A detailed study of the relationship between fatigue crack growth rate and striation spacing in a range of low alloy ferritic steels

It was shown that the measured average fatigue striation spacings predicted the fatigue crack growth rates for low alloy ferritic steels to within a range of +/- 2% to 35% with an overall average error band value of +/- 10.1%. When we consider the fatigue stress range this average error was reduced to only some +/- 4%. This was good news to both failure analysts and other workers involved in the field of component remnant life and life extension since such predicted fatigue stress ranges use real fracture characteristics observed at some point on the actual component fracture surface. These findings were applied to a real cracking problem recently reported in a steam raising plant, viz., a cracked attemperator reducer weld. In this case an NDT assessment indicated that the maximum crack depth was 7 mm while the lower bound critical crack depth was estimated at 10 mm. As such, remnant life assessments can be estimated for a series of fatigue stress ranges through the use of a reported 450 degrees C fatigue crack growth law for C-Mn steels. Remnant life estimates of a 7 mm deep crack for a range of stress ranges varied from 3000 to 4000 starts where the chances of the real remnant life values being greater than the calculated values was only 1 in 2. However when a realistic failure probability which reflected the serious implications of a failure event of E-4 was taken the remnant life values were reduced to around 100 starts or some 6 months of normal service. (C) 2009 Published by Elsevier Ltd.