosity provides additional work to fracture and, thus, an increase in
fracture toughness. Additional information on structure-property relationships for composite materials can be
found in Ref 20 and 21.
Deformation and Strengthening of Glasses
The mechanical behavior of glasses is similar to that of crystalline materials, in that increases in strain rate and
decreases in temperature result in increased strength. The actual mechanisms that are responsible for
deformation, however, differ from those exhibited by crystalline materials.
At low temperatures and low stresses, all glasses deform in a linear elastic manner. Linear elastic strains result
from stretching of the chemical bonds making up the glass structure. Linear elastic deformation is followed by
viscoelastic deformation at intermediate temperature, which is followed by Newtonian viscous flow at elevated
temperatures (i.e., T
g
≤ T ≤ T
mp
). Viscoelastic strains, which are time dependent and recoverable, result from
conformal rearrangements of the basic structural units making up the glass structure (i.e., SiO
4
tetrahedra in
silica-base glasses, individual atoms in metallic glasses, and long chain molecules in organic glasses).
Newtonian viscous flow, which can be correlated with a linear dependence of the applied stress on the applied
strain rate, results from localized slip processes, which result in the permanent displacement of basic structural
units of the material with respect to each other. In glassy polymers, the units are long chain molecules whereas
for metallic or inorganic glasses the basic units are single atoms or SiO
4
tetrahedra, respectively. Elevated
temperature deformation of noncrystalline materials is distributed uniformly throughout the volume of the
material. These facts are often exploited to form glassy materials into useful geometry. As temperature is
decreased, the material becomes stronger, and an alteration in flow behavior occurs. In inorganic glasses,
permanent deformation does not occur at low temperatures. On the other hand, in metallic and organic glasses,
permanent deformation occurs in a heterogeneous fashion involving the cooperative displacement of atoms or
molecules. This process is known as shear banding.
Metallic glasses become softer after shear band formation because the stress required to propagate the band is
lower than the stress required to initiate it. Fracture usually occurs after the propagation of a single shear band.
Conversely, in organic glasses, shear band formation is not followed by instability and fracture. More shear
bands form until the density of bands is so high that they coalesce, resulting in a neck containing highly
oriented molecules. Once the neck forms, it propagates up and down the gage length, leading to an increase in
strength.
References cited in this section
6. H. Baker, Structure and Properties of Metals, Metals Handbook Desk Edition, 2nd ed., ASM
International, 1998, p 85–121
11. H.W. Hayden, W.G. Moffatt, and J. Wulff, Mechanical Behavior, Vol 3, The Structure and Properties
of Materials, John Wiley & sons, 1964, p 111
12. W.D. Kingery, H.K. Bowen, and D.R. Uhlmann, Introduction to Ceramics, 2nd ed., John Wiley & sons,
1976, p 172
13. J.D. Verhoeven, Fundamentals of Physical Metallurgy, John Wiley & Sons, 1975
14. D.J. Green, An Introduction to the Mechanical Properties of Ceramics, Cambridge University Press,
Cambridge, 1998
15. D. Hull and D.J. Bacon, Introduction to Dislocations, Pergamon Press, Oxford, 1984
16. D.T. Grubb, Polymers, Mechanical Properties of, Encyclopedia of Applied Physics, Vol 14, VCH
Publishers, Inc., 1996, p 531–548
17. R.M. Brick, A.W. Pense, and R.B. Gordon, Structure and Properties of Engineering Materials, 4th ed.,
McGraw-Hill, 1977
18. I.M. Ward, Mechanical Properties of Solid Polymers, John Wiley & Sons, 1971
19. T.H. Courtney, Mechanical Behavior of Materials, 2nd ed., McGraw-Hill, Inc., 2000
20. D. Hull, An Introduction to Composite Materials, Cambridge University Press, Cambridge, 1981
21. K.K. Chawla, Composite Materials: Science and Engineering, 2nd ed., Springer-Verlag, New York,
1999
Introduction to the Mechanical Behavior of Nonmetallic Materials
M.L. Weaver and M.E. Stevenson, The University of Alabama, Tuscaloosa
Fatigue and Fracture of Nonmetallic Materials (Ref 22, 23, 24)
Two important aspects of the mechanical behavior of any class of engineering material are the fatigue response
and fracture resistance. Most engineering structures are subject, in some form, to fatigue. Therefore, the ability
of a material to withstand the accumulation of damage from fatigue is very important in terms of the life span of
the component. Additionally, the fracture behavior of a material is important in terms of preventing premature
failure. Considering the importance of these properties, a brief overview of the response of ceramics and
polymers is discussed here and comparison is made to many typical metallic materials.
Fatigue Response of Ceramics and Polymers
When discussing fatigue in terms of brittle solids, it is not necessarily assumed that the term fatigue refers to
cyclic loading conditions. In most cases, fatigue in ceramics is limited to static loading conditions where the
loss of strength with time is the underlying phenomenon. Until recently, it was a common misconception that
cyclic fatigue damage was not observed in ceramics. More recently, the cyclic fatigue lives have been proven to
be shorter than the static fatigue lives, which indicates that there is some effect, albeit an unclear one, that
causes cyclic loading to vary from static loading.
In conventional metals, fatigue is commonly associated with some amount of plastic strain, which is not
prevalent in ceramics and glasses. Since there is no dominance by plastic strains, application of the popular
strain-life method is not possible for ceramic and glass materials at room temperature. For ceramics, the
assumptions of linear elastic fracture mechanics (LEFM) are satisfied, and analysis using this method is more
suitable.
In polymers, traditional fatigue analysis methods, such as those used in metals, can be applied. Polymers,
however, involve a significant complication in the body of hysteresis heating and subsequent softening. This
phenomenon significantly lowers the fatigue life and additionally induces the dominance of plastic flow as a
deformation mechanism, whereas under isothermal conditions fatigue crack propagation dominates. In light of
the additional heating and softening problem, polymers still exhibit a traditional fatigue limit, below which no
cyclic deformation or resultant heating occurs significantly. Additionally, for polymer crystals, fatigue
resistance increases as crystallinity increases (Ref 22, 23, 24).
Fracture Behavior of Ceramics and Polymers
The fracture behavior of materials is important in engineering design because of the high probability of flaws
being present. As such, it is important to understand how a material will tolerate the presence under a flaw
under operating conditions and how a material will resist the propagation of cracks from these flaws.
In ceramics, loading results in deformation in the elastic regime with little or no plastic deformation at fracture.
Conversely, plastics exhibit little or no elastic deformation and a great degree of plastic deformation at final
fracture. While the two materials do not share similar stress-strain curves, they do share low values for fracture
toughness. The fracture toughness of a material is a measure of its ability to resist fracture propagation. Defined
another way, it is the critical stress intensity at which final fracture occurs. In either case, both ceramics and
polymers share low values of this parameter. Table 3 compares the approximate values of several classes of
engineering materials in terms of their fracture toughness.
Table 3 Fracture toughness values for a variety of engineering materials
Material
Fracture toughness (K
Ic
), MPa
Glasses
0.5–1
Most polymers
1–3
Ceramics
3–7
Intermetallics
1–20
Cast irons
10–40
Aluminum alloys
20–50
Titanium alloys
30–90
Steels (all types) 30–200
While polymers and ceramics do exhibit fracture toughness values that are of the same order of magnitude, the
respective modes of fracture are not necessarily the same. In polymers, the fracture process is dominated in
many cases by crazing, or the nucleation of small cracks and their subsequent growth. In ceramics and other
ionic solids, cleavage on electrically neutral planes is often the dominant fracture mode. These two modes are
distinctly different, even though the resistance to fracture for both classes of materials is similar. This brings up
the point that, for most materials, no single parameter can quantify the behavior under a given set of conditions.
In the example of fracture, K
Ic
alone will not always provide sufficient data to understand the fracture process.
Only an intimate knowledge of the materials structure and the fracture toughness will provide a completely
accurate indicator. Besides the material aspects, the fracture process is also statistical in nature (weakest link
theory). While fracture statistics can be complicated to understand fully, a good introduction can be found in
Ref 24. A more detailed description of the fracture process of these classes of materials can be found by
referring to the Selected References.
References cited in this section
22. S. Suresh, Fatigue of Materials, 2nd ed., Cambridge University Press, Cambridge, 1998, p 383–430
23. R.W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, 4th ed., John Wiley &
Sons, 1996, p 664–680
24. J.R. Wachtman, Mechanical Properties of Ceramics, John Wiley & Sons, 1996, p 235–246
Introduction to the Mechanical Behavior of Nonmetallic Materials
M.L. Weaver and M.E. Stevenson, The University of Alabama, Tuscaloosa
References
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
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
6. H. Baker, Structure and Properties of Metals, Metals Handbook Desk Edition, 2nd ed., ASM
International, 1998, p 85–121
7. R.E. Reed-Hill and R. Abbaschian, Physical Metallurgy Principles, 3rd ed., PWS-Kent, Boston, 1992
8. R.H. Doremus, Glass Science, 2nd ed., John Wiley & Sons, 1994, p 109
9. W.G. Moffatt, G.W. Pearsall, and J. Wulff, Structure, Vol 1, The Structure and Properties of Materials,
John Wiley & Sons, 1964, p 104
10. W.G. Moffatt, G.W. Pearsall, and J. Wulff, Structure, Vol 1, The Structure and Properties of Materials,
John Wiley & Sons, 1964, p 106
11. H.W. Hayden, W.G. Moffatt, and J. Wulff, Mechanical Behavior, Vol 3, The Structure and Properties
of Materials, John Wiley & sons, 1964, p 111
12. W.D. Kingery, H.K. Bowen, and D.R. Uhlmann, Introduction to Ceramics, 2nd ed., John Wiley & sons,
1976, p 172
13. J.D. Verhoeven, Fundamentals of Physical Metallurgy, John Wiley & Sons, 1975
14. D.J. Green, An Introduction to the Mechanical Properties of Ceramics, Cambridge University Press,
Cambridge, 1998
15. D. Hull and D.J. Bacon, Introduction to Dislocations, Pergamon Press, Oxford, 1984
16. D.T. Grubb, Polymers, Mechanical Properties of, Encyclopedia of Applied Physics, Vol 14, VCH
Publishers, Inc., 1996, p 531–548
17. R.M. Brick, A.W. Pense, and R.B. Gordon, Structure and Properties of Engineering Materials, 4th ed.,
McGraw-Hill, 1977
18. I.M. Ward, Mechanical Properties of Solid Polymers, John Wiley & Sons, 1971
19. T.H. Courtney, Mechanical Behavior of Materials, 2nd ed., McGraw-Hill, Inc., 2000
20. D. Hull, An Introduction to Composite Materials, Cambridge University Press, Cambridge, 1981
21. K.K. Chawla, Composite Materials: Science and Engineering, 2nd ed., Springer-Verlag, New York,
1999
22. S. Suresh, Fatigue of Materials, 2nd ed., Cambridge University Press, Cambridge, 1998, p 383–430
23. R.W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, 4th ed., John Wiley &
Sons, 1996, p 664–680
24. J.R. Wachtman, Mechanical Properties of Ceramics, John Wiley & Sons, 1996, p 235–246
Introduction to the Mechanical Behavior of Nonmetallic Materials
M.L. Weaver and M.E. Stevenson, The University of Alabama, Tuscaloosa
Selected References
General
• M.F. Ashby and D.R.H. Jones, Engineering Materials 1: An Introduction to Their Properties and
Applications, Pergamon Press, 1980
• M.F. Ashby and D.R.H. Jones, Engineering Materials 2: An Introduction to Microstructures,
Processing and Design, Pergamon Press, 1986
• W.D. Callister, Jr., Materials Science and Engineering: An Introduction, 4th ed., 1997
• T.H. Courtney, Mechanical Behavior of Materials, 2nd ed., McGraw-Hill, 2000
• N.E. Dowling, Mechanical Behavior of Materials, 2nd ed., Prentice Hall, 1999
• W. Hayden, W.G. Moffatt, and J. Wulff, Mechanical Behavior, Vol III, The Structure and Properties of
Materials, John Wiley & Sons, 1965
• W.G. Moffatt, G.W. Pearsall, and J. Wulff, Structure, Vol I, The Structure and Properties of Materials,
John Wiley & Sons, 1965
• R.E. Smallman and R.J. Bishop, Metals and Materials Science, Processes, Applications, Butterworth
Heinemann, 1995
Ceramics and Glasses
• R.H. Doremus, Glass Science, 2nd ed., John Wiley & Sons, 1994
• D.J. Green, An Introduction to the Mechanical Properties of Ceramics, Cambridge University Press,
Cambridge, 1998
• V.A. Greenhut, Effect of Composition, Processing, and Structure on Properties of Ceramics and
Glasses, Materials Selection and Design, Vol 20, ASM Handbook, ASM International, 1997
• W.D. Kingery, H.K. Bowen, and D.R. Uhlmann, Introduction to Ceramics, 2nd ed., John Wiley & Sons,
1976
• W.E. Lee and W.M. Rainforth, Ceramic Microstructures, Chapman & Hall, New York, 1994
Polymers
• N.G. McCrum, C.P. Buckley, and C.B. Bucknall, Principles of Polymer Engineering, 2nd ed., Oxford
University Press, Oxford, 1997
• S.L. Rosen, Fundamental Principles of Polymeric Materials, John Wiley & Sons, 1982
• I.M. Ward, Mechanical Properties of Solid Polymers, John Wiley & Sons, 1971
Intermetallic Compounds
• R. Darolia, J.J Lewandowski, C.T. Liu, P.L. Martin, D.B. Miracle, and M.V. Nathal, Structural
Intermetallics, The Minerals, Metals and Materials Society, Warrendale, PA, 1993
• M.V. Nathal, R. Darolia, C.T. Liu, P.L. Martin, D.B. Miracle, R. Wagner, and M. Yamaguchi,
Structural Intermetallics 1997, The Minerals, Metals and Materials Society, Warrendale, PA, 1997
• G. Sauthoff, Intermetallics, VCH Publishers, New York, 1995
• J.H. Westbrook, Guest Editor, MRS Bulletin, Vol 21 (No. 5), 1996
Composite Materials
• K.K. Chawla, Composite Materials: Science and Engineering, 2nd ed., Springer-Verlag, New York,
1999
• D. Hull, An Introduction to Composite Materials, Cambridge University Press, Cambridge, 1981
Mechanical Testing of Polymers and Ceramics
Introduction
THE MECHANICAL BEHAVIOR of polymers and ceramics differs from that of metallic materials due to
some basic relationships between microstructure and mechanical properties, as described in the preceding
article “Introduction to the Mechanical Behavior of Nonmetallic Materials” in this Volume. This article briefly
reviews the general mechanical properties and test methods for polymers and ceramics. Additional coverage is
also provided in other Sections of this Volume on hardness testing, high-strain-rate testing, fatigue testing, and
fracture toughness. This article does not address the mechanical properties and testing of fiber-reinforced metal-
matrix or polymer-matrix composites, which are a distinct product from that involves more specialized method
for the testing and analysis of mechanical properties (see the article “Mechanical Testing of Fiber-Reinforced
Composites” in this Volume).
Mechanical Testing of Polymers and Ceramics
Mechanical Testing of Polymers
Polymers are high-molecular-weight materials that may exhibit the mechanical behavior of a fiber, plastic, or
elastomer (Fig. 1). The use of a polymer as an elastomer (rubber), a plastic, or a fiber depends on the relative
strength of its intermolecular bonds and structural geometry. Noncrystalline polymers with weak intermolecular
forces are usually elastomers or rubbers at temperatures above the glass transition temperature, T
g
. In contrast,
polymers with strong hydrogen bonds and the possibility of high crystallinity can be made into fibers. Polymers
with moderate intermolecular forces are plastic at temperatures below T
g
. Some polymers, such as nylon, can
function both as a fiber and as a plastic. Other polymers, such as isotactic polypropylene, lack hydrogen bonds,
but because of their good structural geometry, they can serve both as a plastic and as a fiber.
Fig. 1 Typical stress-strain curves for a fiber, a plastic, and an elastomer. Source: Ref 1
This section briefly reviews the mechanical properties and test methods commonly used for these three general
categories of polymers, although most of the emphasis is placed on the mechanical properties of structural
plastics. In terms of general structure, most polymers are amorphous (noncrystalline) materials with a hard
glassy structure below the glass transition temperature (T
g
), and either a viscous or rubbery structure above the
glass transition temperature (Fig. 2a). This is in contrast to low-molecular-weight materials (such as metals)
that typically have crystalline structures. However, the structure of some polymers is regular enough to promote
some crystallization, which may result in a flexible crystalline structure above the glass transition temperature
(Fig. 2b). For example, isotactic and syndiotactic polypropylenes can crystallize, but atactic polypropylene does