Fatigue initiation and propagation in natural and synthetic rubbers

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 Δ T, 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.