Gamma-Ray Defectoscopy of Round Timber

V. I. Gorbunov, A. N. Karmadonov, Yu. I. Litovchenko,
V. A. Grudinin, A. N. Kiselev

Complete automation of the principal technological processes for cutting and sorting round timber under production conditions at the lower yards of timber-industry enterprises and sawmills can be accomplished only on the basis of solving the problem of defectoscopy of tree-length logs and developing the scientific foundations for algorithmizing the bucking process.

The detection of hidden defects in wood in the technological flow by nondestructive testing methods makes it possible to assess objectively the quality of round timber and, on this basis, to carry out optimal cutting of tree-length logs with an increased yield of merchantable and higher-grade wood.

Among the known methods for quality control of materials, the radiation method has a number of advantages over others, which makes it possible to recommend it for industrial quality control of round timber [1]. Essential advantages of the radiation method are the contactless nature and high productivity of the inspection, which is very important in continuous-flow production. In view of the fact that defectoscopes using radioactive isotopes as sources are much more compact than X-ray ones, simpler, more transportable, and more stable in operation, the use of gamma-ray defectoscopy is the most optimal in this control method [2]. Taking into account the characteristics of the object being inspected, it was concluded that, in gamma-ray defectoscopy of tree-length logs, it is necessary to rely only on the differential method of inspection, which to some extent eliminates the influence of inhomogeneity in the material structure and of fluctuations in the thickness being radiographed [1, 3]. One of the most important stages in carrying out automatic wood-quality control is the selection of the optimal geometry for gamma irradiation of round timber. The choice of geometry is connected with the location of the defectoscope in the technological flow, the creation of optimal conditions for detecting internal rot and knots, and the productivity of the inspection. In this, a special role is played by the choice of the distance between the compared sections of the controlled tree-length log. To assess the quality of the wood it is necessary to determine not only the presence of rot or knots, but also their location. Since rot and knots may be located anywhere in the tree-length log, it is necessary to ensure the collection of information on quality along the entire length of the controlled tree-length log.

For these purposes the following irradiation geometry was developed, Fig. 1. The gamma-radiation source 1 is fastened to a metal rod—

with counterweight 3 and rotates in a plane parallel to the longitudinal section of the object. During each revolution of the source, the radiation passes twice through the inspected object and is continuously recorded by the stationary radiation detector 4. The measuring unit records only the radiation flux that has passed through the inspected object, and compares the information from each two adjacent sections. The range of permissible distances between the compared sections lies within \(0.6\,m \leq l \leq 1.4\,m\) [4]. The sensitivity and detectability of defects in gamma defectoscopy of wood are determined only by the difference in density between defective and sound material (on average by a factor of 1.5–2), since the effective atomic number of the principal defects that reduce quality (with the exception of cracks and cavities) differs hardly at all from that of the material itself. Rots reduce the density of the base material by a factor of 1.5–2, while knots usually have a density 2–2.5 times higher than that of sound wood. Even in sound wood the density varies over a wide range for a single species and even within one and the same trunk (Fig. 2), but the densities of sound and affected freshly cut wood in one and the same trunk at a distance of 1–2 m will be different, which is the basis of the present inspection method.

Fig. 1. Geometry of gamma radiography of round timber

Defectoscopy of rots and knots has its own particular features. Rots have a much larger volume than knots and are located mainly in the central part of the trunk, whereas knots are located at the periphery. To reduce the influence of random density fluctuations caused by the presence, in the inspected volume, of bark, small knots, cracks, etc., the inspected volume should be increased. In this case the values of the mean densities of the sections will depend only weakly on the presence of small inhomogeneities, which leads to an increase in sensitivity for detecting rots, but decreases sensitivity for detecting knots. Analysis of gamma-radiation attenuation curves over the cross section of wood shows that, in most cases, rots have a weakly pronounced form (Fig. 3). Therefore, in defectoscopy of rots it is better to compare the integral values of the radiation flux over transverse sections of the wood. The gamma-quantum fluxes behind sound and affected sections of the inspected object have the following form [1, 3]:

\[ \bar{n}_1=\bar{n}_0\int_0^{\tau_1} e^{-\mu_m \cdot \rho_1 \cdot x_1(t)}\,dt, \]

\[ \bar n_2=\bar n_0 \int_0^{\tau_2} e^{-\mu_m \rho_2 x_2(t)}\,dt, \tag{1} \]

where
\(\bar n_0\) is the radiation flux at the detector in the absence of the object;
\(\tau_1\) and \(\tau_2\) are the inspection times for the two sections;
\(\mu_m\) is the mass attenuation coefficient of radiation by the wood substance;
\(\bar\rho_1\) and \(\bar\rho_2\) are the mean wood densities of the inspected sections;

Fig. 2. Graphs of the normal distribution of the density of freshly felled wood by species

\(x_1(t)\) and \(x_2(t)\) are the variable thicknesses of the inspected sections. The mean density of the wood varies depending on the presence and degree of its damage. To simplify expressions (1), the current variable thickness \(x(t)\) may be replaced by the equivalent effective thickness of the object \(x_{\mathrm{eff}}\). Then

\[ \bar n_1=\bar n_0\cdot \tau_1\cdot e^{-\mu_m\bar\rho_1 x_{1\mathrm{eff}}}, \]

\[ \bar n_2=\bar n_0\cdot \tau_2\cdot e^{-\mu_m(\bar\rho_1-\Delta\bar\rho)(x_{1\mathrm{eff}}-\Delta x_{\mathrm{sb}})}, \tag{2} \]

where \(\Delta\bar\rho=\bar\rho_1-\bar\rho_2\) is the difference between the mean densities of the sections,
\(\Delta x_{\mathrm{sb}}\) is the magnitude of the taper of the log diameter. The relative change in the radiation flux beyond the object has the following form:

\[ \frac{\Delta n}{n}=\frac{n_1}{n_2}-1= \frac{\tau_1}{\tau_2} e^{\mu_m\left[(\bar\rho_1-\Delta\bar\rho)(x_{1\mathrm{eff}}-\Delta x_{\mathrm{sb}})-\bar\rho_1x_{1\mathrm{eff}}\right]} -1, \tag{3} \]

i.e., it is a function of the difference in density of the sections, the thickness, the taper, and the mean density of the wood. For small sizes of damage (small density difference), the contribution of the component due to taper amounts to tens of percent, which reduces the sensitivity of the inspection method. In order to increase the sensitivity of the method for detecting rot, the so-called method of cutting out the inspection zone is used. Its essence consists in recording only the radiation flux that has passed through the central part of the section; the radiation that has passed through the edge of the log is cut off, which increases the relative change in the radiation flux. Such a method makes it possible to increase the sensitivity to the detection of rot by a factor of 2–3, depending on the degree of “cutting out” of the inspection zone (Fig. 4). The optimal solution from the point of view of the simultaneous detection of rot and knots is the use of a detector consisting of

of a bank of counters. In this case, information about rot is summed from all counters, while information about knots is taken from each cell separately. With this scanning geometry for round timber, the diameter of the timber is measured with good accuracy, which makes it possible to compensate for the influence of the thickness and taper of stems on the inspection results [5].

Fig. 3. Gamma-radiation attenuation curve in the transverse section of round timber

Herein lie new reserves for increasing the sensitivity of the inspection method. Experimental studies carried out to verify the principal parameters of the gamma-defectoscopy method for round timber showed that third-stage rot is reliably detected down to \(1/4\) of the stem diameter.

Fig. 4. Relative change in the radiation flux behind the object for different degrees of “cutting out” of the inspection zone

Conclusions

1. The developed gamma-defectoscopy method makes it possible to carry out quality control of timber in the technological flow.
2. The sensitivity of the method for quality control of round timber can be substantially increased by compensating for the influence of thickness and taper of stem diameter.
3. The problem of defectoscopy of knots still requires further research.
4. The studies carried out in developing the method of gamma-defectoscopy of timber give grounds to believe that the problem of optimal stem cutting will be solved in the near future.

References

1. A. N. Karmadonov, Yu. I. Litovchenko, V. A. Grudinin et al. On the method of monitoring wood quality. Proceedings of the Institute of Forest and Wood, Siberian Branch, Academy of Sciences of the USSR, “Study of the Properties of Wood and Wood Materials.” Krasnoyarsk, 1969.

2. V. I. Gorbunov, A. N. Karmadonov, Yu. I. Litovchenko et al. The use of radioisotopes for detecting defects in round timber. In: Radiation Defectoscopy. Moscow, 1970.

3. Development of a prototype defectoscope for monitoring the quality of round timber. Report of the Research Institute of Electronic Introscopy under the Tomsk Polytechnic Institute. Tomsk, 1969.

4. A. N. Karmadonov, Yu. I. Litovchenko, V. A. Grudinin, V. N. Ermolin, L. I. Isaeva, V. A. Glazachev. Selection and substantiation of the geometry of radiation quality control of round timber during longitudinal feeding of tree-length logs along a conveyor. Proceedings of the Institute of Forest and Wood, Siberian Branch, Academy of Sciences of the USSR. Krasnoyarsk, 1971.

5. Development of a prototype defectoscope for monitoring the quality of tree-length logs. Report of the Research Institute of Electronic Introscopy under the Tomsk Polytechnic Institute. Tomsk, 1970.