Fluctuating plasticity

The most important remedies to prevent fluctuating plasticity consist therefore of a guarantee of constant dosing and homogeneity conditions in front of the extruder. 

The demand for pipe-coating enamels tends to fluctuate, depending on the success of oil and gas exploration. In the United Kingdom plastic-coated pipes are used on shore. Most production is exported for use in the Middle and Far Eastern regions. 

For convenience, in the previous sections it has been arranged so that the mean stress is zero. However, in many cases of practical interest the fluctuating stresses may be always in tension (or at least biased towards tension) so that the mean stress is not zero. The result is that the stress system is effectively a constant mean stress, a superimposed on a fluctuating stress a a- Since the plastic will creep under the action of the steady mean stress, this adds to the complexity because if the mean stress is large then a creep rupture failure may occur before any fatigue failure. The interaction of mean stress and stress amplitude is usually presented as a graph of as shown in Fig. 2.76. 

A series of utuaxial fatigue tests on unnotched plastic sheets show that the fatigue limit for the material is 10 MN/m. If a pressure vessel with a diameter of 120 mm and a wall thickness of 4 mm is to be made from this material, estimate the maximum value of fluctuating internal pressure which would be recommended. The stress intensity factor for the pressure vessel is given by K = 2hoop stress and a is the half length of an internal defect. 

Plastic s main disadvantages are its lower scratch resistance and, in some systems, comparative intolerance to severe temperature fluctuations. Even if plastic does have less temperature tolerance than glass, most optical systems do not operate in ambient temperatures beyond the thermal limits of plastics or the human body. 

Morphological plasticity and cellular acclimation When plants are exposed to stress as a consequence of resource depletion or climatic fluctuation, many different responses are possible depending upon the species and the nature and severity of the stress. However, as Bradshaw (1965) recognised, stress responses can be classified into two basic types, one of which is morphological and the other physiological. One of the major uses of plant strategy theory is to provide a basis for predicting which of the two mechanisms is likely to be operative in particular species, populations and situations. 

Cations can be seen as acting as ionic crosslinks between polyanion chains. Although this may appear a naive concept, crosslinking can be seen as equivalent to attractions between polyions resulting from the fluctuation of the counterion distribution (Section 4.2.13). Moreover, it relates to the classical theory of gelation associated with Flory (1953). Divalent cations (Zn and Ca +) have the potential to link two polyanion chains. Of course, unlike covalent crosslinks, ionic links are easily broken and re-formed under stress there could therefore be chain slipping and this may explain the plastic nature of zinc polycarboxylate cement. 

The costs, as for all plastics, fluctuate greatly with the crude oil price, and are only given to provide a rough idea. They are generally of the order of 1 per kilogram. 

The costs are generally high and fluctuate greatly with the crude oil price, as for all plastics. 

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