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1-Introduction:
Durability assessment should be an integral part of the engineering design process with elastomers. All polymers, and elastomers in particular, are potentially sensitive to the temperatures, fluids, and mechanical conditions they are likely to encounter in service, and they can undergo changes in properties large enough to cause failure. This sometimes surprises engineers whose professional training in materials science has been restricted to metals and may lead to the false conclusion that polymer engineering components are always unreliable. Most of these changes are understood scientifically and can be predicted if the material has been adequately characterized.
Durability then is defined here as the resistance to any change in properties due to the service environment. Elastomers vary widely in their resistance to specific environments, depending on the material composition. The detailed composition and microstructure of elastomeric materials, which are essentially created during the manufacture of the component, depend on both the ingredients selected for the compound and the forming process used. This again is different from metals, where components are generally formed from materials whose composition is not substantially changed by the forming process. There is at present no accepted standardization of elastomer compounds, and these formulations are usually proprietary secrets of component manufacturers.
In practice, assessment of durability is often qualitative and derived from simply cataloguing changes in standard test properties. Although this does give some guidance, and some such information is included in this chapter, it is nevertheless inadequate for critical engineering components with demands for quantitative life assessment. Such life assessment requires first identifying the properties that will determine the function of the component and then defining acceptable limits within which they may change. Tests may then be devised to measure rates of change in these properties, and these rates may be used for quantitative life prediction.
One of the most important changes that can limit component durability is the growth of cracks in the material. These normally arise due to oscillating or static mechanical conditions of service. Cracks may grow in any mode of deformation, and growth rates may be either slow and stable, or catastrophic. Use of fracture mechanics is the most scientific approach to the characterization of fatigue life, defined by the growth of cracks.
In addition to crack growth, other physical and chemical processes may cause elastomer properties to change with time in service environments. Some of these processes, such as physical creep and stress relaxation and crystallization, are in principle reversible and should not cause permanent damage.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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