Page 304 - Handbook of Materials Failure Analysis

300 CHAPTER 12 A nonlocal damage-mechanics-based approach

400
1T SEB, a/W = 0.53 (P f =0.05)
Fracture toughness K JC (MPa m) 200 1T SEB, a/W = 0.13 (P f =0.50)
1T SEB, a/W = 0.53 (P f =0.50)
1T SEB, a/W = 0.53 (P f =0.95)
1T SEB, a/W = 0.13 (P f =0.05)
300
1T SEB, a/W = 0.13 (P f =0.95)

100

0
–100 –80 –60 –40 –20
Temperature (°C)
FIGURE 12.14
Effect of crack-depth on fracture toughness variation in DBTT regime (shallow vs. deeply
cracked 1T SEB specimen).

been compared with that of the shallow-cracked specimen in Figure 12.14. The frac-
ture toughness values of the shallow-cracked SEB specimen are considerably higher
compared to the deeply cracked SEB specimen and hence, the transition temperature
is also lower for the shallow-cracked specimen. For this reason, shallow-cracked
specimens are less susceptible to DBT compared to the deeply cracked specimens.
This is in line with experimental observation.
As the crack-tip constraint is lower for the shallow-cracked specimens, the mag-
nitude of crack-tip stress field is lower compared to that of the deeply cracked spec-
imens for the same level of loading. This results in lower values of Weibull stresses
and hence, the probability of fracture is lower for a given value of K JC loading in case
of the shallow-cracked specimen. In components, the observed cracks are usually
surface cracks with very small depth compared to the thickness and hence, the data
of shallow-cracked specimens are more appropriate for evaluation of transition tem-
perature in these cases. The master curve as evaluated from deeply cracked speci-
mens cannot be applied directly for safety evaluation of actual components with
very small surface cracks and hence, the modeling technique as presented in this
work will be very useful for such situations.

6.6 EXTENT OF STABLE CRACK GROWTH (BEFORE CLEAVAGE
FRACTURE) IN THE DBTT REGIME
The extent of stable crack growth before cleavage fracture varies drastically with
temperature in the DBTT regime. In the lower-shelf regime ( 100 °C), the fracture
mode is purely cleavage and hence, the extent of stable ductile crack growth is