The following paper is intended to improve the fatigue behaviour of a complex aluminium valve geometry under high internal cyclic pressure loading. The autofrettage process helps to increase the fatigue durability and a simple but efficient design method for this process is pointed out. Based on non-linear material's behaviour, finite element simulations of the crack-free geometry help to determine the minimum and maximum autofrettage pressure to be used, without iterative crack simulations, which would require a lot of computational time. Material tests under inverse plastifications were performed in order to determine the correct material model. The derived design method was verified with simplified specimens subjected to different autofrettage pressure levels and subsequent cyclic fatigue tests.

Threaded connections in an aluminium valve body under high internal swelling pressure are investigated. A static straining process called autofrettage leads to an improved fatigue behaviour of the aluminium component, while normally the threaded connections are unloaded during this autofrettage. But by unloading the thread during autofrettage the first loaded thread flank became the weakest point of this valve component. This effect is analyzed with non-linear finite element simulations, FKM guideline for fatigue assessment and by experimental testing. The analytical and experimental parts match very well and it can be shown that a well-designed autofrettage without unloading the threaded connection is helpful for the aluminium thread and extends its fatigue lifetime, as compressive residual stresses and an equalized stress distribution over the thread flanks can be generated. Finally different materials were chosen for the plug or screw and this effect for cyclic loading is shortly analyzed.