Cyclic flexing

Because of the absence of the methyl pendant group, cto 1,4-polybutadiene is more resistant to abrasion and develops less heat buildup as a result of cyclic flexing than Hevea rubber (polyisoprene NR). Tires made from polybutadiene do not grip the road as well as those made from Hevea elastomers. 

The damping coefficient k,. can be measured from the mechanical loss of tire wall specimen under cyclic flexing (12). It was also shown that the mechanical loss of tire, l.e. the rolling resistance, can be related to the mechanical loss In cyclic flexing (14). Thus, k represents a measure of the tire mechanical loss. 

The development of cardiac assist devices and other instrumentation and components which come in contact with the cardiovascular system requires materials which will perform in a physiological environment. One particularly difficult application is the diaphragm for blood pumps, which requires an elastomer to undergo cyclic deformation and/or flexing while in contact with blood. 

Tyres are very definitely fatigued during use and, as mentioned for fabric/rubber adhesion above, it is very important to carry out dynamic tests to assess bond efficiency. Methods have not apparently been standardised but a variety of procedures have been reported71 79 Some workers have used the same or a similar test piece as in static tests and applied a cyclic tensile stress or strain, whilst others have used some form of fatigue tester operating in compression/shear to repeatedly stress or strain cord/rubber composite, or even to flex samples in the form of a belt. Khromov and Lazareva80 describe a method using test pieces cut from tyres. 

Many types of fatigue tester are used (flexing beams, rotating beams, constant amplitude of cyclic stress or strain, constant rate of increase in amplitude of stress or strain, etc.). 

Dura Flex. [SdioUer] Cyclic nitrogen compd. cellulosic reactant for wash/ wear, crease resistant and shrink control rinishes. 

Refractories in rotary kilns are subject to cyclic mechanical stresses as a result of flexing of the kiln shell as it rotates. They are also exposed to greater thermal shock when the kiln is shut down or re-lit. 

When even higher concentrations of sulfur are used (with the maintenance of constant modulus), flex life decreases. It is possible that this is due to the large amount of cyclic chain modification associated with high levels of sulfur. 

See Fig. 2-4 for the geometrical relationships in a two-dimensional analog of the three-dimensional stress-strain condition. So far the discussion of stress-strain relationships has been on static stress conditions. Cyclical stresses such as are applied in the operation of machine parts, in vibrating structures, and in the rapid flexing of structures that occurs in vehicles produce a different set of stress response problems. 

---