Ahmet Gökay Öztürk 030050143 (5th week)

Liquid-crystal polymers

Liquid crystalline polymers are a kind of polymer that show liquid crystal
phases. They are composed of low molecular mass liquid crystals, which
can be either rod-like or disc-like, or rod- and disc-like together in one. The
constituent blocks may be of very complicated two-dimensional or three
dimensional shape. They may be composed of amphiphilic molecules as
well. According to the way the mesogenic units are incorporated into the
polymers, the liquid crystalline polymers can be classified as main chain liquid
crystalline polymers in which the mesogenic units are connected in the
backbone, or side chain liquid crystalline polymers in which the mesogenic
units are attached to the backbone as side pendants. The mesogenic units
may be incorporated in both ways, that is, a part as the backbone and the
other part as side groups attached to the backbone. This kind of liquid crystalline
polymer is called the combined liquid crystalline polymer.
(Xin-Jiu Wang,Qi-Feng Zhou,LIQUID CRYSTALLINE POLYMERS, pg. 10)


Superplastic Forming


Superplasticity is a property that allows sheet to elongate to quite large strains
without localized necking and rupture. In uniaxial tensile testing, elongations to
failure in excess of 200% are usually indicative of superplasticity. Micrograin superplasticity occurs in some materials with a fine grain size, usually less
than 10 m, when they are deformed in the strain range of 0.00005–0.01/s
at temperatures greater than 0 5Tm, where Tm is the melting point in degrees
Kelvin. Although superplastic behavior can produce strains in excess of 1000%,
superplastic forming (SPF) processes are generally limited to about 100–300%.
The advantages of SPF include the ability to make part shapes not possible
with conventional forming, reduced forming stresses, improved formability with essentially no springback and reduced machining costs. The disadvantages are
that the process is rather slow and the equipment and tooling can be relatively
expensive.
(F.C. Campbell, Manufacturing Technology for Aerospace Structural Materials, pg.51)

CASE HARDENING

Some times special characteristic are required in metal such as hard outer surface and soft, tough and more strength oriented core or inner structure of metal. This can be obtained by casehardening process. It is the process of carburization i.e. saturating the surface layer of steel with carbon or some other substance by which outer case of the object is hardened where as the core remains soft. It is applied to very low carbon steel. It is performed for obtaining hard and wear resistance on surface of metal and higher mechanical properties with higher fatigue, strength and toughness in the core.
(R. SINGH, Introduction to Basic Manufacturing Processes and Workshop Technology, pg.145)

Stress Relieving

Stress relieving is used typically to remove residual stresses which have accumulated from prior
manufacturing processes. Stress relief is performed by heating to a temperature below Ac1 (for
ferritic steels) and holding at that temperature for the required time, to achieve the desired reduction
in residual stresses. The steel is then cooled sufficiently slowly to avoid the formation of excessive
thermal stresses. No phase transformations occur during stress relief processing. Nayar recommends heating to
• 550–650◦C for unalloyed and low-alloy steels;
• 600–700◦C for hot-work and high-speed tool steels.
These temperatures are above the recrystallisation temperatures of these types of steels. Little or no
stress relief occurs at temperatures<260◦C and approximately 90% of the stress is relieved at 540◦C. The maximum temperature for stress relief is limited to 30◦C below the tempering temperature. The results of the stress relieving process are dependent on the temperature and time which are correlated through Holloman’s parameter (P):
P = T (Chj + log t)
where T is the temperature (K), t is the time (h) and Chj is the Holloman–Jaffe constant which is
calculated from:

CHJ = 21.53 − (5.8 × %C)

P is a measure of the ‘thermal effect’ of the process and that processes with the same Holloman’s parameter exhibit the same effect.
Another similar commonly used expression employed in evaluating the stress relief of spring
steels is the Larson–Miller equation:

P = T ( log t + 20)/1000

Stress relieving results in a significant reduction of yield strength in addition to a decrease in
the residual stresses to some ‘safe’ value. Typically heating and cooling during stress relieving is
performed in the furnace, particularly with distortion and crack-sensitive materials. Below 300◦C,
faster cooling rates can be used.
(W. F. Gale, T. C. Totemeier, Smithells Metals Reference Book 8th ed., ch.29, pg.29)