20.2 A ring-shaped seal, made from a viscoelastic material, is used to seal a jo
ID: 592602 • Letter: 2
Question
20.2 A ring-shaped seal, made from a viscoelastic material, is used to seal a joint between two rigid pipes. When incorporated in the joint, the seal is held at a fixed compressive strain of 0.2. Assuming that the seal can be treated as a Maxwell model, determine the time before the seal begins to leak under an internal fluid pressure of 0.3 MPa. It can be assumed that the relaxation time of the material is 300 days and the short-term (instantaneous) modulus of the material is 3 MPa. A viscoelastic polymer that can be assumed to obey the Boltzmann superposition principle is subjected to the following deformation cycle. At a time, t=0, a tensile stress of 20 MPa is applied instantaneously and maintained for 100 s. The stress is then removed at a rate of 0.2 MPa s1 until the polymer is unloaded. If the creep compliance of 20.4 the material is given by 4-exp) to where Fo= 200 s determine (a) The strain after 100s (before the stress is reversed) (b) The residual strain when the stress falls to zero.Explanation / Answer
Solution:
Use formula for maxwell solid: S(t) =S(0) exp(-t/T0)
given S(t) =0.3
S(0)= Modulus*strain =3MPa*0.2 =0.6MPa
0.3 =0.6*exp(-t/T) where T =300 days
-0.693 = t/300
t= 208 days approx/
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