where: [[epsilon].sub.ctot](t) and [[epsilon].sub.rvis](t) are the total strain under an applied load and the strain recovery, respectively, q[[epsilon].sub.c](t)and [[epsilon].sub.r](t) are the creep strain and the remaining contribution from viscoelasticity at time t, [[epsilon].sub.i] and [[epsilon].sub.f] are instantaneous strain
and viscous flow, respectively, [[beta].sub.c] and [[beta].sub.r] are the shape parameter for creep and recovery, respectively, and [n.sub.c] and [n.sub.r] parameters are the typical life for creep and recovery, respectively .
is the pure specific creep function, involving the partly recovered instantaneous strain
Table 2 lists the increment values of total strain (comprising instantaneous strain
and rheological strain) under each level of axial load.
gradually increases with temperature and elastic modulus decreases with temperature.
For example, if we increase the stress from 1000 to 2000 psi, the instantaneous strain
increases by 0.19%.
The complete total energy balance is given by (Symans et al, 2008) [E.sub.1] = [E.sub.s] + [E.sub.k] + [E.sub.D] + [E.sub.H] where, at a given instant in time, t, [E.sub.1] = cumulative input energy; [E.sub.s] = instantaneous strain
energy stored by the structure; [E.sub.k] = instantaneous kinetic energy of the moving mass; ED= cumulative viscous damping energy; [E.sub.H] = cumulative hysteretic energy dissipation.
Under normal condition of loading, the instantaneous strain
at the application of load includes not only the elastic strain but also some creep.
If an elastomer part is subjected to instantaneous strain
(at time t = O) and held constant for a long time (as in the case of many press-fit bushings), the corresponding stress in the stress-time graph decreases with time from its instantaneous value, until it reaches some non-zero value.
Both axial instantaneous strain
and steady strain of the specimen were increased with compression stress; the residual strains were also appeared increase trend with increasing of former stress levels.
where [[epsilon].sub.0] is the instantaneous strain
, a and b are material constants, and the other variables are as for Eq.
It is convenient to include instantaneous strain
in the relation since the instantaneous strain
due to application of load and the subsequent creep could not easily be separated from one another.
Much of the disagreement in the literature regarding a material's behavior at times shorter than [[tau].sub.k] stems from the mechanical issues inherent in a rheometer's imposition of an instantaneous strain
. Three main issues are the focus of much of the discussion.