Criteria for Fatigue Design as Applicable to Rotating Biological Contactors,
CLOW CORP FLORENCE KY
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Fatigue is a localized progressive behaviour involving the initiation and propagation of cracks to final usually sudden fracture. In welded steel structures used for RBcs the initiation stage is non-existent because of the presence of small discontinuities introduced by all welding processes in or near the weldment - although good welding practice will minimize the number and size of these discontinuities. The first part of this paper explains how existing codes handle design criterion for welded structures for fatigue, what is meant by allowable stresses, stress categories, stress range etc. The second part will explain methods of improving fatigue life by controlling one or combinations of variables such as flaw size, residual stresses, stress concentration and will describe practical application of one such method. Two full size RBC shafts manufactured by above method has been successfully tested for 16 million cycles equivalent to 20 years of life.