Improved design of threaded connections by autofrettage in aluminium compounds for cyclic high pressure loading: design calculations and experimental verification

Threaded connections in an aluminium valve body under high internal pulsating 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 becomes the weakest point of this valve component. This effect is analysed with nonlinear finite element simulations, the FKM guideline for fatigue assessment and by experimental testing. The analytical and experimental parts match very well. 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 residual compressive stresses and an equalized stress distribution over the thread flanks can be achieved. Finally, different materials were chosen for the plug or screw, and this material influence for cyclic loading is shortly analysed.