Taha Selman Cakir
030070023
13th week

Fluoropolymer:

The fluoropolymer family consists of polymers produced from alkenes in which one or more hydrogens have been replaced by flourine. The most important members of this family are polytetraflouroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), poly (vinyl fluoride) (PVDF); copolymers of tetrafluoroethylene with ethylene, perfluoropropyl vinyl ether, and perfluoropropene;and the copolymer of ethylene and chlorotrifluoroethylene. The fluoropolymers are obtained mainly by suspension polymerization; emulsion polymerization is also practiced. Commercial polymerization in supercritical carbon dioxide (scCO2) was achieved by DuPont in 2002 for the copolymerization of tetrafluoroethylene with hexafluoropropene and a perfluoroalkyl vinyl ether. The molecular weights of the polymers are high, ranging up to 10^5-10^6 for PTFE, apparently due to the lack of appreciable chain-transfer reactions and the precipitation of growing radicals (leading to greatly decreased termination).

(Odian G. G., Principles of polymerization, Ed. 4th, p. 309)

Aramid:

Aramid was of the first commercial polymeric fibres in which high strength and rigidity were achieved by chain alignment. It is an aromatic polyamide called poly(para-phenylene terephtalamide), with the chemical formula:

It is also known by the tradename Kevlar given by the DuPont Company to its aramid fibre. The aromatic rings provide the molecule with rigidity, and in the production process these stiff molecules are aligned parallel to the fibre axis, thus leading to a modulus of elasticity which can be as high as 130 GPa. A fibre consists of planar sheets of molecules linked together by hydogen bonding. The sheets are stacked together radially to form the fibre. The bonds between the sheets are weak, and therefore the fibre has a low longitudinal shear modulus and poor transverse properties. The properties of the fibre, and in particular the modulus of elasticity, depend on the degree of alignment achieved during production, and therefore aramid fibres can be of different qualities. Aramid, because of its stable chain structure, is reasonably resistant to temperature compared to many other syntetic fibres. However, at temperatures higher than 300° C, the fibre may lose most of its strength, and will also creep considerably. Such effects require special evaluation of the composite, especially from the point of view of fire resistance.

Bentur A., Mindess S., Fibre reinforced cementitious composites, Ed. 1st, p.358)

Reverse Redrawing:

Instead of conventional second draw, reverse (inside-out) redrawing can also be used. In reverse drawing, the pre-formed cup is brought down to the next diameter reduction by being turned inside-out. After reverse redrawing the inner walls of the drawn cup become outer walls.

(Tschätsch H., Metal forming processes, 2006, p. 161)

Critical Resolved Shear Stress:

When a single crystal specimen is stressed in tension, shear stress is termed as resolved shear stress. Let Φ represent the angle between the normal to the slip plane and the applied stress direction and λ be the angle between the slip and stress direction. If the applied force is F, the force operating in the slip direction is given by Fcosλ the projection of specimen crosss sectional area on the slip plane gives an area of A/cosΦ

Resolved shear stress is given by

τ = Fcosλ/(A/cosΦ) = σcosλcosΦ

When the resolved shear stress on the slip plane in the slip direction reaches a required level, the slip begins in the crystal. This is called critical resolved shear stress and is given by

τ_{c =} σ_c cosλcosΦ

where σ_{c is the applied stress necessary to produce the plastic deformation. This is known as Schmid's law.}

(Srinivasan, Engineering materials and metallurgy, p. 32)