1) Polyvinylchloride(PVC)(material)(new)(better)
(plasticisers) to give a tough flexible material not unlike rubber in its properties and the way in which it
could be processed. The large market thus developed soon produced reductions in the cost of the VCM
monomer and the manufacturing process so that it was natural for PVC manufacturers and users to consider
its use for unplasticised applications. PVC in its unplasticised state when correctly fabricated is capable of
producing tough rigid articles of excellent surface appearance and transparency if desired. However, it is
thermally labile and will decompose quite rapidly at or near the temperatures needed to melt the PVC
sufficiently to produce the finished articles. Hence, before the full commercial potential of PVC could be
realised, much progress had to be made improving the basic heat stability of the PVC, developing improved
heat stabilisers and process formulations, and developing equipment capable of fabricating the required
finished products. As a result of this successful development work, it is now possible to produce rigid, i.e.
unplasticised, articles of very considerable commercial importance (pipes, profiles, etc.) which are in
widespread use. Even so, their performance is limited necessarily by the basic properties of PVC. Some of
these basic properties are listed in Table 4.1 for the narrow range of molecular weight (expressed as K-
value) used for rigid applications. Attempts to widen this property range by molecular weight change lead
either to an unprocessable material (at K-values above 70) or a brittle material without the most desired
property of PVC, its toughness (at K-values below 50).
These basic properties of rigid PVC can be extended quite considerably
Polyvinylchloride(PVC)(old)
This is one of the most widely used of all plastics. With the resin mixed with stabilizers, lubricants, fillers, pigments, and plasticizers, a wide range of properties is possible from flexible to hard types, in transparent, opaque, and colored forms. It is tough, strong, with good resistance to chemicals, good low-temperature characteristics and flame-retardant properties. PVC does not retain good mechanical performance above 80°C. It is used for electrical conduit and trunking, junction boxes, rainwater pipes andgutters, decorative profile extrusions, tanks, guards, ducts, etc.
(Frank Kreith, Mechanical Engineering Handbook CRC Press LLC, 1999, pg. 12-28)
2) Vacuum Thermoforming(manufacturing)(new)(better)
Straight vacuum forming utilizes atmospheric pressure to force the heated sheet against the mold
surface where it cools. Although this force is quite limited, about 15 PSI maximum, this is the most
common process used for high volume thin gage products. In this process the heated sheet is placed
over a cavity mold. Contact is made between the sheet and the mold creating a seal. The air in the
cavity is evacuated and atmospheric pressure forces the sheet against the contours of the cavity. Most
vacuum forming machines include a surge tank which is first evacuated so the forming can occur
very quickly in the process.
Fundamentals of Plastics Thermoforming, Peter W. Klein, p:27
Vacuum thermoforming (15:36) (old):
Vacuum forming is a thermoforming process that forms thermoplastic sheets into three-domentional shapes through the application of heat and vacuum. During the vacuum thermoforming process, plastic material is heated (170° C to 200° C) until it becomes pliable, and then is placed over a mould of the requisite shape and drawn in by a vacuum until it takes on the desired shape. The application of a vacuum draws out the air between the mould and the sheet so that the plastic conformsto the mould exactly. This is accomplished through venting holes in the mould that are joined to vacuum lines. The mould also has an integrated water-cooling system that brings the temperature of the plastic to the set temperature needed. Once the curing temperature is reached after the part is formed, air flows back into the mould and seperates the new part from the mould.
(Rao, Manufacturing Technology Volume 1, Ed. 3rd, p. 474)
3) Gray cast iron(material)(new)(better)
Gray cast iron is the most commonly used cast iron and is the least
expensive. It is the easiest to cast and machine. The tensile strength of gray
cast iron ranges from 155 to 400 N/mmz (10 to 26 tonlin.’). The tensile
modulus ranges from 70 to 140 kN/mmZ and the hardness from 130 to
300 DPN.
In nearly all standards for gray cast iron the grades are designated according
to the tensile strength, not composition. In the British standard BS1452, for
example, there are seven grades from 155 to 400 N/mmZ (10 to 26 tonf/h2).
This is the tensile strength measured on a test bar having a diameter of
approximately 30 mm (1.2 in.). The actual strength of a casting will differ
from that of the test bar according to the cross-sectional area (Table 3.2).
Castings are designed to be loaded in compression because the compressive
strength of gray iron is about three times that of its tensile strength.
The recommended maximum design stress in tension is onequarter the
ultimate tensile strength (for cast irons a value up to 185 N/mm2 (12 tonf/
in?)). The fatigue strength is one-half the tensile strength. Notched
specimens show the same value as unnotched specimens. For 220 N/mm2
(14 tonf/h2) grades and above, the fatigue strength of unnotched specimens
is approximately one-third the tensile strength. There is some notch
sensitivity, although much less than is found in steel.
MATERIALS SELECTION DESKBOOK, Nicholas P. Cheremisinoff, Ph.D. , p:55
Gray Cast Iron(old)
Gray cast iron is the most widely used of all cast irons. Pieces of gray cast iron are usually cast ins and molds and then allowed to cool inthe mold. As a rule, if you come across somecast iron, chances are you’re looking at gray cast iron. Why it is called gray cast iron? You guessed it: the fractured metal looks gray.
All gray cast irons contains graphite in the form of flakes. For the most part, gray cast irons aren’t ductile, which means they break instead of bending and elongating. The tensile strength of a gray cast iron can range from 20,000 pounds per square inch to as much as 55,000. (Tensile strength is the amount of force you can apply to something before it tears apart or break) You can weld gray cast iron, and it still retains all its properties.
You can findd gray cast ironin all kinds of common everyday items, from internal combustion engines( especially diesel engines) to pump housings to teh cast iron cookware that, in this welder’s humble opinion, is an absolute must if you are trying to make the perfect batch of cornbread.
(Steven Robert Farnsworth, Welding for Dummies, p. 218)
4) Low Carbon Steels (Mild Steel)(material)(new)(better)
Mild steel (<0.25% carbon) is the most commonly used, readily welded
construction material, and has the following typical mechanical properties
(Grade 43A in BS4360; weldable structural steel):
0 Tensile strength, 430 N/mrn2
0 Yield strength, 230 N/m2
0 Elongation, 20%
0 Tensile modulus, 210 kN/mm2
Hardness, 130 DPN
No one steel exceeds the tensile modulus of mild steel. Therefore, in
applications in which rigidity is a limiting factor for design (e.g., for
storage tanks and distillation columns), high-strength steels have no advantage
over mild steel. Stress concentrations in mild steel structures are relieved by
plastic flow and are not as critical in other, less-ductile steels.
Low-carbon plate and sheet are made in three qualities: fully killed
with silicon and aluminum, semikilled (or balanced), and rimmed steel.
Fully killed steels are used for pressure vessels. Most general-purpose
structural mild steels are semikilled steels. Rimming steels have minimum
amounts of deoxidation and are used mainly as thin sheet for consumer
applications.
The strength of mild steel can be improved by adding small amounts (not
exceeding 0.1 %) of niobium, which permits the manufacture of semikilled
steels with yield points up to 280 N/mmz. By increasing the manganese
content to about 1.5% the yield point can be increased up to 400 N/mm2.
Tlus provides better retention of strength at elevated temperatures and
better toughness at low temperatures.
MATERIALS SELECTION DESKBOOK, Nicholas P. Cheremisinoff, Ph.D. , p:62
Low carbon steels(old)
Low carbon steels, steels that contain less than 0.25% C, make up the highest tonnage of all steels produced in a given year. Structural shapes and beams for building and bridges, plate for line pipe, and automotive sheet applications are just a few major applications for low carbon steels. These applications are driven by manufacturing requirements for good formability and weldability, and performance requirements of good combinations of strengths and fracture resistance for given applications. While early approaches to design of steel structures involved increasing section size of low-strength, low-carbon steels to increase load carrying capacity, recent approaches have been based on developing low carbon steel microstructures of higher strength in order to reduce section size and weight. Higher strengths are increasingly produced in steels with lower and lower carbon contents, an approach that improves formability, weldability, and toughness or fracture resistance. As a result, the last two decades of the twentieth century have seen dramatic changes in the compositions of low carbon steels, theri strength, ductility, and toughness, and the processing approaches for their manufacture.
(George Krauss, Steels: Processing, Structure, and Performance, p.217)
5) Semiconductor(new)(better)(material)
A semiconductor is usually defined rather
loosely as a material with electrical resistivity lying in the range of 10 2
109 ø cm.1 Alternatively, it can be defined as a material whose energy gap (to
be defined more precisely in Chap. 2) for electronic excitations lies between
zero and about 4 electron volts (eV). Materials with zero bandgap are met-
als or semimetals, while those with an energy gap larger than 3 eV are more
frequently known as insulators. There are exceptions to these definitions. For
example, terms such as semiconducting diamond (whose energy gap is about
6 eV) and semi-insulating GaAs (with a 1.5 eV energy gap) are frequently
used. GaN, which is receiving a lot of attention as optoelectronic material in
the blue region, has a gap of 3.5 eV.
The best-known semiconductor is undoubtedly silicon (Si). However, there
are many semiconductors besides silicon. In fact, many minerals found in na-
ture, such as zinc-blende (ZnS) cuprite (Cu2O) and galena (PbS), to name just
a few, are semiconductors. Including the semiconductors synthesized in labo-
ratories, the family of semiconductors forms one of the most versatile class of
materials known to man.
Semiconductors occur in many different chemical compositions with a
large variety of crystal structures. They can be elemental semiconductors,
such as Si, carbon in the form of C60 or nanotubes and selenium (Se) or
binary compounds such as gallium arsenide (GaAs). Many organic com-
pounds, e. g. polyacetylene (CH)n, are semiconductors. Some semiconductors
exhibit magnetic (Cd1 xMnxTe) or ferroelectric (SbSI) behavior. Others be-
come superconductors when doped with sufficient carriers (GeTe and SrTiO3).
Many of the recently discovered high-Tc superconductors have nonmetallic
phases which are semiconductors. For example, La2CuO4 is a semiconductor
(gap 2 eV) but becomes a superconductor when alloyed with Sr to form
(La1 xSrx)2CuO4..
Fundamentals of SemiconductorsPhysics and Materials Properties, PeterY.Yu
Manuel Cardona, p:1
SEMICONDUCTOR(old)
A semiconductor material has a resistivity lying between that of a conductor and that of an insulator. However, in contrast to the granular materials used for resistors, a semiconductor establishes its conduction properties through a complex quantum mechanical behavior within a periodic array of semiconductor atoms, i.e., within a crystalline structure. For appropriate atomic elements, the crystalline structure leads to a disallowed energy band between the energy level of electrons bound to the crystal's atoms and the energy level of electrons free to move within the crystalline structure (i.e., not bound to an atom). This \energy gap" fundamentally impacts the mechanisms through which electrons associated with the crystal's atoms can become free and serve as conduction electrons. The resistivity of a semiconductor is proportional to the free carrier density, and that density can be changed over a wide range by replacing a very small portion (about 1 in 10^6) of the base crystal's atoms with di®erent atomic species (doping atoms). The majority carrier density is largely pinned to the net dopant impurity density. By selectively changing the crystalline atoms within small regions of the crystal, a vast number of small regions of the crystal can be given different conductivities. (Semiconductor Materials S. K. Tewksbury)
SUN KINK: Under extreme heat or heat fluctuations, rail expansion can cause rail buckling or a sun kink. The buckling occurs due to the expansion as a result of high rail temperatures. Buckling causes the track to shift laterally and sometimes vertically, resulting in a deformation that deviates from the normal track alignment. Buckling usually occurs in the afternoon and early evening hours, over the course of a hot day when rail stresses are highest. (Heat Order Issues Technical Memorandum December 15, 2008 Prepared by: Virginia Department of Rail and Public Transportation)
yeni kurallara göre aynı kişiden en fazla 2 kelime yapılabiliyormuş.
ReplyDeletetanımlar düzeltildi
Delete