Effect of media on the stress crack resistance of plastics effectively evaluate
The Kunststoff-Zentrum SKZ (a plastics standards organization) and the testing instrument manufacturer IPT have developed a testing instrument that offers new possibilities for investigating the influence of various media on the stress crack resistance of plastics. In addition to the pure determination of failure times via classical media creep rupture tests, the device also enables the continuous determination of the deformation of the test specimen in a heatable media container, as well as various test modes.
New testing facility at the SKZ to determine the influence of media on the stress crack resistance of plastics. (Photo: Luca Hoffmannbeck, SKZ)
Joint R&D project between SKZ and IPT successfully completed
Stress cracking, i.e. failure under the simultaneous action of mechanical stress and chemical medium, is one of the most common causes of failure for plastic components. The stress cracking test on plastics is described in the ISO 22088 series of standards. These methods are basically ranking methods and usually do not allow a quantitative determination of material properties relevant for the prediction of service life.
Development of an accelerated test concept
Within the framework of a research and development cooperation project funded by the German Federal Ministry for Economic Affairs and Climate Protection (BMWiK) through the Central Innovation Program for SMEs (ZIM), the two partners IPT Institut für Prüftechnik Gerätebau GmbH & Co. KG and the Plastics Center SKZ have developed a testing device that enables various mechanical tests of plastics under the influence of media. Parallel to the development of the device, the SKZ developed a time-lapse test concept in order to make prognoses for long-term behavior (failure times as well as creep deformation after several months and years) on the basis of relatively short-term tests at different test speeds and temperatures.
Conventional tensile tests are also possible
Unlike the test devices for the stress crack resistance of plastics that are already available on the market, which are all stand-alone solutions, the developed test system can be used to implement various load conditions and test modes while simultaneously recording the stress and strain of the test specimen in the heatable media container. In addition to determining failure times using classic media-saturated time tests, the device also allows the recording of time-dependent deformation (creep). Furthermore, relaxation tests at constant strain and recording of the time-dependent stress drop are also possible. In addition to these long-term tests (creep, relaxation), conventional tensile tests with constant pull-off speed or constant strain rate or force increase rate are also possible.
Six autonomous measuring stations
The current version of the testing device has six autonomous measuring stations, enabling parallel measurements with different media, plastics or test conditions (temperature, test type, test parameters). Tests in different media (e.g. alcohols, surfactants, oils, operating fluids – not self-igniting) are possible. This was achieved by manufacturing all parts that come into contact with the media from V4A stainless steel and by equipping the system with an extraction system that draws off any gases that arise. Heating of the media container and circulation of the medium allow measurements up to 95 °C. Measurements with forces up to max. 5 kN (per measuring station) and deformations up to 100 mm are possible on the test system. The strain measurement on the test specimen is carried out using a media-resistant extensometer (accuracy: class 1 according to ISO 9513). The measuring set-up allows various test specimens (e.g. test specimen types 1A, 1B, 1BA, 1BB, 5A, 5B according to ISO 527-2) to be tested.
New testing system now available
The developed testing system is now available to the SKZ for further research activities as well as for testing services. With this expansion of its testing equipment and the expertise built up over more than 50 years in ensuring the quality of plastic products, the SKZ is making an important contribution to ensuring that there will be fewer and fewer cases of damage due to stress cracking in plastic components in the future.
More information about the research area component properties
More information about the Institute for Testing Technology Gerätebau GmbH & Co. KG
