Fatigue Crack Paths 2003

that the crack front shape oscillates between extremes corresponding to the boundaries

between the light and dark areas.

Figure 6. Sections through crack growth surface of tubular welded joint.

Figure 7. Tortuous crack growth surface in a tubular welded joint.

Spot Welds

Manyspot welds are lap joints, loaded in tension, as shown in Fig. 9. Whenspot welded

lap joints are loaded in fatigue, there are two distinct types of crack path [24], as shown

in Fig. 10. At low stress levels maximumprincipal stress dominated (Mode I) fatigue

cracks originate at or near a point A in Figs. 9 and 10(a). These link up into a single

crack and grow through the thickness of a sheet. The resulting crack front family is