Lifetime impact of the surface finish, including the mould partition line, on rubber test pieces and components

Introduction and intention:

Since one of the first methodical investigations in 1940 by Cadwell et al.[1], the interest in the mechanical fatigue of rubber is unabated. The comprehension of the damage mechanisms within a rubber exposed to a dynamic load is fundamental for the design of rubber components. However, after 80 years of research and far more than 400 publications dealing with the fatigue of rubber, many major questions are still open or not even ask yet. In order to progress in rubber fatigue research, simple test piece geometries with defined conditions at the origin of the failure are used. A large number of  articles provide some fatigue results or extensive databases of lifetime data using such test pieces. These kind of experimental databases are often applied for research proposes but also as a foundation for the End-of-Life prediction of components. The goal of the present study is motivated by a former project of the research institution DIK. Within this project, mechanical fatigue tests with different uniaxial test pieces were conducted. These test pieces experience the same strain-state and similar stress-state at their average location of major crack initiation. However, a tremendous difference of several tenfold in number of load-cycles to failure was measured between the test pieces as shown in Fig. 1. Using a reflected light microscope, one could ascertain geometrical differences in the mould partition line as the main reason for the lifetime deviation. In general, it was concluded that fatigue cracks initiating at the mould partition line lead to shorter lifetimes in comparison with the reference test piece with surface-independent failure.

Objective

Main objective of the project is a deeper understanding of the impact of the surface finish, with focus on the mould partition line on the mechanical fatigue of rubber. This impact shall be characterised for two, in their crack initiation behaviour very different, materials, natural rubber against a synthetic rubber (e.g. SBR-based). The first is known as a multi-crack coalescing material e.g. [3] whereas the latter generally fails doe to a single major crack.


Approach

The project objective shall be reached with performing fatigue tests by consciously varying the properties of the surface finish with focus on the mould partition line. These fatigue tests are related to surface independent fatigue results originating from a newly developed fatigue test piece. Differences in the surface finish and geometry of the mould partition lines among the selected test pieces are evaluated and reported using appropriate microscopy technics. Within reason, the differences in surface finish and mould partition line shall be described using finite-element-simulations. Fig. 2 shows a very generic example of how to describe the stress-concentration-effect of a mould partition line with varying thickness. It shows the possible strong impact on the local deformation doe to the geometry of the mould partition line on a microscopic scale. Next to these irregularities in the thickness of the mould partition line, the total thickness and its lifetime impact is an unknown and shall be investigated as well. Subsequent to the study of the mould partition line, the focus can be shifted towards the influence of the general surface roughness on the fatigue behaviour. The  comprehension of the mould partition line effect is the necessary foundation for this step.

Economic benefit

Since fatigue tests with simple test pieces are often performed as a basis for the End-of-Life prediction of components, it is of fundamental interest to develop understanding of the significant impact on the lifetime of the surface finish. This knowledge enables on the one hand a more precise finite-element-method-based lifetime prediction and contains on the other hand a major potential in  improvement of mould design. Thanks to the consideration of the two materials, insights might be gained to a material-tailored mould design; expensive surface treatment only where and if necessary.

Time, project management and contact details

• Funded by industry
• Approx. project duration: 2.5 years
• Expenses: 90 000 EUR/year. However, with a limit of 20 000 EUR/year per participant
• Deadline for response of interest: 01. June 2021

Deutsches Institut für Kautschuktechnologie (DIK) e. V.
Oliver Gehrmann
Eupener Straße 33
30519 Hannover
Email: Oliver.Gehrmann@DIKautschuk.de

[1] S. M. Cadwell, R. A. Merrill, C. M. Sloman, and F. L. Yost. Dynamic fatigue life of rubber. Rubber Chemistry and Technology, (13):304–315, 1940.
[2] O. Gehrmann and N. H. Kröger. Schlussbericht zu IGF-Vorhaben Nr. 20537 N, Untersuchung des Einflusses von Relaxationsphänomenen auf die Lebensdauer technischer Gummiwerkstoffe. 2021.
[3] T. Balutch, B. Huneau, Y. Marco, P. Charrier, and C. Champy. Fatigue behaviour of an industrial synthetic rubber. MATEC Web of Conferences, 165:22004, 2018.