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 the hull construction should therefore have good ‘notch ductility’ at the service temperatures particularly where thick plate In association with the problem of brittle fracture it was not uncommon at one time to hear reference to the term ‘crack arrester’. The term related to the now outdated practice of introducing riveted seams in cargo ships to subdivide the vessel into welded substructures so that any possible crack propagation was limited to the substructure. In particular such a ‘crack arrester’ was usually specified in the sheerstrake/stringer plate area of larger ships. Today strakes of higher notch toughness steel are required to be fitted in such areas. Lloyds Register, for example, require the mild steel sheerstrake and stringer plate at the strength deck over the midship por- tion of vessels of more than 250 metres length to be Grade D if less than
15 mm thick and Grade E if of greater thickness (see Chapter 17).