ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation
Подождите немного. Документ загружается.
not (see the discussion with Fig. 14 in the preceding article “Introduction to the Mechanical Behavior of
Nonmetallic Materials”). Generally, bulky side groups (as in polystyrene) hinder crystallization, while
hydrogen bonding (as in nylons) promotes crystallization (Ref 2).
Fig. 2 Influence of molecular weight and temperature on the physical state of polymers. (a) Amorphous
polymer. (b) Crystalline polymer. Source: Ref 2
Because of the partial or complete noncrystalline structure of polymers, they undergo a change in mechanical
behavior that is not seen in fully crystalline materials. At temperatures well below T
g
, plastics exhibit a high
modulus and are only weakly viscoelastic. At temperatures above T
g
, there is drastic reduction of modulus (Fig.
3), which may be as large as three orders of magnitude. Therefore, the glass transition temperature is the most
important temperature that can be specified for most polymers because in all but highly crystalline polymers, it
represents the temperature above which the polymer loses most of its stiffness and thus its dimensional
stability.
Fig. 3 Shear modulus versus temperature for crystalline isotactic polystyrene (PS), two linear atactic PS
materials (A and B) with different molecular weights, and lightly cross-linked atactic PS
A typical modulus-temperature curve is shown in Fig. 3. At temperatures below T
g
, most plastic materials have
a tensile modulus of about 2 GPa (0.3 × 10
6
psi). If the material is crystalline, a small drop in modulus is
generally observed at T
g
, while a large drop is seen at the melting temperature, T
m
. The T
g
is primarily
associated with amorphous, rather than crystalline, resins or cross-linked thermosets. Resins that are partially
crystalline have at least a 50% amorphous region, which is the region that has a T
g
. If a material is amorphous, a
single decrease is usually seen at temperatures near T
g
. At even higher temperatures, there is another similar
drop in modulus, and the plastic flows easily as a high-viscosity liquid. At this condition, the plastic can be
processed by extrusion or molding.
Mechanical properties are also affected by molecular weight. Most material manufacturers provide grades with
different molecular weights. High-molecular-weight materials have high-melt viscosities and low-melt indexes.
For a commercial product, a melt index is generally an inverse indicator of molecular weight.
When molecular weight is low, the applied mechanical stress tends to slide molecules over each other and
separate them. The solid, with very little mechanical strength, has negligible structural value. With a continuing
increase in molecular weight, the molecules become entangled, the attractive force between them becomes
greater, and mechanical strength begins to improve.
It is generally desirable for material manufacturers to make plastics with sufficiently high-molecular weights to
obtain good mechanical properties. For polystyrene, this molecular weight is 100,000 and for polyethylene this
value is 20,000. It is not desirable to increase molecular weight further because melt viscosity will increase
rapidly, although there are occasional exceptions to this rule. The yield strength of polypropylene decreases
when molecular weight increases. High molecular weight and branching reduce crystallinity. Polymers with
high intermolecular interaction, such as hydrogen bonding, do not require high molecular weight to achieve
good mechanical properties. With low molecular weight, viscosity is very low, which is commonly observed
for polyamides.
Typical T
g
temperatures and some general thermal properties of selected plastics are listed in Tables 1, 2, and 3.
It should also be understood that glass transition temperatures are not distinct transition temperatures like phase
transformations, because the transition occurs over a range of temperatures. In a thermoplastic polymer such as
polystyrene, the change that occurs gradually over the T
g
region eventually leads to a complete loss of
dimensional stability. In a thermoset (network) polymer such as epoxy, the change is less severe, but
nonetheless produces significant softening and loss of mechanical properties. The value of T
g
may also depend
upon the method of measuring viscoelastic transition. Thus, T
g
for a polymer represents roughly the center of
the transition region.
Table 1 Transition and continuous-use temperatures of general purpose plastics
Glass transition
temperature (T
g
)
Melting
temperature (T
m
)
Continuous use
temperature
Plastic
°C °F °C °F °C
°F
Polystyrene (PS)
Atactic (amorphous)
100–105 212–220
(a)
(a)
45
110
Isotactic (crystalline) 100–105 212–220 240 265 45
110
Polyvinyl chloride (PVC)
Rigid PVC
75–105 170–220 212 415 …
…
Plasticized PVC
(b)
(b)
… … …
…
Chlorinated PVC 110 230 212 415 …
…
Polyethylene (PE)
High-density
-90 or -20 -130 or -5 137 280 85
185
Low-density -110 or -20 -165 or -5 115 240 85
185
Polypropylene (PP)
Atactic (amorphous)
-6 21 165–175 330–350 105
220
Isotactic (crystalline) -18 0 165–175 330–350 105
220
Syndiotactic -4 25 165–175 330–350 105 220
(a) Amorphous.
(b) Varies with plasticizer content
Table 2 Transition and continuous-use temperatures of selected structural thermoplastics
Glass transition
temperature (T
g
)
Melting
temperature (T
m
)
Continuous use
temperature
Plastic
°C °F °C °F °C
°F
Acetals (polyoxymethylene, POM) -85 -120 163–175 325–345 90
195
Acrylics (polymethyl methacrylate, PMMA)
Syndiotactic
3 35 105–220 220–250 90
195
Isotactic 3 35 45 115 90
195
Polyamides (PA)
Nylon 6
50–70
(a)
120–160
(a)
225 440 95
200
Nylon 12 46 115 180 360 …
…
Nylon 6/6 57–80
(a)
135–175
(a)
265 510 105
220
Nylon 6/10 50 120 219 425 …
…
Nylon 6/I 142 290 210 410 …
…
Polycarbonate (PC) 150 300 265 510 120
250
Polyesters
Polycaprolactone (PCL)
40 105 60–70 150–160 …
…
Polybutylene terephthalate (PBT) 60–70 140–160 225–235 440–455 …
…
Polyethylene terephthalate (PET) 78–80 170–175 260–265 500–510 … …
(a) Varies with moisture content
Table 3 Glass transition and continuous-use temperatures for selected thermoset plastics
Glass transition temperatures (T
g
)
Continuous use temperature
Plastic
°C °F °C
°F
Amino resins None
(a)
100
210
Polyurethane 135 275 90–120
190–250
Polyester 110 230 120–150
250–300
Epoxy 60–175 140–347 120–290
250–550
Phenolic 300 570 120–175
250–350
Polyimide 315–370
(a)
600–698
(a)
260–315 500–600
(a) Dry
Glass transition temperatures are also influenced by moisture absorption and the intentional addition of
plasticizers. Absorbed moisture invariably lowers the T
g
, and the more moisture is absorbed, the lower the
transition temperature. This is consistent with the role of water as a plasticizer, which is why absorbed moisture
can reduce the strength of plastics. Plasticizers are low-molecular-weight additives that lower strength and T
g
.
The lowering of transition temperatures by plasticizers can be quantitatively described by various mixing
formulas (Ref 3, 4), which can be quite useful for predicting the loss of properties due to absorbed moisture.
References cited in this section
1. R. Seymour, Overview of Polymer Chemistry, Engineering Plastics, Vol 2, Engineered Materials
Handbook, ASM International, 1988, p 64
2. A. Kumar and R. Gupta, Fundamentals of Polymers, McGraw-Hill, 1998, p 30, 337, 383
3. L.E. Nielson, Mechanical Properties of Polymers, Van Nostrand Reinhold, 1962
4. F.N. Kelly and F. Bueche, J. Polym. Sci., Vol 50, 1961, p 549
Mechanical Testing of Polymers and Ceramics
Mechanical Testing of Plastics
Engineering plastics are either thermoplastic resins (which can be repeatedly reheated and remelted) or
thermoset (network) resins (which are cured resins with cross links that depolymerize upon exposure to
elevated temperatures above T
g
). Typical mechanical properties of various thermoplastic and thermoset resins
are briefly summarized in Tables 4 and 5. Engineering plastics are not as strong as metals, but due to the lower
density of plastics, the specific strengths of structural plastics are higher than those of metallic materials. This is
shown in Table 6, which compares the range of mechanical properties of plastics with those of other
engineering materials. These data show that glass-filled plastics have strength-to-weight ratios that are twice
those of steel and cast aluminum. In addition to glass fillers, other types of additives (such as plasticizers, flame
retardants, stabilizers, and impact modifiers) can also modify the mechanical properties of plastics.
Table 4 Room-temperature mechanical properties of selected thermoplastics with glass filler
Tensile
strength
(a)
Tensile
modulus
(a)
Flexural
strength
(b)
Flexural
modulus
(b)
Izod impact
strength
notched
(c)
Compressive
strength
(d)
Thermoplastic Glass
fiber
content,
wt%
MPa
ksi
Tensile
elongation
at
break
(a)
, %
kPa psi MPa
ksi GPa
10
6
psi
J/m ft · lbf/in. MPa
ksi
… 46 6.7 2.2 320 46 97 14.0
3 0.45 11 0.2 97
14.0
20 76 11 1.0 760 110 107 15.5
7 0.96 53 1.0 111
16.1
30 93 13.5
1.0 900 130 117 17.0
8 1.22 53 1.0 120
17.4
Styrene
40 103 15 1.0 1100
160 121 17.5
10 1.47 59 1.1 122
17.7
… 72 10.5
3.0 390 56 103 15.0
4 0.55 27 0.5 103
15.0
20 90 13 2.0 860 125 129 18.7
8 1.10 53 1.0 134
19.5
30 107 15.5
1.5 1000
145 155 22.5
10 1.52 53 1.0 141
20.5
Styrene-acrylonitrile (SAN)
40 119 17.2
1.5 1240
180 161 23.4
12 1.80 53 1.0 148
21.5
… 48 7 8.0 210 30 72 10.5
3 0.38 240 4.5 69
10.0
20 90 13 3.0 620 90 117 17.0
6 0.80 80 1.5 86
12.5
30 105 15.2
3.0 690 100 128 18.5
7 1.00 75 1.4 107
15.5
Acrylonitrile-butadiene-styrene
(ABS)
40 110 16 2.0 1030
150 145 21.0
9 1.30 69 1.3 118
17.1
… 40 5.8 5.1 240 35 83 12.0
2 0.33 213 4.0 52
7.5
Flame-retardant ABS
20 76 11 2.0 510 74 107 15.5
5 0.71 64 1.2 97
14.0
… 32 4.7 15.0 130 19 41 6.0 2 0.30 27 0.5 34
5.0
20 59 8.5 3.0 380 55 55 8.0 4 0.60 43 0.8 41
6.0
30 62 9 3.0 450 65 59 8.5 6 0.80 59 1.1 45
6.5
Polypropylene (PP)
40 69 10 2.0 520 75 62 9.0 7 1.00 69 1.3 48
7.0
… 32 4.7 15.0 130 19 41 6.0 2 0.30 27 0.5 41
6.0
20 76 11 3.0 410 60 83 12.0
4 0.60 75 1.4 69
10.0
Glass-coupled PP
40 97 14 2.0 550 80 131 19.0
7 1.00 85 1.6 90
13.0
… 30 4.3 9.0 100 15 38 5.5 2 0.22 69 1.3 28
4.0
20 48 7 3.0 410 60 62 9.0 4 0.55 75 1.4 34
5.0
30 69 10 2.0 590 85 76 11.0
6 0.80 91 1.7 48
7.0
Polyethylene (PE)
40 76 11 2.0 760 110 86 12.5
7 1.00 91 1.7 55
8.0
… 61 8.8 60.0 280 41 90 13.0
3 0.37 69 1.3 36
5.2
20 83 12 2.0 830 120 110 16.0
7 1.00 53 1.0 83
12.0
Acetal (AC)
30 90 13 1.8 930 135 114 16.5
8 1.20 43 0.8 83
12.0
… 55 8 200.0 280 40 88 12.8
2 0.34 11 0.2 90
13.0
Polyester
20 117 17 5.0 690 100 152 22.0
6 0.85 80 1.5 110
16.0
30 131 19 4.0 1030
150 179 26.0
8 1.20 96 1.8 124
18.0
40 152 22 3.0 1380
200 207 30.0
10 1.50 107 2.0 138
20.0
… 61 8.9 60.0 280 40 101 14.7
3 0.38 48 0.9 100
14.5
Flame-retardant polyester
30 131 19 3.0 1100
160 176 25.5
9 1.30 69 1.3 124
18.0
… 81 11.8
200.0 280 40 103 15.0
3 0.40 53 1.0 90
13.0
20 128 18.5
3.0 690 100 159 23.0
6 0.80 80 1.5 148
21.5
30 155 22.5
3.0 900 130 186 27.0
8 1.10 117 2.2 159
23.0
Nylon 6
40 185 26.8
2.0 970 140 207 30.0
9 1.30 160 3.0 159
23.0
… 85 12.3
60.0 290 42 110 16.0
3 0.40 53 1.0 90
13.0
Flame-retardant Nylon 6
30 152 22 3.0 900 130 228 33.0
9 1.35 91 1.7 16
2.3
… 79 11.4
300.0 130 19 103 15.0
1 0.19 53 1.0 34
4.9
20 138 20 3.0 830 120 193 28.0
6 0.85 64 1.2 159
23.0
30 179 26 2.0 1030
150 259 37.5
9 1.30 107 2.0 165
24.0
Nylon 6/6
40 214 31 2.0 900 130 293 42.5
11 1.60 139 2.6 172
25.0
… 67 9.7 35.0 130 19 90 13.0
1 0.18 53 1.0 34
4.9
Flame-retardant Nylon 6/6
30 148 21.5
2.0 830 120 172 25.0
7 1.00 85 1.6 159
23.0
… 61 8.8 150.0 200 29 86 12.5
2 0.29 53 1.0 76
11.0
20 124 18 4.0 690 100 193 28.0
6 0.90 59 1.1 131
19.0
Nylon 6/12
30 152 22 4.0 900 130 221 32.0
8 1.10 128 2.4 152
22.0
… 62 9 110.0 240 34.5
93 13.5
2 0.34 160 3.0 86
12.5
10 90 13 5.0 480 70 110 16.0
4 0.60 107 2.0 124
18.0
20 110 16 5.0 620 90 138 20.0
6 0.80 117 2.2 138
20.0
30 131 19 4.0 900 130 165 24.0
8 1.20 128 2.4 145
21.0
Polycarbonate (PC)
40 152 22 3.5 1170
170 193 28.0
10 1.40 144 2.7 148
21.5
… 70 10.2
75.0 250 36 106 15.4
3 0.39 32 0.6 97
14.0
20 131 19 3.0 620 90 138 20.0
5 0.75 64 1.2 138
20.0
30 148 21.5
3.0 830 120 155 22.5
7 1.00 75 1.4 155
22.5
Polysulfone (PSU)
40 165 24 2.0 1240
180 172 25.0
9 1.25 107 2.0 172
25.0
Polyphenylene sulfide (PPS) 40 138 20 1.5 1410
205 234 34.0
12 1.80 80 1.5 172 25.0
(a) ASTM D 638 test method.
(b) ASTM D 790 test method.
(c) ASTM D 256 test method with 6.35 mm (¼ in.) bar.
(d) ASTM D 695 test method
Table 5 Mechanical properties of fiberglass-reinforced thermoset resins
Tensile
strength
Compressive
strength
Flexural
strength
Resin
MPa ksi
Elongation,
%
MPa ksi MPa ksi
Izod
impact
strength,
J/mm
Hardness,
HRM
Polyester 173–
206
25–
30
0.5–5 103–
206
15–
30
69–
276
10–
40
0.1–0.5
70–120
Phenolic 35–69 5–10 0.02 117–
179
17–
26
69–
415
10–
60
0.5–2.5
95–100
Epoxy 97–
206
14–
30
4 206–
262
30–
38
138–
180
20–
26
0.4–0.75
100–108
Melamine 35–69 5–10 … 138–
241
20–
35
103–
160
15–
23
0.2–0.3
…
Polyurethane
31–55 4–8 10–650 138 20 48–62 7–9 No break 28 HRM-60
HRR
Table 6 Range of mechanical properties for common engineering materials
Elastic modulus Tensile strength Maximum
strength/density
Material
GPa 10
6
psi MPa ksi (km/s)
2
(kft/s)
2
Elongation
at break, %
Ductile steel 200 30 350–800 50–120 0.1 1
0.2–0.5
Cast aluminum alloys 65–72 9–10 130–300 19–45 0.1 1
0.01–0.14
Polymers 0.1–21 0.02–30 5–190 0.7–28 0.05 0.5
0–0.8
Glasses 40–140 6–20 10–140 1.5–21 0.05 0.5
0
Copper alloys 100–117 15–18 300–1400 45–200 0.17 1.8
0.02–0.65
Moldable glass-filled polymers 11–17 1.6–2.5 55–440 8–64 0.2 2
0.003–0.015
Graphite-epoxy 200 30 1000 150 0.65 1.3 0–0.02
The testing of plastics includes a wide variety of chemical, thermal, and mechanical tests (Table 7). The
following sections briefly describe the test methods and comparative data for the mechanical property tests
listed in Table 7. In addition, creep testing and dynamic mechanical analyses of viscoelastic plastics are also
briefly described. For more detailed descriptions of these test methods and the other test methods listed in Table
7, readers are referred to Ref 5 and an extensive one-volume collection of ISO and European standards for
plastic testing (Ref 6).
Table 7 ASTM and ISO mechanical test standards for plastics
ASTM
standard
ISO
standard
Topic area of standard
Specimen preparation
D 618 291
Methods of specimen conditioning
D 955 294-4
Measuring shrinkage from mold dimensions of molded thermoplastics
D 3419 10724
In-line screw-injection molding of test specimens from thermosetting
compounds
D 3641 294-1,2,3
Injection molding test specimens of thermoplastic molding and extrusion
materials
D4703 293
Compression molding thermoplastic materials into test specimens, plaques, or
sheets
D 524 95
Compression molding test specimens of thermosetting molding compounds
D 6289 2577
Measuring shrinkage from mold dimensions of molded thermosetting plastics
Mechanical properties
D 256 180
Determining the pendulum impact resistance of notched specimens of plastics
D 638 527-1,2
Tensile properties of plastics
D 695 604
Compressive properties of rigid plastics
D 785 2039-2
Rockwell hardness of plastics and electrical insulating materials
D 790 178
Flexural properties of unreinforced and reinforced plastics and insulating
materials
D 882 527-3
Tensile properties of thin plastic sheeting
D 1043 458-1
Stiffness properties of plastics as a function of temperature by means of a
torsion test
D 1044 9352
Resistance of transparent plastics to surface abrasion
D 1708 6239
Tensile properties of plastics by use of microtensile specimens
D 1822 8256
Tensile-impact energy to break plastics and electrical insulating materials
D 1894 6601
Static and kinetic coefficients of friction of plastic film and sheeting
D 1922 6383-2
Propagation tear resistance of plastic film and thin sheeting by pendulum
method
D 1938 6383-1
Tear propagation resistance of plastic film and thin sheeting by a single tear
method
D 2990 899-1,2
Tensile, compressive, and flexural creep and creep-rupture of plastics
D 3763 6603-2
High-speed puncture properties of plastics using load and displacement
sensors
D 4065 6721-1
Determining and reporting dynamic mechanical properties of plastics
D 4092 6721
Dynamic mechanical measurements on plastics
D 4440 6721-10
Rheological measurement of polymer melts using dynamic mechanical
procedures
D 5023 6721-3
Measuring the dynamic mechanical properties of plastics using three-point
bending
D 5026 6721-5
Measuring the dynamic mechanical properties of plastics in tension
D 5045 572
Plane-strain fracture toughness and strain energy release rate of plastic
materials
D 5083 3268
Tensile properties of reinforced thermosetting plastics using straight-sided
specimens
D 5279 6721 Measuring the dynamic mechanical properties of plastics in torsion
Source: Ref 5
Tensile Properties
The chemical composition and the long-chain nature of polymers lead to some important differences with
metals. These differences include significantly lower stiffnesses, much higher elastic limits or recoverable
strains, a wider range of Poisson's ratios, and time-dependent deformation from viscoelasticity. Thermoplastics
also exhibit a unique variety of post-yield phenomena. For example, Fig. 4 is a typical stress-strain plot for
aluminum and polyethylene. The aluminum sample necks and extends to 50% strain. The polyethylene sample
necks and extends to 350% strain as a consequence of the long-chain nature of polymers. The polyethylene also
shows a stiffening due to chain alignment at the highest strains. This postyield stiffening involves shear
deformation as described in Ref 6.
Fig. 4 Typical stress-strain curves for polycrystalline aluminum and semicrystalline polyethylene. Both
materials neck. In polyethylene, chain alignment results in stiffening just before failure. Source: Ref 7
The ultimate tensile strengths of most unreinforced structural plastics range from 50 to 80 MPa (7–12 ksi) with
elongation to final fracture much higher than metals (Fig. 5). It is very common to see large differences
between metals and plastics in the amount of recoverable elastic strain. In metals, the amount of recoverable
elastic strain is determined by the amount of strain that can be put into one of the metallic bonds before
breaking. This amount of strain is typically less than 1%. In elastomers, the amount of recoverable strain can be
500% or more.
Fig. 5 Tensile stress-strain curves for copper, steel, and several thermoplastic resins. Source: Ref 8
When recoverable strain is this large, the individual polymer chains must be prevented from flowing past each
other during deformation. This is easily accomplished by cross links tying the chains together (Ref 9). Even in
glassy polymers, in which internal energy effects are evident in the elasticity, the recoverable strain is limited
by the strain required to break the weaker and longer-range van der Waals bonds. Because these bonds can be
stretched farther than metallic bonds, it is possible to have a recoverable strain in glassy polymers of 5% or
more. At such large strains, it is possible for the assumptions of small-strain elasticity to break down. Any
standard test procedures based on small-strain elasticity may have to be modified to account for large elastic
strains.
The mechanical behavior of polymers is also time dependent, or viscoelastic. Therefore, data based on short-
term tests have the possibility of misrepresenting the tested polymer in a design application that involves long-
term loading. The magnitude of the time dependence of polymers is very temperature dependent. Well below
the T
g
, glassy or semicrystalline polymers are only very weakly viscoelastic. For these polymers, test data based
on a time-independent analysis will probably be adequate. As the temperature is increased, either by the
environment or by heat given off during deformation, the time dependence of the mechanical response will
increase.
Under viscoelastic conditions, one method useful for obtaining long-term design data is the time-temperature
superposition principle. This principle states that the mechanical response at long times at some particular
temperature is equivalent to the mechanical response at short times but at some higher temperature (Ref 8). By
determining shift factors, it is possible to determine which temperature to use in obtaining long-term data from
short-term tests. This is essentially true for linear viscoelastic behavior in the absence of a phase change.
The short-term tensile test (ASTM D 638 and ISO 517) is one of the most widely used mechanical tests of
plastics for determining mechanical properties such as tensile strength, yield strength, yield point, and
elongation. The stress-strain curve from tension testing is also a convenient way to classify plastics (Fig. 6). A
soft and weak material, such as PTFE, is characterized by low modulus, low yield stress, and moderate
elongation at break point. A soft but tough material such as polyethylene shows low modulus and low yield
stress but very high elongation at break. A hard and brittle material such as general-purpose phenolic is
characterized by high modulus and low elongation. It may or may not yield before break. A hard and strong
material such as polyacetal has high modulus, high yield stress, high ultimate strength (usually), and low
elongation. A hard and tough material such as polycarbonate is characterized by high modulus, high yield
stress, high elongation at break, and high ultimate strength.
Fig. 6 Tensile stress-strain curves for several categories of plastic materials
Because of the diversity of mechanical behavior, the tension testing of plastics is subject to potential
misapplication or misinterpretation of test results. This is particularly true for thermoplastics, which have some
important differences with thermoset plastics. Compared to thermoset resins, thermoplastics exhibit more
disruption or changes in the secondary bonding between the molecular chains during tension testing. This leads
to a variety of postyield phenomena, such as the stiffening observed in polyethylene (Fig. 4). Another example
is shown in Fig. 7. At the yield point the average axis of molecular orientation in thermoplastics may begin to
conform increasingly with the direction of the stress. The term draw is sometimes used to describe this
behavior. There is usually a break in the stress-strain curve as it begins to flatten out, and more strain is
observed with a given increased stress. The result is that the giant molecules begin to align and team up in their
resistance to the implied stress. Frequently, there is a final increase in the slope of the curve just before ultimate
failure (Fig. 7). The extent to which this orientation takes place varies from one linear thermoplastic to the next,
but the effect can be quite significant. Even the smallest amount of the teaming-up effect imparts greatly
improved impact resistance and damage tolerance. In thermoplastics, there is much more area under the stress-