1.5Visco-elastic fluid behaviour
In the classical theory of elasticity, the stress in a sheared body is directly
proportional to the strain. For tension, Hooke’s law applies and the coefficient
of proportionality is known as Young’s modulus, G
where dx is the shear displacement of two elements separated by a distance
dy. When a perfect solid is deformed elastically, it regains its original form on
removal of the stress. However, if the applied stress exceeds the characteristic
yield stress of the material, complete recovery will not occur and ‘creep’ will
take place–that is, the ‘solid’ will have flowed.
At the other extreme, in the Newtonian fluid the shearing stress is proportional
to the rate of shear, equation (1.1). Many materials show both elastic
and viscous effects under appropriate circumstances. In the absence of the
time-dependent behaviour mentioned in the preceding section, the material is
said to be visco-elastic. Perfectly elastic deformation and perfectly viscous
flow are, in effect, limiting cases of visco-elastic behaviour. For some materials,
it is only these limiting conditions that are observed in practice. The
elasticity of water and the viscosity of ice may generally pass unnoticed! The
response of a material depends not only its structure but also on the conditions
(kinematic) to which it has been subjected; thus the distinction between ‘solid’
and ‘fluid’ and between ‘elastic’ and ‘viscous’ is to some extent arbitrary and
subjective
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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