AE04.pdf

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2 5 4 7 8 30 (1.2) 20 (0.8) 10 (0.4) 0 (0) 0 20 40 60 80 100 120 140 160 0 (0.8) (1.6) (2.4) (3.2) (4.0) (4.8) (5.6) (6.4) 1 Circumferential position in Y axis, mm (in.) Longitudinal position in X axis, mm (in.) F IGURE 15. Two-dimensional source location using eight-hit event definition, with same transducer locations as in Fig. 13. 2 5 4 7 8 30 (1.2) 20 (0.8) 10 (0.4) 0 (0) 0 20 40 60 80 100 120 140 160 0 (0.8) (1.6) (2.4) (3.2) (4.0) (4.8) (5.6) (6.4) 1
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example where the fourth hit is coming from one of the transducers in the second ring of transducers (consisting of 4, 5 and 6) and relieving the problem associated with all of the hits that define the event being in one plane. A common reaction would be to use all of the hits that are available for the overdetermined source location calculation. In this case, the software has a limit of eight hits. The results of using eight hits to calculate the pencil graphite break locations are shown in Fig. 15. In this case, all 94 pencil graphite breaks are located but the source positions are not as accurate as those in Fig. 14. The greatest location error is for those events that were generated in the first 1.0 m (40 in.) and the last 1.0 m (40 in.) along the X axis. In these cases, the seventh and eighth hits are coming from the other end of the cylinder where the stress wave generated by the pencil graphite break experiences considerable attenuation to the point where detection may be caused by a different wave mode than those detected by the first six transducers. This attenuation means that the seventh and eighth hits contain considerable timing measurement error compared with the first six hits. Instead of helping, the averaging effect increases the source location error. In fact, for this example, the best setup is to use only the first six hits as shown in Fig. 16. 134 Acoustic Emission Testing Circumferential position in Y axis, mm (in.) Longitudinal position in X axis, mm (in.) F IGURE 16. Two-dimensional source location using six-hit event definition, with same transducer locations as in Fig. 13. 30 (1.2) 20 (0.8) 10 (0.4) 0 (0) 0 20 40 60 80 100 120 140 160 0 (0.8) (1.6) (2.4) (3.2) (4.0) (4.8) (5.6) (6.4) 2 5 4 7 8 1
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A sampling of digital waveforms associated with various hits can be analyzed along with the conventional hit features recorded during a test. Information can be extracted from these waveforms to help locate active sources. In plate and shell structures, sources are located by reviewing the different wave modes that propagate within a structure. For example, Fig. 17 shows the lamb wave dispersion curves for group velocities in a carbon steel plate. It can be seen that below 2 mm·MHz, only three wave modes can propagate, the first symmetrical mode and the first two asymmetrical modes. At 2 mm·MHz, these three modes all travel at about the same velocity of 3250 m·s –1 (7270 mi·h –1 ). It can also be seen that, between 0 and 0.3 mm·MHz, the first symmetrical mode is nondispersive. The same can be said for the first asymmetric mode between 1.0 and 1.5 mm·MHz. Waveform based source location can be achieved by digital filtering. The technique that can be derived from this selective filtering can be seen in the following example.
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