Challenges in cryogenic mechanical testing of unidirectional composites – special focus on matrix dominated properties.
Topic(s) : Special Sessions
Co-authors :
Michael SCHEERER (AUSTRIA), Zoltan SIMON , Michael MARISCHLER , Heinrich SCHODERAbstract :
Due to its high specific stiffness and specific strength composite materials are widely used in industries where weight optimization plays a crucial role such as the space industry where the launch costs of 1 kg may reach 10.000s of euro. One key component of launchers are cryogenic propellant tanks containing liquid oxygen and liquid hydrogen. Liner-less all-composite tanks show the potential to further decrease the weight of composite propellant tanks by around 25% compared to conventional metal-lined tanks. On the other, hand without a liner microcracks in the polymer matrix might lead to leakage of the propellant, pressure loss and in worst case explosion under working condition. Such microcracks can appear by the combined thermal and mechanical stresses in the composite structure arising from the exposure to the propellant temperature, the internal pressure and the launch loads. Therefore, reliable material data under cryogenic conditions are required for fail safe composite cryogenic propellant tanks. The authors of the paper perform cryogenic mechanical testing down to temperatures of liquid helium on different type of materials ranging from metals, over plastics and composites to honeycomb panels since more than 20 years. During that time most of the uni-directional tensile samples tested perpendicular to the fiber direction failed at the tab region leading to a potential underestimation of the transverse tensile strength important for the design of composite structures against matrix cracking.
Within this paper critical aspects of composite coupon testing under cryogenic conditions are discussed. The investigations focus on matrix dominant properties such as tensile properties perpendicular to the fiber direction of uni-directional composites and the in-plane shear behaviour. The following parameters were analysed: Shape and material of the tab material used for load transfer between the coupon and the fixation, shape of the coupons – straight and dog-bone, different top and bottom layer cross ply architectures in 90° tensile samples.
Multi-scale finite element analyses of the behaviour of the tabbed coupons were performed to assess the thermo-mechanical behaviour of the coupons on macro- and microscale when loaded under cryogenic conditions. Out of these analyses stress concentration regions will be identified and evaluated with respect to potential early failure that arise from the design of the coupon samples. Special care was laid on the usage of top and bottom cross-ply layers of different angles to avoid early failure around the tab region of the coupons. The results of the FE analyses were verified by experimental results performed on the different coupon designs at cryogenic conditions at -196°C in liquid nitrogen and at -269°C in liquid Helium. As no single failure mode is present in coupons with top and bottom cross ply layers, the appearance of transverse matrix cracking was assessed via online Acoustic Emission monitoring.
Within this paper critical aspects of composite coupon testing under cryogenic conditions are discussed. The investigations focus on matrix dominant properties such as tensile properties perpendicular to the fiber direction of uni-directional composites and the in-plane shear behaviour. The following parameters were analysed: Shape and material of the tab material used for load transfer between the coupon and the fixation, shape of the coupons – straight and dog-bone, different top and bottom layer cross ply architectures in 90° tensile samples.
Multi-scale finite element analyses of the behaviour of the tabbed coupons were performed to assess the thermo-mechanical behaviour of the coupons on macro- and microscale when loaded under cryogenic conditions. Out of these analyses stress concentration regions will be identified and evaluated with respect to potential early failure that arise from the design of the coupon samples. Special care was laid on the usage of top and bottom cross-ply layers of different angles to avoid early failure around the tab region of the coupons. The results of the FE analyses were verified by experimental results performed on the different coupon designs at cryogenic conditions at -196°C in liquid nitrogen and at -269°C in liquid Helium. As no single failure mode is present in coupons with top and bottom cross ply layers, the appearance of transverse matrix cracking was assessed via online Acoustic Emission monitoring.