Strength degradation mechanisms

Bhatt RT, Hull DR (1998) Strength-degrading mechanisms for chemically-vapor-deposited SCS-6 silicon carbide fibres in an argon environment. J Am Ceram Soc... 

Strength, particularly where the material will be subjected to fatigue loading or is susceptible to degrading mechanisms such as hydrogen-induced cracking ... 

Composite materials, such as glass, carbon and aramid reinforced plastics, are principally used for their mechanical rigidity and strength. The most significant degradation mechanisms are therefore also mechanical or have mechanical consequences. Even when... 

Much recent ongoing work has focused on the chemical degradation mechanisms and revealed that radical attack is the root cause of the membrane decomposition. However, there is still a lack of fundamental understanding of the mechanisms for the degradation of mechanical strength, which is related to the membrane failure mode responsible for the sudden death behavior of fuel cells. 

For example, assume a mechanical strength indicator, in this case the strain-to-failure of membrane, decreases from about 100% for the control samples to about 4% after chemical degradation. The exposure time is taken to be about 48 h. For this degradation mechanism, we assume that the degradation is linear with time as a first-order approximation and write the strain-to-failure (ef) as... 

Gong, J.H., Guan, Z.D., Jiang. D.C. (1992), Analysis of strength degradation of indented specimens due to thermal shock , in Bradt, R.C., Hasselman, D.P.H., Munz, D., Sakai, M., Shevchenko, V.Y., Fracture Mechanics of Ceramics, Plenum Press, New York, 605-610. 

Both benzoxazole hydrolysis and hydrolysis of intermediate products such as benzamide or benzoate potentially cause tensile strength degradation in PBO, by disrupting the conjugated rigid rod stmcture and by chain scission, respectively. The former mechanism appears to be the primary mode of degradation in the laboratory-aged armor the latter mechanism appears to be predominant in the case of the failed field body armor. 

Apart from these three degradation mechanisms, rearrangements of the fractions formed may take place. A polymer does not undergo only one pyrolysis route always, but multiple routes may be taken simultaneously. The type of reaction is totally governed by the strength of bonds in the molecules. The lowest energy path will be favored. 

The solubility of the drug in the ointment base is approximately 0.05% w/w. It is present both in solution and in suspension in the higher concentration ointments. The relative proportions in the solution and suspension states depend on the ointment strength. Various reactions will be occurring simultaneously, especially if the degradation mechanism differs between the solution and suspension phase. 

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