Non-audible acoustic emission characterization of Reticulitermes termites in pine wood

All tests were carried out on a solid wood sample of the species Pinus nigra with dimensions of 225 × 60 × 40 mm, in which subterranean termites Reticulitermes grassei Clèment, 1977—considered the most frequent termite genus in peninsular Spain (Gaju-Ricart et al. 2002, 2018)—were artificially inoculated.

Termites are insects with hemimetabolous development characterized by incomplete metamorphosis. They are considered eusocial insects, living in colonies with a close relationship among individuals (Laine and Wright 2003). Part of the evolutionary success of termites owes to the functioning of the termite mound as a super-organism where everyone collaborates in its survival. Basically, three castes are distinguished: primary reproductive (originated from nymphs with wing buds), workers, and soldiers, each having unique characteristics (Gaju-Ricart et al. 2015). In the present work, a distinction is made between soldiers and workers (Fig. 1). Soldier termites have a hypertrophied head, larger than that of workers, and highly developed jaws that they use to defend the colony. The soldiers, however, cannot feed themselves because of the size of their jaws. The worker termites represent a persistence of the juvenile state, having the appearance of nymphs that have not developed wing buds. The most significant characteristic of both soldiers and workers is that they lack eyes and body pigment. Only the soldier head is yellowish in color, because it is more chitinized. Eusociability entails the care by the workers of the rest of the individuals in the colony, to whom they transfer predigested food (trophallaxis). They also carry out the cleaning of the termite mound to avoid diseases, as well as body cleaning (grooming), among workers and the other members of the colony. The individuals of each colony can be identified through a mixture of cuticular hydrocarbons, which in addition to offering them protection by waterproofing their bodies to prevent water loss, serve to distinguish them from termites belonging to other colonies. Recognition is achieved when two termites touch each other’s antennae.

Four different tests were carried out: in test 1, only wood without termites was analyzed; in test 2, only soldier termites were inoculated; in test 3, only worker termites were inoculated; and in test 4, both soldier and worker termites were inoculated, as detailed in “Description of the tests”. In the tests where only one type of termite was studied, inoculation was done manually. The termite type was selected and introduced into a perforation previously made in the sample with the help of tweezers. In test 4, inoculation occurred naturally. Initially, the termites were placed in a plastic container with a quantity of moistened substrate and a small amount of wood from the same sample to ensure food for the termites (Fig. 2a). This container was connected to the wooden test sample by means of a tube. From the container, the termites freely reach the wood sample at the inoculation point (Fig. 2b).

During all the tests, an attempt was made to guarantee the survival of the termites, ensuring protection from light, ventilation, a temperature of 26 ± 2 °C, and a moisture content greater than 70 ± 5%.

Acoustic emission (AE) testing is an effective and passive NDT (non-destructive testing) method for monitoring the behavior of materials or structures subjected to a change in the strain field. The generated elastic wave propagates through the material until it reaches the surface, where it can be captured by a piezoelectric sensor. The scheme of the method and the instrumentation used are shown in Fig. 3.

Transient acoustic emission signals can be generically characterized as shown in Fig. 4. The traditional characteristics of these signals are the peak amplitude (A, dB), defined as the voltage of the transient; the duration (D, µs) defined by the time interval between the first and the last time the signal crosses the detection threshold; and the rise-time (RT, µs) defined as the interval of time between the first threshold crossing and the maximum amplitude. The threshold is established by the user and must be higher than the background electromagnetic noise existing at the measurement place.

All tests were monitored by means of acoustic emission using Vallen Systeme^® AMSY-6 equipment and two Vallen Systeme^® VS45 piezoelectric multiresonant sensors placed on the extreme faces of the specimen, as shown in Fig. 5. The frequency response of the sensors used is shown in Fig. 6. The sensors were placed on a silicone grease base to ensure proper transmission of the acoustic waves between the wood and the sensor. In addition, a clamp was placed to ensure that the sensor-sample coupling remained stable throughout the duration of the test and to minimize the transmission of possible mechanical noise from the ground. A 30.1 dB threshold, 5 MHz sampling frequency, 34 dB preamplification and [95, 580] kHz digital input filters were used for the acquisition.

After mounting the sensors, installation of the AE instrumentation, the two sensors and the two acquisition channels were verified by mine breaks (PLBs, 0.5 mm 2H leads), i.e. standardized Hsu-Nielsen source. It was also verified that there was no background noise in the laboratory. After these verifications and before the inoculation of the termites, the AE activity was recorded in each of the samples, confirming that no AE activity was recorded or that it was insignificant and comparable to the results obtained in the specimen without termites.

The four tests carried out and monitored with acoustic emission (Fig. 5) are detailed as follows.

First, measurements were taken to determine the propagation velocity of the acoustic waves in the wood sample. For this purpose, 5 PLBs were performed on the wood sample without termites at the same point where the termite inoculation would occur. The wave propagation velocity can be determined by taking the difference between the arrival times at each sensor and the distance between the inoculation point and each sensor. The mean value obtained for the 5 PLBs was 470 ± 17 cm/ms. Figure 7 shows an example of the acoustic emission signal recorded during the test and the frequency response of the first 25 µs of signal, with a peak at 127 kHz, coinciding with the first resonance frequency of the sensor.

During Test 1 a cluster of several signals concentrated in time was observed, as shown in Fig. 8 and marked as red square symbols. Figure 9 shows an example of these signals and their frequency spectrum. As can be observed, these signals have a very small duration and rise-time, which suggests that they were related to electromagnetic noise generated in the room or by the acquisition equipment. Because of their well-defined characteristics, it was possible to apply filtering to remove them automatically. Figure 10 shows the plot of rise-time versus duration and peak amplitude versus duration for all recorded signals. A polygonal filter (Fig. 10a) defined by D < 9 µs and RT < 5 µs was used to remove the signals identified with a red square. Signals within this polygon were automatically removed for subsequent analysis.

The remaining signals that passed this filter have the shape seen in Fig. 11 (signal in time and its spectrum). These signals exhibit wide variation in their main characteristics of duration and rise-time, but have similar amplitudes, as seen in Fig. 10. Owing to this, it was impossible to characterize them and establish a filter that automatically eliminated all of them. However, the activity of these signals was very low, 1.2 signals per hour. Therefore, their presence does not significantly affect the tests with termites. These signals were probably generated by spurious mechanical activity during acquisition, which was impossible to determine, due to its inaudible nature.

Figure 12 offers the plots of amplitude and number of events versus time, peak amplitude versus duration, peak amplitude versus rise-time, and rise-time versus duration of the signals recorded in each of the three termite tests. That is, soldiers only (test 2), workers only (test 3) and soldiers and workers (test 4), respectively. In these three tests, the polygonal D-RT filter defined for the test without termites was applied automatically. It is clearly seen that the number of signals (AE activity) recorded in the test with soldiers and workers (third column) is considerably higher, due to the longer test duration and the higher number of inoculated termites. It should be noted that the activity recorded in the test with soldiers and workers is mainly associated with the workers, found in greater proportion: 200 workers as compared to 5 soldiers. In all three tests, the signals are grouped in a single cluster of the amplitude-duration graph.

Figure 13 shows the activity recorded in test 4 with soldier termites and workers over a 24-h period. As seen, the activity is not constant throughout the day; a cyclic activity was observed, with periods of higher and lower activity having a periodicity between 20 min and 1 h. In addition, the highest activity was detected at night.

Observing the activity of worker and soldier termites, no relevant differences were found in terms of amplitude, duration or rise-time characteristics that could differentiate between the signals generated by each. The acoustic activity generated by a soldier is due to the beating of its head against the wood, whereas that generated by a worker is due to the feeding-excavating process (Eggleton and Tayasu 2001; Evans et al. 2005; Lehrer 2013). Figures 14 and 15 respectively show transient signals and their frequency spectrum generated by a soldier during test 2 and by a worker during test 3. It can be observed that the signals are similar in both the time and frequency domains, which made it impossible to discriminate them.

In test 4, with soldiers and workers, two clusters of signals are seen in the amplitude-rise time graph (Fig. 12). Cluster 1 accumulates the largest number of signals, corresponding to a rise-time of below 100 µs and higher amplitudes. Cluster 2, with rise-time even higher than 400 µs and lower amplitudes than the first one, corresponds mostly to cascade signals, probably due to several termites generating noise simultaneously. Figure 16 shows an example of a transient signal corresponding to each cluster.

Using the previously measured propagation velocity, a linear localization of the signals recorded in the three tests with termites was performed. The results are shown in Fig. 17. The distance at which the termites were inoculated, 7.5 cm from sensor 1, is indicated by a dashed line. The distribution of located AE activity along the sample is shown with a solid line. As can be seen, in the test with soldiers, both the maximum activity and the maximum intensity (amplitude) were located right at the inoculation point. In the test with worker termites, the maximum intensity of acoustic emission was concentrated very close to the inoculation point, although the distribution of activity registered its maximum slightly shifted towards sensor S1. In the test with termites and workers, the maximum intensity of acoustic emission was also concentrated very close to the inoculation point, although the activity distribution recorded its maximum slightly shifted towards sensor S1, similarly to the test with worker termites. In addition, the located activity was 260 times higher than in the other tests, owing to the longer duration of the test and the larger number of termites. It is noteworthy that there are signals with very high amplitudes of almost 80 dB.