Environmental Engineering Reference
In-Depth Information
the subsequent inspection of the structure can be carried out by controlling
the parameters of operational impacts on the structure (mechanical,
thermal, thermomechanical, corrosion, radiation). Non-destructive inspection
should be carried out only on the basis of the indications of the control
of the parameters of operational impact. This organisation scheme of
inspection has already been discussed in Ref. 97 within the framework of
the Integrated System of Operational Control (ISOC) where it was named
the 'resource control'.
The advantages of such an organisation of in-service inspection are
evident, since the control of the parameters of operational impacts can often
be organised in a simple and less expensive way in comparison with NDT.
In addition, staff are exposed to smaller radiation doses.
Organisation of ISI in aviation can serve as a prototype of the above-
described organisation of inspection where the volume and frequency of
NDT is determined depending on the number of hours spent by aircraft in
the air, the number of takeoffs and landings, etc.
Combination of different physical NDT methods
The results of strength analysis carried out with the residual defectiveness
of the structure taken into account can be used to define conditions for
optimum combinations of various NDT methods. For example, new cracks
can nucleate in pressure vessels and piping of nuclear power plants, oil and
gas industry and other similar structures only by the mechanism of fatigue,
thermal fatigue, corrosion fatigue, corrosion cracking, intergranular or
crevice corrosion. Obviously, such a crack can nucleate in the vast majority
of cases only at the surface of the metal structure. Therefore, dye penetrant,
magnetic particle, eddy current inspection and other methods for detecting
defects on the surface of the metal of the structure should be the principal
methods used during operation.
Methods for inspecting the state of subsurface layers of metal of
structures should be used in conjunction with the methods of surface
inspection at the stages of input and preoperational inspection to reduce
the probability of the existence of technological defects to an acceptable
level. During operation, these methods should be used only on the basis of
the results of strength analysis conducted with the data on actual operational
effects and the residual defectiveness of the structure when there is a risk
of rapid crack propagation from the technological defects remaining in the
structure.
NDT and hydraulic strength testing
The issue of the usefulness of hydraulic tests (HT) of strength by the pressure
exceeding the working pressure is the subject of frequent discussions. The
negative impact of HT is evident, as each cycle of mechanical stress leads
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