Page 104 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Summary

actual operations, heat is lost to the environment, to tools and dies, and to lubricants
or coolants used, if any. If the deformation process is performed rapidly, the heat
losses will be relatively small over that brief period. If the process is carried out slow-
ly, the actual temperature rise will be only a fraction of the calculated value.

SUMMARY

° Many manufacturing processes involve shaping materials by plastic deformation;
consequently, such mechanical properties as strength (yield strength, Y, and ulti-
mate tensile strength, UTS); modulus of elasticity, E; ductility (total elongation
and reduction of area); hardness; and the energy required for plastic deformation
are important factors. These properties, in turn, depend, to various extents, on
the particular material and on its condition, temperature, deformation rate, sur-
face condition, and environment.
° The tensile test is the most commonly used test to determine mechanical proper-
ties; from these tests, true stress-true strain curves are constructed that are needed
to determine the strength coefficient (K), the strain-hardening exponent (11), the
strain-rate sensitivity exponent (m), and the toughness of materials.
° Compression tests are subject to inaccuracy due to the presence of friction and to re-
sultant barreling of the specimen. Torsion tests are conducted on tubular specimens
subjected to twisting. Bending or flexure tests are commonly used for brittle mate-
rials to determine their modulus of rupture or the transverse rupture strength.
° Several hardness tests are used to determine the resistance of a material to perma-
nent indentation or scratching. Hardness is related to strength and wear resist-
ance of a material, but it is, itself, not a fundamental property.
° Fatigue tests indicate the endurance limit or fatigue limit of materials-that is, the
maximum stress to which a material can be subjected without fatigue failure,
regardless of the number of cycles. Some materials have no endurance limit; and
instead, their allowable stress must be reported with respect to the number of
loading cycles.
° Creep is the permanent elongation of a component under a static load maintained
for a period of time. The specimen eventually fails by rupture (necking and
fracturing).
° Impact tests determine the energy required to completely break a specimen. This
energy is called the impact toughness of the material. Impact tests are also useful
for determining the transition temperatures of materials.
° Failure and fracture constitute an important aspect of a material’s behavior when
it is subjected to deformation in manufacturing operations. Ductile fracture is
characterized by plastic deformation preceding fracture, and it requires a consid-
erable amount of energy. Brittle fracture can be catastrophic, because it is not
preceded by plastic deformation; it requires much less energy than ductile frac-
ture. Impurities, inclusions, and voids play a major role in the fracture of metals
and alloys.
° Residual stresses are those that remain in a workpiece after it has been plastically
deformed and then has had all external forces removed. Surface tensile residual
stresses are generally undesirable; they may be reduced or eliminated by stress-relief
annealing, further plastic deformation, or by relaxation over a period of time.