Cyclic loadings and crystallization of natural rubber: An explanation of fatigue crack propagation reinforcement under a positive loading ratio

doi:10.1016/j.msea.2010.09.079

Materials Science and Engineering: A

Volume 528, Issue 3, 25 January 2011, Pages 1078-1086

https://doi.org/10.1016/j.msea.2010.09.079 Get rights and content

Abstract

Natural rubber is known to have excellent fatigue properties. Fatigue crack propagation studies show that, under uniaxial tension loading, fatigue crack growth resistance increases with the loading ratio, even if the peak stress increases. Studies dealing with crack initiation confirm this trend. If strain induced crystallization is believed to play a major role in this reinforcement process, it is not clear yet by which mechanism this reinforcement takes place. Using SEM investigation, it is shown here that the reinforcement process is associated with strong crack branching in the crack tip region. From experimental results it is shown that under particular reinforcing loading condition a cyclic strain hardening process can be observed on the natural rubber which is able to overcome classically observed softening effects. A cumulative strain induced crystallization process is proposed to explain the stress ratio effect on fatigue crack initiation and propagation properties of natural rubber.

Research highlights

▶ Crack branching extending in a fractal-like behavior across the scales. ▶ Crack branching reduces significantly the crack growth rate. ▶ Cyclic hardening can be observed on natural rubber under particular loading conditions. ▶ Cyclic hardening is induced by accumulation of strain induced crystallisation. ▶ Crack branching is induced by accumulation of strain induced crystallisation.

Introduction

Natural rubber's resistance to crack growth and its ability to withstand large strains without permanent deformation are two of the main reasons for its extensive use in many industrial applications. Most of them involve significant static and cyclic loading. The need for appropriate multiaxial fatigue life criteria has become crucial over the past 10 years. A good understanding of the micromechanisms involved in the fatigue crack initiation process are essential for the establishement of physically motivated criteria. In the case of natural rubber, in addition of damage mechanisms, specific reinforcing mechanisms still have to be understood. Indeed, under particular loading conditions (under uniaxial loading these particular conditions correspond to positive stress ratio tests), natural rubber shows an exceptional fatigue resistance so that damage mechanisms only cannot fully describe the fatigue behavior of natural rubber. A reinforcing mechanism must be taken into account in order to propose a fatigue life criterion suitable for all types of loading. The aim of this paper is not to propose such a criterion but rather to understand the origin of this mechanism so that a physically based criterion can be proposed (see [1], [2] for a multiaxial fatigue life prediction method taking into account reinforcing mechanisms on natural rubber). The particular strength of natural rubber, as opposed to synthetic rubber on which such reinforcing mechanisms are not observed, has been attributed to its ability to crystallize upon stretching. Strain induced crystallization is believed to play a major role in fracture properties of rubber by inducing strong microstructural changes in the crack tip region. Strain induced crystallization of natural rubber has been largely investigated using numbers of techniques such as X-ray diffraction [3], [4], [5], [6], infra-red spectrometry [7], [8], birefringence [9], low-angle light scattering [10], Raman [11], [12], or nuclear magnetic resonance [13], [14]. These studies have resulted in a better understanding of the link between microstructural features (cross-link density, chemical composition, fillers, see [6] for more details), temperature and strain on natural rubber crystallization. The work of Trabelsi et al. [3], Siesler [7] and Toki et al. [4], [5] give interesting results concerning the effect of cyclic deformation on strain induced crystallization, which is of prior importance when dealing with fatigue loading. However, the aim of these studies was not to link the strain induced crystallisation to the reinforcement processes observed under fatigue loading so that the loading frequencies were several orders of magnitude below the one usually encountered in fatigue tests, the type of strain cycles did not correspond to cases where reinforcement processes are observed, and finally the link with fatigue crack initiation mechanisms was not assessed. For these reasons no satisfactory answer to the reinforcement process observed under cyclic loading has yet been proposed.

In this paper we present the crack initiation mechanisms observed under reinforcing conditions and propose an explanation for the reinforcement under cyclic loading and its relation to strain induced crystallization.

Section snippets

Strain induced crystallization and fracture properties

Strain induced crystallization is due to the particular steric purity of the polyisoprenne backbone (in our case 99.9% polyisoprenne cis-1,4). By stretching, the conformational entropy decreases, which increases the ability of natural rubber to crystallize. Strain induced crystallization of natural rubber has been largely investigated using a number of techniques as stated in the previous section. The morphology of the strain/stress induced crystallization has been reported to have many

Material

The elastomer used in the present study was a vulcanized natural rubber material filled with 23 parts of reinforcing carbon black (N772, N330) per hundred parts of rubber. Formulation and mechanical properties are given in Table 1, Table 2. Thin strips of material were cut from 150 mm × 150 mm × 2 mm rubber sheets. Those specimens were used for one-dimensional cyclic tensile tests presented in Section 4.2.3. Before testing, the material was pre-conditioned 100 cycles at the maximal elongation (in our

Uniaxial fatigue results

One-dimensional fatigue results have been reported in Fig. 3(a) in a Haigh diagram with two parameters, the stress amplitude and mean stress. The dashed lines represent optimized iso-contour lines using a simple bi-linear model. It is interesting to note that the fatigue life sensitivity to the loading ratio R is in agreement with that observed by Cadwell in his early studies. At constant stress amplitude, increasing the mean stress increases the fatigue life for positive loading ratio tests.

Cumulative effect and crack branching

The aim here is to better understand from the previous results the microstructural evolution in the crack tip loading under relaxing and non-relaxing conditions. Under relaxing conditions the evolution of crystallinity follows the experimental results obtained by Toki [4] and can be modeled as shown in Fig. 11(a). At each cycle the crack tip is fully unloaded so that strain induced crystallization disappears and the crystallinity level restarts from zero. Under non-relaxing conditions, it is

Conclusions

Using SEM observations it was shown that the excellent fatigue resistance properties under non-relaxing conditions are related to strong crack branching extending in a fractal-like behavior during both the initiation and the propagation process. The crystallization process was studied using WAX diffraction. A cyclic hardening has been observed on a natural rubber using non-conventional stress–strain paths. This cyclic hardening was understood as the result of a cumulative process of

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Cyclic loadings and crystallization of natural rubber: An explanation of fatigue crack propagation reinforcement under a positive loading ratio

Abstract

Research highlights

Introduction

Section snippets

Strain induced crystallization and fracture properties

Material

Uniaxial fatigue results

Cumulative effect and crack branching

Conclusions

Int. J. Fatigue

Int. J. Fatigue

Polymer

Polymer

Spectrochim. Acta A

Polymer

Polymer

Int. J. Fatigue

Macromolecules

Macromolecules

Appl. Spectrosc.

Macromol. Chem. Macromol. Symp.

Macromolecules

J. Polym. Sci.

J. Polym. Sci. B

J. Chem. Phys.

Rubber Chem. Technol.

Proc. R. Soc. A