Holpzaphel_-_Nonlinear-Solid-Mechanics-a-Contin

(or contraction) J = J 9 {8) is assumed to be governed by relation (7.104). The specific heat capacity and the linear expansion coefficient are given by c0 = 1.83 · 103Nm/kgK

= 22.333 · 10-5 1{-1 (se.e CYR [1988]), respectively. We assume that for the

reference state the specimen and its environment constitute a system, which is in thermodynamic equilibrium. The homogeneous temperature field is given by the value e·o= 293.15K (= 20°C).

Solution. The relationship between the nominal stress P and the temperature change l) = e - 80 is given explicitly by the theoretical solution (7.109), with volume change (7.104), and by experimental results due to ANTHONY et a.L [1942]. The respective diagram for various fixed values of/\ is depicted in F.igu~~ 7.6.

The figure is supplemented by a curve indicating the evolution of the temperature change iJ with respect to the stretch ratio ,.\. Since the adiabatic process is assumed to be reversible the energy balance equation (4.142) reduces to 81) = 0. Consequently, the entropy (r/ = canst) possessed by the ·material remains constant, which indicates that the deformation process is also js~ntropic. The condition rJ gives the

relation between the temperature change 19 and the stretch ,.\, which is based upon the constitutive equation for the entropy, as derived in eq.. (7. ll 0), with eq. (7.l04). The analytical solution is completed by experimental data due to JOULE [1859].

The qualitative fit to the experimental data given by JOU.LE [1859] and ANTHONY et al. [.1942] is satisfying; nevertheless, the physical properties of the vulcanized rubber strip used in Joule's and Anthony's experiments .are only partly documented and may differ from those given above.

One of the very first works reporting simple observations regarding therma] effects due to deflections of vulcanized rubber bands was published by GOUGH [1805].

The :first work which explored experimentally the crucial physical properties of elastic

.india-rubber was .presented by JOULE [1859]. The phenomena of structural thermoelastic heating (or cooling) of stressed rubber-Hke materials is called the Gough-Joule effect. A wide range of.publications followed over more than one century, which mostly deal with theoretical formulations ·of more or less approximate .theories. The history of these investigations is discussed in the classical book by FLORY U953, Section XL "I a], an overview is given by TRELOAR [1975, Chapters 2, 5], CHADWICK [1974] and

PRICE [.1976, and .references therein]. For a detailed .account of .the relevant results s·~ also ·OGDEN [1992b].

Numerical .analyses for this type of adiabatic (isentropic) simple tension test within the finite ele·ment context were given by MIEHE [1995b] and HOLZAPFEL and SIMO

[l 996b]. The algorithmic .solution procedure reduces .merely to one step, which is ~.o~cerned with solving a mechanical problem with lsentr~pic elasticities as in~i~ated e?CpliCitly through expression (7.58). During the solution process the entropy c~ns:~raint condition 81} = 0 .must be enforced, which means that the entropy at each time-step

is unchanged. As a direct result the temperature and the displacement field follow according to Figure

Thermoelastic inve.rs·ion point. For a small stretch ratio .,\ the rubber band indi-

c.ates an initial cool.ing effect (see Figure 7.-6), which increases first with deformation an.d ..changes to a heating effect at a certain ..minimum point, which entered .into the literature as the so-called thermoelastic inversion point. The associated extension, 1.abe~ed as Aim.. , characterizes a configuration of the sample in which the acting force is i~-_dependent of the temperature.

· ·· To gain more insight in the interesting thermoelastic inversion phenomena the fo]- lowing observations are emphasized:

(i)An analytical investigation of the reversible adiabatic (ise.ntropic) process - characterized by constitutive relation (7.110), with -r7 = const -- leads, within an

adequate linearization process1 to an explicit expression for the stretch at the "in-

(see JAMES and GUTH [1943] among many others). With given data thermoelastic inversion takes place for.an extension of about Ainv = 1.0616 at which the maximum temperature drop occurs (see Fi.gure 7.6).

A second point of interest is at ·1? = 0, where a heating process takes place. Nate that this behavior is contrary to that for a metallic spring, which cools continu- ousIy on stretching within the ·elastic domain .. An approximate solution for A at

·tJ = 0 may be de.rived from eq. (7.110) using an adequate linearization process. We obtain a quadratic equation in.-\ i.e. ,,\2 2~\~.•v ~ 0 (see and

GUTH [ 1943]). Remarkably, the stretch .ratio at iJ = 0 is only influenced by the location of the thennoelastic inversion point; with the given data the nu.merical value is/\ :;:: 1.1257 (see Figure 7.6).

(ii) In view of eq.. (7 .109) it is crucial to remark that .for a fixed A the stress is a nonlinear function with respect to the temperature. The nonlinearity is clearly caused by the volume thennal expansion term J = J 6 (8) characterized by eq. (7.104).

As a direct consequence the term Je governs the change in the slope oP/80 of the stress-temperature curves, as seen in Figure 7.6. If the elongation is small the slope is negative and the defonnation behavior is dominated by thermal ex·pansion (energetic contribution). For larger strain ranges the slope DP/88 becomes positive. It emerges that the reversal in the slope of the weak non.linear stresstemperature plot .is indicated at ,,\ = /\inv., which can be shown using eq. (7 ..109)

along with condition 8P/D8 = 0.

352 7 Thermodynamics of ·Materials

At the thermoelastic inversion point thermal expansion and entropy contraction balance. Hence, thermoelastic inversion is governed clearly by

sion, which is :expressed by re]ation (7.104), Le. J = exp[3a0 (8 - -80 )]. As can be seen from (7.112}, thermoelastic inversion depends basically on the linear expansion coefficient n 0 :/; 0. Alternatively, .it is interesting to note that the .inversion pojnt also occurs at the same value of extension by fixing the load of a strip of rubber and increasing the temperature. A connection of the mentioned observations may be found within the context of the thermodynamic Maxwell relations (7.39) and (7.40), which motivates the inclusion of the two diagrams within one figure (Figure 7.6) (see also the theoretical study by OGDEN [ l992b]).

."··:,,.,...,..,..,_,.,,,,.,,.•.,_.~,,,,,,,:,., __~~·,:,,,,, ..,_,...,..,,.,,..,.,.,..,.,.,..,... ,..... ,.,,,,_,,.,.,,..,.,,....,..,.. ,,,,,,_ ....,,,,,_,_....• ,.......,.,,.,.,,,,.,,,,,,.. :,·····,···,•:••,,,.,.,.,.,.... ,,,..... ,.,,,,,... ,,.... ,...,, ... ,.... :,........ ,....,,_.:,.,,,.;,......,.·: ... ,..,,........, .. ,_,....,, .......·.. ,_..:,.,.,,............ ,..... ,..... ,.,.: .., .......

We consider here the one-dimensional case of finite thermoelasticity. A .rod (with uniform cross-section) is imagined as being stretched to .X in the direction of its (1;-)axis, with the associated kinematic .relation :c = A.X (uniform non-isothermal deformation). The rod capable of supporting finite thermoelastic deformations admits the stretch ratio A and the temperature 8 as inde.. pendent m.echanical and thermal variables.

From the Helmholtz free-energy function \JI = w(.A, 8) (measured per unit reference volume), which is assumed to ·exist, we are able to deduce the stress and entropy functions .P = P-(,.\, 8) and r1 = r1(/\, 8), respectively. They are nonlinear scalar...

valued functions which de.pend on the two scalar variables ...\ and 8 and have the same form as (7.1.08). Note that the -constitutive relation (7.108).J. determines the nominal Stress p by keeping the temperature 8 fixed While (7.108h determines the entropy T} at a fixed stretch ...\.

For .a more profound understanding of the constitutive relations it is beneficial to note the differential mathematical relationship between the nominal stress P and the

..entropy 'T/. For the continuous function '11 the well-known property

µ 2 (8.0 ) = -0.125µ 0 , so that Jl 1 (80 )/tt2 (80 ) = -7 (see ANAND [1986]), with the shear .modulus JLo = 4.2.25 .. 105N /m2 in the reference ·Con.figuration. For the neo-Hookean model (1V = l) assume a 1 = 2 and /ti (8.0 ) = JLo and for the Varga model (J.V = 1), n1 = 1 andµ .. (80) = 2µ 0 .

(a)Based -on the Ogden, Mooney-Rivlin, neo-Hookean and Varga material models derive the temperature ·evolution tJ = e - -Go of the rubber band

due to the stretch ratio /\.

(b)Derive a relationship between the nominal stress P and the stretch ratio A for a non-isothermal deformation process. Show the difference compared

with the (classical) solution, which is based on the isothermal theory (set 8 = 8 0 ). Note that for an isothermal deformation process the free energy

\JI reduces to the strain energy, as given in (6..119) and eqs. (6.1.27)-(6..129).

Figures 7.7 and 7.8 show a comparison between the thermoelastic Ogden, Moo- ney-Rivlin, neo-Hookean and Varga .material .models. Observe the sharp upturn in the stress at high elongations ("\ > 6) for the Ogden material model as indicated in Figure 7.8. This fact may partly be explained physically within more advanced theories obeying Langevin distribution function (non...Gaussian statistical theory),. as discussed by, for .example, TRELOAR [1975., Chapter 6]. The observed rise of stress is mainly caused by limited extensibility of the polymer chains themselves and by strain-induced crystallization (see FLORY f.l 976] for further insi.ght).

4. A bicycle wheel with spokes made of .rubber bands is mounted with the axle horizontal. The spokes are in tension so that the rim .is kept in place. An electric heat plate is placed on the right-hand side of the whe.el (see Figure 7.9) so that the spokes on one side of the wheel are heated. As a consequence, the bicycle wheel starts to rotate counterclockwise, as lon.g as heat is induced. Explain this effect.

This amusing device may be viewed as a Carnot thermal engine, in which rubber alone constitutes the working substance. The Carnot thermal engine is alternately subjected to two adiabatic and two isothermal processes. Investigate a hypothetic.al thermodynamic Carnot cycle for rubber-like .materials. In partic- ular, discuss the temperature change fJ of a rubber band which occurs during the four processes as a function of the stretch A and the ·entropy 17. For a detaiJed ex- position of the relevant results the reader is referred to the paper by HOLZAPFEL

and SIMO .[l 996b].

Wheel ···

Electric

heat plate

Spokes made of .rubber

Figure 7!'9 A wheel with rubber spokes starts lo ro~ale counterclockwise when heated on one side.

7.9 Thermodynam,ics with Internal Variables

In this section we link together finite elasticity and non-equilibrium thermodynamics.

We consider the thermodynamics of continuous media within the large strain regime and apply the theory of .finite .thermoviscoelasticity..

We use a thermodynamic approach with internal variables which leads to a very general description of materials involving irreversible (dissipative) effects, such as damage, relaxation and/or creep and plastic deformations. It generalizes finite thermoelastic.ity, as outlined in Section 7.3, in the sense that additional thermodynamic variables {known as internal variables) are incorporated with the aim of representing

.the irreversible mechanism of the (inelastic) structural material behavior. A_general discussion of constitutive models with internal variables was -emphasized in Section 6..9.

A fully coupled three-dimensional thermomechanical model for viscous materials is examined. It is .particularly suited for the thermoviscoelastic behavior of dissipative elastomeric (rubber-like) materials under varying temperatures at finite strains. Con- stitutive equations for the stress, the entropy and the internal variables are specified.

Finite thermoviscoelasticity. We define a Helmholtz free-energy function (mea- sured per unit reference volume) as

(7..121)

The .thermodynamic state is completely characterized through the set of independent variables (F, -8, ei, ... ,~m.), i.e. the deformation gradient F., the absolute temperature

35"8 7 Thermodynami-cs of Materials

8 and rn additional internal variables e0 , n = 1, ... , 1n. For the case of thermoviscoe.Jasticity the tensor variables ea- represent the thermov.iscoelastic contribution to the material response. Note that the variables en may also represent damage and/or plllstic mechanisms.

A material which is characterized by the free energy {7 .12 l) for any point and time we ca.II a ,thermoviscoelastic material. The behavior of a thermoviscoelastic material is assumed to be governed by a· = l_, ... , ·m. relaxation (retardation) processes with given relaxation (retardation) times r 0 (8) E (O_, oo), a: = 1, ... , ni, which are, in general, temperature dependent.

The next aim is to derive the complete set of the constitutive equations for the first Piola-Kirchhoff stress tensor P and the entropy 1J (per unit reference volume) -in the general form. For that purpose we follow the standard methods presented in Sections 6.9, 7.3 and use the Clausius-Planck form of the second law of thermodynamics

(4.153), Le.. the internal dissipation ·inequality Vint = P : F - W- ·r18 > 0 (the

Clausius-Planck form assumes non-negative entropy production due to conduction of

change in the thermodynamic state., that is the Gibbs relati-on for thermoviscoelastic materials, namely

where ea, (j'= 1, ... ' m., denote the internal strain rates.

Since the rates F and ecan be chosen arbitrarily we find the constitutive equations

for the first Piola-Kirchhoff stress P and the entropy 17, and we deduce a remainder inequality

o=l

The inequality Vint > 0 characterizes the internal dissipation in the viscous material which generates heat in an irreversible manner.

The defined tensor variables .80:, ft = 1, ... , m., correspond to the internal tensor