Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

For example, 150 mm (6 in.) diam, 8100 mm (320 in.) long shafts used in
pressure rolls in papermaking machinery developed fatigue cracks in their
500 mm (20 in.) long threaded end sections after long and severe service.
These cracks were detected and measured at 3-month intervals, using a
contact-type straight-beam search unit placed on the end of each shaft,
without removing the shaft from the machine.
When the cracks were found to cover over 25% of the cross section of a shaft,
the shaft was removed and replaced. In another case, fatigue cracking in a
weld joining components of the shell of a ball mill 4.3 m (14 ft) in diameter by
9.1 m (30 ft) long was monitored using contact type angle-beam search units.
The testing was done at 3-month intervals until a crack was detected; then it
was monitored more frequently. When a crack reached a length of 150 mm (6
in.), milling was halted and the crack repaired.