Experimental Demonstrations of Capability
8.9 Implications for structural integrity
Fig. 8.15 Comparison of the accuracy of TOFD and amplitude-based ultrasonic techniques [after Ammirato and Willetts, 1989].
a second choice, followed by the 6 dB drop method.
The fact that standard deviations for the various sizing methods have been quoted could be taken to imply that the sizing errors are random and distributed according to the Normal distribution. This would imply that errors could be reduced by repeated measurements, so that, for instance, the 6 dB drop method could equal the accuracy of TOFD if repeated 20 times. This is probably far from the truth, since a large part of the spread in sizing results is likely to be related to the range of defect types and geometries employed in the trial. The use of the standard deviation is a convenient way of indicating the spread of measurements, rather than a precise statement about the distribution of results.
8.9 Implications for structural integrity
The defects of most concern in typical engineering structures are those which could lead to failure. This usually means planar, crack-like defects orientated perpendic-ular to the principal stresses. Fracture mechanics criteria of one form or another can be used to give precise definitions of a critical defect size for a given material strength and loading. These criteria can be chosen from linear elastic fracture me-chanics or can include elastic-plastic analysis and will not be discussed in any more detail here. One common set of rules for assessing defect significance, based on lin-ear elastic fracture mechanics, is Section XI of the Boiler and Pressure Vessel Code of the American Society of Mechanical Engineers [ASME, 1974,1977,1983]. Silk
carried out a study to determine whether Time-of-Flight Diffraction data obtained with the then current Zipscan equipment were compatible with the requirements of the ASME-XI code [Silk, 1987b]. The study evaluated the Time-of-Flight Diffrac-tion technique, in its basic search and sizing role, against the requirements of the ASME-XI code and a modified version of the code which is close to the European industrial norm. Silk concluded that ‘Operation in conformity with ASME-XI is ex-pected to be possible for all internal defects and for all defects which lie deeper than 30% of the specimen thickness in steel specimens exceeding 12 mm in thickness. Op-eration in conformity with the modified code is expected to be possible for all defects in steel specimens exceeding 10 mm in thickness.’ These conclusions applied only to the basic use of Time-of-Flight Diffraction, as it was commonly applied in the field with Zipscan equipment. When the technique was used to size known defects, or in specialised uses such as nuclear pressure vessel inspections, higher precision could be achieved by tailoring the equipment and methods for the specific task.
As well as engineering codes of practice, such as ASME-XI, which concern the acceptability of defects of different sizes, locations and orientations, other ways of assessing defect significance and the relationship between the reliability and preci-sion of non-destructive testing techniques and structural integrity have been devel-oped. Marshall [1982] and Lucia and Volta [1983] used probabilistic analyses to determine the size range, aspect ratio and location of the flaws which have the great-est influence on integrity of the pressure vessel of a PWR during possible accident sequences. As we have already seen in Section 8.4, the greatest contribution to the vessel failure rate is expected from cracks in a limited size range, which depends both on the chance that cracks in that size range will be present and the chance that the material properties in the region of the crack will be such that the crack is of a critical size for some possible transient stress. The cracks contributing most to the predicted failure rate, under conditions appropriate to large loss-of-coolant ac-cidents (LOCAs) or steam line breaks, are those planar, crack-like flaws orientated perpendicular to the principal stresses (i.e. lying in planes perpendicular to the pres-sure retaining surfaces) and with a through-wall extent of between 10 and 50 mm [Cameron, 1984]. The most important locations of the cracks are the nozzle-to-shell weld, nozzle corners and the belt-line welds, and these are, therefore, the geometries appropriate for test-block exercises. For conservatism in predictions of the failure rate of vessels, it is usually assumed that the cracks are all surface breaking or at least would be classed as surface-breaking cracks according to proximity rules such as given by ASME [1974,1977,1983]. If the assumption that the cracks are all near the surface is not valid then the estimated failure rates of vessels decrease by at least three orders of magnitude [Lucia and Volta, 1983].
The hazard presented by the failure of a component should determine the reli-ability required of that component. If the component is required to survive various possible excess transient stresses, for example, then non-destructive testing may well be used to identify flawed components before any catastrophic failure occurs.
A defect is classified as unacceptable if it poses a threat to the integrity of the structure and acceptable if it does not. Basing his assessment on probabilistic
frac-8.9. Implications for structural integrity 179
ture mechanics applied to analysis of the expected failure rate of PWR pressure ves-sels, Marshall [1982] suggested a target to be achieved for the reliability of classi-fying defects according to their through-wall extent. The through-wall extent is, of course, precisely what the Time-of-Flight Diffraction technique measures, whereas, for other inspection techniques, it may only be derivable indirectly. Marshall’s target was a 50% chance of detecting and correctly classifying a defect of 6 mm through-wall extent coupled with a 95% chance of detecting and correctly classifying a defect of through-wall extent 25 mm. This is now believed to be a conservative estimate of the reliability of ultrasonic techniques but nevertheless yields a failure rate of 10−7per vessel year for a PWR pressure vessel. In general, probabilistic fracture-mechanics assessments have assumed that a single parameter of the defect, through-wall size, governs the likelihood of vessel failure. However, this parameter is not what is measured most readily by most ultrasonic inspections except by Time-of-Flight Diffraction.
Even if the chance of failing to correctly classify a defect were as low as 10−4 independent of defect through-wall extent, then the failure rate of the pressure vessel would only decrease to a little below 10−8per vessel year. Thus there is a limit to the advantage that can be gained by increasing the inherent capability of inspection tech-niques. There are many assumptions in these analyses which are beyond the scope of our present brief discussion. For more detail the reader is referred to Marshall [1982] and Cameron [1984]. Section A.12 of the Appendix discusses these points in a little more detail.
The important point is that targets outlined above define a scale for how reliable inspection should be for the pressure vessel of a pressurised water reactor. As we have shown, Time-of-Flight Diffraction can achieve much greater accuracy and re-liability than this target. The fact that such performance is also possible with well designed conventional pulse-echo methods gives confidence that diverse techniques are available in situations where the highest performance is demanded.