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Maderas. Ciencia y tecnología

versión On-line ISSN 0718-221X

Maderas, Cienc. tecnol. v.4 n.2 Concepción  2002

http://dx.doi.org/10.4067/S0718-221X2002000200003 

Maderas. Ciencia y tecnología. 4(2):133-139, 2002

ARTICULO

NONDESTRUCTIVE EVALUATION OF WOOD USING ULTRASONIC TECHNIQUE

EVALUACIÓN NO DESTRUCTIVA DE LA MADERA CON ULTRASONIDO

Fabiana G. R. Oliveira1, Janaína A. O. de Campos1 , Everaldo Pletz2 ,Almir Sales3
1Post-Doctorando en Estructuras de la EESC/USP. São Carlos. Brasil.
2Profesor Titular de la EESC/USP. São Carlos. Brasil
3Doctorando en Ciencias e Ingeniería de Materiales – IFSC/IQSC/EESC – USP. São Carlos. Brasil

Corresponding author:almir@ufscar.br


ABSTRACT

This paper reports on a study of the application of ultrasound waves in wood with the purpose of evaluating the mechanical properties of Pinus taeda. Tests were carried out to determine the properties of density at 12% moisture content (ρap,12%) and modulus of elasticity in static deflection (MOE). Ultrasound tests were also performed to find the dynamic modulus of elasticity (Ed), for purposes of comparison with the static findings. The results and analysis lead us to conclude that the nondestructive ultrasound test can be employed to obtain reliable evaluations of the mechanical properties of lumber.

Keywords: ultrasound, nondestructive evaluation, wood.


RESUMEN

En este trabajo se presenta un estudio sobre la utilización de ultrasonido como una técnica para la evaluación de las propiedades mecánicas de la madera. En el mismo, se utilizó madera de la especie Pinus taeda, se determinó la densidad (ρap,12%) y el módulo de elasticidad en la flexión (MOE). Con el ensayo ultrasónico se determinó el módulo de elasticidad dinámico (Ed). Los resultados y análisis de las correlaciones entre los valores dinámicos y estáticos permiten concluir que la aplicación del ultrasonido puede ser utilizada con confiabilidad en coníferas.

Palabras claves: ultrasonido, evaluación no destructiva, madera.


INTRODUCTION

Wood, being a natural material, has highly variable properties originating, among other factors, from edaphic and climatic conditions, so care must be taken to ensure that appropriate sampling and test procedure are used.

The characterization of wood, however, can be done effectively using nondestructive methods that do not require the extraction of test specimens, since the element or structure, rather than a specimen, is evaluated. Nondestructive evaluation is defined as the science of identifying the physical and mechanical properties of an element of a given material without altering its final application capacity, Ross et al. (1998).

The basic hypothesis for the nondestructive evaluation of wood was first propounded by Jayne (1959), who proposed that the energy storage and dissipation properties of wood, which can be measured by nondestructive means, are controlled by the same mechanisms that determine the static behavior of this material. At a microscopic level, energy storage properties are controlled by cell orientation and structural composition, which are factors that contribute to the material’s static elasticity. These properties can be observed as vibration oscillation frequency or sound velocity transmission. Thus, measurements of the deterioration rates of free vibrations or acoustic wave attenuation are used to examine the property of energy dissipation in wood.

According to McDonald et al. (1990), the nondestructive evaluation of homogeneous and isotropic materials such as steel, plastics and ceramics can detect faults and variability originating from the manufacturing process. In wood, these irregularities occur naturally and their influence on the mechanical properties can be evaluated by nondestructive methods.

Oliveira et al. (2002) stated that nondestructive methods offer several advantages over conventional wood characterization methods, such as the possibility of evaluating the structural integrity of an element without extracting test specimens, faster analysis of large populations, and versatility to adapt to standardized production line routines.

According to Kabir et al. (2002), nondestructive evaluation is an important tool for the characterization of wood and can be used industrially to improve process quality control by ensuring a greater uniformity of the raw material and its by-products.

Another application of the nondestructive methods is the in situ evaluation of structures, i.e., in loco evaluation, allowing for their maintenance or rehabilitation through a mapping of the deteriorated areas, which permits evaluations to be made of their structural integrity without the need to remove part of the structure.

Ross (1999) describes the use of several technologies, such as X-rays, chemical analysis, vibrational properties and sound wave transmission, that are used to evaluate wood nondestructively.

Ultrasound is characterized by frequencies above 20000 Hz. Its use offers several advantages, including the low cost of equipment compared to that of automatic classification machines and it is the relatively simple to use, both of which favor the method’s easy dissemination to the wood industry, Oliveira (2001).

The mechanical behavior of wood is determined by the characterization of nine constants, i.e., three Young moduli of elasticity, three transverse moduli of elasticity, and three Poisson coefficients. Young’s modulus is the main elastic parameter and has been the subject of extensive investigation.

The terms of the rigidity matrix (C) can be determined by ultrasonic measurements, while those of the flexibility matrix (S) are determined by static tests. Wood is considered an orthotropic material having three axes of elastic symmetry (longitudinal, radial and transversal). The rigidity matrix for this material consists of nine independent constants: six diagonal terms (C11, C22, C33, C44, C55 and C66) and three nondiagonal terms (C12, C13 and C23).

The three Young moduli of elasticity are proportional to the corrected rigidity of Poisson’s coefficients. Thus, for long specimens (considered infinite) the correction factor is 1, in which case CLL can represent the dynamic modulus of elasticity (Ed).

According to Wang and Chuang (2000), the relation of the elasticity modulus is (C)-1 = (S) and the elasticity modulus relating to the flexibility matrix is E11 = 1/S11, using the corrected values of Poisson’s coefficient. When the elements in question are longer than 50cm, it can be assumed that the experimental values correspond to the modulus of elasticity, without the need for corrections (CLL = Ed).

The application and measurement consists of positioning two accelerometer transducers on the material to be evaluated. The ultrasonic wave is introduced into the material by one of the transducers and recorded up by the other transducer, with the time reading – in microseconds – performed by the ultrasound instrument itself. The recorded times are used to calculate the dynamic modulus of elasticity, based on equation (1).

Ed=ρ x ν2 x 10 -6

(1)

where:
Ed: dynamic modulus of elasticity (MPa);
r : density of the wood (kg/m3);
v: velocity of the longitudinal wave (m/s).

The factors that influence the propagation of ultrasonic waves in wood are physical properties of the substrate, geometrical characteristics of the species (macro and microstructures), conditions of the medium (temperature, humidity) and the procedure utilized for the measurements (frequency and sensitivity of the transducers, their size, the position and dynamic characteristics of the equipment), Bucur and Bönhke (1994).

McDonald et al. (1990) state that high correlations have been found between the moduli of elasticity obtained from acoustic wave (Ed) and static deflection (MOE) techniques. It is more difficult to correlate the MOR with Ed, since the presence of defects and the slant of the fibers exert a more significant effect on the MOR than on the longitudinal velocity of the wave. Because the defects of the wood affect the grain deviation, any method that is sensitive to this will have a high potential for determining the wood’s strength.

Halabe et al. (1995) obtained low correlation coefficients (r2) for regressions between the MOR and the Ed for the southern pine species. Low r2 values are also due to the fact that the stress induced in wood during dynamic tests is very slight, i.e., dynamic measurements are based on the mechanical properties only at the elastic limit. The modulus of rupture (MOR) occurs with higher stress and is above the elastic limit, thus resulting in a poor correlation with nondestructive test parameters. It should also be noted that the MOR behavior is significantly affected by the presence of knots at the ends.

The values of the modulus of elasticity obtained through the ultrasound method are usually higher than those found with static deflection. According to Halabe et al. (1995), this is because wood is a viscous elastic and highly impact absorbent material. In the vibration of a wood species, the restored elastic force is proportional to the displacement and the dissipative force is proportional to the velocity. Therefore, when force is applied for a short time, the material shows a solid elastic behavior while, with longer applications of force, its behavior is equal to that of a viscous liquid. This behavior is more evident in static bending tests (long duration) than in ultrasonic tests. Thus, the modulus of elasticity determined by the ultrasound method is usually greater than that obtained in static deflection testing.

The purpose of this study was to obtain correlations between dynamic tests by ultrasound (Ed) and static bending tests (MOE) on structural dimensions beams, with the aim to analyze the feasibility of applying the ultrasonic method to evaluate the properties of wood of Pinus taeda.

EXPERIMENTAL PROCEDURE

Thirty-three beams of the Pinus taeda lumber, with dimensions of 5cm x 20cm x 440cm, were used. The beams were dried in the open air to 12% moisture content. Drying was followed by periodic weighing of samples taken from the test planks.

Tests were carried out to evaluate the following physical and mechanical properties:

    density at 12% moisture content

    :

    ρ ap,12% (kg/m3)
    modulus of elasticity in static deflection

    :

    MOE (MPa)
    dynamic modulus of elasticity

    :

    Ed (MPa)

To minimize possible variations caused by strain introduced in the bending test, the dynamic modulus testing was undertaken in-grade strength testing.

The dynamic tests were carried out using ultrasound SYLVATEST equipment with 22 kHz transducers. Two readings were made of the wave propagation velocity, and the dynamic modulus of elasticity (Ed) was calculated based on the mean of the two readings and the density at 12% moisture content, according to equation (1). Medicinal gel was applied at the tops of the tested beams to ensure good contact between the wood and the transducers. This gel provides better transmission of the ultrasonic wave at the interface, reducing interference in the signal.

For the bending test, the beams were initially placed in the position of the axis of least inertia so as to simplify the test format; however, the modulus of elasticity (MOE) values obtained were very low because the beams had a very large number of knots. The bending test was therefore repeated, with the beams placed in the position of the axis of greatest inertia.

RESULTS AND DISCUSSION

Table 1 shows the mean values and the coefficients of variation of the properties determined in the static and dynamic tests on Pinus taeda beams. As expected, the dynamic MOE was higher than the static testing MOE.

Table 1 – Mean value and variation coefficients

 

Density
(kg/m3)

Velocity
(m/s)

Dynamic MOE
(MPa)

Static MOE
(MPa)

Mean

476

3790

6942

5749

CV (%)

7,9

8,9

22,1

26,5


The mean values found for the static properties studied here are compatible with those normally found in experiments on conifers cultivated in Brazil, Rocco Lahr (1990).

The variation coefficient obtained for the dynamic modulus of elasticity was also close to the values reported in the literature, Bucur (1995).

The results of the dynamic tests (Ed) were 20% higher than those of the static tests (MOE). According to Bartholomeu (2001), the Ed term is 22% higher than the static methods when no correction is made by Poisson’s coefficient.

Linear regressions were made of each pair of dynamic and static moduli of elasticity values obtained for each beam, as shown in the graph below.

Figure 1 – Static MOE as a function of dynamic MOE

CONCLUSIONS

The lumber tested in this study showed extremely variation, with a large number of knots associated grain deviation, which increased the variability of the results. It should be pointed out that the behavior of the ultrasonic device was highly satisfactory, proving to be sensitive to the anatomical differences of the beams and producing coherent results.

The coefficient of determination obtained (r2 = 0,80) was highly significant, allowing us to conclude that the nondestructive ultrasound technique can be used to evaluate the mechanical properties of structural wood. Hence, this nondestructive method could be employed in the classification of structural lumber.

ACKNOWLEDGEMENTS

The authors thank FAPESP (São Paulo State Research Support Foundation) for its financial support and LaMEM (Laboratory of Wood and Wooden Structures) for its help in the development of this work.

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