Modern materials such as elastomers and their compounds are becoming increasingly important in many aspects of day-to-day life. In other areas such as machine engineering, the automotive industry and aerospace, they have been used for years. The reason for this is that these polymers can withstand huge, non-linear physical strain without plastic deformation. Elastomer components are often used in areas where they will be subjected to major dynamic loads. So it is crucial to evaluate the characteristics and properties of this material before it enters the development process.
Materials have developed so quickly in recent years that the existing analyses and test machines are no longer up to the job, especially when trying to understand impact behavior. This applies as much to damage analysis as the design of components. We already know a great deal about mechanical material properties under slow strain, but the picture is still very vague with respect to fast speeds and high deformation rates which can only be looked at using expensive crash test stands with highly elaborate equipment. Understanding material properties in detail at high test speeds allows you to improve components in a targeted manner and thus make them much safer and more reliable. The data generated during testing is also needed to establish the parameters of FEM calculations used to design components for high load rates.
As part of a research project, Coesfeld GmbH & Co. KG and the Westsächsische Hochschule Zwickau in the west of Saxony developed a new, ground-breaking, high-speed test stand for capturing the impact properties of elastic materials. In partnership with the Steinbeis Research Center Applicationoriented Material-, Production-, and Process- Technology, the company has been examining a variety of elastomer materials made by European producers.
The new testing machine makes it possible to test materials at speeds between 2 and 50 m/s releasing energy between 10 and 4500 J. At the same time, samples can be tempered to examine the influence of temperature on material properties. The company is currently taking live measurements of a broad variety of elastomers to gain a comprehensive understanding of impact behavior at high rates of loading.
For example, conventional elastomer compounds with a variety of properties were examined at a test speed of 30 m/s to check their falling weight impact properties. A number of the elastomers examined, such as natural rubber (NR) or thermoplastic elastomers (TPE) are more elastic and the direction of the force adheres to excellent linearity. Nitrile rubber/Styrene-butadiene rubber or ethylene propylene diene mono- mer rubber (EPDM) can take higher loads than TPE and NR before giving way to the impact although elasticity is around half the level of the other samples.
Instrument-based checking of material reactions to falling weight impact on the new machine makes it possible to capture the impact behavior of elastomer materials in detail and thus provide new insights into the development of advanced, premium quality materials – opening the door to new applications.
Of course the mechanical properties of synthetic materials are strongly influenced by their close dependence on other testing constraints, such as the phasing of the load, the rate of the load, temperature and time. The test method outlined here allows you to examine the deformation properties of materials offering high distensibility in conditions very similar to actual strain environments.
Dipl.-Ing. Alexandru Söver
Prof. Dr.-Ing. Lars Frormann
Steinbeis Research Center Applicationoriented Material-, Production-, and Process-Technology (Zwickau)