A Quicker Method to Measure Rubber Ageing
The properties of rubber-like plastics change due to temperature and the impact of force over the course of their use. In order to make predictions about service life, the activation energy of thermal ageing is determined by stress relaxation. A multichannel test bed should now speed up the process.
For the product development of vehicle tyres, for example, it is important to be able to make predictions about the service life of the elastomers under certain conditions. Determining the activation energy of thermal ageing is essential, and the stress relaxation method can make this easy and cost-effective. However, the measuring has until now taken several weeks to months to complete, depending on the temperature selected. With the new test bed, which has been developed at the Fraunhofer Institute for Structural Durability and System Reliability, it is possible to simultaneously test the activation energy at six different temperatures and to improve both the efficiency and the quality of service life predictions.
The stress relaxation method is particularly suitable for predicting the service life of elastomers. The material is stretched at a higher temperature by a certain amount, usually by 10%. This stretching is then maintained and the tensile stress of the material is continuously measured. The stress decreases over time, meaning it relaxes. This is due to ageing processes in the sample: physical and chemical cross-links are degraded, and polymer chains are severed.
The multichannel measuring station consists of six test chambers which can be adjusted independently from one another. The researchers from Darmstadt can investigate samples in each of the chambers with a constant temperature and a particular displacement. A power sensor records the constantly falling tensile force over the entire duration of the test. This parallel measurement of the ageing process enables a time saving of several weeks in comparison to single-channel test beds.
Algorithms for determining the activation energy
An analysis algorithm helps to determine how high the activation energy is. The researchers developed this on the basis of what is known as a master curve construction. The individual curves of the different temperatures are combined with suitable displacement factors to form a master curve. By applying and adjusting the logarithmic displacement factors in an Arrhenius diagram, the scientists are able to determine the activation energy.
The method can be used to calculate the displacement factors at any temperature and to predict the corresponding force-time curves. This allows an estimate of the service life of the materials to be made. It appears that the activation energy of stress relaxation corresponds to that of very complex cyclical fatigue experiments.