Non-Contact Velocity Measurement of Japanese Cedar Columns Using Air-Coupled Ultrasonics

The ultrasonic wave velocities of Japanese cedar columns were measured using a non-contact method. An air-coupled ultrasonic wave was propagated through the axial and lateral directions of wood. The velocities in the axial direction (VL) showed the minimum values around the pith. The averaged VL increased from 3600 m/s towards the outside of measurement area and attained the maximum values (=4010 m/s). The velocities in the lateral direction (VRT) showed no tendency among measurement points. The averaged VRT was 1450 m/s. The velocities obtained using the non-contact method showed a significant positive relationship with those obtained using the contact method. The averaged ratio of VL to VRT was measured to be approximately 2.2 to 2.8. These ratios were in agreement with those from a contact method. These findings suggest that it is possible to measure the velocity in Japanese cedar columns with the non-contact method by using aircoupled ultrasonics.


Introduction
Japanese cedar is a popular tree cultivated in Japan.The distribution area of Japanese cedar is the largest among planted forestry species, accounting for 26% of the country's total.In addition, 64% of roundwood production of Japanese cedar is used for sawnwood.The Plan to Create Dynamism through Agriculture, Forestry, and Fishe-ries and Local Communities was enacted in 2013.Forestry will become a growth industry through the creation of new wood demand and the building of a stable and efficient supply scheme [1].As a result, demand for Japanese cedar as an engineering wood is expected to increase significantly in the future.Therefore, it is important to understand the usage of ultrasonic techniques to perform nondestructive evaluation of Japanese cedar.Hasegawa et al. demonstrated that the ultrasonic wave velocities along the longitudinal direction of Japanese cedar exhibited strong correlations with the tracheid length and microfibril angle with a significant level of p < 0.01 [2].In addition, Mori et al. evaluated the surviving strength of bending of Japanese cedar damaged by termite [3].It was possible to evaluate the surviving strength using the ultrasonic velocity in the longitudinal direction.Hasegawa et al. investigated the within-tree variation of the acoustoelastic behaviors in Japanese cedar to clarify the possibility of nondestructive stress measurement [4].
Recently, air-coupled ultrasonic waves have been studied for use in the quality control of sawing timber and the maintenance of posts and beams in a wooden construction [5]- [9].This technique makes it possible to evaluate the current state of the wood without contacting the wood.Vun et al. evaluated the relationships between moisture content and ultrasonic wave velocity in red pine [5], while Dahmena et al. measured the elastic constants of an olive wood plate [6].To the best of our knowledge, there is no report for evaluating Japanese cedar by using air-coupled ultrasonics.
In this study, we tried to measure the ultrasonic wave velocity in Japanese cedar columns with a non-contact method.The air-coupled ultrasonic wave was propagated through the axial and lateral directions in wood.The validity of the ultrasonic wave velocities using the non-contact method was experimentally investigated.In addition, the velocities obtained using the non-contact method were compared with those obtained using the contact method.

Ultrasonic Measurement
An air-coupled ultrasonic wave was propagated through air and specimens of wood, as shown in Figure 1.Ultrasonic wave velocities were measured by using a pulser-receiver (JPN-10CKN, Japan probe Co. Ltd., Japan), a preamplifier, and monolithic composite transducers of type 14 × 20 mm with a natural frequency of 200 kHz (Japan probe Co. Ltd., Japan).The propagation directions of the ultrasonic wave corresponded to the axial (longitudinal) and lateral (radial or tangential) directions in wood.Ultrasonic velocity was measured at nine points on the surface of cross and tangential sections, respectively (Figure 2).The room temperature (t) and velocity measurement were recorded at the same time.The ultrasonic velocity (V) was calculated using Equations ( 1) and (2).
( ) ( ) 331.5 0.61 where L is the propagation distance of wood, T is the propagation time with a wood sample, T a is the propagation time without a wood sample, and V a is the velocity in air.In addition, the ultrasonic wave velocities were measured using the contact method.The measurement equipments were the same as those used in the non-contact method.The ultrasonic velocity (V c ) was calculated using Equation (3).
( ) where L is the propagation distance of wood, T c is the propagation time with a wood sample, and T 0 is the propagation time without a wood sample.

Receiving Waveform for Air-Coupled Ultrasonics
Figure 3 shows the receiving waveform in air at 110 mm distance between transducers.The propagation time was determined using a zero-crossing method.The propagation time showed about 330 µs. Figure 4 shows the examples of receiving waveforms in axial and lateral directions of wood.The propagation time in wood is smaller than that in air, because an ultrasonic velocity in wood is faster than that in air.The averaged propagation time in axial and lateral directions was about 60 µs and 100 µs, respectively.The propagation time was ranked in the ascending order of lateral and axial directions.These propagation times were substituted for Equation (1) in order to calculate the velocities.As shown in Figure 3, some small signals that were observed before the receiving waveform are used to determine the propagation time.The small signals may have some important information for ultrasonic propagation characteristics.Further research is required to study these small signals.

Ultrasonic Wave Velocities in the Axial and Lateral Directions
Table 2 shows the ultrasonic wave velocities in the axial and lateral directions for three test specimens (S1, S2, S3).The average values of velocity in the axial direction (V L ) were 3964 m/s, 3515 m/s, and 3468 m/s, respectively.Figure 5 shows the values of ultrasonic wave velocity each position in the axial and lateral directions.The minimum values for V L show 3597 m/s, 3234 m/s, and 2733 m/s, respectively.Their values existed around   the pith.On the other hands, the maximum values show 4414 m/s, 3816 m/s, and 3796 m/s, respectively.Their values existed in the outside point of measurement area.In general, the tracheid length gradually increases toward the outside and attains a constant value.An ultrasonic wave dissipates acoustical energy when it occurs at the end of a fiber [10].Hasegawa et al. demonstrated that V L for Japanese cedar and Japanese cypress were significantly related to the tracheid length [2].Tracheid length seems to affect V L .On the other hand, the average values of velocity in the lateral direction (V RT ) were 1412 m/s, 1606 m/s, 1331 m/s, respectively.The ratios of V L to V RT ranged from 2.2 to 2.8.Distinct relationship among measurement points such as the axial directions was not observed.As shown in Figure 2, the growth ring structure significantly influences the wave propagation.In the center of the test specimens, the propagation path coincides with the radial direction.However, at the edge of the test specimens, the propagation path coincides with the radial and tangential directions.As a result, the ultrasonic beam is deflected and shifted laterally with respect to the incident angle [11].In addition, for the measurement V RT , the distance between the test specimen and sensor was within the near field distance.As a result, there is a large variation of V RT in wood.In the future, we will determine the optimal measurement condition such as near-field distance for Japanese cedar column.
As shown in Table 2, the velocities obtained using the contact method (V c ) were larger than those obtained using the non-contact method (V) at a significant level of 5%.The correlation coefficients in the axial and lateral directions were 0.48 and 0.40, respectively.The ratios of V L to V RT ranged from 2.4 to 2.5.The ratios of velocity obtained using the contact method were similar to those obtained using the non-contact method.Vun et al. demonstrated that the velocities measured using the non-contact method showed higher values than those measured using the contact method for an oriented strand board [12].Raffaella et al. reported that the non-contact ultrasonic velocity of food items was significantly higher than the velocity measured using the contact method (P < 0.05) [13].Results in this study were not in agreement with those in the previous study [12] [13].Further research is needed to clarify the reason V c shows larger values than V.
The values of velocity in this study were smaller than those in the previous study, which used a contact method, as shown in Table 2.An ultrasonic wave velocity changes with the specimen dimensions and natural frequency [10].Bucur demonstrated that the ratios of V L to V R in European wood species ranged from 2.4 to 3.4 for softwood [10].For Japanese cedar, Hasegawa et al. [2] and Mishiro [14] reported that the ratios of velocity in the longitudinal direction to that in the radial direction were 2.3; the ratios of velocity in the longitudinal direction to that in the tangential direction were 3.1.The ratios for a non-contact method were almost the same as those for a contact method.The findings in this study suggest that the ultrasonic wave velocities in Japanese cedar could be measured with a non-contact method by using air-coupled ultrasonics.

Conclusion
Ultrasonic wave velocities in the axial and lateral directions on Japanese cedar columns can be measured with a non-contact method by using air-coupled ultrasonics.As compared, the velocities were also measured by using the contact method.The velocities in the non-contact method were smaller than those measured using the contact method.The ratios of velocity were the same as those in a contact method.These results in the present study have suggested that the air-coupled ultrasonic wave is a useful tool for non-contact and nondestructive evaluation in Japanese cedar.Most importantly, this study could be considered as the first step toward the application of air-coupled ultrasonics to the nondestructive evaluation in Japanese cedar columns.In the future, the need is envisaged to measure an ultrasonic velocity in a full-sized lumber and to explore the possibility of non-contact and nondestructive evaluation in existing wood constructions using air-coupled ultrasonics.

Figure 3 .
Figure 3. Waveform of ultrasonic wave in the air at 110 mm distance.

Figure 4 .
Figure 4. Waveforms of ultrasonic wave in the axial and lateral directions of wood specimens.

Figure 5 .
Figure 5. Distributions of ultrasonic wave velocity in the axial and lateral directions of wood specimens.

Table 1 .
Density and moisture content of test specimens.

Table 2 .
Ultrasonic wave velocities in axial and lateral directions.