TOOLED THICK COMPOSITES by ARVEN H. SAUNDERS III ...

3.7 Viscosity
Viscosity is the property of a fluid to flow when pressure is applied. Numerous resin
viscosity models have been proposed, including that by Ciriscioli and Springer. The model will
utilize the Ciriscioli viscosity model for both composite materials used in this research:
V
= V
m
e
( E / RT + KV α )
where V is viscosity in Pa-secs, and Vm, Ev/RT, and KV are material constants.
3.8 Gutowski Effective Stress and Resin Pressure
v
Gutowski [1986,1997] explored the relationship between the applied pressure and the
resin pressure during consolidation of a laminate. The applied pressure was found to be shared
between a stress that compacts the fibers and the resin pressure. The aggregate of all the
fibers constitute a fiber bed, able to carry a substantial load as the fibers are increasingly
packed together. At any time the total through the thickness stress σ is shared by the fiber bed
and the resin, maintaining the following equilibrium equation:
σ = σ +
where σ is the effective stress in the fiber bed and P is the resin pressure. The effective stress
is given by
σ =
30
P
V f
( −1)
V0
As
1 1
( − )
V V
where As is the fiber stiffness, Vf is the current fiber volume fraction, V0 is the fiber volume
fraction at t=0, and Va is the maximum allowable fiber volume fraction. The effective stress
applied to the fiber bed is a function of the fiber volume fraction Vf, thus the ability of the fiber
bed to sustain a load increases like an increasingly stiff spring. When pressure is applied
initially, Vf is at the low end of its range, and all applied pressure is applied to the resin. As
consolidation continues, the resin pressure and resulting flow increase the Vf so an increasing
f
a
4