Analysis of residual stress relaxation of aluminum alloys EN AW 6061/-82 T6 under cyclic loading

in Fatigue and Fracture of Engineering Materials and Structures (2021)

Stress relaxation describes the reduction of stress under static or cyclic loading at a constant strain level. Several processes induce intentionally residual stresses, for example, autofrettage of thick ... [more ▼]

Stress relaxation describes the reduction of stress under static or cyclic loading at a constant strain level. Several processes induce intentionally residual stresses, for example, autofrettage of thick-walled pressurized tubes to improve their fatigue life. This well-known process induces residual compressive stresses at the critical inner surface by using a single static but controlled over- loading internal pressure. Relaxation of residual stresses due to cyclic loading in service would endanger the effectiveness of autofrettage and could finally lead to unexpected fatigue failure. In this study, strain-controlled experiments up to 500,000 load cycles and amending nonlinear finite element simulations were done for the aluminum alloys EN AW 6061 T6 and EN AW 6082 T6 to study potential cyclic stress relaxation in four-point bending tests after con- trolled single static plasticization for residual stress generation. This analysis identifies almost stable residual stresses for both materials under different cyclic strain-controlled load levels. [less ▲]

Optimization assisted redesigning a structure of a hydrogen valve: the redesign process and numerical evaluations

in International Journal on Interactive Design and Manufacturing (2020)

This study introduced the redesign process of an automotive hydrogen valve. The process relied on the structural optimization approach, which used to build up the new valves having promising stiffness and ... [more ▼]

This study introduced the redesign process of an automotive hydrogen valve. The process relied on the structural optimization approach, which used to build up the new valves having promising stiffness and the lowest possible weights. To achieve the goals, the study was proposed to be taken place via the three main stages. These stages included topology optimization, lattice optimization, as well as numerical evaluations. The achieved results firstly indicated that the two newly designed valves possessed longer life and lower mass than the original valve. Especially, the topology optimized one could withstand more than 5E4 working cycles in the pre-treated condition before the first crack would be nucleated. The results also pointed out the influences of the pre-treatment pressure on the fatigue performance of the hydrogen valve. Within the examined ranges of the pressure, increasing the pressure’s magnitudes tended to shorten the fatigue life of the topology optimized valve. Additionally, the results highlighted the impact of the employed materials on the estimated fatigue life of such a non-treated structure. In the highlights, the considered steel valves could function normally far beyond 1.5E5 working cycles while the aluminum valves would have an initial crack formation prior to reaching 3E3 cycles. The results also suggested that further practical evidence is needed to not only confirm whether the selected printed aluminum is among the promising candidate materials of the hydrogen valve but also to support the described evaluations. [less ▲]

Untersuchung der Ermüdungsfestigkeit von komplexen innendruckbelasteten Aluminiumgeometrien und der Lebensdauersteigerung durch Autofrettage

Doctoral thesis (2014)

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 ... [more ▼]

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. [less ▲]