Fatigue initiation and propagation in natural and synthetic rubbers

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Abstract

Elastomeric matrix composites are usually reinforced by mineral particles such as carbon black and sometimes by long metallic or organic fibers. In the absence of fiber, rubbers can be considered as nanocomposites. In service conditions, the fatigue damage of rubbers is a combination of: (a) mechanical damage; (b) chemical damage; (c) thermal damage. Experience shows that, in cyclic loading, rubbers are damaged to the point of formation of one or several cracks which then propagate. As for metal, it is recommended to study separately initiation of cracks and then their propagation. Generally speaking, the fatigue resistance is affected by chemical transformation such as crystallisation. It means that compression loading is an important factor. To show this effect, an axisymetric hour-glass shape specimen (D=25 mm) is proposed to test rubbers. A large effect has been found of the mean stress on the fatigue strength depending on the chemical composition of the materials and of crystallisation transformation if it appears. The crack growth rate is studied using linear fracture mechanics as proposed earlier (Rivlin RS, Thomas AG. Rupture of rubber: I. Characteristic energy for tearing. J Polymer Sci 1953;10(3):291). In this case, the strain energy release rate G is substituted for the concept of tearing energy T. The application of fracture mechanics to elastomers generates some difficulties because of the important deformability. In order to apply a tension–compression loading a thick edge notched specimen is recommended with two lateral grooves (W=150 mm, B=25 mm). For low values of ΔT, a threshold can de defined depending on the R ratio. It is shown that for a high R ratio the fatigue crack would not propagate if the crystallisation transformation is high. In contrast, if R=−1, the threshold disappears. A finite element simulation of stresses and strains is presented in order to get a better explanation of the experimental results.

Introduction

It is well known that the specific fatigue strength of long fiber reinforced polymer matrix composites is excellent compared with other materials. Considering short fiber or particulate reinforced polymer composites the characterization of fatigue strength is more difficult. It is not possible to compare the fatigue resistance of rubbers with other composite materials for two reasons: (1) the deformation of elastomeric composites is extremely large in contrast to many polymeric matrices; (2) the chemical composition and the chemical microstructure have a drastic effect on fatigue mechanisms, much more important than for other polymeric matrices.

In fact, rubbers have to be approached as nanocomposites because the first reinforcement of an elastomeric matrix is related to several networks due to sulfur vulcanization and carbon black networks, for example. Many mineral constituents are also involved. A key problem to understand fatigue of rubbers is the transformation from an amorphous to crystalline phase. In another words, an elastomeric matrix is not chemically stable under cyclic loading and the fatigue damage of rubbers is a combination of mechanical damage, chemical damage and thermal damage.

From the mid 20th century, many papers have been published about the fatigue cracking of rubbers—by Rivlin and Thomas in 1953 [1], Lindley in 1972 [2], Lake in 1983 [3], Stevenson in 1973 [4], Gent in 1992 [5], and Kakavas in 1996 [6].

In this paper, the same approach is used for modelling the fatigue behaviour of NR, CR and SBR rubbers. Natural rubber is assumed to be a reference.

However, in order to point out the principal mechanisms, observation of damage is done a three levels: macro, meso and micro scales. The mesoscopic scale is probably the most fruitful way to study damage related to the chemical microstructure. In order to observe the transformation from an amorphous to a crystalline phase, X-ray computed tomography is used. The attenuation of the X-ray intensity reveals the existence of crystallites, but also detects voids, inclusions and local damage.

Fatigue of rubbers is presented in this paper in five sections:

  • chemical microstructure of rubbers

  • loading effect

  • environment effect

  • thickness effect

  • complex loading

Section snippets

Chemical structure and crystallization

Natural rubber vulcanized and reinforced by carbon black, at room temperature is amorphous without loading. During elongation, this rubber is supposed to be transformed from amorphous to a crystallite phase. All the mechanical properties, including fatigue strength, are affected by the crystallization even if the transformation concerns a small amount of molecules. The hardness morphous of NR is 50 shore A.

To show this effect, some tests are carried out with axisymmetric hour-glass shape

Environmental effect

When fracture mechanics concepts are applied to fatigue cracking, we have to remember that cyclic damage of rubbers depends not only on the mechanical loading (including frequency and R ratio) but also on temperature and thermal dissipation, and finally on the environment [6], [7]. The oxygen content of air is known to play an important role in the fatigue of rubbers.

A coherent approach is proposed for fatigue cracking of rubbers, considering the main mechanical parameters and the influence of

Thickness and stress state effect

The application of linear fracture mechanics to elasto-plastic materials shows that toughness and fatigue crack resistance are much higher for plane stress conditions, that is to say in thin materials.

This effect is more complicated for rubbers. Reports in the literature [5] state that the fatigue crack growth rate is higher in thin materials such as NR or SBR. It means that there is no plane stress effect in rubbers. Fig. 9 presents the thickness effect in natural rubbers. Between 1 and 3 mm

Complex loading

In order to identify the principal parameters and mechanisms of the fatigue of rubbers under complex loading several typical tests are discussed.

  • compression–compression fatigue

  • tension–tension with high hydrostatic pressure

  • tension–torsion fatigue

  • compression–torsion fatigue

Conclusion

The study of fatigue crack growth and damage mechanisms in rubbers has led to the following conclusions:

  • 1.

    Fatigue damage depends on three basic mechanisms: chemical (composition, crystallization), environmental (oxygen) and mechanical (stretching, triaxial stresses).

  • 2.

    A mean stress in tension improves the fatigue behavior by crystallization of the stretched bonds for exclusively tension cycles. On the other hand, a minimum stress in compression seriously damages the material.

  • 3.

    Fatigue crack growth of

References (7)

  • R.S Rivlin et al.

    Rupture of rubber: I characteristic energy for tearing

    J. Polymer Sci.

    (1953)
  • P.B Lindley

    Energy for crack growth in model rubber components

    J. Strain Anal.

    (1972)
  • C.J Lake

    Prog. Rubber Technol.

    (1983)
There are more references available in the full text version of this article.

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