Co-authors​ :

 Nassos SPETSIERIS (UNITED KINGDOM), Michael GOWER , Richard SHAW , Peter LOVELOCK , Stefanos GIANNIS (UNITED KINGDOM) 

The aviation industry’s transition to using hydrogen as a sustainable fuel is undoubtedly pushing the boundaries when it comes to materials that can withstand the cryogenic temperatures that liquid hydrogen (LH2) must be sustained at. To develop such materials, it is imperative that testing and characterisation methods are fit-for-purpose and accurately measure the required properties at the associated environmental conditions. For decades, there has been a lot of research generated around the nuances of cryogenic mechanical testing of materials, mainly for the space/aerospace industries, as well as for characterising superconductive materials for particle physics. The typical test setups mainly consist of a load frame (commonly a universal test machine) and the device that is responsible for cooling the test space down to the desired cryogenic temperature. Regarding the latter, a standard environmental test chamber can be used in conjunction with liquid nitrogen evaporative cooling to achieve temperatures as low as ~100K. For reaching lower temperatures, a cryostat operating with liquid nitrogen or helium is typically required. When it comes to mechanical testing however, the method of the load introduction to the specimen is also critical. Ensuring that the specimen is well aligned with regards to the load direction and that the load is exerted fully on the specimen without any relative motion or compliance throughout the fixture is of outmost importance. Gripping the specimen is especially challenging at cryogenic temperatures, as the fixtures and jigs typically used for room temperature testing, are not suitable for low temperature use. Therefore, there are numerous issues that can be observed when testing with non-cryogenic rated equipment at cryogenic temperatures. The most common, would be the slipping of specimens in the grips because of trapped moisture that has iced, as well as loosening of any threaded and load bearing connections because of thermal contraction of the different components that are commonly made from different materials. In this work, a novel gripping system has been developed, allowing for tensile testing of fibrous composite materials at cryogenic temperatures. Two different design approaches have been proposed and their unique benefits, when it comes to optimising the test procedure, are hereby presented. The performance validation is done in two tests setups. The first consists of an environmental chamber where gaseous nitrogen is used to achieve a target temperature of 110K, while the second one is in a liquid flow cryostat where the test space is submerged in liquid nitrogen at 77K. The requirements, and modifications necessary for reaching 20K are being investigated, with heater elements being introduced to bring liquid helium to a gaseous state and achieve the temperature of liquid hydrogen (~20K).