6.3 Perzyna Hardening Model
377
σ
(, vp
) = ∂ ψ(, vp , hi , hk ) = E [ − vp ],
(6.211a)
σ
vp (, vp
) = ∂ vp ψ(, vp , hi , hk ) = −E [ − vp ],
(6.211b)
σ
hi (
hi
) = ∂ hi ψ(, vp , hi , hk ) = H hi
,
(6.211c)
σ
hk (
hk ) = ∂ hk ψ(, vp , hi , hk ) = K hk
.
(6.211d)
Note that the total stress σ applied to the rheological model (that enters the equilibrium condition) coincides identically with the energetic stress, σ
≡ σ, and, due
to the serial arrangement of the elastic spring and the mixed-hardening viscous frictional slider, also with the negative of the energetic visco-plastic stress, −σ
vp ≡ σ.
Furthermore, for the specific Perzyna mixed hardening model the (total) dissipation potential π consists of a convex and smooth viscous contribution, the viscous
dissipation potential π v , together with a convex and non-smooth plastic contribution,
the plastic dissipation potential π p , i.e.
π(˙ vp , ˙
hi , ˙
hk ) = π v (˙ vp ) + π p (˙ vp , ˙
hi , ˙
hk ).
(6.212)
Thereby the viscous and plastic contributions π v and π p to the (total) dissipation
potential π are chosen as
π v (˙ vp ) =
1
2
η |˙ vp |
2
(6.213)
π p (˙ vp , ˙
hi , ˙
hk ) = [σ y + H hi ] |˙ vp | − H hi ˙
hi + K hk [˙ vp − ˙
hk ].
Observe that (i) π = π v + π p does not depend on ˙
, thus the dissipative stress
σ
= σ − σ
≡ 0 vanishes identically, and that (ii) π v is positively homogenous of
degree two in ˙
vp and obviously smooth at the origin ˙
vp = 0, and that (iii) π p is
positively homogenous of degree one in {˙ p , ˙
hi , ˙
hk } and obviously non-smooth at
the origin {˙ p , ˙
hi , ˙
hk } = {0, 0, 0}. As a consequence of the additive structure of
the (total) dissipation potential the dissipative visco-plastic stress σ
vp consists of a
viscous contribution, the dissipative viscous overstress σ
v , and a plastic contribution,
the dissipative plastic stress σ
p , i.e.
σ
vp (˙ vp , ˙
hi , ˙
hk ) = σ
v (˙ vp ) + σ
p (˙ vp , ˙
hi , ˙
hk ).
(6.214)
Thereby the dissipative viscous overstress σ
v computes as partial derivative of the
viscous dissipation potential with respect to its conjugated variable
σ
v (˙ vp ) = ∂ ˙
vp π v (˙ vp ) = η ˙
vp ,
(6.215)
whereas the dissipative plastic stress σ
p and the dissipative isotropic- and kinematichardening stresses σ
hi and σ
hk compute as some sub-derivatives of the plastic dissipation potential with respect to their conjugated variables
377
σ
(, vp
) = ∂ ψ(, vp , hi , hk ) = E [ − vp ],
(6.211a)
σ
vp (, vp
) = ∂ vp ψ(, vp , hi , hk ) = −E [ − vp ],
(6.211b)
σ
hi (
hi
) = ∂ hi ψ(, vp , hi , hk ) = H hi
,
(6.211c)
σ
hk (
hk ) = ∂ hk ψ(, vp , hi , hk ) = K hk
.
(6.211d)
Note that the total stress σ applied to the rheological model (that enters the equilibrium condition) coincides identically with the energetic stress, σ
≡ σ, and, due
to the serial arrangement of the elastic spring and the mixed-hardening viscous frictional slider, also with the negative of the energetic visco-plastic stress, −σ
vp ≡ σ.
Furthermore, for the specific Perzyna mixed hardening model the (total) dissipation potential π consists of a convex and smooth viscous contribution, the viscous
dissipation potential π v , together with a convex and non-smooth plastic contribution,
the plastic dissipation potential π p , i.e.
π(˙ vp , ˙
hi , ˙
hk ) = π v (˙ vp ) + π p (˙ vp , ˙
hi , ˙
hk ).
(6.212)
Thereby the viscous and plastic contributions π v and π p to the (total) dissipation
potential π are chosen as
π v (˙ vp ) =
1
2
η |˙ vp |
2
(6.213)
π p (˙ vp , ˙
hi , ˙
hk ) = [σ y + H hi ] |˙ vp | − H hi ˙
hi + K hk [˙ vp − ˙
hk ].
Observe that (i) π = π v + π p does not depend on ˙
, thus the dissipative stress
σ
= σ − σ
≡ 0 vanishes identically, and that (ii) π v is positively homogenous of
degree two in ˙
vp and obviously smooth at the origin ˙
vp = 0, and that (iii) π p is
positively homogenous of degree one in {˙ p , ˙
hi , ˙
hk } and obviously non-smooth at
the origin {˙ p , ˙
hi , ˙
hk } = {0, 0, 0}. As a consequence of the additive structure of
the (total) dissipation potential the dissipative visco-plastic stress σ
vp consists of a
viscous contribution, the dissipative viscous overstress σ
v , and a plastic contribution,
the dissipative plastic stress σ
p , i.e.
σ
vp (˙ vp , ˙
hi , ˙
hk ) = σ
v (˙ vp ) + σ
p (˙ vp , ˙
hi , ˙
hk ).
(6.214)
Thereby the dissipative viscous overstress σ
v computes as partial derivative of the
viscous dissipation potential with respect to its conjugated variable
σ
v (˙ vp ) = ∂ ˙
vp π v (˙ vp ) = η ˙
vp ,
(6.215)
whereas the dissipative plastic stress σ
p and the dissipative isotropic- and kinematichardening stresses σ
hi and σ
hk compute as some sub-derivatives of the plastic dissipation potential with respect to their conjugated variables