Fatigue Crack Growth in Welded Beams

The fatigue behavior of welded steel beams is evaluated using the fracture mechanics concepts of stable crack growth. A fracture mechanics model for cracks originating from the pores in the web-to-flange fillet weld is developed. Estimates of the stress-intensity factor are made that numerically describe the initial flaw condition. With the final crack size known, a theoretical crack-growth equation was derived from the fatigue test data of the welded beams. The derived relationship compares well with actual crack-growth measurements on a welded beam and available data from crack growth specimens. The regime of crack growth where most of the time is spent growing a fatigue crack in a structural element is shown to correspond to growth rates below ten to the minus six in. per cycle. Little experimental crack growth data is available at this level. It is concluded that the fracture mechanics concepts can be used to analyze fatigue behavior and to rationally evaluate the major variables that influence the fatigue life of welded beams.