The parameters determining the characteristics of foams are the ratio of bulk density of foam to that of unfoamed matrix ,and the cellar structure of foam. The foam can be open cell or closed cell or a mixture of the two.
Figure(1) shows typical forms of compressive stress strain graph for polymeric foams. Over the initial straight line part of the graph the cell wall just bend under the action of the applied stress. The next stage is when the walls elastically buckle, often giving a plateau of deformation, the deformation is still elastic and finally the cell walls suffer irrecoverable buckling collapse.
With cushion materials the foam is required to give continually increasing resistance to increasing load and so plateau in stress strain curve is not required.so the form shown in A is required.
Foams used for packaging need to absorb the energy ivolved when packaging droped and so a plateau is highly desirable like that indicate in B.
where Vs the fraction of the bulck volume of the foam that is solid matrix
V_(s=) v_s/(v_s+v_g )
Where v_s is the volume of the foam that is solid
v_g is the volume of the foam that is gas, the bulck density?_fof the foam is
?_f=m_(s+m_g )/(v_s+v_g )
V_s=(v_s ?_f)/(m_s+m_g )
m_g is very small in comperation with m_s
V_s=?_f/?_s
E_f=?_f/?_s
(?_f/?_s )n=
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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