Ensayo de esclerometría en el colegio nacional 7221 La Rinconada. 102

If a defect has straight horizontal upper and lower edges, its length in the scan di-rection can be measured by first fitting the shaped cursor to the left-hand tail of the indication and then to the right-hand tail and noting the movement between these two positions. This technique is particularly effective for near-surface defects, be-cause the hyperbolic signal arcs are narrow in the scan direction, so there is little ambiguity in positioning the cursor. For defects at considerable depth, the arcs are broader and the measurements consequently less accurate. In this case, more ac-curate length measurements may be derived from data processed by the synthetic aperture focusing technique (SAFT) (see Section 5.8.1).

If the defect edges are curved or sloping, good length measurements may still be obtained in many cases, provided the procedure described in the next section is followed. If the defects are very irregular in shape, it may be that SAFT processing would deliver better accuracy of length measurement but no convincing systematic demonstration of SAFT on this type of defect has been published thus far.

5.7.1 Using the shaped cursor for defect length measurement

To demonstrate that good results can be obtained on realistic defect shapes, we in-clude here in Figure 5.9 a simulation due to Hawker and Burch [1999], showing successive steps in measuring the profile of a far-surface crack by careful matching of the shaped cursor against the signal indication. The point to emphasise here is that where the cursor curve touches the signal indication curve, the slopes must match.

For all such points, the position of the centre of the cursor is marked (being the po-sition of the diffracting edge which produced that portion of the signal). The locus of the marked points traces out the profile of the diffracting edge and, if it is sensibly complete, gives an accurate indication of the whole extent of the defect. Another useful technique illustrated here is that of fitting the cursor to the tails on the back-wall echo at each end of the region where it is obscured. This allows one to estimate the full length of the crack where it opens to the back surface.

5.7. Measurement of defect length 99

Fig. 5.9 Simulated derivation of a defect profile using shaped cursor. The upper fig-ure shows the successive positions of the hyperbolic cursor used in deriva-tion of the profile. At each point, the cursor touches a defect signal or back-wall echo at a point where the slopes match. The lower figure shows the actual defect shape in the block with the measured points superposed.

The defect used in this demonstration was such that it gave a continuous signal indication over its whole length and every part of the diffracting edge contributed to the indication. The majority of real defects would fall into that category but it is possible to imagine ‘pathological’ defects which would be much more difficult to profile. How this may arise is described in the next section.

5.7.2 Effects of defect shape on apparent defect length

Diffracted waves arise from all the insonified parts of the edges of a defect but signals will be detected only when the contributions from different parts are sufficiently close in phase for constructive interference to occur. From Fermat’s principle, this will occur whenever the path length from the transmitter to the receiver via a point on the defect edge is approximately stationary with respect to variations in the position

’Active’ region

Rectangular planar defect Sections

through isochronal surfaces

Inspection surface

Probes lie on a line passing through this point

Fig. 5.10 Isochronal surfaces for a rectangular defect located midway between the transmitter and receiver.

of the point on the defect edge.

Let us consider the standard Time-of-Flight Diffraction probe arrangement of two probes facing each other on a horizontal inspection surface and, further, let us suppose that the pulse is a single half cycle. For contributions from different edge points to add, they must have transit times which differ by less than the pulse dura-tion. Let us divide transit time into units of one half cycle and associate an isochronal surface or isochrone with each integral time point. The isochrones are then ellipsoids of revolution with the probe indices as foci. The only regions of these isochrones rel-evant to signal production (active regions) are those which lie within both ultrasonic beams. A particular defect edge will produce a noticeable signal if it follows the active region of an isochrone closely.

Consider a planar defect lying in the vertical plane which is equidistant from the two probes; this plane cuts the isochrones in a set of circles centred on the point in the inspection surface which lies on the line joining the probes. If a long rectangular defect lies directly between the probes, and perpendicular to the line joining the probe centres, its top and bottom edges pass through a horizontal active region of the isochrone and thus produce strong signals, while its outside vertical end edges are either nearly normal to the isochrones, or are outside the active region, and so produce a negligible resultant signal. This situation is illustrated in Figure 5.10.

Suppose now that the probes are scanned parallel to the defect plane so as to approach and pass over the defect. The top and bottom signals will remain constant over most of the defect length, falling by 6 dB at the points where the defect ends are aligned with the beam centreline. At these points the signals should be showing slight extra delay and this will increase, giving rise to the characteristic signal curves, as the scan passes beyond the defect. Thus, for a rectangular defect, the length of the

5.7. Measurement of defect length 101

’Active’ region Sections

through isochronal surfaces

Inspection surface

Probes lie on a line passing through this point

Defect edge in ’glint’ position Defect edge after small displacement

Fig. 5.11 Isochronal surfaces for a semi-circular defect located symmetrically be-tween the transmitter and receiver (solid line) and with its centre laterally displaced (broken line).

top and bottom signals in the D-scan image will give a good indication of the defect length and length measurements made either by 6 dB drop or cursor fitting should be reasonably accurate.

Let us replace the rectangular defect with one of a rather special shape, a surface-breaking semi-circular crack. As Figure 5.11 shows, at almost every scan position the defect edge crosses several isochrones and the signal will be destroyed by de-structive interference. When the centre of the semi-circle lies on the line joining the probes, however, the whole defect edge lies parallel to an isochrone and a very large signal will result. This effect is most clearly demonstrated for very wide-beam probes but even for conventional probes the effect is striking, as shown in Figure 5.12.

This tendency to produce a strong glint or flashpoint at the symmetrical position and weak or negligible signals elsewhere applies whenever a section of the lower edge of a defect approximates a portion of a semi-circle centred on the inspection sur-face. Typical defects showing this effect are semi-elliptical surface-breaking cracks.

Note, however, that the total length of a surface-breaking crack can be estimated from the scan distance over which the lateral wave is blocked. For a defect of el-liptical shape, in an arbitrary orientation with respect to the transmitter and receiver, there are up to four flashpoints on the defect edge, generally three on the lower edge and one on the upper edge of the defect. The curvature of the edge causes focusing of the diffracted rays, described in the theory by caustics — regions of (theoretically)

Fig. 5.12 A glint or flashpoint from a semi-circular defect edge.

infinite amplitude.

The signal patterns produced by defects of other shapes can be worked out by similar arguments to those used above. A buried crack with irregular edges would tend to produce top and bottom signals appearing intermittent on the scan image. In attempting to characterise the defects from the appearance of such signals, it must be borne in mind that discontinuous signals do not necessarily arise from discontinuous defects.

One method of obtaining additional detectable signals in a conventional scan is to carry out further scans with the probes skewed so that the active region moves out to the side of the vertical plane through the probes [Atkinson, Birchall and Plevin, 1989;

Highmore and Rogerson, 1988]. SAFT processing of data collected with wide-beam probes should also be effective.