Ultrasonic techniques for nondestructive testing of standing trees

Ultrasonic techniques for nondestructive testing of standing trees

Ultrasonics 43 (2005) 237–239 www.elsevier.com/locate/ultras Ultrasonic techniques for nondestructive testing of standing trees V. Bucur q * Centr...

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Ultrasonics 43 (2005) 237–239 www.elsevier.com/locate/ultras

Ultrasonic techniques for nondestructive testing of standing trees V. Bucur

q

*

Centre de Recherches Forestie`res de Nancy, INRA, Laboratoire d’ Etudes et Recherches sur le Mate´riau Bois, LERMAB UMR 1094, 54280 Champenoux, France Available online 14 July 2004

Abstract The aim of this study is to assess wood quality of standing trees with ultrasonic velocity methods. The ultrasonic techniques currently used are: scattering based techniques that use travel time and other wave parameters for measuring the elastic constants of wood, and ultrasonic tomographic imaging techniques which seek to provide a high resolution picture of the defect. Scattering based techniques were firstly used as nondestructive techniques with increment cores of 5 mm diameter. On these specimens measurements with bulk waves––longitudinal and shear were used to detect the influence of sylvicultural treatment (pruning) on wood quality and also to detect defects (slope of grain, wavy figures). High resolution imaging techniques were developed for imaging abnormalities induced by biological attacks of fungi or insects in standing trees. In this case, ultrasonic tomography refers to cross-sectional imaging of trees from data collected by illuminating the tree from different directions. Ultrasonic images were reconstructed from the time of flight. The images were reconstructed from 120 measurements in situ and through the bark with direct transmission technique, using 1 MHz transducers. The resolution is 5 cm.  2004 Elsevier B.V. All rights reserved. Keywords: Trees; Defect detection; Wood; High resolution imaging

1. Introduction Quality assessment and defect detection on wood of standing trees is one of the major task of forestry. Nondestructive evaluation of the properties of standing trees has it origin in the necessity to solve practical problems without the destruction of the integrity of trees. For this purpose, two main approaches are possible. The earliest method was to take an increment core from the tree, (which is considered nondamaging for its life) and to develop different nondestructive techniques for the study of main physical characteristics of wood. The requirements of forestry genetics to capture breeding opportu-

q This article is based on a presentation given at the Ultrasonics International 2003. * Tel.: +33 3 83 68 48 46; fax: +33 3 83 68 44 98. E-mail addresses: [email protected], [email protected] lermab.uhp-nancy.fr (V. Bucur).

0041-624X/$ - see front matter  2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ultras.2004.06.008

nities for the improvement of wood quality, determined recent work on methods for measuring stiffness of young trees with different techniques such as static bending testing, stress wave, resonance frequency, ultrasound. Acoustical methods were also used for understanding the phenomena of juvenile wood formation in softwoods, detection of decay, evaluation of wood quality of young hybride softwoods. Presently, our interest is focused on ultrasonic techniques developed for wood quality assessment on standing trees, performing nondestructive measurements either directly on trees in situ, or, on increment cores bored from living trees. The defects in standing trees can be classified such as: defects induced by different irregularities from natural growth patterns (grain deviation, knots, pitch pockets, etc.); and, abnormalities induced by biological attacks of fungi, insects, etc. The various stages that are usually taken into consideration during ultrasonic inspection of the trees are:

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detection, localization, characterization and decision to act on it, if the defect is important enough. The ultrasonic techniques currently used are: scattering based techniques that use the travel time, and other wave parameters, and ultrasonic tomographic imaging techniques which seek to provide a high resolution picture of the defect [1,2].

2. Scattering based techniques These techniques are related to the measurement of the velocity of propagation of ultrasonic waves on trees and increment cores. In what follows we describe the techniques used for the detection of the slope of the grain on standing trees, for the detection of the pruning treatment and of the presence of wavy structure on trees and increment cores. There is a permanent need to determine the slope of the grain from the very early age of the tree to the mature age to help capture genetic breeding opportunities to produce better structural lumber. The slope of grain can be defined as the angle between longitudinal anatomical elements and the axis of the stem. Excessive grain angle has important effects on shrinkage, rate of moisture movement, strength, machining properties of lumber, plywood, veneer, etc. The precise determination of the slope of grain on a tree can be performed with an array of transducers, mounted on the periphery of the trunk. The elliptical pattern of wood propagation allow us to say that it is possible to determine the slope of grain with three measurements of velocity. The agreement between the ultrasonic measurements and the grain angle with optical method is given by the correlation coefficient r = 0.95 for measurements on 50 trees of different species. The effect of pruning on wood quality is related to the improvement of the cylindrical shape of the trees, the reduction of the juvenile, the reduction of wood shrinkage, the improvement of mechanical properties, the absence of knots, etc. The improvement of wood mechanical properties can be correlated with the increasing of the ultrasonic velocity in longitudinal and radial direction of the tree. The increase of the velocity in the radial direction is 25% and of the velocity on longitudinal direction of 8.6%. The velocity in the radial direction is more related to the uniformity of annual rings induced by the pruning while the increase of the velocity in longitudinal direction is related to the increase of fiber length. It can be noted that there is less dispersion for values measured on pruned trees than in control trees. The superimposition of the velocity values measured in control and pruned tree is in the range 5000–6500 m/s. The quality assessment on an increment core is related to its mechanical properties. On an increment core,

the elastic constants can be determined with the ultrasonic velocity method. Ultrasonic velocity measured on an increment core is in good agreement with other physical properties of wood determined with microdensitometric X-ray technique. The pruning can improve the wood quality expressed by the values of the dilatation terms of the stiffness matrix if wood is considered as an orthotropic material. The highest influence was observed on the longitudinal direction L, for stiffness CLL which increased by 30% in the case of 50% height pruning. The shear terms are less affected by the sylvicultural treatment. The detection of wavy structure of maple using ultrasonic technique was possible on standing trees and on increment cores. Ultrasonic velocity of surface waves was measured at 1.30 m height following 4 main directions, corresponding to N, S, E W. The difference between the measured values of velocities are between 16% and 75%. A more sophisticated statistical treatment using variance analysis for velocity data, has shown that there is an important statistical difference between normal and wavy maple trees, but the measurement orientation has no significant effect. From the same trees, increment cores of 10 mm diameter were bored and laboratory measurements were performed using shear wave transducers. The wavy pattern results from an undulation of fibers and abundant medullary rays. The shear velocity VRL propagation in R direction and polarization in L direction) seems to give a pertinent indication about the presence of wavy figures in radial direction. In the R direction the waves find a tubular structure formed by the ray cells and the modulation of the shear wave polarization is along the fibers. In the wavy structure the rays are much more abundant than in normal maple and probably for this reason that the shear velocities are higher.

3. Ultrasonic tomography Wood quality assessment of standing trees can be observed with ultrasonic diffraction tomography As for Xray computed tomography, ultrasonic tomography refers to the cross-sectional imaging of an object from data collected by illuminating the tree from different directions. Different types of ultrasonic waves can be used, but the most convenient are bulk longitudinal waves. Ultrasonic images can be reconstructed from all characteristic parameters of the wave: time of flight, amplitude, frequency spectra of the waveform, the phase, etc. The energy distribution and energy flow are important parameters for enhancing the image contrast. There are three main types of algorithms that can be used to form tomographic images from ultrasonic data: transform techniques, iterative techniques and direct inversion techniques.

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16 points were used for ultrasonic velocity measurements. Finally the image was reconstructed from 120 measurements in situ and through the bark. To improve the reading on the received signals, a frequency analysis of the signal was performed. Iso-velocity zones were defined for 300 m/s, and represented with a characteristic color. The range of measured velocities values was between 400 and 2350 m/s. Fig. 1 shows the 3 D ultrasonic reconstructed image of the internal structure of a beech standing tree, of 55 cm average diameter, having an important decay zone. The image resolution is between 4 and 5 · 10 2 m. A good agreement between the ultrasonic reconstructed images and the real images after the harvesting of the tree was obtained. Data acquisition and treatment is relatively long (45 min for a tree of 55 cm diameter), but improvements could be obtained with an array of several transducers permitting the fan-beam capture.

4. Concluding remarks

Fig. 1. 3D reconstructed image of internal structure of a beech, with an important central zone of decay [3].

On trees, the equipment for ultrasonic imaging, can operate in contact by through transmission mode, using frequencies ranging from 50 kHz to 1 MHz. The increasing of frequency determines the increasing of the image resolution, but, unfortunately, in the same time the increasing of signal attenuation. For this reason a good compromise is required between frequency and image resolution. The measurements were performed at three height levels: at 0.36 m, at 0.84 m and at 1.38 m over the ground, with a portable apparatus (SATTEC, AU 80, France). At each height, on each tree circumferences

Imaging of the internal structure of standing trees is possible using both scattering based techniques that use the travel time, and high resolution ultrasonic tomography based on the measurement of ultrasonic velocity with longitudinal bulk waves. 3D images were reconstructed with 2D images obtained at different heights of the tree with a resolution of about 4 cm. This resolution is convenient for the inspection and defect detection of standing trees from parks and public gardens as well as for poles and other wooden structural elements.

References [1] V. Bucur, Nondestructive Characterization and Imaging of Wood, Springer Verlag, Heidelberg, 2003. [2] R. Martinis, Analisi e sviluppo di technique non invasive per la valutazione di carie in alberi dÕalto fusto, Ph.D. thesis, Universita` degli Studi di Firenze, Dipartimento di Biotecnologie, Agrarie. [3] R. Martinis, V. Socco, L. Sambuelli, O. Schmitt, V. Bucur, Tomographie ultrasonore pour les arbres sur pied, Ann. Forest Sci. 61 (2) (2004) 157–162.