Hyperelastic material models: Static modelling of elastomers by elastic

There are two rather different approaches to study the rubber elasticity. On the one hand, the statistical or kinetic theory attempts to derive elastic properties from some idealized model of the structure of vulcanized rubber. On the other hand, the phenomenological theory treats the problem from the viewpoint of continuum mechanics. This approach constructs a mathematical framework to characterise hyperelastic behaviour so that stress analysis and strain analysis problems may be solved without reference to microscopic structure or molecular concepts.

The statistical-thermodynamic theory of a molecular .— This theory was originally developed by Kuhn and Grün [17]; additional contributions were made by James and Guth [18] and also Flory and Rehner [19]. Treloar [20]

comprehensively reviewed the molecular theory of rubber-like elasticity, the base of which is on the fundamental statistical property of the elastomer molecules and the network entropy of deformation.

The work of deformation per unit volume is:

) 3 2 (

1 ₂

3 2 2 2

1   

 NkT   

W Eq. 1.60

where N is the number of network chains in a unit volume, k is the Boltzmann’s constant, i are the principal extension ratios and T is the temperature in Kelvin. Additionally, developing this theory summarized in e.a. [21], the next relation can be imposed:

T

physical constant G, the shear modulus, which may be determined from the degree of crosslinking in the rubber.

Substituting Eq. 1.30 in Eq. 1.62,

 

This theory predicts a simple relationship between the stress and the strain. In simple shear, the shear stress is linearly related to the shear strain by the shear modulus G. The form of the relationships is similar to all elastomers that are only scaled by the magnitude of the distance between cross-links. It predicts well the initial elastic modulus at small strains. But it breaks down as the chain extension approaches strains of 50% for an unfilled elastomer. The high strain behaviour caused by the effects of a finite extensibility is of course neither predicted nor the marked nonlinearity is at moderate strains. The phenomenological theories, to be discussed next, are not restricted by any particular physical interpretation. They largely concentrate on trying to

represent the high strain behaviour of unfilled elastomers with attempts to extend these ideas to represent the behaviour of filled elastomers.

Rivlin [22] has shown that the statistical theory is the natural extension of Hooke’s law of large deformations, hence the material that obeys it is called Neo-Hookean.

Replacing Eq. 1.24 by Eq. 1.62,



³



2 1

1

 G I

W ^{Eq. 1.64}

Deviations from the theory are apparent, especially in uniaxial extension, where, at low strains (below about 50%), the measured modulus is too high in relation to its value at moderate strains (up to 400%). At even higher strains, a rapidly rising modulus is seen, which is also not predicted by the theory presumably due to the finite extensibility of the chains.

Phenomenological theory of rubber-like elasticity.— Before the deviations of the statistical theory, a general treatment of the stress-strain relation of rubberlike solids, that began with Mooney [23] and was further developed by Rivlin [24], shown from the concept of an ideal elastic solid, assuming that the material is only isotropic in elastic behaviour in the un-strain state; no volume change occurs on deformation (the energy cannot be dissipated). Its mechanical behaviour may be described by means of an ESED function or Helmoltz free energy of deformation per unit volume of material referred to the undeformed state, which is a single-valued function of the state of deformation.

Based on the symmetry considerations, appropriate measures of strain - regardless the choice of axes - are given by three strain invariants I1, I2, and I3,

W  W ( I

₁

, I

₂

, I

₃

)

_.

When the material is unstrained,

I

_1,

I

_{2 and}

I

₃ take the values 3, 3 and 1 respectively. It can be shown that if the linear stress-strain relations of classical elasticity are to be applied for a sufficiently small deformation of the material,

W can be approximated with any desired degree of accuracy by a power series undeformed state is considered to be that in which the strain energy is zero.

So the function is simplified; consequently:

j

Mooney [23] developed the first phenomenological theory in 1940 prior to the development of the statistical theory:

)

Typically this expression and other similar stored energy functions were written in terms of the three strain invariants. The initial value of the Young modulus and the shear modulus can be calculated:



_{10 01}



The Mooney ESED contained two elastic constants C10 and C01 Eq. 1.67 and was simplified by the Neo-Hookean, as given in Eq. 1.70, when C10=G/2 and C01=0 or in terms of the strain invariants as



₁ 3



10 

C I

W Eq. 1.70

This model lies in the fact that the statistical theory of rubber elasticity arrives at the same strain energy function Eq. 1.64. (Treloar [20]) and Yeoh [25]

showed that only a small strain range could be fitted to a carbon black filled elastomer. The Mooney expression appears to be unsuitable for modelling the behaviour of filled elastomers. It has also been pointed by Charlton and Yang [9] that the Mooney constants determined from tensile data are inadequate to predict the behaviour in other modes of deformation.

Tschoegl [26] suggested that the failure of the Mooney-Rivlin equations arises from not taking enough terms of the Rivlin series Eq. 1.66. James and Green [27] fitted test data to highly carbon black filled elastomers with various high order expansions of the Rivlin series. They reported that a third order deformation expansion with 5 terms, Eq. 1.71, gave better predictions beyond the range of the input data than the expansion of a higher order or of a higher number of terms:

Gregory [28] noted that a simple relationship existed between stress/strain data obtained in uniaxial tension, uniaxial compression and simple shear.

Other empirical relationships for W have been developed by Varga, Ogden, Valanis-Landel [29–31]. These diverge from the Rivlin type of relationship in that some discard the principle that the strain invariants I¹ and I² are even-powered functions of the extension ratios and some are written in terms of strains or extension ratios rather than in terms of strain invariants. Based on extension ratios, Ogden [30] proposed the next relation for incompressible elastomers,

 

either positive or negative. This representation includes the statistical theory

) 2

(₁ and the Mooney equation (₁2,₂ 2) as special cases. Ogden [30] showed that a three-term expression is required to represent tension, pure shear and equi-biaxial extension for an unfilled elastomer, containing six adjustable parameters. The degree of agreement with the experiment is quite satisfactory for unfilled elastomers. Ogden’s formulation has the merit to be mathematically simple, although the magnitudes of a large number of independent constants have to be determined- since all the terms in the equation have an identical form. According to Sawyers and Rivlin [32] the Ogden model is a special case of the Rivlin ESED and Treloar [20] affirmed that the two formulations are equivalent. In the same way, Valanis-Landel [31]

proposed the next relation based in extension ratios,

 

Gent [33] developed a function that describes reasonably the whole range of strains especially the large strain behaviour with the upturn in the stress-strain behaviour that is due to the finite extensibility of the chains. It would give some confidence in the use of a model if the parameters had some molecular/physical significance. In this respect, the function may be written as:



where Im is the limiting value of I1 corresponding to the deformation when the network is fully stretched.

The previous relation could be written as:

E and Jm are physical constants: E is the small strain tensile or Young modulus and Jm denotes a maximum value for



I₁3



. At small strains, this equation reduces to Eq. 1.60 from the statistical theory where C10 is equal to the shear modulus G orNkT. This ESED function is claimed to have the advantage that it reduces the description of the stress-strain behaviour of an elastomer to two parameters having a clear physical meaning.

The general observation that can be made for unfilled materials is that W/I1

 W/I2 and the examination of data published by Fukahori and Seki [34]

also supports the contention that for filled elastomers W/I2 by comparison with W/I1 was numerically close to zero. If it could be assumed that W/I2

was equal to zero, then the difficulties of measuring the relationships for I2

could be ignored, and filled elastomer characterisation would be significantly simplified. This approach was originally suggested by Gregory [28] who observed, on the basis of measurements of the stress-strain behaviour of carbon filled natural-rubber elastomers, that a simple relationship existed between shear, tension and compression data for a number of different compounds. Davies et al [35] confirmed this observation with carbon black filled materials. The observation on filled materials showed that the mechanical behaviour, that could be described using the first strain invariant, could only be true if W/I1 was independent of I2 and if the magnitude of W/I1 was significantly greater than W/I2. From this consideration, the review shows that there is no unique solution. The choice of a function will depend on the particular situation. The first one will generally need to be accurate at small and moderate strains (<100%) unlike the second one which would be required to predict moderate and large strains accurately.

Yeoh [25], using the work of Gregory [28], proposed to take only the first three

This approach predicted the stress strain behaviour of filled elastomers well at large strain. The use of this function has been shown to permit the prediction of stress/strain behaviour in different deformation modes from data obtained in one simple deformation mode. But this leads to unstable functions predicting physically unrealistic behaviour under conditions outside the range of the experimental data. The initial value of the Young modulus and the shear modulus can be calculated as follows:

10

Yeoh’s model [25] reported good ability to predict multi-axial data, including comparison with the published biaxial data of James and Green [27] for filled elastomers. Conceptually, this proposed function is a model with a shear modulus varying as a second-degree polynomial in I₁3. The variation of the shear modulus in the case of carbon black elastomers is a fall of the modulus with increasing strain and arise at large deformations due to finite extensibility.

This characteristic behaviour can be modelled if C is negative while ₂₀ C and ₁₀ C are positive. 30

Additional experimental evidence and those recent works by Othman and Gregory [36], Davies et al [35], Gregory et al [37], Yeoh and Fleming [38] have also suggested that it is appropriate and more reliable to make the ESED a function of I1 for filled materials. Any inaccuracy resulting from making these

simplifying assumptions may not be too severe a limitation, as elastomers are only imperfectly elastic.

In the same way, another ESED function based on the previous consideration was developed by Arruda-Boyce [39]:

 

G0 and λm are material parameters which represent the initial shear modulus and the locking stretch at which the strain/stress curve of the model stiffens significantly respectively. This function is also called the eight-chain model because it was developed based on a representative brick element where eight chains emanate from the centre of the cube to each corner. The values of Ci arise from statistical treatment of non-Gaussian Chains:

2

Mullins effect and Phenomenological hyperelasticity.—

The ESED functions consider the energy cannot be dissipated (section 1.2.2).

Two of the earlier constitutive models that account for softening have been developed by Simo [40] and Godvinjee and Simo [41] although these models are not generally available in finite element codes.

The problem described above is compounded as the hysteresis loop changes systematically on each cycle due to stress softening. Hawkes et al [42] have attempted to solve this problem by mathematically representing the stress-softening phenomenon with a strain energy function of the following form:

) 1

(n W f

W_n  Eq. 1.81

where W₁ and W are the ESED functions on the first and _n n^thnumber of cycles, and f(n) is a decreasing function of n. This model developed for a constant maximum cyclic strain condition is obviously limited to this particular test condition and cannot be applied as a general case. In a general situation, the strain energy function depends not only on the previous number of cycles that the specimen has endured but also on the whole strain amplitude history.

Ogden and Roxburgh [43] proposed a pseudo elastic mathematical approximation which allows the prediction of the decrease in material stiffness modifying the initial value given by hyperelastic ESED material models. The model is a maximum load modification to the nearly and fully-incompressible hyperelastic constitutive models already available. In this model, the virgin material is modelled using one of the available hyperelastic potentials, and the Mullins effect modifications to the constitutive response are proportional to the maximum load in the material history. The Ogden-Roxburgh’s model results in a scaled stress given by

𝑆_𝑖𝑗 = 𝜂𝑆_𝑖𝑗⁰ Eq. 1.82

where η is a damage variable which is defined as follows:

𝜂 = 1 −1

𝑟𝑒𝑟𝑓 [1

𝑚𝑊_𝑚− 𝑊₀] Eq. 1.83

where Wm is the maximum previous strain energy and W0 is the strain energy for the hyperelastic material.

Some earlier models were proposed e.g. Miehe [44] and Miehe and Keck [45].

Newer proposal is implemented in ANSYS software [15]: The modified Ogden-Roxburgh pseudo-elastic model results in a scaled stress given by

𝜂 = 1 −1

𝑟𝑒𝑟𝑓 [𝑊_𝑚− 𝑊₀

𝑚 + 𝛽𝑊_𝑚] Eq. 1.84

The modified Ogden-Roxburgh damage function requires and enforces the three damage material constants r, m, and β.

The material constants are selected to ensure 𝜂𝜖(0,1] over the range of application. This condition is guaranteed for r > 0, m > 0, and 𝛽 ≥ 0; however, it is also guaranteed by the less stringent bounds r > 0, m > 0, and (m + βWm)

> 0. The latter bounds are solution-dependent, so you must ensure that the limits for η are not violated if β < 0.

1.2.2 Hysteresis. Energy dissipation of strained/cycled elastomers: Mullins,

In document Dynamic stiffness and damping prediction on rubber material parts, FEA and experimental correlation (Page 39-49)