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)
(
F
)
0
;
for
F
0
(15.25)
)
(
F
)
F
;
for
F
!
0
In the above equations K is a material parameter describing its time dependent
behaviour (viscosity).
A visco-plastic analysis proceeds in time steps and a visco-plastic strain increment is
computed at each time step by:
w
' '
w
H
P
H
t
(15.26)
P
t
where '
t
is a time increment. The initial stresses for the computation of the residual are
(15.27)
V
' '
ı D
H
0
P
P
The time increment '
t
cannot be chosen freely but has to satisfy certain stability
conditions to prevent oscillations
5
.
V
V
()
n
'
V
p
V
n-1)
V
V
Figure 15.4
Explanation of the concept of visco-plasticity
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