Page 102 - Failure Analysis Case Studies II

87

Fig. 7. Close-up of hairline subcritical cracks (arrows) in another vertical weld of the failed tank. The weld was adjacent
to that which failed, occurring about the same height, towards the centre of the panel. The cracks were found during
re-inspection of the tank, by dusting the welds with fine talcum powder, and represent sub-critical cracks formed during
loading the tank with caustic soda. The weld itself is about 2.5 mm wide.

second weld, where they were seen to bisect the complete weld, being almost perfectly aligned with
the major weld axis (Fig. 7). This crucial piece of evidence allowed the cracks to be identified and
distinguished from the striations caused by bead smoothing. The latter marks tended to wander
slightly in direction, while the hairline cracks were perfectly straight in their alignment.
Low power examination in an optical microscope showed that the weld possessed no less than
about 10 hairline cracks, varying in length from about 3 mm to more than 40 mm, the longest one
being situated at the approximate centre of the panel. Direct measurement of crack width from
these plates show the cracks to be about 0.3 mm wide at the top surface. Many of the cracks also
had small parts which appeared to be slightly wider than the rest of the crack, especially towards
their centres.