Hyperbolic structure for a simplified model of dynamical perfect plasticity

0.39 MB·

by  Jean-Francois Babadjian

HYPERBOLIC STRUCTURE FOR A SIMPLIFIED MODEL OF DYNAMICAL
PERFECT PLASTICITY
JEAN-FRANC¸OISBABADJIANANDCLE´MENTMIFSUD
6
1
0 Abstract. Thispaperisdevotedtoconfronttwodifferentapproachestotheproblemofdynam-
2 icalperfectplasticity. InterpretingthismodelasaconstrainedboundaryvalueFriedrichs’system
enablesonetoderiveadmissiblehyperbolicboundaryconditions. Usingvariationalmethods,we
n
showthewell-posednessofthisprobleminasuitableweakmeasuretheoreticsetting. Thanksto
a
thepropertyoffinitespeedpropagation,weestablishanewregularityresultforthesolutionin
J
shorttime. Finally,weprovethatthisvariationalsolutionisactuallyasolutionofthehyperbolic

5 formulationinasuitabledissipative/entropicsense,andthatapartialconversestatementholds
1 underanadditionaltimeregularityassumptionforthedissipativesolutions.

]
P
A
1. Introduction
.
h
t Friedrichs’ systems are linear symmetric hyperbolic systems of the form
a
m n
∂ U + A ∂ U =0 in Rn (0,T),
[ t i xi ×
 i=1
1 U(t=0X)=U ,
0
v
3 where U : Rn (0,T) Rm is the unknown of the problem, A1,...,An are symmetric m m
5 matrices, and U× :Rn →Rm is a given initial data. They appear in a number of physical syst×ems
0
8 such as the wave equa→tion or systems of conservation laws. In particular, the system of three-
3
dimensional linearized elasto-dynamics can be put within this framework (see [22]) where U is a
0
vector of size 9 (with three components for the velocity, and six for the symmetric 3 3 Cauchy
.
1 ×
stress),andA ,A ,A areexplicit9 9matricesdependingontheLam´ecoefficientsofthematerial.
1 2 3
0 Problems of continuum mechani×cs are usually settled in a bounded domain Ω Rn, which
6 ⊂
requires to impose a boundary condition. Of course, it is a difficult issue in hyperbolic equations
1
: since the initial condition is transported through the characteristics up to the boundary, where
v
the value of the solution might thus be incompatible with the prescribed boundary data. In other
i
X words,onehastoimposeboundaryconditionsonlyonapartoftheboundarywhichisnotreached
r by the characteristics (see e.g. [9, 28, 24] in the case of scalar conservation laws, or [17] for one-
a
dimensional nonlinear systems). In Friedrichs’ seminal work [20], the following type of boundary
conditions are considered
(A M)U =0 on ∂Ω (0,T), (1.1)
ν
− ×
where A = A (x) := n A ν (x) (ν(x) is the outer normal to Ω at the point x ∂Ω), and
ν ν i=1 i i ∈
M = M(x) is a m m matrix, for x ∂Ω, satisfying the following algebraic conditions (in the
× P ∈
non-characteristic case, i.e., when A is non-singular,see also [26] for more details)
ν
M +MT is non-negative,
Im(A M) Im(A +M)= 0 , (1.2)
 ν ν
− ∩ { }
Rm =Ker(Aν M) Ker(Aν +M).
− ⊕

Date:January18,2016.
1
2 J.-F.BABADJIANANDC.MIFSUD
The fact that the symmetric partof M is supposedto be non-negativeis a way to ensure that the
L2(Ω)-normofthe solutiondecreasesintime, andthusthishypothesisisrelatedtothe uniqueness
of the solution. In other words,the non-negativity of M +MT is connected with the dissipativity
of the equation. The two other assumptions are related to the existence of a solution.
Unfortunately, the previous formulation necessitates to define properly the trace of U on the
boundary, which might not be desirable if one is interested in weak solutions in Lebesgue-type
spaces (in the spirit of e.g. [28, 24] for a L -theory of boundary value scalar conservation laws).
∞
In [26], a generalL2-theory forsuch boundaryvalue Friedrichs’systems has beenintroduced. The
so-called dissipative solutions are defined as functions U L2(Ω (0,T);Rm) satisfying, for all
constant vector κ Rm and all ϕ W1, (Ω (0,T)) with∈ϕ 0,×
∞
∈ ∈ × ≥
T n T
U κ2∂ ϕdxdt+ A (U κ) (U κ)∂ ϕdxdt
| − | t i − · − xi
Z0 ZΩ i=1Z0 ZΩ
X
T
+ U κ2ϕ(0)dx+ Mκ+ κ+ϕd n 1dt 0, (1.3)
0 −
| − | · H ≥
ZΩ Z0 Z∂Ω
where κ+ = κ+(x) is the orthogonal projection of κ onto the linear space Ker(A +M) ImA .
ν ν
∩
As discussed in [26], this formulation naturally discriminates the admissible boundary conditions.
In particular, in this case, the matrix M has to be a non-negative symmetric matrix (see Subsec-
tion3.3). Notethatthis kindofdissipativeformulationisreminiscentinhyperbolicequations(see
e.g. [23]in the caseof scalarconservationslaws). Moreover,the family of functions U U κ2,
where κ Rm, can be thought of as the analogue of the Kruˇzkov entropy functions i7→n [2|3].−Th|e
∈
term dissipative refers to the decreasing character of the L2-norm of the solution which prevents
conservation of the energy, and to a special class of boundary conditions for hyperbolic systems
(see [10] and [26, Section 4]).
A number of mechanical problems, such as in elasto-plasticity or generalized non-newtonian
fluids, involve a convex constraint. For that reason, it becomes relevant to ask whether one can
incoporate convex constraints within a generaltheory of Friedrichs’systems. In [16], this problem
has been addressed in the full space Ω=Rn. The authors define a notion of dissipative solutions
(from which the previous formulation (1.3) in [26] has been inspired) which are shown in [6] to
be the (unique) limit of a sequence of viscosity solutions for a regularized diffusive model where
the constraintispenalized. TheformulationofgeneralconstrainedFriedrichs’systemsinbounded
domains becomes therefore a natural extension. However, there might be non trivial interactions
between the constraint and the boundary condition (see (1.8) below), which makes the problem
difficulttoaddressinitsfullgenerality. Thisisthereasonwhy,inthispaper,wefocusourattention
to the meaningful particular case of dynamical perfect plasticity.
Tobemoreprecise,weconsiderasimplifiedtwodimensionalproblemofanti-planeshearelasto-
plasticity (see Subsection 3.2 for a formal derivation from three-dimensional small strain elasto-
plasticity), where the displacement field u : Ω [0,T] R is scalar valued and the stress σ :
Ω [0,T] R2 is vector-valued. General consi×deration→s of continuum mechanics state that the
× →
equation of motion
u¨ divσ =0 in Ω (0,T)
− ×
must be satisfied. Then, following standard models of perfect plasticity, the stress is constrained
to remain inside a fixed closed and convex set of R2. For simplifity, we assume that
σ 1. (1.4)
| |≤
Moreover,the displacement gradient decomposes additively as
u=e+p,
∇
HYPERBOLIC STRUCTURE FOR A SIMPLIFIED MODEL OF DYNAMICAL PERFECT PLASTICITY 3
where e and p:Ω [0,T] R2 stand for the elastic and plastic strains, respectively. The elastic
× →
strain is related to the stress by means of a linear relation, and, again for simplicity, we set
σ =e. (1.5)
Finally, the plastic variable evolves through the so-called flow rule, which stipulates that
p˙ =0 if σ <1,
| | (1.6)
( pp˙˙ =σ if |σ|=1.
| |
This system must be supplemented by initial conditions on (u,u˙,σ,p) and boundary conditions.
The mathematical analysis of dynamical elasto-plastic models has been performed in [5, 7] (see
also [30, 31, 4, 14] in the static and quasi-static cases).
From the hyperbolic point of view, this problem can be interpreted as a constrainedFriedrichs’
system. Formally it can be put within a hyperbolic formulation of the type (1.3) (see Subsection
3.3) where U =(u˙,σ) R3, and the 3 3 symmetric matrices A , A are given by
1 2
∈ ×
0 1 0 0 0 1
− −
A = 1 0 0 , A = 0 0 0 .
1 2
−   
0 0 0 1 0 0
−
   
The dissipative formulation is exactly given by (1.3), except that the constant vector κ must
belongtothecontraintsetK =R B,whereB istheclosedunitballofR2 (see(1.4)above). The
×
hyperbolic vision of this problem motivates our choice of boundary conditions. It turns out that,
in the unconstrained case (i.e. the wave equation), admissible dissipative boundary conditions in
the sense of (1.1)–(1.2) are all of the form
σ ν+λ 1u˙ =0 on ∂Ω (0,T), (1.7)
−
· ×
for some λ > 0 (see Lemma 3.1). This choice will be a posteriori justified by the fact that the
variationalanddissipativeformulationsareessentiallyequivalent. Notethatthistypeofboundary
condition, is quite unusual in solid mechanics. These are not of Robin type since it involves the
velocity u˙, and not the displacement u. It is closer to Navier’s no-slip boundary condition rather
found in fluid mechanics problems.
Thegoalofthispaperconsiststhusinstudyingthisparticularmodel(inanyspacedimensionand
with a source term) related to dynamical perfect plasticity from both variational and hyperbolic
points of view. First of all, using variational methods we establish a well-posedness result for this
model. To this aim, we regularize the problem by considering a elasto-visco-plastic model where
the constitutive law (1.5) is replaced by a Kelvin-Voigt visco-elastic law
σ =σ˜+ε u˙,
∇
where ε > 0 is a viscosity parameter, and stress constraint (1.4) together with the flow rule (1.6)
are replaced by a Perzyna visco-plastic law
σ˜ P (σ˜)
B
p˙ = − ,
ε
where P denotes the orthogonal projection operator onto B. The equation of motion and the
B
boundary condition are thus to be modified into
u¨ div(σ˜+ε u˙)=0 in Ω (0,T),
− ∇ ×
((σ˜+ε u˙) ν+λ−1u˙ =0 on ∂Ω (0,T).
∇ · ×
The well-posedness of this regularized model is presented in Section 4.
In Section 5, we prove the existence and uniqueness of a (variational) solution for the original
model by means of a vanishing viscosity analysis as ε 0. However, since in the limit, the stress
→
satisfies the constraint σ 1, the original boundary condition (1.7) cannot be satisfied at points
| |≤
4 J.-F.BABADJIANANDC.MIFSUD
of the boundary where u˙ > λ. Therefore, a relaxation phenomenon occurs (see Proposition 5.1)
| |
which implies that the boundary condition (1.7) relaxes as
σ ν+λ−1Tλ(u˙)=0 on ∂Ω (0,T), (1.8)
· ×
where T (z) = min( λ,max(z,λ)) is the truncation of z R by the values λ. It shows an
λ
− ∈ ±
interesting interaction which imposes the boundary condition to accomodate the constraint. Note
that since λ (0,+ ), the important cases of Dirichlet and Neumann boundary conditions are
∈ ∞
prohibited by this formulation. In Subsection 5.5, we show by means of asymptotic analysis that
the Dirichlet (resp. Neumann) boundary condition can be recovered by letting λ 0 (resp.
→
λ + ).
→ ∞
As usual in plasticity, the solution happens to concentrate, leading to a bounded variation
solutionforthedisplacement,andameasuresolutionfortheplastic strain. InSection6,usingthe
property of finite speed propagation, we prove a new regularity result in plasticity which states
that, provided the data are smooth and compactly supported in space, the solution is smooth as
well in short time. The argument rests on a Kato inequality (Proposition 5.12) which states a
comparison principle between two solutions associated to different data. The fact that the data
is compactly supported in Ω together with the finite speed propagation property ensures that, in
short time, the boundary is not reached by the solution so that the boundary condition can be
ignored,andonecanargueasinthefullspace. Toourknowledge,itseemstobethefirstregularity
result in dynamical perfect-plasticity, and its generalization to more general vectorial models will
be the object of a forthcoming work.
Finally,inSection7,weestablishrigorouslinksbetweenthevariationalandhyperbolicformula-
tions. We show that any variational solutions generate dissipative solutions. Conversely,provided
the solution of the hyperbolic problem are smoother in time, variationalsolutions can be deduced
from the dissipative formulation.
2. Mathematical preliminaries
2.1. General notation. If a and b Rn, we write a b for the Euclidean scalar product, and we
denote by a =√a a the associated∈norm. Let B :=·x Rn : x 1 be the closed unit ball in
Rn, and P| |be the o·rthonormalprojection onto B, i.e{., P∈ (σ)=| σ|/≤σ }if σ =0, and P (σ)=0 if
B B B
| | 6
σ =0. It is a standard fact of convex analysis that the function
σ P (σ)2
B
σ | − |
7→ 2ε
is convex, of class 1, and that its differential is given by σ σ P (σ) /ε. In addition, its
B
C 7→ −
convex conjugate is p p +εp2/2, and in particular,
7→| | | | (cid:0) (cid:1)
σ P (σ)
p= − B σ p= p +εp2.
ε ⇐⇒ · | | | |
We write Mn n forthesetofrealn nmatrices,andMn n forthatofallrealsymmetricn n
× × sy×m ×
matrices. Given a matrix A Mn n, we let A := tr(AAT) (AT is the transpose of A, and trA
×
isitstrace)whichdefinesthe∈usualEuclidean|n|ormoverMn n. Werecallthatforanytwovectors
p ×
a andb Rn, a b Mn n stands forthe tensorproduct, i.e., (a b) =a b forall1 i,j n,
× ij i j
and a ∈b:=(a⊗b+∈b a)/2 Mn n denotes the symmetric ten⊗sor product. ≤ ≤
⊙ ⊗ ⊗ ∈ sy×m
2.2. Functional spaces. Let Ω Rn be an open set. We use standard notation for Lebesgue
⊂
and Sobolev spaces. In particular, for 1 p , the Lp(Ω)-norms of the various quantities are
≤ ≤ ∞
denoted by .
p
We writek·k(Ω;Rm) (or simply (Ω) if m=1) forthe space of boundedRadonmeasures inΩ
with values iMn Rm, endowed with tMhe norm µ(Ω), where µ (Ω) is the total variation of the
| | | |∈M
HYPERBOLIC STRUCTURE FOR A SIMPLIFIED MODEL OF DYNAMICAL PERFECT PLASTICITY 5
measureµ. The Lebesgue measure inRn is denoted by n, and the (n 1)-dimensionalHausdorff
L −
measure by n 1.
−
We denotHe by H(div,Ω) the Hilbert space of all σ L2(Ω;Rn) such that divσ L2(Ω). We
∈ ∈
recall that if Ω is bounded with Lipschitz boundary and σ H(div,Ω), its normal trace, denoted
byσ ν, iswelldefinedasanelementofH 1/2(∂Ω). Iffurth∈erσ H(div,Ω) L (Ω;Rn),itturns
− ∞
· ∈ ∩
out that σ ν L∞(∂Ω) with σ ν L∞(∂Ω) σ (see [2, Theorem 1.2]). Moreover, according
to [13, The·ore∈m 2.2], if Ω is ofkcla·ssk2, then≤fokr akll∞ϕ L1(∂Ω),
C ∈
1
lim σ(y εsν(y)) ν(y) (σ ν)(y) ϕ(y)d n−1(y)ds=0, (2.1)
ε→0Z0 Z∂Ω − · − · H
where ν denotes the outer u(cid:0)nit normal to ∂Ω. (cid:1)
ThespaceBV(Ω)offunctionsofboundedvariationinΩismadeofallfunctionsu L1(Ω)such
thatthedistributionalgradientDu (Ω;Rn). Wereferto[1,18,21]foradetailed∈presentation
∈M
of this space. We just recall here few facts. If Ω has Lipschitz boundary, any function u BV(Ω)
∈
admits a trace, still denoted by u L1(∂Ω), such that Green’s formula holds (see e.g. Theorem
∈
1, Section 5.3 in [18]). Moreover,according to [3, Theorem 4], if Ω is further of class 1, for every
C
ε>0, there exists a constant c (Ω)>0 such that for every u BV(Ω),
ε
∈
u L1(∂Ω) (1+ε)Du(Ω)+cε(Ω) u 1. (2.2)
k k ≤ | | k k
Moreover,if Ω is of class 2, one has
C
1
lim u(y εsν(y)) u(y) d n 1(y)ds=0. (2.3)
−
ε 0 | − − | H
→ Z0 Z∂Ω
Conversely, Gagliardo’s extension result (see [21, Theorem 2.16 & Remark 2.17]) states that if
Ω Rn is a bounded open set of class 1, and g L1(∂Ω), for each ε>0, there exists a function
⊂ C ∈
u W1,1(Ω) such that
ε
∈
u =g on ∂Ω,
ε
 uε 1 ε g L1(∂Ω), (2.4)
k k ≤ k k
 uε 1 (1+ε) g L1(∂Ω).
k∇ k ≤ k k
Let us finally mention a variant of the usual approximation result for BV functions [2, Lemma

5.2].
Proposition 2.1. Let Ω Rn be a bounded open set with Lipschitz boundary, u BV(Ω) and
σ L2(Ω;Rn). There exi⊂sts a sequence (u ) W1,1(Ω) such that u ⇀ u weakly∈* in BV(Ω),
j j
D∈u σ (Ω) Du σ (Ω) and u =u on ∂Ω⊂for all j N.
j j
| − | →| − | ∈
2.3. Generalized stress/strain duality. According to [2, Definition 1.4], we define a duality
pairing between stresses and plastic strains as follows.
Definition 2.2. Let u BV(Ω) L2(Ω) be such that Du = e+p for some e L2(Ω;Rn) and
p (Ω;Rn), and let σ∈ H(div,∩Ω) L (Ω;Rn). We define the distribution [σ∈p] (Ω) by
∞ ′
∈M ∈ ∩ · ∈D
h[σ·p],ϕi=h[σ·Du],ϕi− σ·eϕdx for all ϕ∈Cc∞(Ω),
ZΩ
where [σ Du] is given by
·
[σ Du],ϕ = u(divσ)ϕdx u(σ ϕ)dx for all ϕ (Ω).
h · i − − ·∇ ∈Cc∞
ZΩ ZΩ
Remark 2.3. Using an approximation procedure as in [2, Lemma 5.2], one can show that [σ p]
·
is actually a bounded Radon measure in Ω satisying
[σ p] σ p in (Ω).
| · |≤k k∞| | M
6 J.-F.BABADJIANANDC.MIFSUD
A slight adaptation of [2, Theorem 1.9] shows the following integration by parts formula.
Proposition 2.4. Let u BV(Ω) L2(Ω), σ H(div,Ω) L (Ω;Rn) and ϕ W1, (Ω). Then,
∞ ∞
∈ ∩ ∈ ∩ ∈
ϕd[σ Du]+ uϕ(divσ)dx+ u(σ ϕ)dx= (σ ν)uϕd n 1.
−
· ·∇ · H
ZΩ ZΩ ZΩ Z∂Ω
3. Description of the model
3.1. Small strain elasto-plasticity. To simplify the presentation of the model, we consider the
physical three-dimensional case. We assume that the reference configuration of the elasto-plastic
body under consideration occupies the volume R3. In the framework of small strain elasto-
plasticity, the natural kinematic variable is theBd⊂isplacement field u : [0,T] R3 (or the
velocity v := u˙). Denoting by Eu := (Du+DuT)/2 : [0,T] MB3 ×3 the lin→earized strain
B× → sy×m
tensor, small strain elasto-plasticity assumes that Eu decomposes additively as
Eu=e+p, (3.1)
where e and p : [0,T] M3 3 stand for the elastic and plastic strains, respectively. The
B × → sy×m
elastic strain is related to the Cauchy stress tensor σ : [0,T] M3 3 by means of Hooke’s
lawσ =Ce,whereC isthe symmetric fourthorderelastiBcit×ytensor→. Forsey×mxample,inthe isotropic
case, one has
σ =λ(tre)I +2µe, (3.2)
where λ and µ are the Lam´e coefficients satisfying µ > 0 and 3λ+ 2µ > 0. In a dynamical
frameworkandinthe presenceofexternalbody loadsf : [0,T] R3, the equationsofmotion
B× →
are a system of partial differential equations which writes as
u¨ divσ =f in (0,T). (3.3)
− B×
Plasticity is characterized by the existence of a yield zone in the stress space beyong which the
Cauchystress is notpermitted to live. The stresstensor is indeedconstrainedto belong to a fixed
nonempty, closed and convex subset of M3 3. In the case of Von Mises plasticity, the constraint
sy×m
only acts on the (trace free) deviatoric stress σ :=σ (trσ)I/3, and reads as
D
−
σ k, (3.4)
D
| |≤
where k >0 is a critical stress value. The evolution of the plastic strain is described by means of
the flow rule and is expressed with the Prandtl-Reuss law
p˙ =0 if σ <k,
D
| | (3.5)
( pp˙˙ = σkD if |σD|=k.
| |
Thesystem(3.1)–(3.5)issupplementedbyinitialandboundaryconditionswhichwillbediscussed
later.
3.2. Anti-plane shear. Denoting by (e ,e ,e ) the canonical basis of R3, we assume that is
1 2 3
invariant in the e direction so that = Ω R, where Ω R2 is a bounded open set. We Balso
3
B × ⊂
suppose that the displacement is anti-plane u(x ,x ,x ) = u(x ,x )e for some scalar function
1 2 3 1 2 3
u:Ω [0,T] R, so that computing the linearized strain yields
× →
Eu=(∂ u)e e +(∂ u)e e
x1 1⊙ 3 x2 2⊙ 3
corresponding to pure shear strain. We thus assume that the elastic and plastic strains conserve
this special structure so that
p=p e e +p e e , e=e e e +e e e ,
1 1 3 2 2 3 1 1 3 2 2 3
⊙ ⊙ ⊙ ⊙
HYPERBOLIC STRUCTURE FOR A SIMPLIFIED MODEL OF DYNAMICAL PERFECT PLASTICITY 7
for some functions e , e , p and p : Ω [0,T] R. Denoting by e = (e ,e ) and p = (p ,p ),
1 2 1 2 1 2 1 2
× →
the additive decomposition (3.1) now reads as
u=e+p.
∇
Computing the Cauchy stress according to (3.2) yields a pure shear stress σ = 2µe, so that
denoting by σ :=(σ ,σ ) its only nonzero components, we have
13 23
σ =µe.
We also assume that the body load is compatible with the anti-plane assumption, i.e., f = fe ,
3
for some f :Ω [0,T] R, so that the equations of motion (3.3) becomes a scalar equation
× →
u¨ divσ =f in Ω (0,T).
− ×
Finally, the stress constraint (3.4) now reads as σ k/√2, and the flow rule (3.5) is given by
| |≤
p˙ =0 if σ <k/√2,
| |
( pp˙˙ = √k2σ if |σ|=k/√2.
| |
In order to simplify notation, we assume henceforth that µ = 1 (so that σ = e) and k = √2.
The simplified model of plasticity thus consists in looking for functions u : Ω [0,T] R,
σ :Ω [0,T] R2 and p:Ω [0,T] R2 such that the following system holds in×Ω (0,T→):
× → × → ×
u=σ+p,
∇
u¨ divσ =f,
−
|p˙σ=|≤01, if σ <1, (3.6)
| |
p˙ =σ if σ =1.
Note that the flow rule can be equiva|pl˙e|ntly be written a|s|
σ p˙ = p˙ . (3.7)
· | |
which expresses Hill’s principle of maximal plastic work.
We supplement the system with initial conditions on the displacement, the velocity, the stress
and the plastic strain
(u,u˙,σ,p)(0)=(u ,v ,σ ,p ).
0 0 0 0
The precise mathematical formulation of this model will be the object of Section 5. In particular,
the flow rule (3.7) will have to be interpreted in a suitable measure theoretic sense according to
the generalized stress/strain duality introduced in Definition 2.2.
Thediscussionofadmissibleboundaryconditionsistheobjectofthefollowingparagraphs,once
the hyperbolic structure of the system will be described.
3.3. Dissipative formulation of the model. In this section, we perform formal manipulations
onthe system(3.6)in orderto write it in a differentform, moreappropriateto describe hyperbol-
icity. To do that, we denote by U := (u˙,σ), and observe that the first two equations of (3.6) can
be written as
∂ U +A ∂ U +A ∂ U +P =F, (3.8)
t 1 x1 2 x2
where F =(f,0,0), P =(0,p˙ ,p˙ ) and
1 2
0 1 0 0 0 1
− −
A = 1 0 0 , A = 0 0 0 . (3.9)
1 2
−   
0 0 0 1 0 0
−
   
8 J.-F.BABADJIANANDC.MIFSUD
Taking the scalar product of (3.8) with U, yields
1 1 1
∂ U 2+ ∂ (A U U)+ ∂ (A U U)+P U =F U,
2 t| | 2 x1 1 · 2 x2 2 · · ·
while, for every constant vector κ =(k,τ) K :=R B, taking the scalar product of (3.8) with
∈ ×
κ leads to
∂ (U κ)+∂ (A U κ)+∂ (A U κ)+P κ=F κ.
t · x1 1 · x2 2 · · ·
Substractingboth previousequalities,andusingthat P (U κ)=p˙ (σ τ) 0 accordingto the
· − · − ≥
flow rule written as (3.7), we infer that
2
∂ U κ2+ ∂ A (U κ) (U κ) 2F (U κ). (3.10)
t| − | xi i − · − ≤ · −
i=1
X (cid:0) (cid:1)
We then multiply the previous inequality by a test function ϕ (Ω ( ,T)) with ϕ 0,
∈ Cc∞ × −∞ ≥
and integrate by parts to obtain
T 2 T
U κ2ϕ˙dxdt+ A (U κ) (U κ)∂ ϕdxdt
| − | i − · − xi
Z0 ZΩ i=1Z0 ZΩ
X
T
+ U κ2ϕ(0)dx+2 F (U κ)ϕdxdt 0,
0
| − | · − ≥
ZΩ Z0 ZΩ
which is precisely the formulation of constrained Friedrichs’ systems as defined in [16] without
taking care of boundary conditions since ϕ vanishes in a neighborhood of ∂Ω (0,T).
×
Inordertoaccountforboundarycondition,wefollowanapproachintroducedin[26]. Following
the pioneeringwork[20], we areformallyinterestedindissipativeboundary conditionsofthe form
(A M)U =0 on ∂Ω (0,T), (3.11)
ν
− ×
where A = A ν +A ν (ν = (ν ,ν ) is the outer normal to ∂Ω), and M M3 3 is a boundary
ν 1 1 2 2 1 2 ×
∈
matrix satisfying the following algebraic conditions
M =MT,
M is non-negative,
 (3.12)
KerAν ⊂KerM,
R3 =Ker(A M)+Ker(A +M).
ν ν
−
Note that in the non-characteristic case (i.e. when Aν is non-singular), conditions (3.12) imply
those(1.2)introducedbyFriedrichs(see[26]foradetaileddiscussion). Thus,multiplyinginequality
(3.10)by a testfunction ϕ (Rn ( ,T))withϕ 0,andintegratingby parts,wegetthat
∈Cc∞ × −∞ ≥
T 2 T
U κ2ϕ˙dxdt+ A (U κ) (U κ)∂ ϕdxdt+ U κ2ϕ(0)dx
| − | i − · − xi | 0− |
Z0 ZΩ i=1Z0 ZΩ ZΩ
X
T T
+2 F (U κ)ϕdxdt A (U κ) (U κ)∂ ϕdxdt 0. (3.13)
· − − ν − · − xi ≥
Z0 ZΩ Z0 Z∂Ω
According to [26, Lemma 1], we have that
R3 =KerA Ker(A M) ImA Ker(A +M) ImA .
ν ν ν ν ν
⊕ − ∩ ⊕ ∩
For each κ R3, we denote b(cid:0)y κ the projection o(cid:1)f κ(cid:0)onto Ker(A M) I(cid:1)mA . Using the
± ν ν
∈ ± ∩
(strong) boundary condition (3.11), we have that U Ker(A M), or still U+ = 0. The
ν
∈ −
HYPERBOLIC STRUCTURE FOR A SIMPLIFIED MODEL OF DYNAMICAL PERFECT PLASTICITY 9
algebraic conditions (3.12) together with [26, Lemma 1] thus yield
Aν(U κ) (U κ)= M(U κ)+ (U κ)++M(U κ)− (U κ)−
− · − − − · − − · −
= Mκ+ κ++M(U κ) (U κ) Mκ+ κ+.
− −
− · − · − ≥− ·
Inserting in (3.13), we get that for all constant vector κ K and all ϕ W1, (Ω (0,T)) with
∞
∈ ∈ ×
ϕ 0,
≥
T 2 T
U κ2ϕ˙dxdt+ A (U κ) (U κ)∂ ϕdxdt
| − | i − · − xi
Z0 ZΩ i=1Z0 ZΩ
X
T T
+ U κ2ϕ(0)dx+2 F (U κ)ϕdxdt+ Mκ+ κ+ϕd 1dt 0. (3.14)
0
| − | · − · H ≥
ZΩ Z0 ZΩ Z0 Z∂Ω
The previous family of inequalities defines a notion dissipative solutions U L2(Ω (0,T);K) to
∈ ×
the dynamicalelasto-plasticproblem. Note thatit is meaningfullwithin a L2 theoryofFriedrichs’
systems(assuggestedby(3.14))sincethetraceofU ontheboundary∂Ω (0,T),whichisnotwell
×
defined, is not involved in this definition (see also [28, 24] for an L -theory of initial/boundary
∞
value conservation laws).
3.4. Derivation of the boundary conditions. The well-posedness of this kind of dissipative
formulations in the full space Ω = R2 has been established in [16]. On the other hand, among
the results of [26], it is shown the existence and uniqueness of a solution to this problem in the
unconstrained case (K = R3), and the dissipative boundary condition (3.11) is proved to be
satisfied in a suitable weak sense.
In order to formulate more precisely the admissible boundary conditions in our particular sit-
uation, we need to characterize all boundary matrices satisfying the required algebraic conditions
(3.12).
Lemma 3.1. Assume that A and A are given by (3.9) and ν R2 satisfies ν = 1. The
1 2
∈ | |
following assertions are equivalent:
(1) A matrix M M3 3 satisfies (3.12);
×
∈
(2) There exists λ (0,+ ) such that
∈ ∞
λ 1 0 0
−
M = 0 λν2 λν ν . (3.15)
 1 1 2
0 λν ν λν2
1 2 2
 
Proof. It is immediate to check that any matrix M of the form (3.15) with λ > 0 fullfills all
conditions (3.12). Conversely, assume that
d a b
1
M =a d2 c∈M3sy×m3
b c d
3
 
satisfies (3.12). Since
0 ν ν
1 2
− −
A = ν 0 0 ,
ν 1
− 
ν 0 0
2
−
 
we get that
KerA = (v,σ)=(v,σ ,σ ) R3 :v =0 and σ ν =σ ν +σ ν =0 .
ν 1 2 1 1 2 2
∈ ·
(cid:8) (cid:9)
10 J.-F.BABADJIANANDC.MIFSUD
Denoting by ν := σ R2 :σ ν =0 the linear space of dimension 1 in R2, condition KerA
⊥ ν
∈ · ⊂
KerM reads as
(cid:8) ν σ(cid:9) R2 :aσ +bσ =0 =:E ,
⊥ 1 2 1
⊂ ∈
ν σ R2 :d σ +cσ =0 =:E ,
 ⊥ ⊂(cid:8) ∈ 2 1 2 (cid:9) 2
ν⊥ ⊂(cid:8)σ ∈R2 :cσ1+d3σ2 =0(cid:9) =:E3.
Consequently, we obtain that the dimension of the linear spaces E , E and E is larger than or
 (cid:8) (cid:9) 1 2 3
equalto1. If dimE =1,then ν is orthogonalto E , while if dimE =2, thena=b=0. Inboth
1 1 1
cases, one can find µ R such that
1
∈
(a,b)=µ ν.
1
Arguing similarly for E and E , there exist µ and µ R such that
2 3 2 3
∈
(d ,c)=µ ν, (c,d )=µ ν,
2 2 3 3
so that
d µ ν µ ν
1 1 1 1 2
M = µ ν µ ν µ ν .
1 1 2 1 2 2
 
µ ν µ ν µ ν
1 2 3 1 3 2
UsingthatM issymmetric,wemusthaveµ ν =µ ν . Sinceν =1,theneitherν =0orν =0.
2 2 3 1 1 2
| | 6 6
Suppose without loss of generality that ν =0, and define λ=µ /ν , then
1 2 1
6
d µ ν µ ν
1 1 1 1 2
M = µ ν λν2 λν ν .
 1 1 1 1 2
µ ν λν ν λν2
1 2 1 2 2
Using next that M is non-negative, it follows that for all (v,σ) R3,
∈
v v
M =d v2+2µ vσ ν+λ(σ ν)2 0,
σ · σ 1 1 · · ≥
(cid:18) (cid:19) (cid:18) (cid:19)
which ensures that d 0, λ 0. In fact, if d = λ = 0, the previous expression can easily be
1 1
≥ ≥
made negative so that either d >0 or λ>0 (since the case µ =0 is impossible).
1 1
From the conditions KerA KerM anddimKerA =1, we obtain that dimKer(A M) 1
ν ν ν
anddim Ker(A M) Ker(A⊂+M) 1. ThelastconditionR3 =Ker(A M)+Ker±(A +≥M)
ν ν ν ν
− ∩ ≥ −
then implies that dimKer(A M) = 2 (since A is neither non-negative, nor non-positive).
ν ν
(cid:0) ± (cid:1)
Computing
d ( µ 1)ν ( µ 1)ν
1 1 1 1 2
± ± − ± −
A M = ( µ 1)ν λν2 λν ν ,
ν ±  ± 1− 1 ± 1 ± 1 2 
( µ 1)ν λν ν λν2
± 1− 2 ± 1 2 ± 2
we infer that  
v d v+(µ +1)σ ν = 0,
(Aν −M) σ =0 ⇐⇒ (1µ1+1)1v+λσ·ν = 0.
(cid:18) (cid:19) (cid:26) ·
Observe that, since dimKer(A M)=2, we obtain that
ν
−
d v+(µ +1)σ ν =0 (µ +1)v+λσ ν =0
1 1 1
· ⇐⇒ ·
d λ (µ +1)2 =0,
1 1
⇐⇒ −
and similarly, since dimKer(A +M)=2,
ν
d v+(µ 1)σ ν =0 (µ 1)v+λσ ν =0
1 1 1
− · ⇐⇒ − ·
d λ (µ 1)2 =0,
1 1
⇐⇒ − −
which implies that µ =0 and d λ=1, hence λ>0. (cid:3)
1 1
Remark 3.2. A similar characterizationresult can be proved in any dimension n 2, i.e., when
the matrices M belong to M(n+1) (n+1). ≥
×