Viscoelasticity from Mechanical Point of View
The classical theory of elasticity deals with the properties of elastic solids. Hook’s law is applied where stress is always directly proportional to strain in small deformation but independent of the rate of strain.
The elastic solid has a definite shape and is deformed by external forces into a new equilibrium shape. On removal of these external forces it reverts exactly to its original form.
The solid stores all the energy that it obtains from the work done by the external forces during deformation. This energy is then available to restore the body to its original shape when these forces are removed.
The classical theory of hydrodynamics deals with properties of viscous liquid. Newton’s law is applied where the stress is always directly proportional to the rate of strain but independent of strain itself. A viscous liquid, on the other hand, has no definite shape and flows irreversibly under the action of external forces.
If both strain and rate of strain are infinitesimal, system may exhibit behavior, which combines liquid like and solid like characteristics; Materials whose behavior exhibits such characteristics are called viscoelastic.
It is difficult to classify polymers as particular types of materials such as a glassy solid or a viscous liquid, since their mechanical properties are so dependent on the conditions of testing, e.g. the rate of application of load, temperature, amount of strain.
A polymer can show all the features of a glassy brittle solid or an elastic rubber or a viscous liquid depending on the temperature and time. At low temperatures, or high frequencies of measurement, a polymer may be glass-like with young’s modulus of (109—1010) N/m 2 and will break or flow at strains greater than (5%).
At high temperatures or low frequencies, the same polymer may be rubber-like with a modulus of (106—107) N/m2, withstanding large extensions (approximately 100%) without permanent deformation. At still higher temperatures, permanent deformation occurs under load, and the polymer behaves like a highly viscous liquid.
In an intermediate temperature or frequency range, commonly called the glass transition range, the polymer is neither glassy nor rubber-like and it shows an intermediate modulus.
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 .
الرجوع الى لوحة التحكم
|