R&D testing in extreme temperatures has long been part of ensuring required longevity and performance of automotive materials. That is particularly so regarding plastics, the behavior of which depends on temperature and strain rate. That testing usually involves the use of a thermal chamber, but these can have drawbacks. So, a team of scientists at the Fraunhofer Institute for Structural Durability and System Reliability (LBF) in Darmstadt, Germany, set about developing an alternative that could aid the development of plastics for everything from trucks to e-bikes.
Now in operation, it utilizes technology based on compressed air cooled by liquid nitrogen to deliver “highly performing” results, LBF claims, particularly regarding digital image correlation (DIC). The new method provides flexible testing of a variety of component sizes, together with different load types, at temperatures down to minus 40°C/F.
Enabling fewer iterations
Full understanding of behavior of plastics across the broad spectrum of temperatures in both the laboratory and in actual service is an R&D must. To this end, Fraunhofer’s new method puts samples in the flow of compressed air mixed with liquid nitrogen (-195°C/-319°F) that is described as allowing only a few ice crystals to form on a test sample’s surface.
The gas mixture from a cold reservoir also ensures a more constant temperature of air flow than when applying liquid nitrogen directly. The technology incorporates a controller and switching element, the cold reservoir, a nitrogen tank and supply line. Unlike a thermal chamber, there is no glass pane between recording camera and test sample that otherwise might tarnish, freeze or form air vortices when a pane is heated.
According to engineer Axel Nierbauer, a member of Fraunhofer LBF’s research team, the technology will benefit automotive OEMs and suppliers by reducing, through more exact material parameters, the iterations in simulation and later in physical test parts. “For the material models on which the simulation is based, the material behavior is required as precisely as possible, so that the results of that simulation come as close as possible to reality,” he told SAE International.
For automobiles, the parameters of all possible operating conditions are required so that the safety of the vehicle is guaranteed in both cold and hot climates. Nierbauer noted that the strain rate dependency of plastics represents another large influence on mechanical behavior. “Plastics show a different behavior with static loads or dynamic loads as they occur in a crash. The evaluation of all these influences is necessary for a safe vehicle design,” he said.
While dynamic tensile tests at low temperatures are not new, Fraunhofer’s new technology represents a novel approach to a flexible characterization of low-temperature behavior at high strain rates, Nierbauer claimed. It was developed to offer fresh possibilities regarding visual observation of samples during testing, and different sample geometries and clamping designs.
Inside the thermal chamber
When using a thermal chamber in dynamic tests with optical strain recording by DIC, Nierbauer noted that there are disadvantages due to having one or more glass panes between the sample and the camera. “In the case of a single pane, the strong temperature gradient causes precipitation of moisture, or even the formation of ice crystals on the pane – or heat shimmer when the pane is heated,” he explained. “In the case of multiple panes, the reflections between them and the decreasing transmission are more of a problem.”
This has a negative effect on the image quality. And above a certain degradation, the quality of the results of the gray scale correlation also deteriorates. During high speed testing, the number of available images for DIC is usually restricted by the available frame rate of the high-speed cameras, he said. This makes it even more critical if some of these images are rendered unusable due to the above reasons, making the results prone to scatter.
The usual advantage of thermal chambers does not apply to dynamic testing. Nierbauer provided details: “A large room is tempered in chambers, so that the sample can be tempered homogeneously even with large elongation. However, dynamic tests are carried out with short samples and high pull-off speeds. This leads to low elongation at break and our temperature control, which is spatially limited due to its concentrated air flow, can be ideally used. A cooled air stream is directly applied to the sample.”
The slim design of the unit in the sample area enables it to be monitored with a thermal camera and a camera for DIC, without the usual disadvantages of a thermal chamber mentioned above. This gives much more reproducible results and a higher reliability during the test execution, he said. Moreover, the usual restrictions on test geometries and clamping setups that must be considered with thermal units are not applicable.
Responding to industry needs
The improvement of strain-evaluation results from DIC tests has always been a field of research at LBF. “Especially in the case of high strain rates and low temperatures we are eager to improve results and reliability of the available testing methods,” Nierbauer said, adding that the new concept was “based on the demand of our industry partners for more reliable dynamic material parameters.”
According to Nierbauer, to further develop and productionize the technology, The Fraunhofer Institute is working with a variety of companies including automotive OEMs and suppliers. “In the context of increasing sustainability demands across the transportation sector, the investigation spectrum also comprises alternative modes of urban transport such as the e-bike or other small modes of transport,” he said.Continue reading »