Environmental Engineering Reference
In-Depth Information
5.1.3 Methods for studying the detectability of
non-destructive testing
To compare different inspection methods, an apriori initial inspection
method can be developed and compared with the test method. The
initial method should give the largest amount of information about the
discontinuities. In cases where the initial method gives complete information
on the discontinuities it can also be referred to as the reference method.
Before the advent of technologies for production of test samples, the
only apparently reference method which provided complete information
about the real defectiveness of the structure was the method of cutting
layers from a structure, combined with metallographic studies (method of
opening the defects).
The method of opening the defects is costly and time consuming. Also,
in this case there is a likelihood of not detecting cracks,. Therefore, it is
sometimes useful in assessing the relative detectability to apply the most
economical and widespread NDT method as the initial method. In some
cases, the initial method can be the same as the test method but with
different sensitivity of the apparatus and different configuration.
New possibilities in the study of the detectability of NDT are offered
by the test specimens with artificial, i.e. specifically embedded, hidden
discontinuities. In this case, defects of almost any type, size, location
and orientation can be formed in the sample. With the well-developed
manufacturing technology of test samples it is not necessary to use the
reference and initial inspection methods, since the information about
discontinuities is known in advance.
Studies using the test samples also have drawbacks, since in practice
it is not always possible to produce defects in accordance with the
requirements. In addition, the test samples may contain unplanned
defects due to imperfect welding. In this case, information about the
real defectiveness can be obtained on the basis of design data on the
discontinuities as well as the NDT results obtained by different methods
(e.g., ultrasonic and radiographic) and different NDT inspectors.
The reliability of NDT can also be estimated by the analysis of testing
results. In this case it is not necessary to use the the initial inspection
method and preliminary information about the discontinuities in the
structure. This method of analysis is described below. The calculation
method also has disadvantages.
The most efficient methods of determining detectability are, apparently,
the methods based on the application of the test samples. In addition,
the same test samples can be used to assess the qualifications of NDT
inspectors, as trainers, to assess the effectiveness of the NDT methods and
means.
The ASME rules of (XI) have legalised the use of test samples for
evaluation of inspection means and methods as well as to assess the skill
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