Dynamic Plastics Testing at Low Temperatures
Temperature and strain rate affect how plastics behave. When designing components, it is therefore important to be aware of how the polymers used will behave not only in laboratory conditions but also in their subsequent operating conditions.
To this end, the scientists at the Fraunhofer Institute for Structural Durability and System Reliability LBF have expanded the dynamic testing possibilities of the Institute's own high-speed tensile testing machine with a device which enables plastics to be tested without a thermal chamber even at low temperatures; it has been proven to work down to -40 degrees Celsius.
The new device will allow plastics to be tested even below room temperature. It can also optically measure strain using DIC/GSC (digital image correlation/greyscale correlation). This measures a 2D strain field on the sample, which is then used to derive longitudinal and transverse strains. The research team generates the low temperatures with compressed air cooled by liquid nitrogen, with the sample positioned in this airflow. A thermal camera monitors the surface temperature, and measurement starts as soon as the desired temperature is reached. According to the LBF, the benefit of mixing compressed air with nitrogen is that the compressed air is dry and only a few ice crystals form on the surface of the sample. In addition, the gas mixture from the cold reservoir ensures a more constant airflow temperature than if the nitrogen were supplied directly.
From a granulate bag to a material card
The new cooling device consists of a controller and a switching element, a cold reservoir, a nitrogen tank and a supply line to the sample. Eliminating the thermal chamber is beneficial because it means there is no pane between the camera and the sample, which could mist up or freeze or form air vortexes if the pane is heated. This is not only advantageous for the DIC, and therefore results, but also allows flexible testing of different component sizes and different load types.
The new cooling device supplements the Fraunhofer LBF's capabilities in temperature ranges relevant for applications. Within this spectrum, it is not only possible to carry out the tests themselves but also to make use of the experience of those at the Institute in order to implement the entire process from a single source, from the delivery of a granulate bag to a validated material card.
By developing these testing procedures, the Darmstadt-based Fraunhofer LBF is helping many industries to design safe plastic components. These include the automotive industry, automotive suppliers, the aerospace industry and manufacturers of raw materials and consumer products such as sports equipment, garden tools and electric hand tools.