Holpzaphel_-_Nonlinear-Solid-Mechanics-a-Contin

280· 6 Hyperelastic Materials

behavior. Under the action of a constant deformation (strain), the Maxwell model is supposed to produce instantaneously a stress response by the spring which is followed by an exponential stress relaxation due to the dashpot. On the other hand, the KelvinVoigt mode·1 is supposed to produce no immediate deformation for a constant load (stress).. However, in a Kelvin-Voigt model a deformation (strain) will be ·created with time according to an exponential function. Within the realm of non-equilibrium thermodynamics the viscoelastic deformation mechanisms of these material models are not reversible.

The rate of decay of.the stress and strain in a viscoelastic process is characterized by the so-called relaxation time T E (0, oo), with dimension of time, known from linear viscoelasticity. The parameter r associated with a creeping process is often referred to as the retardation time.

The constitutive equations introduced hitherto are no longer sufficient to describe dissipative materials. The vast majority of constitutive models that are used to approximate the physical behavior of real nonlinear inelastic :materials are developed on the basis of internal variables.

.In this chapter we remain within an isothermal framework, in which the temperature is assumed to be constant (8 = 8 0 ). Hence, we postulate a Helmholtz free-energy function wwhich defines the thermodynamic state .by the observable variable F and a set of additional internal ·history variables e0' a = 1, ... , m, to be specified for the particular problem. We write

(6..230)

whe.re the second-order tensors ea, a = 1, .... , 'm,, represent the '..dissipation .mechanism of the material. They are linked to the irreversible relative movement of the material inside the system and describe the deviations from ·equilibrium (see, .for example, VALANIS [1972]). An assumption of the form (6.230) can easily ·be adjusted to describe a rich variety of poroust viscous or plastic materials. The actual number of the phenomenological internal variables needs to be chosen for each different material and may vary from one theory and (boundary) condition to another; for example, the size of the specimen under observation. However, the definition of internal variables should be chosen so :that they somehow replicate the underlying internal microstructure of the material (even though they are introduced as macroscopic quantities)..

In general, the internal variables may take on scalm; vector or tensor values. Here the internal variables are all denoted by second-order tensors.

In order to particularize the Clausius-Planck inequality of the form (4.154) to the

free energy 'lll at hand, we must differentiate (6.230) with respect to time. By means of the chain rule we obtain ~(F, e1, ... 'em)= awI8F ·: F+ E:=l awIae.(t : ~n~ and

finally, with the expression for the stress power Wint = P : Fper unit reference volume,

choices of the tensor variable F, we deduce a physical expression for the first PiolaKirchhoff stress P and a remainder inequality governing the non-negativeness of the

.internal dissipation Vint (required by the second law of thermodynamics). We have.

o:=l

which must hold at every point of the continuum body and for all times during a thermodynamic process. In (6.232h we have defined the internal (second-orde~) tensor variables Sa, et = 1, ... , m, which are related (conjugate) to e0 through the internal constitutive equat.ions

The additional constitutive equations (6.233) restrict the free energy wand relat~ the gradient of the free energy wwith respect to the internal variables ea to the associated internal variables 8 0 , a = 1, ... , rn. Note that the presence of additional variables in the free energy (6.230) justifies additional constitutive equations. A physical motivation of restriction (6.233) may be given by several examples, one of whic.h, stemming

from linear viscoelasticity, is presented on p. 286; in particular, see eq. (6.251 ).

In constitutive equations (6.232)J. and (6.233) the tensor variables F and .ea are

associated with the thermodynamic forces P and 8 0 , respectively. A constitutive model which is characterized .by the set of equations (6.231)-(6.233) is called an internal variable .model.

For the case in which the .internal variables e0 are not needed to characterize the thermodynamic state of a system, then, the internal dissipation Vint in (6.232h is zero (the material is considered to be perfectly ·elastic) and a11 relations from previous sec~ tions of this chapter may be applied. In order to describe materials without dissipative character, the set of equations {6.23.1 )-(6.233) simply reduces to (6.3) and (6.1)1.

Evolution equations and The derived set of .equa-

tions (6..232) and (6.233) must be complemented by a kinetic relation, which describes the evolution of the involved internal variable eo and the associated dissipation .mechanism. Consequently, suitable equations of evolution (rate equations) are required in order to describe the way an irreversible process evolves.

The only restriction on these equations is thermodynamic admissibility, i.e. the

satisfaction of the fundamental inequality (6.232h characterizing local entropy production. The m.issing equations for the evolution of the .intern.al variables e0 may be written, .for example, as

The evolution of the system is described by Ao, n = 11 ••• , rn, which are tensor-valued functions of .1 + m, tensor variables.

Every syste·m will tend towards a state of thermodynamic equilibrium, which implies that the observab]e and internal variables reach equilibrium under a prescribed stress or strain; they remain constant at any particle of the system with time. Hence, the behavior at the equilibrimn state may be considered as a limiting case and does not

depend upon time.

In view of eq. (6.234), the definition of an equilibrium state now requires the addi-

tional conditions

Hence, en may be seen as the rate ofchan.ge with whic:h e(t) tends toward its equilib-

0

rium.

In an elastic continuum, every state .is an equilibrium state. The internal dissipation Vint at equilibrium is zero, which characterizes, for instance., a perfectly elastic material, as pointed out in Section 6. .1.

6.10 Viscoel.astic Materials at Large Strains

Many materials of practical interest appear to behave in a markedly viscoe.lastic manner over a certain range of stresses and times. The mechanical..behavior of, for example, thermoplastic elastomers (actually rubber-like materials) or some other types of natural and syntheticpolymers are associated with relaxation and/or creep .Phenom- ·ena, which are important design factors (see, for example, MCCRUM et al. [1997], SPERLING [1992] and WARD and HADLEY :[.1993]). Problems that involve relaxation and/or creep effects determine irreversible process,es and belong to the realm of equilibrium thermodynamics. For a detailed introduction of the 1inear and nonlinear theory of viscoelasticity the reader is referred to the book by CHRISTENSEN ['1982]. Experimental investigations are documented by, for example, SULLIVAN [1986], LION

[1996] and M.IEHE and KECK [2000].

In the following we characterize the thermodynamic state of such problems explic-

"itly by means of an internal variable model as introduced .in the previous section. A description sole'ly via external variables is also possible; but it emerges that such types of formulation are not preferred for numerical realizations using the finite element

method.

Num·erous viscoelastic materials can often not be modeled adequately within limits by means of a linear theory. Here we postulate a three-dimensional v·iscoelastic model suitable for finite strains and small perturbations away from the equilibrium state. In

contrast to several theories of viscoelasticity (see, for exam.pie, the pioneering paper· by GREEN and TOBOLSKY [1946]) the present phenomenological approach is not restricted to isotropy. For theories that account for finite perturbations away from the equilibrium state, the reader is referred to, for ·ex.ample, KOH and ERINGEN [.1963],

HAUPT fl 993a, b] and REESE and GOVJNDJEE [.1998a].

Additionally we foJJow a phenomenological approac·h that does not consider the

.underlying molecular structure of the physical object.

In particular, we choose an approach which applies the concept of internal variables motivated by S"IMO [1987] and followed by, for example, Gov·1NDJEE and .S-JMO [I 992b, 1993], HOLZAPFEL

[1996a], KALISKE and ROTHERT (1997] and SIMO and HUGHES [l.998, Chapter 10]. Our study is based on the theory of compressible hyperelasticity within the is<?ther~

mal regime, as discussed in Section 6.4. We postulate a decoupled representation of the Helmholtz free-energy function \JI. The free energy uses the multiplicative ~ecom­ position of the deformation gradient into dilational and volume-.preserving parts. Our present approach is in contrast to that which uses ·the multiplicative decomposition of the deformation gradient into elastic (rate-independent) and permanent (viscous) parts (see S.IDOROFF [ l974] and LUBLINER [.1985] among others).

The change of \JI within an isothermal elastic process from the reference to the

current configuration is given as

o=l

valid for some closed time interval t E [O., T] of interest. We assume that each contribution to the free energy \JI must satisfy the normalization condition (6A), i.e.

(6.237)

A material which is characterized by the free energy (6.236) for any point and time we

call a vis·coclastic material.

The first two terms in (6.236), i..e. w:1( J) and W~{C), are strain-energy functions per unit reference volume and characterize the ·equilibrium state of the sol.id. They can be identified as the terms presented by eq. (6.85) describing the volumetric elastic response and the isochoric elastic response as t -+ oo, respectively. In fact, the superscript (• )00 characterizes functions which represent the hyperelastic behavior of sufficiently slow processes.

284 6 Hypere1astic Materials

The additional third term in (6.236), i.e. the 'dissipative' potential E~~.1 T °'is

responsible for the viscoelastic contribution and extends the decoupled strain-energy function (6.85) to the viscoelastic regime. The scalar-valued functions Y ch a: = 1, ... , m, represent the so-called ·configurational free energy of the viscoelastic solid and characterize the .TJ011-equilibrium state, i.e. the behavior of relaxation and creep.

Motivated by experimental data we assume a time~dependent chan.ge of the system caused purely by isochoric defonnations. Hence, the volumetric response remains

fully elastic and the configurational free energy is a function of the ·modified right Cauchy-Green tensor C and a set of strain-like internal variables (history variables) not accessible to direct observation, here denoted by r 0 , a: = 1, ... , m. Each hidden tensor variable r 0 characterizes the relaxation and/or creep behavior of the material. They are ·considered to be (inelastic) strains ·akin to the strain measure C, with r a = I, a: = 1,,. .. , rn, at the (stress-free) reference configuration.. The viscoelastic behavior is, in particular, modeled by n = 1, ... , m viscoelastic processes with corresponding relaxation times (or retardt1tion times) 'Ta E (0, oo.), a: = 1, ... , 1n.

ated constitutive -equations describing viscoelastic behavior at finite strains we specify postulate (6.236).

Following arguments analogous to those which led from (6.230) to eqs. (6.232) and

(6.233), we obtain physical expressions for the (symmetric) second Piola-Kirchhoff stress S and the non-negative internal dissipation (local entr~py production) Vint in the forms

Starting from the d~~oupled free energy (6.236), .a straightforward computation leads to an additive split of S, as already derived for purely elastic compressible hyper-

elastic materials (see Section 6.4). We have

o=l

of the isochoric contributions. In eqs. (6.239h and (6.240) the q~antities

determine volumetric and isochoric contributions, which we take to be fully ·elastic. In relation (6.241.h the .(fourth-order) projection tensor IP = l[ - ic-1 ® C furnishes the deviatoric operator in the Ltigrangian description. Note that for these elastic contributions we may apply the framework of compressible hyperelasticity and adopt relations (6.88)-(6.91) by using \11~1 and w: instead of Wvol and Wiso·

In (6.240) we have introduced additional internal tensor variables Q0 , a = .1, ... ,·m., which may be interpreted as non..equiHbrium stresses in the sense of non-equilibrium thermodynamics. Note that the symbol Q has al.ready been used and must not be confused with the orthogonal tensor. As can be seen from (6.240) the isochoric second Piola-Kirc.hhoff stress is decomposed into an equilibrium p.art and a non-equilibrium part characterized by the e·lastic response of the system s: and the viscoelastic re-

By ·analogy with (6.90) we have defined the relationship

of the so-called fictitious non-equilibrium stresses Q0 • As can be seen from (6242),

Q0 is the deviatoric projection of Q0 times J-2/ 3, with projection tensor JP,

Motivated by the (mechanical) equilibrium equations for the linear viscoelastic

solid (see the following Example 6.10., in particular, eq. (6.251)), we conclude further that Q0 are variables related (conjugate) to r 0 , a = 1, ... , 11i, with the internal constitutive equations

a=l,~ .. ,-m. (6.244)

These conditions restrict the configurational free energy L::~1 To in view of (6.242)1. Hence, the internal dissipation Vint in-eq. (6.238)2 equivalently reads Vint ·=L::=.l Qo: :

r0 j2 > 0.

The condition for thermodynamic equilibrium (compare with eq. (6.235)) implies that fort -->- oo the stresses in eq. (6.240) reach equilibrium, which means that Q =

0

-28'I0 /8I'a..lt·-HX> =0, a= 1, .. . ,m, and hence, Q.Q characterize the current ·'distance from e.quilibrium '. Consequently, the dissipation at equilibrium .is zero as seen from (6.238h and (6.244). In other words, .at thermodynamic equilibrium the material responds as perfectly elastic; general finite elasticity is recovered.

Let a be the total stress applied to the generalized Maxwell model and c be an ex- which measures the total linear strain due to the stress. By equilibrium,

the total stress applied to the device .is found to be

o:=l

(see Figure 6..6), where the definition of the stress at equilibrium, i.e. a 00 = E 00 e, .is to be used. The internal variables ·qo., o: = 1., ... , rn, are the non...equilibrium .stresses in the dashpot of the a-Maxwell element characterizing the dissipation mechanism of the viscoelastic model.

The stresses qm n: = 1., ... ,-ui, .acting -on each dashpot are related to the associated

equation by Newton, i.e. q = r1n )', n == 1, ... , ni. On the other hand, the stress in the

0 0

spring of the a-Maxwell element is determined by q0 = E0 (e - /a) (see Figure ·6.6). Consequently, the stresses (not necessarily at equilibrium) acting on each dashpot .is

for the internal variables with.in the one-dimensional and linear regime, where the def:...

inition of the relaxation time (or retardation time) Tu: = T/c.J E 0 > 0, a = 1, ... , rn, is to be used.

Since <Jo and i'o are the stresses and the strain rates acting on each dashpot, we are in the position to define the rate of work dissipated within the considered device. By

means of (6.246hT the internal dissipation takes on the form

motivated strain energy 't/1 determines the energy stored elastically in the springs of .the device, as illustrated in Figure 6.6. The strain energy v 0 = va(e, /ci:) is responsible

288 6 Hyperelastic Mat-erials

for the viscoelastic contribution and is related to the .a-relaxation (retardation) process

with relaxation (retardation) time Ta E (0, oo).

Differentiation of '1/J with respect to the total strain c gives the total stress a applied

to the device. On comparison with (6.245) we conclude that

(6.250)2 (or (6..246)) the associated non-equilibrium stresses q0 in the dashpots. The resulting .internal constitutive equations read

which, when substituted into (6 ..248)i, .gives the intern.al dissipation Vint expressed through the strain energy, i.e. Vint = -

Note that the general stress relation (6.239) with definition (6.240) may be identified as the three-dimensiona·.1 and nonlinear version of the linear rheological model (6.249), which, in view of Figure 6.6, decomposes the stresses .in equilibrium and non- equilibrium parts. In .addition, the internal constitutive equations (6.244) and definition (6.242h may be considered as the three-dimensional generalization of (6.251) and

(6.250h and also its extension to the finite strain regime. II

Evolution equations and their solutions. In order to describe the way a viscoelastic process evolves it is necessary to specify complementary equations ofevolution so that the local entropy production, i.e. the inequality (6.238)2, is satisfied. .In particular, we look for a law which governs the internal variables Q0 , a = 1, ... , 1n, introduced as isoclzoric non-equilibrium stresses. We require that the evolution -equations have a physical basis and provide a good approximation to the observed physical behavior of real materials in the large strain regime. In addition, we require that they are suitable for efficient time integration algorithms that are accessible for use within a finite element procedure.

We motivate the evolution equations for the three-dimensional and nonlinear deformation regime by reference to the relationship (6.247). Having th.is in mind, an obvious