Mechanical degradation stress effect

Mechanical degradation describes the breakdown of molecules in the high flow rate region close to a well as a result of high mechanical stresses on the macromolecules. This short-term effect is important only in the reservoir near the wellbore (and also in some of the polymer handling equipment, in chokes, and so on). 

Mechanical degradation of polymer is much more severe at higher flow rates, longer flow distances, and lower brine permeabilities of porous media. In a lower-permeability porous medium, the average pore throat diameter is smaller, and the stress acting on the polymer is larger. Thus, it is more probable for the polymer chains to be broken and the viscosity to be more heavily reduced. Similarly, we can understand the effects of flow rate and flow distance. 

It is accepted knowledge that the mechanical property of materials is subject to the influence of the environment in which the materials stand. The susceptibility of materials to corrosion is also affected by the mechanical stress. These synergetic effects of corrosion and mechanical degradation are of practical importance in the industrialized society and are among the most critical issues for corrosion science. 

Whereas at low temperatures, elements in substitutional solid solution are supposed to contribute to the athermal component of flow stress, and interstitial elements to the thermal barriers, as the temperature rises all alloying species become more or less mobile and associate themselves with atmosphere effects to extents that depend on solute-atom dif-fusivities [Ros73]. Chemical effects such as oxidation and hot-salt stress corrosion may limit the service temperature of a titanium alloy in some applications in others, mechanical degradation such as high-temperatrue creep will limit the service-temperatiue range. 

It is often difficult to investigate mechanical degradation mechanisms experimentally, because the effects of shear and heat input due to shearing cannot be separated, and the low heat conductivity of the melt impairs uniform temperature distribution. Moreover, increased temperatures lead to an increase in thermal and thermal-oxidative degradation as well as to a simultaneous reduction in viscosity, and thus to more rapid relaxation and lower influence of the exerted stresses on degradation [20]. 

Initiation of polyolefin mechanical degradation increases with the applied stress [33], yet the effect is pronounced if a limit of stress is exceeded and for a deformation of 5-12%. 

H2 embrittlement is a consequence of H2/metal interactions. It has been established that severe mechanical degradation occurs and is manifested in a decrease of fracture resistance. From a materials-classification standpoint, one may distinguish between microstructurally stable materials, in which the metal and solute H2 interact, and those which require attention to phase stability. Stress-induced hydride formation and cleavage mechanism is the main H2 embrittlement mechanism for pure-metal H2 permeation membranes. The H2 permeation metals Pd, IVB (Ti, Zr, Hf) and VB (V, Nb, Ta) can all easily form hydride when exposed to H2 at relatively low temperatures (Buxbaum and Markerb, 1993 Buxbaum and Kinney, 1996). Alloying with other metals can stabilize the structure and improve the embrittlement effect (Yamaura et al, 2004 Yukawa et al, 2008). 

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