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S. Seichter, V. Archodoulaki, T. Koch:
"Measurement set-up to investigate the crack nucleation and crack growth approach for the estimation of the fatigue life of rubber";
Talk: DVSPM 2015, Gmunden; 2015-05-11 - 2015-05-13; in: "Danube Vltava Sava Polymer Meeting - DVSPM 2015, Proceedings of a Conference on Polymer Science", M. List, C. Paulik, Z. Major, I. Teasdale, O. Brüggemann (ed.); Trauner Verlag, Linz (2015), ISBN: 978-3-99033-491-1; 81.

Due to its properties rubber is often used for applications where cyclic loading is applied. It is very important to know how long rubber parts can withstand those loadings and many different approaches can be found to predict the fatigue life of rubber under cyclic loading.
The models can be classified in two approaches. The first one is the crack growth approach. In 1964 Gent et al. described a method to estimate the life time of rubber dumbbell specimen from the crack growth behaviour of tensile strips made of the same material. This relationship has been often used and modified to give a better fit to the experimental data. A second approach is the crack nucleation approach. The fatigue lives of specimens are detected in terms of a fatigue life parameter and the results are used to predict the fatigue life for different loadings or for applications where a history of different loadings is questioned. Calculations are done using Finite Element Methods, but also analytical equations are derived, for example by Wang et al.
Although many different models already exist, the optimal model is still subject of research. For further investigation and development it is very crucial to experimentally validate the existing models. Furthermore, most of the previous studies worked with elastomers consisting of one or maximal two polymer types. In the present work application-relevant rubber compounds are investigated containing SBR, NR and BR and also different types of carbon black.
Two testing machines which apply axial load on the samples are used. An electrically driven machine is used to measure the fatigue life of rubber compounds. The observation of the samples is realised by an optical system which takes pictures of the surface every N cycles and by recording the electric current consumed by the drive to execute the maximal elongation. Due to the optical measurement system superficial cracks of the sample can be detected. A hydraulic testing machine is used to measure fatigue life under high loads and to determine the crack growth behaviour of pure shear samples. It is equipped with an optical measurement system and an image processing system which is used for the crack growth measurements. The hydraulic testing machine is also equipped with a position sensor and a load cell to record the hysteresis curve.
For both experimental set-ups the same specimen shape is used in order to provide a better coherence between the fatigue life and crack growth measurements since the crack growth behaviour of rubber is strongly dependent on the specimen geometry.
In this work the measurement set-up and the different approaches to determine the fatigue life are presented. The first results show that the sample holders have a detrimental influence on the crack initiation. In order to exclude such effects the designs of the specimens and the sample holders have been optimized.
Besides the evaluation of the measurement set-up the experiments also show the influence of the compound composition on material properties like heat build-up, hysteresis, crack growth and subsequently fatigue life.

Fatigue life estimation, crack growth, measurement set-up