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Feng, H., Fang, Y., Li, J., and Li, G. (2015). "Detecting longitudinal position of internal log hole using an impact-echo method," BioRes. 10(4), 7569-7579

Abstract

This paper presents a new methodology for detecting the longitudinal location of the hidden hole in a log using impact-echo testing. The hole and the end surface of the log produced two different amplitude peaks in the frequency spectrum after a mechanical tap on the top of the log. The ratio of the two frequencies was used to estimate the longitudinal position of the hole. The major advantage of this method is that it avoids measuring the travel velocity of the stress wave, which is sensitive to many factors and then poses a formidable challenge to the realization of inspection. Experimental studies were carried out using Pine logs with mechanically drilled holes and a Cinnamomum camphora log with natural hole. The results indicated that the estimated positions were in good agreement with the actual position of the hole. The impact-echo testing can be applied to detect the longitudinal position of the internal hole in log.



Full Article

Detecting Longitudinal Position of Internal Log Hole Using an Impact-Echo Method

Hailin Feng,a Yiming Fang,a,*Jian Li,and Guanghui Li b

This paper presents a new methodology for detecting the longitudinal location of the hidden hole in a log using impact-echo testing. The hole and the end surface of the log produced two different amplitude peaks in the frequency spectrum after a mechanical tap on the top of the log. The ratio of the two frequencies was used to estimate the longitudinal position of the hole. The major advantage of this method is that it avoids measuring the travel velocity of the stress wave, which is sensitive to many factors and then poses a formidable challenge to the realization of inspection. Experimental studies were carried out using Pine logs with mechanically drilled holes and a Cinnamomum camphora log with natural hole. The results indicated that the estimated positions were in good agreement with the actual position of the hole. The impact-echo testing can be applied to detect the longitudinal position of the internal hole in log.

Keywords: Non-destructive testing; Log; Internal hole detection; Longitudinal position determination; Impact-echo testing

Contact information: a: School of Information Engineering, Zhejiang A & F University, Hangzhou City, P. R. China; b: Zhejiang Provincial Key Laboratory of Forestry Intelligent Monitoring and Information Technology, Hangzhou City, P. R. China; *Corresponding author: ymfang@zafu.edu.cn

INTRODUCTION

Wood is a valuable engineering material with a high strength to weight ratio and is widely used as a structural element. Holes in the wood, caused by insects, loose knots, cracks, or fungus, frequently lead to poor mechanical performance. Consequently, the hole should be detected before further use or processing of the wood. Visual inspection is the easiest and lowest cost method for log inspection if hole is present in the wood. However, this method is ineffective with hidden holes because there may be no external evidence of their presence. The desire for wood-based products in industry has driven extensive research to detect hidden hole in logs using a non-destructive method.

The acoustic technique has been attracting attention and developing rapidly (Roohnia et al. 2011; Sohi et al. 2011). In principle, different internal conditions, that is, sound, deterioration, or the decay of logs often result in a modified propagation velocity of the acoustic wave (Wang 2013). Hence, the flight time of the acoustic wave, together with the prior knowledge of the logs, provides internal information about the logs (Li et al. 2014). In our previous work, tomographic images were reconstructed from the time of flight (TOF) data with the implementation of the interpolation algorithm. Information concerning the size, shape, and position of the internal defect was presented in an intuitive manner (Feng et al. 2014). However, the described work mainly focused on the inspection of the cross-sectional modes. Therefore, these methods cannot be directly employed to estimate the longitudinal position of the internal hole in a log.

The impact-echo method is a popular technique for non-destructive testing. Stress waves are excited from a short duration mechanical impact. The frequency spectra obtained from the recorded waveforms are affected by the conditions of the test sample. Depending on the emerging amplitude peak in the frequency spectrum, the presence and location of internal defects can be determined. This approach has been widely employed to detect defects in many disciplines. It was revealed that the axisymmetric void, non-axisymmetric void, and neck could be detected in the frequency domain (Kim et al. 2002). Wang et al. (2012) evaluated Poisson’s ratio using the materials: concrete, steel, stainless steel, and brass with the impact-echo testing method. Experimental data yielded satisfactory results. The work done by Kee et al. (2011) demonstrated that the impact-echo method is a practical option for consistent and rapid in-situ evaluation of reinforced concrete bridge decks.

However, until now, only a small number of successful applications in wood testing have been mentioned in the literature. Conventional impact-echo based methods depend upon knowledge of the propagation velocity of the stress wave, which presents a challenge to realize a precise testing of logs. In contrast to metal, wood is a complex, inhomogeneous material. The propagation of the stress wave is affected by many natural features, such as moisture content, knots, texture angle, earlywood, and latewood (Wang et al. 2004). It becomes very difficult to obtain an accurate value of the velocity. Thus, there is a strong need to investigate novel approach with which no prior information concerning velocity is required.

In this paper, a novel method is proposed aiming to eliminate the effects of travel velocity on the results of log inspection. Although the method is still based on the echo signals, a different system is used for processing the signals. Unlike conventional approaches, this method does not involve any determination of frequency displacements or travel time. More attention is paid to the changes in magnitude spectrum. Moreover, the travel speed of the stress wave is not involved in the computation. Accordingly, the detection is no longer affected by the inhomogeneity, anisotropy and natural variability of wood.

Principle of the Impact-Echo Method

The principle of the impact-echo method is illustrated in Fig. 1. When a hammer strikes the top of the log, a longitudinal stress wave (P wave) is introduced to the log and travels through the log. This wave will be reflected at the end of the log, whereupon it returns in the reverse direction. While being reflected several times, the recording is transmitted by a receiver sensor attached on the top of the log. Then, the obtained signal, which indicates the reverberation of the wave, is processed using Fast Fourier Transform (FFT). If an internal hole is present, the heterogeneities will result in internalinterfaces. The stress wave will be reflected both on the interfaces of hole and the end of the log. Subsequently, additional peaks appear in the frequency spectra. By interpreting the change in the frequency domain, one can estimate the distance between the hole and the top, represented as in Fig. 1. (Carino 2001; Yeh and Liu 2008)

Fig. 1. Schematic illustration of the impact-echo technique

Frequency Spectrum of the Sound Log

Wang et al. (2003) suggested that a defect-free log can be treated as a cylindrical orthotropic form with a taper. The stress wave speed () in a log is dependent upon the longitudinal elastic modulus of the log and the log’s density (Eq. 1):

(1)

The order resonance frequency of the echo-wave can be expressed as follows (Lin 1997) (Eq. 2),

(2)

where denotes the area of the cross section, is the length, and is the stiffness ratio (Lin 1997).

According to Newton’s third law and the force balance relationship, equals approximately zero (Liu 2012). Hence, a simpler approximation of as follows is used (Eq. 3):

(3)

When equals 1, the fundamental frequency can be achieved as follows:

(4)

Hole Location Determination with Spectra Analysis

When an internal hole is present, the stress wave is reflected on the surface of the hole beside the end surface of the log. The reverberations on the hole surface are shown in Fig. 1 with a dashed line. Then a new component is added to the received signal, and a new peak emerges in the magnitude spectrum. For convenience, the new peak frequency is defined as . In principle, the period of the new component is two times of the travel time. The distance between the top of the log and the surface of the hole, as shown in Fig. 1, is defined as . Hence, can be determined by (Eq. 5):

(5)

From Eq. 5, it can be inferred that the frequency is relevant to the distance between the hole and the top of the log. Given the travel velocity C, the value of d can be calculated with the frequency . Therefore, Eq. 5 has been widely used in impact-echo testing of homogeneous materials. However, such an approach does not work well in the inspection of wooden products. The travel velocity of stress wave, which plays key role in the calculation, is sensitive to microstructure and physical properties of wood. This sensitivity causes difficulties and lower precision in the impact-echo testing of log.

In this work, Eq. 5 is used indirectly aiming to avoid measurement of C. Generally, the wave travels at a constant speed in the same log. Combining Eqs. 4 and 5, the value of can be estimated by using the ratio of to (Eq. 6),

(6)

where is the estimated value of . From Fig. 1, it can be observed that the waves with frequency of travel longer before being reflected than those of . This means that their periods are longer. Then is definitely larger than . Therefore the estimated will be within the range from 0 to .

In Eq. 6, should be the fundamental frequency of the defect-free log. In general, it is different from the fundamental frequency measured with a defective log because an internal hole consequently leads to a longer travel time of the stress wave (Wang 2013). However, for a given defective log, it is hard to obtain the corresponding fundamental frequency for the defect-free condition. In this work, the fundamental frequency obtained with the test sample was used as a substitute for the fundamental frequency of the sound log.

Note that what can be estimated using Eq. 6 is the distance between the top of the log and the nearest surface of the hole, as shown in Fig. 1. By conducting testing on the other side of the log, the distance between the end and the hole on the other side can also be obtained. Other information regarding longitudinal positions, such as geometrical center, can be determined with further calculations. For simplicity, this paper mainly focused on the testing at one side because of the similarity of the testing.

EXPERIMENTAL

Materials and Experimental Setup

Test specimen

Three pine logs, as shown in Fig. 2, were employed in this study, aiming to evaluate the performance of the method. They were numbered from 1 to 3 and the sizes were Ф 0.22 2.1 m (No. 1), Ф 0.25 2.0 m (No. 2), and Ф 0.25 2.1 m (No. 3).

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Fig. 2. Photo of the pine logs

An acoustic tomography device provided by Fakkop Enterprise of Hungary was employed to remove the suspicion of having internal defects. With a few taps on each sensor evenly spaced along the log, acoustic images can be developed. Figure 3 shows the inspection results. In the images, three color schemes (green, red, and blue) were used to represent the internal conditions of logs, which corresponded to “sound”, “decayed”, and “hollow”, respectively. Obviously, the used logs were quite healthy because the images were dominated by green color.

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(a) (b) (c)

Fig. 3. Acoustic images of the pine logs developed by Fakkop tomography equipment: (a) Image of the No.1 log; (b) Image of the No.2 log; (c) Image of the No.3 log

Cinnamomum camphora log, shown in Fig. 4(a), was also employed in this study, aiming to further evaluate the performance of the method. The size of the log was Ф 0.18 1.6 m. The visual inspection of the sample did not show any indication of there being a hole over the entire log. However, from Fig. 4(b), it can be seen that there existed an internal hole because a blue area appeared in the Fakkop image. The hole can also be located from Fig. 4(b). It was about 1.0 m apart from the A side of the log and 0.4 m from the B side.

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