the crack grows and after it has become of a size such that stress
concentration effect reduces, the stress intensity will govern the
growth. This phenomenon has been observed by Dowling [21] and
Smith and Miller [22]. The crack growth rate increases in depth
direction after 2c/a more than 5.
Prediction of crack growth in piping components with defect
having aspect ratio less than or equal to 2 may not be possible with
Paris law. In order to show the difficulty in prediction of fatigue
crack growth life in pipes with low crack aspect ratio, crack growth
data of the pipe (SSPW-6-1) have been analyzed considering the
crack growth rate for different a/t ratios. Plots of da/dN with DKrms,a
and dc/dN with corresponding DKrms,c have been shown in Fig.14(a)
and (b) respectively. Figures show that there is large variation in
crack growth rate in length and depth direction for different ranges
of a/t ratio. Paris constants evaluated from the CT specimens give
unique value which can be used for the calculation of piping
components with increasing crack growth rate with increase in
crack depth or length. In this case notch concentration effect should
be considered for the prediction of crack growth life. Therefore, it is
difficult to predict the FCG life of SSPW-6-1 pipe with initial notch
aspect ratio of 2, using the growth constants (C & m) derived from
CT specimens. However, the predictions are comparable for SSPW-
6-3 test which has initial aspect ratio as 4 (Fig. 7). The reasonable
prediction of fatigue crack growth life of a piping component with
crack of aspect ratio lower than 4 needs more investigations.
Table 7 summarizes the crack growth life predicted by schemes
B and C for all the analyzed cases except SSPW-6-1. The comparisons
have been made with respect to experimental data to determine
their relative degree of closeness. These comparisons are
either based on the number of cycles (N) keeping crack size same or
crack depth size (a) keeping number of cycles same. The first
criterion (‘N’) is based on comparing the number of cycles for
a specific crack size (column 5 of Table 7). This criterion is suitable
for true error estimation when 1st beach mark has more number of
cycles than that predicted up to a/t as 0.8 as in cases of SSPW 6-3
and SSPW 6-7 pipe tests. The second criterion (‘a’) is based on
comparison of crack size for a given number of cycles. This criterion
is useful when slower crack growth occurs during beach mark
loading as in case of SSPW 12-10 test. Therefore making