ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation
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Overview of Mechanical Properties and Testing for Design
Howard A. Kuhn, Concurrent Technologies Corporation
References
1. Cambridge Engineering Selector, Granta Design Ltd., Cambridge, UK, 1998
2. G.E. Dieter, Engineering Design: A Materials and Processing Approach, McGraw Hill, 1991, p 1–51, p
231–271
3. Metals Handbook, American Society for Metals, 1948
4. F.B. Seely, Resistance of Materials, John Wiley & Sons, 1947
5. Properties and Selection of Metals, Vol 1, Metals Handbook, 8th ed., American Society for Metals,
1961, p 503
6. Modern Plastics Encyclopedia, McGraw Hill, 2000
7. M.F. Ashby, Materials Selection for Mechanical Design, 2nd ed., Butterworth-Heinemann, 1999
8. H. Davis, G. Troxell, and G. Hauck, The Testing of Engineering Materials, 4th ed., McGraw Hill, 1982,
p 314
9. G. Carter, Principles of Physical and Chemical Metallurgy, American Society for Metals, 1979, p 87
10. M.A. Meyers and K.K. Chawla, Mechanical Metallurgy, Prentice-Hall, Edgewood Cliffs, NJ, 1984, p
626–627
11. W.J. Taylor, The Hardness Test as a Means of Estimating the Tensile Strength of Metals, J.R. Aeronaut.
Soc., Vol 46 (No. 380), 1942, p 198–202
12. George Vander Voort, Metallography: Principles and Practices, ASM International, 1999, p 383–385
and 391–393
13. R.T. Shield, On the Plastic Flow of Metals under Conditions of Axial Symmetry, Proc. R. Soc., Vol
A233, 1955, p 267
14. H. Chandler, Ed., Hardness Testing, 2nd ed., ASM International, 1999
15. G.E. Dieter, Mechanical Metallurgy, 2nd ed., McGraw Hill, 1976, p 49–50, 79–80, 379, 381, 385
16. J.H. Faupel and F.E. Fisher, Engineering Design, John Wiley & Sons, 1981, p 102, 113, 230–235, 802
17. D.J. Wulpi, Understanding How Components Fail, ASM International, 1966, p 27
18. T. Baumeistes, Ed., Marks' Mechanical Engineers' Handbook, 6th ed., McGraw-Hill, 1958, p 5–106
19. R.W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, 2nd ed., John Wiley
& Sons, 1983, p 240, 287, 288, 436–477
20. W.C. Young, Formulas for Stress and Strain, 5th ed., McGraw-Hill, 1975
21. S.P. Timoshenko and J. Goodier, Theory of Elasticity, 3rd ed., McGraw Hill, 1970, p 418–419
22. O.C. Zienkiewicz, The Finite Element Method in Engineering Science, 4th ed., McGraw Hill, 1987
23. K.H. Heubner, et al., The Finite Element Method for Engineers, 3rd ed., John Wiley & Sons, 1995
24. ABAQUS/Standard, Example Problems Manual, Vol 1, Version 5.7, 1997
25. A.A. Griffith, Trans. ASM, Vol 61, 1968, p 871
26. A.F. Liu, Structural Life Assessment Methods, ASM International, 1998
27. J.M. Barsom, Engineering Fracture Mechanics, Vol 7, 1975, p 605
28. M.E. Fine and Y.-W. Chung, Fatigue Failure in Metals, Fatigue and Fracture, Vol 19, ASM Handbook,
ASM International, 1996, p 63
29. M.R. Mitchell, Fundamentals of Modern Fatigue Analysis for Design, Fatigue and Fracture, Vol 19,
ASM Handbook, ASM International, 1996, p 227–249
30. N.E. Dowling, Estimating Fatigue Life, Fatigue and Fracture, Vol 19, ASM Handbook, ASM
International, 1996, p 250–262
31. D.L. McDowell, Multiaxial Fatigue Strength, Fatigue and Fracture, Vol 19, ASM Handbook, ASM
International, 1996, p 263–273
32. D. Woodford, Design for High-Temperature Applications, Materials Selection and Design, Vol 20,
ASM Handbook, ASM International, 1998, p 580
33. L. Samuels, Metals Engineering: A Technical Guide, ASM International, 1988, p 161, 168
34. G. Koch, Stress-Corrosion Cracking and Hydrogen Embrittlement, Fatigue and Fracture, Vol 19, ASM
Handbook, 1996, p 486
35. P. Timmins, Failure Control in Process Operations, Fatigue and Fracture, Vol 19, ASM Handbook,
1996, p 479
36. P. Timmins, Solutions to Hydrogen Attack in Steels, ASM International, 1997
Overview of Mechanical Properties and Testing for Design
Howard A. Kuhn, Concurrent Technologies Corporation
Selected References
• M. Ashby, Materials Selection for Mechanical Design, 2nd ed., Butterworth-Heineman, 1999
• N. Dowling, Mechanical Behavior of Materials: Engineering Methods for Deformation, Fracture, and
Fatigue, Prentice Hall, 1999
• R.C. Juvinall and K.M. Marshek, Fundamentals of Machine Component Design, 2nd ed., John Wiley &
Sons, 1991
• J.E. Shigley and L.D. Mitchell, Mechanical Engineering Design, 4th ed., McGraw-Hill, 1983
Mechanical Testing for Metalworking Processes
Serope Kalpakjian, Illinois Institute of Technology
Introduction
AN IMPORTANT ACTIVITY in metalworking facilities is the testing of incoming raw materials for
characteristics that ensure the integrity and quality of the products made. Several traditional as well as
specialized tests are now available to assess the quality of materials, in bulk or sheet form, in order to predict
their behavior in metalworking operations.
Because of the generally complex nature of the processes involved, the identification and quantification of
appropriate parameters to predict performance and failure during processing continue to be challenging tasks.
The metalworking industry has, by and large, depended on cumulative and long practical experience rather than
on the continuous reporting of research findings in the technical literature. A notable exception is the
automotive industry, particularly in sheet metal forming.
While such practical experience has been indispensable to the successful production of quality products, major
efforts and investigations continue to be made to arrive at a comprehensive understanding of the underlying
principles of the behavior of metals in deformation processing. The necessity for such an approach is self-
evident, even though it is clear that there are, as yet, no simple criteria fully responsive to all metals and alloys,
operations, and processing conditions.
This article generally reviews the state of knowledge in this subject. A more detailed discussion of various
aspects related to specific topics and processes are given in ASM Handbook, Volume 14, Forming and Forging,
and the other references cited in this article.
Mechanical Testing for Metalworking Processes
Serope Kalpakjian, Illinois Institute of Technology
Workability and Formability
A simple definition of workability is “the maximum amount of deformation a metal can withstand in a
particular process without failure” (Ref 1). The term is generally applied to bulk deformation processes, such as
forging, rolling, extrusion, and drawing, in which the forces applied are predominantly compressive in nature.
In contrast, formability is usually applied to sheet-metal forming processes, in which the forces applied are
primarily tensile. These definitions can also include undesirable conditions such as poor surface finish, sheet
wrinkling, or lack of die fill in forging.
Although some definitions include the relative ease with which a metal can be shaped, the general definition of
workability does not include forces or energies involved in processing. The reason is that forces and energies
are related primarily to the strength of the workpiece material, tribological factors (friction, lubrication, and
wear), and the size and capacity of the metalworking equipment.
The maximum amount of deformation has a different meaning depending on the particular metalworking
process. For example, in bending, it is the minimum bend radius; in deep drawing, it is the maximum ratio of
blank-to-punch diameters. In power spinning of tubular or curvilinear shapes, maximum deformation is the
reduction in thickness per pass.
It is generally recognized that there are two basic types of failure in metalworking processes:
• Local or total separation of the metal: Surface cracking in upsetting or open-die forging, internal
cracking in extrusion or drawing, and necking and subsequent tearing of sheet metals during forming
• Buckling: Upsetting of slender workpieces and wrinkling in sheet-metal forming operations
As a general guide to workability, some suppliers of metals have prepared tables or charts showing the relative
workability or formability ratings (using letters, numbers, or terms such as excellent, good, fair, and poor).
While such ratings are based on cumulative and proven experience on the plant floor and can indeed be useful,
their application is somewhat limited due to the fact that ratings generally do not apply to specific processes and
conditions and are not quantitative.
The behavior of a metal in an actual forming operation may be predicted from mechanical test results. Test
specimens are cut from the same blank and, as much as possible, subjected to the same conditions (such as state
of stress, temperature, and strain rate) as in the particular metalworking operation. Few metalworking processes
can be simulated by such simple testing, however. Consequently, much effort has been expended toward the
design of new test methods to simulate actual processing conditions. The alternative is, of course, to perform
the actual process itself at a smaller scale, in a laboratory environment.
Reference cited in this section
1. G.E. Dieter, Ed., Workability Testing Techniques, ASM International, 1984, p 16, 33, 49, 61, 63, 163,
202, 206
Mechanical Testing for Metalworking Processes
Serope Kalpakjian, Illinois Institute of Technology
Mechanical Behavior of Metals Influencing Workability
This section presents a brief review of the common material parameters that can have a direct or indirect
influence on workability and product quality. The parameters described in this section are most commonly
obtained in tension tests (Ref 2, 3, 4). All discussions pertain to metals and their alloys.
Strength. The strength of a metal is defined in terms of quantities such as yield stress, ultimate tensile strength,
and breaking stress. Although easily determined from normal stress-normal strain curves (including shear
stress-shear strain curves) or obtained from published literature and handbooks, these quantities basically
influence the stresses, forces, energies, and temperature rise during processing.
Although strength is not directly relevant to workability, it can indirectly indicate some measure of workability.
For ductile metals, for example, observing the difference between the yield and tensile strengths can indirectly
indicate a measure of workability. The closer the magnitude of these two stresses, the more work hardened the
metal is, and, hence, the lower its ductility, that is, the narrower the stress-strain curve. The dependence of
strength on orientation in a bulk workpiece or sheet metal can, of course, also influence material behavior (as
discussed in subsequent sections). A common example is the formation of ears in deep-drawn cups due to the
planar anisotropy of the sheet.
Ductility. Two traditional and common measures of ductility have been the tensile elongation and the tensile
reduction of area, quantities that are readily available in handbooks and from material suppliers. However, these
quantities depend on gage length and cross-sectional area of the specimen. Total elongation and tensile
reduction of area both increase with increasing cross-sectional area of the specimen. Also, because necking is a
local phenomenon, total percent elongation depends on gage length; as expected, percent elongation decreases
with increasing gage length.
Relationships between specimen length and cross section have been established and standardized in different
countries. In the United States and according to ASTM standards, for example, the gage length-to-diameter
ratio is 4.0 for round and 4.5 for sheet specimens.
Hardness can be defined as resistance to permanent indentation. The influence of hardness may be summarized
as follows:
• It can be related to the strength of the metal and, thus, indirectly, to its ductility (i.e., the higher the
hardness, the stronger the metal is and, generally, the lower its ductility is).
• The type of indentation profile is a measure of strain hardening, (i.e., the sharper the profile and its
raised ridge around the indentation, the more strain hardened the metal is). Hardness is, thus, an indirect
measure of the capacity of the metal for plastic deformation in metalworking.
• It affects the frictional and wear characteristics in forming operations.
The strain-hardening exponent, also called the work-hardening exponent, is a measure of how rapidly the metal
becomes stronger and harder as it is strained (worked). This exponent is typically obtained from the true stress-
true strain curve of the metal (often derived from engineering stress-engineering strain curves) and is expressed
as a power law:
σ = Kε
n
(Eq 1)
where σ is the true stress, K is the strength coefficient, ε is the true strain, and n is the strain-hardening
exponent.
At room temperature, for magnitude of K typically ranges from about 200 MPa for soft aluminum to about
2000 MPa for superalloys; it decreases as the temperature increases. Depending on the metal and its condition,
the values of n typically range from 0.05 to 0.5 (Table 1).
Table 1 Typical values for strength coefficient, K, and strain-hardening exponent, n, (Eq 1) at room
temperature
K Material
MPa
ksi
n
Aluminum
1100-O
180 26
0.20
2024-T4 690 100
0.16
5052-O 210 30
0.13
6061-O 205 30
0.20
6061-T6 410 59
0.05
7075-O 400 58
0.17
Brass
60-39-1 Pb, annealed
800 116
0.33
70-30, annealed 895 130
0.49
85-15, cold rolled 580 84
0.34
Bronze (phosphor), annealed 720 104
0.46
Cobalt-base alloy, heat treated
2070
300
0.50
Copper, annealed 315 46
0.54
Molybdenum, annealed 725 105
0.13
Steel
Low-carbon annealed
530 77
0.26
1045, hot rolled 965 140
0.14
1112, annealed 760 110
0.19
1112, cold rolled 760 110
0.08
4135, annealed 1015
147
0.17
4135, cold rolled 1100
160
0.14
4340, annealed 640 93
0.15
17-4 P-H, annealed 1200
174
0.05
52100, annealed 1450
210
0.07
302 stainless, annealed 1300
188
0.30
304 stainless, annealed 1275
185
0.45
410 stainless, annealed 960 139
0.10
Source: Ref 3
It can be shown that the exponent n also gives a direct indication of the uniform elongation of the metal; that is,
the extent to which the metal can be stretched before it begins to neck (plastic instability). The relationship is
given by the following:
ε
necking
= n
(Eq 2)
for simple tension. For example, in sheet-metal stretching, if the sheet is 1 m long and n = 0.2, the sheet, thus,
can be stretched uniaxially to a true strain of 0.2 (to 1.22 m) before it begins to neck.
The magnitude of the strain-hardening exponent also has an effect on the maximum reduction per pass in rod
and wire drawing. The higher the value of n is, the higher the strength of the metal exiting the die and, hence,
the smaller is the final cross-sectional area to which the metal can be reduced by drawing.
Strain-Rate Effects. The strength exhibited by a metal also depends on the rate at which it is being deformed.
For simple tension, this relationship is given by the following:
σ = C
m
(Eq 3)
where C is the strength coefficient, is the true strain rate, and m is the strain-rate sensitivity exponent. The
magnitude of C at room temperature typically ranges from as low as approximately 10 MPa for aluminum to
about 1000 MPa for titanium (Table 2); it decreases with increasing temperature.
Table 2 Approximate range of values for the strength coefficient, C, and strain-rate sensitivity exponent,
m (Eq 3)
C Material Temperature, °C
MPa ksi
m
Aluminum 200–500 82–14 12–2
0.07–0.23
Aluminum alloys 200–500 310–35 45–5
0–0.20
Copper 300–900 240–20 35–3
0.06–0.17
Copper alloys (brasses) 200–800 415–14 60–2
0.02–0.3
Lead 100–300 11–2 1.6–0.3
0.1–0.2
Magnesium 200–400 140–14 20–2
0.07–0.43
Steel
Low-carbon
900–1200 165–48 24–7
0.08–0.22
Medium-carbon 900–1200 160–48 23–7
0.07–0.24
Stainless 600–1200 415–35 60–5
0.02–0.4
Titanium 200–1000 930–14 135–2
0.04–0.3
Titanium alloys 200–1000 900–35 130–5
0.02–0.3
Ti-6Al-4V
(a)
815–930 65–11 9.5–1.6
0.05–0.80
Zirconium 200–1000 830–27 120–4 0.04–0.4
As temperature increases, C decreases and m increases. As strain increases, C increases, and m may increase or
decrease. Also, m may become negative within certain ranges of temperature and strain.
(a) At a strain rate of 2 × 10
-4
s
-1
.
Source: Ref 3
The magnitude of the exponent m depends not only on the particular material but depends also with great
sensitivity on temperature. The higher the temperature, the higher the magnitude of the exponent m is. At room
temperature, m can approach zero, and at elevated temperatures, it can approach unity. Consequently, strain-
rate and temperature effects in metals are generally studied and reported simultaneously.
Strain-rate sensitivity is an important parameter in the elongation and ductility of metals. As m increases, total
elongation increases and the post-uniform elongation (elongation after the onset of necking) also increases (Fig.
1 and 2). This behavior is typically exhibited by certain very fine grained alloys (10 to 15 μm) where total
elongations up to 2000% at strain rates on the order of 10
-4
to 10
-2
s
-1
are obtained at certain temperature ranges.
This phenomenon is known as superplasticity.
Fig. 1 The effect of the strain-rate sensitivity exponent, m, on the total elongation for various metals and
alloys. Source: Ref 3
Fig. 2 The effect of strain-rate sensitivity exponent, m, on the post-uniform elongation for various metals
and alloys. Source: Ref 3
Typical superplastic metals are zinc-aluminum, titanium, some aluminum alloys, nickel alloys, and iron-base
superalloys. Superplastic forming has become important in fabricating sheet metal structures and various other
components, particularly for aerospace applications.
Temperature effects generally are to decrease strength and increase workability. However, due to factors such
as weakening of the grain boundaries (hot shortness), increased temperature can adversely influence
workability and product quality.
Other related effects of temperature include (a) the blue brittleness range in steels, (b) surface oxidation, and (c)
lubricant behavior, which, because it affects the tribological behavior at die-workpiece interfaces, can influence
metal flow in dies (die filling, laps, defect formation).
State of stress in plastic deformation can have a major influence on workability. For example, even a very
ductile metal can behave in a brittle manner when subjected to high levels of triaxial (hydrostatic) tensile
stresses. Center-burst (chevron) cracking of solid rods in drawing and extrusion, plane-strain drawing of sheet
or plate, and tube drawing or spinning are due to the high hydrostatic tensile stress component at the centerline
of the workpiece during plastic deformation.
Another example of the importance of the state of stress is the favorable influence that normal compressive
stress has on the maximum shear strain before fracture. Note (Fig. 3) that for torsion testing on steels, the shear
strain to fracture rapidly increases as the compressive stress (normal to the cross section of the specimen)
increases.
Fig. 3 The effect of axial compressive stress on the shear strain at fracture in torsion for various steels.
Source: Ref 3
Effects of Hydrostatic Pressure. A highly beneficial state of stress is the environmental hydrostatic pressure; it
has a major influence on the ductility of metals in metalworking operations. An otherwise brittle material can
become ductile when plastically deformed under a state of high hydrostatic pressure, as has been observed in
tension tests performed in highly pressurized chambers (Fig. 4).
Fig. 4 The effect of hydrostatic pressure on the tensile ductility for various metals. Source: Ref 3
While with most metals ductility increases gradually with hydrostatic pressure, in others, such as zinc and its
alloys (hcp structure) very ductile behavior occurs abruptly with a rapid transition from brittle behavior over a
narrow pressure range. Hydrostatic extrusion is the most common example of the beneficial use of hydrostatic
pressure.
References cited in this section
2. G.E. Dieter, Mechanical Metallurgy, 3rd ed., McGraw-Hill, 1986
3. S. Kalpakjian, Manufacturing Processes for Engineering Materials, 3rd ed., Addison-Wesley, 1997, p
44, 45, 50, 398, 399, 409, 416, 438
4. J.S. Schey, Introduction to Manufacturing Processes, 3rd ed, McGraw-Hill, 1999
Mechanical Testing for Metalworking Processes
Serope Kalpakjian, Illinois Institute of Technology
Mechanical Testing Methods and Sample Preparation
The material behavior characteristics already outlined in this article are typically determined by mechanical
testing methods that are described in detail in other Sections of this Volume. Those aspects of testing that are
particularly relevant to workability and quality control for metalworking processes are described later in this
article (Ref 1, 2, 4, 5, 6).
Tension Test. Partly because of its relative simplicity, the tension test has been and continues to be the most
common mechanical testing method. From the test results, true stress-true strain curves are constructed, with a
correction made for necking of the specimen (Bridgman correction factor due to triaxial tensile state of stress in
the necked region) (Ref 2, 3, 4). From these curves most of the material behavior characteristics can be
determined easily.
The shape of the specimens may be round, sheet, or plate, and they are prepared and tested according to various
international and national standards specifications (such as ASTM, ISO, JIS, and DIN). As described earlier,
reporting of test results must specify specimen shape and dimensions, as well as parameters such as strain rate
and temperature.
Plane-Strain Tension Test. A modification of the simple uniaxial tension test is the plane-strain tension test, in
which the specimen has two deep grooves across its width (Fig. 5). This geometry restricts the deformation to
length and thickness of the specimen, but not to its width (hence the term plane strain).
Fig. 5 Schematic of a plane-strain tension test specimen. Source: Ref 1
Compared to the simple tension test, in the plane-strain tension test the same metal exhibits a lower fracture
strain. The plane-strain tension test attempts to simulate metalworking processes in which the workpiece (in
whole or in part) is subjected to plane-strain conditions, typically in sheet metal forming operations. (See the
section “Plane-Strain Compression Test” that follows.)
Compression Test. Unlike tension tests, the compression (upsetting) test has significant difficulties because of
the friction at the platen-specimen interfaces, the surface characteristics of the contacting bodies, and the nature
of lubrication. When conducted with care, reliable true stress-true strain curves can be obtained. It has been
shown that for ductile metals, the true stress-true strain curves in tension and compression are the same (Ref 3).
Barreling is an inevitable phenomenon in compression tests, although it can be minimized by effective
lubrication and use of ultrasonic vibration of the platens. For specimens with high length-to-diameter ratio,
there is a tendency for double barreling, near each end of the specimen.
The extent of deformation of the workpiece in compression tests (using either round or square specimens)
indicates some measure of ductility in bulk deformation processes, particularly in forging and for metals with
limited ductility. A crack usually develops on the cylindrical surface of the specimen, due to what are known as
secondary tensile stresses. Depending on the state of stress and lubrication conditions, the direction of the crack
may be at 45° to the long axis of the specimen, but it can be longitudinal as well, particularly if seams (a defect
developed during prior processing) are present on the cylindrical surfaces of the specimen.
Compression tests at elevated temperature can be difficult to perform because of the heat loss to the dies.
However, compression testing is commonly used as a simple measure of workability of metal, particularly in
forging and similar bulk deformation processes.
Although test specimens for compression are typically prepared with relatively smooth surfaces, longitudinal
notches are sometimes machined on the cylindrical or square surfaces (notched bar upsetting test). The purpose