Physico–statistical models based on the residual defectiveness of structural materials - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 152

accuracy in assessing their equivalent area. The use of the 'tandem' scheme
(curve 3) provides better results.
The dependence of the detectability of defects on the material is
summarised in Fig. 5.33. The graph was plotted using the results of the
third direction of the PISC-III programme. In general, it can be assumed that
the detectability of defects decreases with increasing heterogeneity of the
micro-and macrostructure of the material and with increasing grain size. The
detectability of defects is adversely affected by structural heterogeneity.
Increased curvature of the surface of the component and more complicated
shape of the welded joint and of the structure reduce the reliability of NDT.
Table 5.8 shows the generalised rough estimates of detectability of
discontinuities in relation to the physical inspection method and type of
defect.
Apparently, the strongest influence on the reliability of inspection
is exerted by the human factor. Studies carried out by A.K. Gurvich,
V.F. Luk'yanov, B. Margitroid and others have shown that the results
of inspection are significantly influenced by the skill, work experience,
long breaks in work, psychophysiological factors (stability of attention,
temperament, etc.), social factors, incentives to work, working conditions,
and the moral quality of NDT inspectors. The scatter in the results of
inspection under laboratory conditions because of the influence of the human
factor reaches tens of percent. Radiographic testing is also characterised
by a significant difference between the test results obtained by different
operators working under the same conditions (Fig. 5.35) 85 .
In some cases, due to dishonest attitude or lack of incentive to work the
detectability of defects can fall to zero even when using efficient inspection
methods.

5.2 Residual defects as the most important
characteristic of the state of the structure.
Methods of determination

￿ ￿ ￿ ￿ ￿

As shown in the previous section, in NDT there is always a finite probability
of sending to service a structural element with a defect of the metal. The set
of all missed defects sent to service due to imperfection of the methods and
means of NDT is called the residual defectiveness. If he missed (undetected)
defect reaches the critical size, the whole structure or its element can be
destroyed. Since no information is available about the missed defect (the
defect was not detected by the NDT methods), fracture occurs suddenly.
Consequently, the question of obtaining information about the characteristics
of the residual defects is a crucial issue in assessing the strength and
operating life of structural elements.