Chapter 6 Lamb wave scanning laser source enhancements for the
6.3 Defect depth estimation using scanning laser source near-field en-
Near-field enhancements for a scanning laser source have previously been re- ported to vary as a function of the defect depth for square based notched defects for Rayleigh wave propagation[23], however, this defect structure does not give a good representation of a true defect. The v-shaped defects studied here provide a more realistic defect structure, with an opening angle relative to the horizontal of less than 90◦, making them more representive of the defect opening for the real open stress corrosion cracks discussed in section 1.4.2. A variation in the near-field enhancement as a function of defect depth for these defects gives a means of esti- mating the severity of the defect[116].
A sonogram (as described in section 2.1.5) was produced at each scan posi- tion as the laser source was moved over the defect, and the theoretical arrival times of the supported Lamb modes were overlaid onto the sonogram, so as to aid in the identification of individual modes, with examples shown for a 75% through-thickness defect in figure 6.3. A large increase in the magnitude of the frequency content of several wave modes is seen in the sonograms when the laser source is directly over the defect (figure 6.3b), and the emergence of several higher order wave modes that are not visible when the source is far away from the defect (figure 6.3a) can be seen, as has been previously reported[240]. The enhancement observed for the scanning laser source approach with Lamb waves is significantly larger than that seen for the scanning laser detection case (chapter 5), which is the opposite to the behaviour observed for the case of Rayleigh wave enhancements at angled defects (chapter 4).
(a) No defect region
(b) Defect near-field
Figure 6.3: Sonograms with calculated Lamb wave arrival times overlaid for the case in which the laser source is far away from the defect (a), and for direct illumination of the defect (b) for a 75% through-thickness defect.
To allow for a direct comparison with the results in chapter 5 the peak magnitudes from the sonograms for the same regions of the A0 and S0 modes were investigated as a function of source position. The arrival times were adjusted slightly from the scanning laser detection experiments to account for a small difference in the source to detector distance between the two sets of experiments. The regions of interest studied were between 1.05≤fd ≤1.65 MHz.mm and 15.7≤t ≤17.9µs for the A0 mode, and 2.25≤fd ≤2.70 MHz.mm and 28.9≤t ≤30.5 µs for the S0 mode, labelled A and B respectively on figure 6.3a.
As in chapter 5 the peak value of the magnitude from the chosen regions of the sonogram at each scan position was measured, and is referred to as the frequency magnitude. These regions were chosen for the fact that at the times and frequency-thicknesses chosen the incident wave consists of a single wave mode, thereby simplifying the interpretation of the signals.
(a) S0 tracking
(b) A0 tracking
Figure 6.4: Experimental peak frequency magnitude tracking for the S0 (a) and A0 (b) fundamental waves as the laser spot source is passed over a 75% through- thickness defect in a 1.5 mm thick sheet.
As the laser spot source is scanned over the v-shaped defects, enhancements are observed in the sonograms. A defect free sonogram is shown in figure 6.3a, and an enhanced sonogram shown in figure 6.3b for a 75% through-thickness defect. For the enhanced position the higher order Lamb wave modes have enhanced magni- tude at certain frequencies when compared to the positions at which the source is far away from the defect, with modes being generated that are not present (to any significant extent) when the laser source is a distance away from the defect[240], this can be seen in figure 6.3b. There is also an increase in the magnitude of the fre- quency content for the fundamental waves, such as those studied in regions A and B on figure 6.3.
An example of the variation in the peak magnitude and the subsequent en- hancement is shown for a 75% through-thickness defect for both the S0 mode, figure
Figure 6.5: Enhancement factors as a function of defect depth for v-shaped defects in aluminium plates for the S0 and A0 wave modes.
6.4a and the A0 mode, figure 6.4b. Here the negative source positions correspond to the region of the scan in which both the laser source and detector are on the same side of the defect. Large enhancements are seen for both wave modes, with a large increase in the frequency magnitude,AEnhanced, as the laser source passes over the defect, and a steady level when no defect is present,AN oDef ect. The enhancement factor, Ef, for this defect is obtained from equation 5.1. The peak magnitude can be seen to increase by a factor of 2.48 for the A0 mode and 6.65 for the S0 mode when the source is over the defect. These enhancements are significantly larger than those observed when the laser detector is passed over the defect, which were 1.90 for the A0 and 1.85 for the S0 mode.
The shape of the enhancement peak is again different between the two wave modes, with the S0 mode exhibiting a single enhancement peak, and the A0 mode a double-peaked structure. This suggests that the mechanisms that are responsible for the enhancement (as described in section 6.4) have a different degree of influence over the enhancement for different wave modes. The cause of the differences in the enhancement peak shapes are explained in section 6.5.
The variation in the enhancement factors in the chosen regions of the S0 and A0 wave modes as a function of the defect depth, h, is shown in figure 6.5. The overall magnitude of the enhancements is larger than the equivalent scanning laser detector enhancements for the same defect depths, shown in figures 5.9 and 5.10. This is contrary to the case for Rayleigh wave enhancements, in which the
laser detector enhancements were shown in chapter 4 to be larger than those for the scanning laser source for angled surface-breaking defects[95].
For both wave modes the enhancement factors increase with increasing defect depth, with the S0 mode experiencing much larger enhancements than the A0 mode. The general increase as a function of increasing defect depth allows the enhancement factors in figure 6.5 to be used as a means to identify the depth of a defect; if the enhancement factor is found for both wave modes for a v-shaped defect of unknown depth the values obtained can be used to estimate the defect depth from figure 6.5. For example if a scan over a defect of an unknown depth yields an enhancement factor of 4.31 in the S0 region and 1.87 in the A0 region, then from figure 6.5 the defect depth can be estimated to be 50% of the through-thickness.
For defect depths between 40% and 60% of the full thickness, for both wave modes there exists a region in which the enhancement factors are very similar over a range of defect depths, introducing some ambiguity into the depth estimate. In order to reduce this uncertainty this data could be used in conjunction with scan- ning laser detection enhancements (chapter 5) to improve the probability that the correct defect depth is estimated.