The study of wood knots using acoustic nondestructive testing methods
Introduction
Acoustic nondestructive testing methods applied to wood primarily use the change of sound speed and sound resistance to obtain information about the mechanical characteristics of the wood under consideration and of the location of potential internal defects. Basically one can distinguish three kinds of methods: the ultrasonic pulse method, the acoustic emission technique and the resonance method.
In the early 80′s, Dunlop adopted the ultrasound pulse method to detect defects in wood, and to discriminate the different conditions of wood rot [1]. Shi measured the sound speed of wood along three orthogonal axes and related it – among other parameters – to their moisture content and compression strength [2]. At the end of the 80′s, researchers at EPFL developed an instrument to measure the dynamic modulus of wood based on the longitudinal sound speed [3]. Ross et al. used the difference in time-of-flight measurements to evaluate the presence of rot, cavities and knots in wood [4]. Schafer investigated the energy decay of sound transmission in wood by means of computer simulations in 2000 [5], based on a theory in which the decay of the energy is caused by internal friction and scattering upon transmission of ultrasound through wood.
In the domain of acoustic resonance, Nakayama was the first to introduce and confirm the merit of the resonance method in wood in 1975 [6]. About a decade later, Sobue measured the shear modulus of wood by recording the signal of a complex vibration [7]. Ous reviewed nondestructive evaluation (NDE) methods based on vibrational impulse response of wood and summarized different modes of vibration of logs [8]. Murphy investigated the analytical and numerical solutions for a simply supported vibrating beam, and obtained consistent results for different sample lengths [9]. Kamaguchi et al. related the conditions for transverse vibration to the moisture content of wood [10]. Tonosaki et al. exploited longitudinal and bending vibrations to study the inhomogeneous mechanical characteristics of wood for musical instruments [11].
Concerning NDE on the wood beams, Sandoz measured ultrasonic wave velocity in spruce and fir beams, and correlated the ultrasonic longitudinal wave speed to the modulus of elasticity (MOE) [12]. Diebold examined NDE methods including transverse vibration and ultrasonic inspection on several kinds of wood, and found the ultrasonic inspection had the lowest correlation to the bending strength [13]. Duju applied six methods of measuring MOE to five different timber species, he found the calculated MOEs are correlated to the values of modulus of elasticity (MOR) well [14]. Sandoz and Benoit utilized the ultrasonic wave speed and attenuated energy to characterize the MOE and MOR of wood, Energy damping of the wave was found to be directly dependent upon local singularities such as knots and grain angle [15]. Pires estimated the MOEs of wood using stress wave, transverse vibration, and ultrasound, ultrasound and stress-wave inspection were similarly correlated to those determined from static bending tests [16]. In China, research and use of the resonance method for wood NDE started about several years later. Chen introduced the resonance method by impact excitation in 1991 [17]. Zhong et al. analyzed the static bending modulus of wood using spectral analysis in 1996 [18]. Hu adopted a combination of bending vibration, longitudinal wave propagation as well as resonance to measure the dynamic modulus of chipboards [19]. Later, Cui examined the relation between the dynamic modulus of wood samples and the number of knots contained in these samples by the resonance method [20], while Wang and Yang adopted the ultrasound transmitting method and employed an FFT analysis of the received signals for wood containing holes [21]. The evaluation methods are mostly based on frequency. For transverse vibrations in wood samples of uniform cross-section under free constraint conditions, the resonance frequency spectrum can be related to the modulus and density of the wood material. Internally present knots can significantly affect these two physical properties. Therefore, measuring and comparing the harmonic frequencies of wood beams of the same size by impact excitation can be used to distinguish the presence and characteristics of knots.
The above literature review shows that most of the prior research concerns experiments on wood without defects, in order to identify the wood type and to determine its mechanical parameters. There is little work on assessing wood that contains knots as a kind of defect. Some researchers, e.g. Cui [20], have studied the numbers of knots in wood samples based on the resonance method, however, they neither discussed the size and location of those knots explicitly, nor did they compare this method to a more versatile method based on sound speed. In fact, in this paper, given the fact that the knot’s location affects the accuracy of the sound speed method, we will treat the knot’s position as a primary variable quantity, and use this information to adopt and combine the resonance and sound speed methods to evaluate the modulus of wood. Next, we analyze the obtained results in order to assess the reliability of the knots’ evaluation methods. As part of the conclusion, we formulate advice about some practical measurement procedures at different conditions.
Section snippets
Brief review of the theoretical models for modulus evaluation
In the present study, parallelepiped samples of wood with orthogonal symmetry, as shown in Fig. 1, have been used. The considered beams are long and slender, and have length l, width b (≪l) and thickness h (≪l). The (lengthwise) axial direction corresponds to the axis L in Fig. 1. The samples will be subjected to two types of measurements: longitudinal sound speed measurements along the axial direction, and resonance measurements induced by transverse vibrations perpendicular to the axial
Experimental samples
The experiments were performed on parallelepiped Pinus Sylvestris samples, which is a kind of soft wood. In order to control the variability, we selected two four meter long wood sticks, with rectangular cross-sections, and with the long axis parallel to the axial direction of the tree’s growing orientation, they are both the sapwood cut from the trees. The long sticks were cut into several smaller parts and divided into two groups according to the parental stick origin. As the two sticks were
Results for the first group of samples
For the first group of beams, the comparison of the dynamic moduli inferred from the resonance method (i.e. flexural modulus Ebend, cfr Eq. (4)) and by the sound speed measurements (i.e. compressive modulus E, cfr Eq. (1)) is shown in Fig. 7. The wood samples having number 1–7 are free of knots (and serve as reference samples), sample number 8 and 9 have one knot, and sample number 10–14 contain two knots. The knots of the beams in this group are all in or near the middle of a cross-section
Conclusion
In this paper, we adopted two methods based on sound speed and resonance measurements to infer the dynamic compressive and bending modulus of parallelepiped wood sticks containing knots. A correlation analysis of the two moduli was performed on samples whose knots are located in the middle of the cross-section of the beams or not. When the knots are in the middle of the beam’s cross-section, both methods provide reliable evaluations of the number of knots. When the knots are off-center, it is
Acknowledgements
Project supported by the National Key R & D Program, China (Grant No. 2016YFF0203000), National Natural Science Foundation of China (Nos. 11474160, 11774167), Fundamental Research Funds for the Central Universities (No. 020414380001), Key Laboratory of Underwater Acoustic Environment, Institute of Acoustics, Chinese Academy of Sciences (No. SSHJ-KFKT-1701) and AQSIQ Technology R & D Program, China (Grant No. 2017QK125).
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