Pencil-lead break

Fig. 6.4. Sketch of the concrete beam with eight sensors (left) and recorded acoustic emission signal from a pencil lead break (right). The source was between sensor 4 and sensor 5. The amphtude of the signals shows that the source was closer to sensor 5.

As discussed in Chapter 3, Breckenridge et al. [1981] developed a capacitance-type sensor of very flat response, by which they detected AE waves due to a break of glass capillary shown in Fig. 7.4. Later, the capillary break was replaced by the pencil-lead break by Hsu [1978]. As compared Fig. 7.3 with Fig. 7.4, first time, they showed that AE wave detected by the flat-type sensor due to the step-function force is actually identical to Lamb s solution due to the surface pulse. It was also demonstrated that Lamb s solution due to a buried pulse could be obtained by applying the force at the bottom of the block in Fig. 3.10. Thus, it is clarified by them that the displacement observed by the flat-type sensor due to capillary break or pencil-lead break is identical to G 33(x,yo,t). This implies that Green s function of the specimen can be empirically obtained by just applying the pencil-lead break and recording the displacements. 

The simulation of AE waves due to a point force was also conducted by FEM due to pencil-lead breaks on a thick plate (Hamstad 1996). Remarkable agreement was also reported between experimental data and computed. Thus, Eq. 7.15 is applicable to theoretically compute AE waves due to a point force provided that Green s functions are obtained. 

Hamstad MA, Gary J, O Gallagher A (1996) Far-Field AE Waves by Three-Dimensional Finite Element Modeling of Pencil-Lead Breaks on a Thick Plate. Journal of AE, 14(2) 103-114... 

After the sensors are attached onto a specimen, a test is conducted as illustrated in Fig. 8.6. AE waves are recorded due to a pencil-lead break and then relative coefficients Cs are obtained from,... 

Quarles [1988] used pencil lead breaks on wood blocks of various species to study the effects that grain angle and moisture content might have on AE event rates. The conclusion was that there were significant differences between species, but only minor differences in the moisture content effects within species. Both Quarles [1990] and Sakui [1990] confirmed that AE signal attenuation was relatively constant up to fiber saturation point (the moisture content corresponding to saturated cell walls, but no water in the cell cavity). At moisture contents above fiber saturation point much more significant attenuation was observed. 

The accuracy of localized events with known source position, i.e. artificial sources on a surface like pencil lead breaks, can be plotted as an error vector r, connecting the calculated with the known position. Fig. 16.5 (a). By converting the absolute values of the error vectors into a density function an image of localization accuracy can be generated that allows for better interpretation, Fig. 16.5 (b). 

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