Environmental Engineering Reference
In-Depth Information
a
b
9.11
A pipe section (60-96 between spacing grids 2 and 3 (a)
and the form of corrosion defects on the microsection, ×5 (b).
characteristics of initiation and crack growth were obtained from the results
of regular periodic in-service inspection (ISI). A model of crack growth
and size distribution of cracks in the final cycle of operation (FCO), based
on statistical approaches, was proposed. Because the ISI data differ from
physical data (i.e. actual defects), a simple method of predicting the actual
number of cracks on the basis of the ISI results was proposed. The actual
number of cracks and crack growth can be easily determined on the basis of
simulations using the Monte Carlo method. At the same time, the probability
distribution of the number of cracks and their maximum size in FCO are
determined. Satisfactory results were obtained in comparing the forecast
with the FCO data.
Model for estimating crack growth
The two approaches used for statistical evaluations of structural
strength were combined in Ref. 29. The first approach is based on the
deterministic laws of fracture mechanics in which elements of statistics are
introduced
126
. This approach is useful for analysis of some local issues. But
its use is associated with difficulties when considering large structures due
to the large number of variables (load, mechanical properties, crack size,
etc.) and the complexity of their theoretical description.
Another approach is based on statistical distribution functions for the
most important variables
127
. Parameters of distribution functions can be
obtained from the analysis of real data for some of the most important
variables (crack size, time of initiation of cracks, crack growth rate,
etc.). This method cannot predict the impact of changes in non-destructive
testing systems because it does not address the physical mechanisms of ISI
on the test results.
Combination these two approaches allows us to develop a modified
methodology that uses statistical distribution functions of the crack growth
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