Physico–statistical models based on the residual defectiveness of structural materials - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 165

5.3.2 Method for determining the probability of failure,
the probability of leakage and the probability of
existence of hidden defects from the probabilistic
part of residual defectiveness

The method can be applied to a specific product (or group of m similar
products) for which it is necessary to ensure manufacturing quality,
reliability and safety by the given NDT method during inspection by the
operator with the knowm skill with subsequent repair of identified defects.
Fracture mechanics methods determine the critical size of defects in
the service refime for this product χ cr and the maximum allowable size of
defects in service [χ] d.s (norms defects in the product), defined the valid
regulations and / or Technical Instructions (TI) for manufacturing (e.g., in
nuclear technology - the statutory procedure 104 described in section 4.1).
The characteristic size of the defect χ is, for example, either the linear size
of the defect or a combination of linear sizes of the defect, or the defect
area, or the defect volume. It should be noted that the defects that determine
the quality are the defects whose sizes range from the minimum size of
defects that can be detected (search fize) (search) to the size of defects
that are allowed in manufacture and larger defects; defects determining
reliability - these are defects whose sizes are in the range from rejection
in manufacture or service to the acceptable size in service and larger; the
defects that define safety - from the allowable in service to the critical
size and larger.
The methods are illustrated in Fig. 5.42-5.45.
Fig. 5.42 shows the schematisation of a defect in a pipeline by an
ellipse with minor semiaxsis a and major semiaxis c. Figure 5.43 shows
a set of critical size (curve 3), defects with the size allowable in service
(curve 2) and the allowable size in manufacture (curve 1). Figure 5.44 is
a histogram of defects detected in the component and the curves of initial
and residual defectiveness are also shown. The curves of initial and residual

￿ ￿ ￿ ￿ ￿

5.42 Schematic representation of the longitudinal and transverse
integrity defects of pipeline metal in its longitudinal and transverse
sections.