Chemistry Reference
In-Depth Information
120
10
8
6
4
2
80
40
0
10
20
0
10
20
(a)
Strain (%)
(b)
Strain (%)
12
Elo
ngation at break
10
2.0
8
1.6
6
1.2
4
0.8
2
0.4
0
0 0 0 0
0
200
400
600
800
(c)
Strain (%)
(d)
Strain (%)
FIGURE 1.2
Stress-strain curves. (a) Synthetic fiber, like nylon 66. (b) Rigid, brittle plastic, like
polystyrene. (c) Tough plastic, like nylon 66. (d) Elastomer, like vulcanized natural rubber.
10
6
Fibers have high initial moduli which are usually in the range 0.5
to
3
10
3
MN/m
2
). Their extensibilities at break are often
lower than 20%. If a fiber is stretched below its breaking strain and then allowed
to relax, part of the deformation will be recovered immediately and some, but not
all, of the remainder will be permanent (
Fig. 1.3b
). Mechanical properties of com-
mercial synthetic fibers do not change much in the temperature range between
2
10
6
psi (3
10
3
2
to 14
3
3
3
50
C and about 150
C (otherwise they would not be used as fibers).
[As an aside, we mention that fiber strength (tenacity) and stiffness are usually
expressed in units of grams per denier or grams per tex (i.e., grams force to break
a one-denier or one-tex fiber). This is because the cross-sectional area of some
fibers, like those made from copolymers of acrylonitrile, is not uniform. Denier
and tex are the weights of 9000- and 1000-m fiber, respectively.]
Plastics generally have intermediate tensile moduli, usually 0.5
10
5
to
3
10
3
MN/m
2
), and their breaking strain varies from a
few percent for brittle materials like polystyrene to about 400% for tough,
10
5
psi (3.5
10
2
4
to 3
3
3
3
