Page 87 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials

temperature, such as lead and tin. Typical ranges of m for metals are up to 0.05 for
cold-working, 0.05 to 0.4 for hot-working, and 0.3 to 0.85 for superplastic materi-
als (see below).
The magnitude of the strain-rate sensitivity exponent significantly influences
necking in a tension test. With increasing m, the material stretches farther before it
fails; thus, increasing m delays necking. Ductility enhancement Caused by the high
strain-rate sensitivity of some materials has been exploited in superplastic forming
of sheet metal (Section 16.10).

Superplasticity. The term superplasticity refers to the capability of some materials
to undergo large uniform elongation prior to necking and fracture in tension. The
elongation ranges from a few hundred percent to as much as 2000%. Common non-
metallic materials exhibiting superplastic behavior are bubble gum and glass (at ele-
vated temperatures) and thermoplastics. As a result, glass and thermoplastics can
successfully be formed into complex shapes such as beverage bottles and lighted adver~
tising signs. Among metals exhibiting superplastic behavior are very fine grained
(10 to 15 /rm) titanium alloys and alloys of zinc-aluminum; when heated, they can
elongate to many times their original length.

2.2.8 Hydrostatic Pressure Effects
Various tests have been performed to determine the effect of hydrostatic pressure on
mechanical properties of materials. Test results at pressures up to 3.5 GPa indicate
that increasing the hydrostatic pressure substantially increases the strain at fracture,
both for ductile and for brittle materials. This beneficial effect of hydrostatic pressure
has been exploited in metalworking processes, particularly in hydrostatic extrusion
(Section 15 .4.2), and in compaction of metal powders (Section 17.3).

2.2.9 Radiation Effects
In view of the use of many metals and alloys in nuclear applications, extensive stud-
ies have been conducted on the effects of radiation on mechanical properties.
Typical changes in the properties of steels and other metals exposed to high-energy
radiation are increased yield stress, tensile strength, and hardness, and decreased
ductility and toughness. Radiation has similar detrimental effects on the behavior of
plastics.

2.3 Compression

Many operations in manufacturing, particularly processes such as forging, rolling,
and extrusion (Part III), are performed with the workpiece subjected to compressive
stresses. The compression test, in which the specimen is subjected to a compressive
load, gives information that is useful for estimating forces and power requirements
in these processes. This test is usually carried out by compressing a solid cylindrical
specimen between two well-lubricated flat dies (platens). Because of friction between
the specimen and the platens, the specimen’s cylindrical surface bulges, an effect is
called barreling (see Fig. 14.3). Note that slender specimens can buckle during this
test; thus, the height-to-diameter ratio of the solid cylindrical specimen is typically
less than 3:1. (See also Section 14.4 on heading.)
Because of barreling, the cross-sectional area of the specimen changes along its
height, and obtaining the stress-strain curves in compression can be difficult.