Abstract :
[en] Introducing gaseous high-pressure hydrogen storage for fuel cell vehicles requires safe light-weight valves for the automotive gas management. In contrast to thin-walled pressure vessels, there are no calculation or design guidelines available due to the huge variety of possible geometries and integrated functions. However, hydraulic cyclic pressure tests are compulsory for a finite-life fatigue strength certification of hydrogen valves.
The valve body, linking different functional devices with each other via internal bore intersections, is the most critical part since the sharp-edged bore crossings cause high stress concentrations which distinctively limit the fatigue strength of such internally pressurized parts. Due to demands for light-weight design and manufacturing advantages it is aluminium which should be used as an appropriate material for the valve body. However, its fatigue properties need to be proved.
Because of the complex valve body geometry, local fatigue evaluation concepts are initially applied to a simplified internally pressurized bore intersection and compared to the results of tested samples under pulsating pressure. However, those tests revealed an early crack initiation and a fast spreading of cracks in the aluminium under cyclic load and, thus, the requirements of the applicable testing regulation are not fulfilled.
The present work focuses further on a method to increase the fatigue life by inducing residual compressive stresses at the areas of high stress concentrations. Here, the so called autofrettage, which is typically used for internally pressurized geometries, is a promising technology since it induces residual compressive stresses at the hotspots due to a unique static overload pressure with a distinctively higher pressure level than the subsequent cyclic pressure during operation. Although this is a well-known method, its potential for aluminium is not understood sufficiently. This is also the cast for the geometry dependent choice of a suitable autofrettage pressure range which is still inadequately clarified for the herein studied complex valve body geometry. An efficient design method based on a non-linear finite element method is derived from and applied to the valve body geometry. In order to perform the non-linear simulations, additional information about the plastification behaviour for reverse loading is necessary and being derived from uniaxial material tests. Fatigue testing of the valve body under cyclic pressure load shows a highly increased fatigue life and a design rule for the choice of an appropriate autofrettage pressure is verified.
Besides sharp-edged bore intersections, high stress concentrations are also existent at the threads in the aluminium valve body, leading to an early crack initiation and a fast crack growth. In contrast to the typical implementation of the autofrettage process, it is shown that also even a unique static overload on the threads leads to an increased fatigue life. Thus, the end plugs should be used during the autofrettage process which causes residual compressive stresses at the thread root and a stress homogenisation over the carrying threads. These effects are studied with the help of non-linear finite element simulations considering the detailed thread geometry, the non-linear material behaviour and frictional contact.
This leads to the conclusion that the effects of autofrettage as a method to increase the fatigue life by inducing residual compressive stresses for valve bodies for high-pressure hydrogen applications are being analysed in detail. As a result, it can be stated that with an appropriate selection of the autofrettage pressure and the suitable implementation of the process towards the valve body geometry, the required number of pressure cycles according to the applicable regulation can be successfully achieved.
