A brittle fracture is distinguishable from a ductile failure by the lack of deformation at the edge of the tear, and its bright granular appearance. A ductile failure has a dull grey appearance. The brittle fracture is also distinguished by the apparent chevron marking, which aids location of the fracture initiation point since these tend to point in that direction.
The factors which are known to exist where a brittle fracture may occur must be considered if this to be avoided. Firstly the design of individual items of ship structure must be such that sharp notches where cracks may be initiated are avoided. With welded structures as large as a ship the com- plete elimination of crack initiation is not entirely possible owing to the existence of small faults in the welds, a complete weld examination not being practicable. Steel specified for tFatigue Failures
Unlike brittle fracture, fatigue fracture occurs very slowly and can in fact take years to propagate. The greatest danger with fatigue fractures is that they occur at low stresses which are applied to a structure repeatedly over a period of time (Figure 8.5). A fatigue crack once initiated may grow unno- ticed until the load bearing member is reduced to a cross-sectional area which is insufficient to carry the applied load. Fatigue failures are associated with sharp notches or discontinuities in structures, and are especially pre- valent at ‘hard spots’, i.e. regions of high rigidity in ship structures.
With the growth in size of oil tankers, bulk carriers and container ships there has been increasing use of higher yield strength steels in their hull structures. The classification societies have subsequently placed special emphasis on analysis of the fatigue performance of these larger structures, usually over a 25 year life cycle, as part of their approval process.