Experimental and theoretical-numeric investigations of life estimates of pressure vessel components under cyclic stress

Fatigue strength assessment of pressure vessel as a rule is based on calculations, often on the basis of the relevant regulations. A detailed reference database of experimental reference values of typical pressure vessel structures has never been established because of the high cost and effort involved. For this reason, experimental verification is particularly important. In this investigation, a typical pressure vessel stress with a high fatigue hazard both in the welded and unwelded components was analyzed: A stud placed in the center of a plane slab was welded to it by an automatic welding process in accordance with the current specifications for the welded seam. No finishing process was applied on the weld. In the transition region between the plane slab and the cylindrical vessel, a groove was made in accordance with specifications, which was then analyzed as an unwelded component region with regard to its fatigue characteristics. Dynamic internal pressure load was used for simulating starting and shutdown processes to fatigue failure. In the experiment, fatigue cracking was observed both in the groove and in the transition zone of the weld. The experimentally defined load cycles were used as a reference value for assessing the accuracy of the various theoretical methods, with particular regard to the calculation procedure according to the specifications of the current technical regulations. It was found that accurate knowledge of the geometry of the real component is of decisive importance as compared to the acceptance of calculated permissible load cycles, which are reference values subject to high statistic scatter. (orig.)