Evaluation of the mechanical performance of a composite multi-cell tank for cryogenic storage: part II - experimental assessment

Kurzfassung

Cryogenic fuel containment at tank configurations that can conform to a prescribed space is of significant importance for hypersonic aircrafts. To ensure a safe flight, continuous monitoring of temperature, strain and damage initiation/propagation of the tank is crucial. The possibilities to perform these tasks are strongly dependent on the way the tank itself is designed and manufactured. Therefore, the study presented here focuses on both the analysis and full experimental evaluation of an innovative multi-spherical composite-overwrapped pressure vessel under hydrostatic testing at ambient conditions and pressure cycling with a cryogenic medium (LN2). The tested tank (scaled to 44 [l]) consists of four partly merged quasi isotropically reinforced spherical cells, where the intersections are strengthened by additional UD straps; the central cylindrical cavity is reinforced by an inserted hollow tube (aluminum). The design is characterized by the equal shell strain principle between the spheres and sphere connecting areas (intersections). During hydro-burst testing at a high displacement rate, the strain and damage progression was monitored with Digital-ImageCorrelation (DIC) and Acoustic Emission (AE) techniques. The effect of LN2 filling, pressure cycling and draining on the composite overwrap temperature gradient and related strains was additionally obtained with Fiber Bragg Gratings (FBGs) and thermocouples. The findings of this study were compared to a FEM-based thermo-mechanical model coupled with a progressive failure analysis (PFA) as presented in part I of this study (FE) [14]. The results of the hydrostatic tests completely verified the analytical and FE-based equal strain design principle where the predicted failure took place at the vicinity of the hollow center tube in a leak-before-burst sequence. In addition, stress-strain measurements in the cryogenic regime verified the suitability of the involved stiffness and CTE fitting functions (FE model). As a general conclusion, the most important finding here is the absence of damage in the composite shell; this may be regarded as a positive indication about the suitability of conformal Type IV multi-spherical COPVs for cryogenic storage. It is however believed that a careful consideration of the temperature-dependent strain field remains a critical parameter. Ideally, this strain field should be as homogeneous as possible to avoid CTE induced stress concentrations.