Dissertations and theses : Doctoral thesis
Engineering, computing & technology : Mechanical engineering
Repplinger, Christian mailto [University of Luxembourg > Faculty of Science, Technology and Medicine (FSTM) > Department of Engineering (DoE) >]
University of Luxembourg, ​​Luxembourg
Docteur en Sciences de l'Ingénieur
Maas, Stefan mailto
[en] High-pressure hydrogen storage systems for fuel cell vehicles require a safe, compact, and light-weight design, especially the on-tank valves (OTV). These valves are directly connected to the high-pressure tank and manage the filling process of gaseous hydrogen up to a nominal pressure of 700 bar, the storage of the high-pressurized gas in the tank, and the supply of the fuel cell. The OTV enclosure implements several different technical devices, which are mostly sealed with radial elastomeric O-rings within one complex aluminum valve body.
This work’s first part analyzes and optimizes the fatigue life of the aluminum alloy (EN AW 6061 T6) OTV-body by defining the optimum autofrettage pressure before identifying the most significant effects influencing the sealing behavior of high-pressure loaded elastomeric O-rings at low temperatures in the second part. Both parts are of great benefit for safe product development considering the challenging technical requirements, such as a maximum pressure of 1050 bar and a temperature range of +85 °C to -60 °C, needed for the application of the OTV in a fuel cell vehicle.
The aluminum valve body includes several channels and bore intersections which are pressure-loaded in operation. This complex untreated valve geometry does not achieve the technical requirements of 150 000 pressure cycles without a failure. Determining the optimum autofrettage pressure for this complex aluminum valve body enables an improvement of the lifetime for this internally highly pressurized component. The autofrettage process induces residual compressive stress after the release of a single static overload pressure, leading to plastic deformation at the inner wall, whereas the outer pulsating operating pressure range. Due to the complex geometry of the aluminum valve body, a detailed elastic-plastic finite element analysis is used to determine the optimum autofrettage pressure. Three load steps are simulated in a non-linear way based on experimental stress-strain curves. The FKM-guideline is used to assess fatigue life and crack initiation with detailed subsequent experimental verification. Even if small cracks occur, residual compressive stresses prohibit crack growth (non-propagating or dormant crack). This is analytically verified by fracture mechanical considerations (crack closure effect) and is proven via internal fatigue pressure testing up to 500 000 load cycles. Crack propagation is analyzed by optical inspections with a microscope, computer tomography, and numerical determination.
The autofrettage process intentionally induces residual compressive stresses. Relaxation of these residual stresses due to cyclic loading in service would endanger the effectiveness of autofrettage and could ultimately lead to unexpected fatigue failure. Therefore, strain-controlled experiments up to 500 000 load cycles and amending non-linear finite element simulations are done for the aluminum
alloy EN AW 6061 T6 to study potential cyclic stress relaxation in four-point bending tests after controlled single static plasticization for residual stress generation.
The elastomeric O-ring seals must ensure functionality at high-pressure and a wide range of temperatures. The elastomeric material`s performance is especially limited at low temperatures. Geometrical and material effects are analyzed and assessed by numerical simulations and experiments. An accurate material model is necessary to present the complex material behavior and its influences on the sealing behavior. Therefore, an elastomeric seal material modeling guideline is developed to present the most significant effects. The mechanical material behavior of elastomers depends on time, temperature, and pressure. A thermo-rheologically simple (TRS) visco-hyperelastic material model is defined with the time-temperature superposition principle (TTSP) and used for the finite element simulations. This material model is validated with several material tests. The numerical analyses are especially useful in highlighting the individual influences of machining tolerances, different thermal expansion coefficients, limited recovery, and stress relaxation of the elastomer. The appearance of compressibility or volume swelling and their impacts on the sealing behavior are also explained. Experimental leakage tests are done for several O-ring dimensions from -60 °C to +23 °C up to a maximum pressure of 970 bar. The effect of machining tolerances and the necessity of a back-up ring are analyzed in a first test session. A second test session compares a relatively thin O-ring to a thicker one and investigates the sealing behavior of different geometries of the gland and the back-up ring. These geometrical design optimizations lead to a clear improvement of the sealing behavior.

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