Chapter 2

MATERIALS AND PROCESSES

61

 

 

2

 

F I G U R E 2 - 22

Tensile Test Stress-Strain Curves of Three Thermoplastic Polymers (From Fig. 5.18, p. 161 in N. E.

Dowling, Mechanical Behavior of Materials, Prentice-Hall, 1993, with permission) minum, 8 for steel, and 13 for lead. So, even though the absolute strengths of polymers are low, their specific strengths are respectable due to their low densities.

 

Polymers are divided into two classes, thermoplastic and thermosets. Thermoplastic polymers can be repeatedly melted and solidified, though their properties can Table 2-7

degrade due to the high melt temperatures. Thermoplastics are easy to mold and their Families of Polymers

rejects or leftovers can be reground and remolded. Thermosetting polymers become cross-linked when first heated and will burn, not melt, on reheating. Cross-linking cre-Thermoplastics

ates connections (like the rungs of a ladder) between the long-chain molecules which Cellulosics

wind and twist through a polymer. These cross-connections add strength and stiffness.

Ethylenics

Another division among polymers can be made between filled and unfilled com-Polyamides

pounds. The fillers are usually inorganic materials, such as carbon black, graphite, talc, Polyacetals

chopped glass fibers, and metal powders. Fillers are added to both thermoplastic and Polycarbonates

thermosetting resins, though they are more frequently used in the latter. These filled compounds have superior strength, stiffness, and temperature resistance over that of the Polyphenyline oxides

raw polymers but are more difficult to mold and to fabricate.

Polysulfones

Thermosets

A confusing array of polymers is available commercially. The confusion is in-Aminos

creased by a proliferation of brand names for similar compounds made by different manufacturers. The generic chemical names of polymers tend to be long, complex, and Elastomers

hard to remember. In some cases a particular polymer brand name has been so widely Epoxies

used that it has become generic. Nylon, plexiglass, and fiberglass are examples. Learn-Phenolics

ing the generic chemical names and associated brand names of the main families of en-Polyesters

gineering polymers will eliminate some of the confusion. Table 2-7 shows a number Silicones

of important polymer families. The mechanical properties of a few of these that have Urethanes

significant engineering applications are included in Appendix A.

 

 

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62

MACHINE DESIGN -

An Integrated Approach

 

 

Ceramics

Ceramic materials are finding increasing application in engineering, and a great deal of 2

effort is being devoted to the development of new ceramic compounds. Ceramics are among the oldest known engineering materials; clay bricks are ceramic materials.

Though still widely used in building, clay is not now considered an engineering ceramic.

Engineering ceramics are typically compounds of metallic and nonmetallic elements.

They may be single oxides of a metal, mixtures of metallic oxides, carbides, borides, nitrides, or other compounds such as Al2O3, MgO, SiC, and Si3N4, for example. The principal properties of ceramic materials are high hardness and brittleness, high temperature and chemical resistance, high compressive strength, high dielectric strength, and potentially low cost and weight. Ceramic materials are too hard to be machined by conventional techniques and are usually formed by compaction of powder, then fired or sintered to form bonds between particles and increase their strength. The powder compaction can be done in dies or by hydrostatic pressure. Sometimes, glass powder is mixed with the ceramic and the result is fired to melt the glass and fuse the two together. Attempts are being made to replace traditional metals with ceramics in such applications as cast engine blocks, pistons, and other engine parts. The low tensile strength, porosity, and low fracture toughness of most ceramics can be problems in these applications. Plasma-sprayed ceramic compounds are often used as hard coatings on metal substrates to provide wear- and corrosion-resistant surfaces.

 

 

Composites

 

 

Most composites are man-made, but some, such as wood, occur naturally. Wood is a composite of long cellulose fibers held together in a resinous matrix of lignin. Man-made composites are typically a combination of some strong, fibrous material such as glass, carbon, or boron fibers glued together in a matrix of resin such as epoxy or polyester. The fiberglass material used in boats and other vehicles is a common example of a glass-fiber reinforced polyester (GFRP) composite. The directional material properties of a composite can be tailored to the application by arranging the fibers in different juxtapositions such as parallel, interwoven at random or particular angles, or wound around a mandrel. Custom composites are finding increased use in highly stressed applications such as airframes due to their superior strength-to-weight ratios compared to the common structural metals. Temperature and corrosion resistance can also be designed into some composite materials. These composites are typically nei-Table 2-8

ther homogeneous nor isotropic as was discussed in Section 2.3.

Iron and Steel Strengths

It is interesting to note that if one calculates the theoretical strength of any “pure”

 

elemental crystalline material based on the interatomic bonds of the element, the pre-Form

Sut kpsi (MPa)

dicted strengths are orders of magnitude larger that those seen in any test of a “real”

 

material, as seen in Table 2-8. The huge differences in actual versus theoretical strengths Theoretical 2 900 (20 E 3)

are attributed to disruptions of the atomic bonds due to crystal defects in the real mate-Whisker

1 800 (12 E 3)

rial. That is, it is considered impossible to manufacture “pure anything” on any realis-Fine wire

1 400 (10 E 3)

tic superatomic scale. It is presumed that if we could make a “wire” of pure iron only Mild steel

60 (414)

one atom in diameter, it would exhibit its theoretical “super strength.” Crystal “whis-Cast iron

40 (276)

kers” have been successfully made of some elemental materials and exhibit very high tensile strengths which approach their theoretical values (Table 2-8).

 

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