This paper was presented at the 10th International Symposium on Nondestructive Testing of Wood 26.-28.09.1996 in Lausanne CH.
TOCs of the proceedings
In terms of wood quality evaluation, several NDT methods are allowable in function of the final target. For local density evaluation, ray methods are nowadays leading [1, 2]. For basic elastic property evaluation, ultrasonic stress wave method, working in a range of high frequency (1 MHz and more) is adopted. For mechanical evaluation on larger specimen, like industrial size timber, both transverse vibrations  and propagation ultrasound methods are more effective [4, 5]. In the last ten years, ultrasonic uses have been focused on final material, like stabilised solid wood, or wood based products.
In this study, the ultrasonic method integrated in the portative equipment Sylvatest®, has been used at several steps of the industrial wood transformation. Both transversal decay detection applied to standing trees, and log grading showing the grading effect before sawing on the final solid wood products are presently discussed. For industrial applications in the sawmill, the Sylvamatic concept, automation of Sylvatest® in the industrial process is also detailed.
For any species, the modulus of elasticity on the radial direction is roughly constant. The ration ER/P is a technical characteristic for one species, and can be accepted roughly constant as well, allowing a characteristical speed of ultrasound for every species. This referential speed is than the basic parameter for living tree detection, obtained by differentiation of the measured speed with regard to the referential, as eq. (2):
Because of the influence of the moisture content and temperature on the elastic wood properties, two constants, kMC, kT, will affect the measured speed, taking into account both the thermic and the hydric wood gradients (reference MC up to fibre saturation point and T = 20°C).
Measurements of the radial axis are done on two orthogonal axis, and the final result is expressed as an average of at least two readings. Coupling and geometrical considerations are mentioned on figure 1.
In order to improve the wood quality grading in the several steps of the industrial wood transformation, ultrasonic readings have been supported on green logs. A sample of logs was followed through the different industrial transformation steps (sawmill, glued laminated timber producer) and the final product. 12 glued laminated timber beams, GLT, have been tested in four points bending in the laboratory, in order to have a quantification of the mechanical performances.
The log sample has been graded in five groups, from group one, weak quality, to group five, top quality. The logs had a length of 5 m, and a diameter from 30 cm to 50 cm. The grading has been done based on two ultrasonic readings, on the longitudinal log axis. The sample studied have been built integrating nine logs from group one, nine logs from group three, and nine logs from group five. In the sawmill, the sample has been cut into planks, which were stabilised in terms of moisture content, before they were sent to the glulam producer, for GLT beam preparation.
Before gluing together the GLT beams, the constitutive planks have been measured. The results of the speed of ultrasound found for each plank in each group is given in figure 2.
Based on the 12 GLT beams (four beams in each group), the efficiency of the grading at the log level has been characterised. Mechanical and ultrasonic results obtained on this beam sample are given in table 1. It appears clearly that the log grading is very efficient on the final wood product quality.
|Group||Number|| Log |
| Dry plank|
| Group 1
||Beam 11||-||5339 ||10'226
|| Beam 12 ||-||5427 ||11'491
||Beam 13||- ||5344 ||10'735
||Beam 14||- ||5254 ||10'820
|| Average group 1 ||- ||4362 ||5343|| 10'818
|| Group 3 ||Beam 31||-||5576||12'836
||Beam 32||-||5616 ||12'692
||Beam 33||- ||5652 ||12'571
||Beam 34||- ||5582 ||11'573
||Average group 3||- ||4878 ||5622 ||12'418
||Group 5 ||Beam 51||-|| 5837|| 14'579
||Beam 52 ||- ||5786 ||14'330
||Beam 53 ||- ||5856 ||14'268
||Beam 54 ||- ||6011 ||17'152
||Average group 5||- ||5286 ||5873 ||15'082|
Used in structural system including several components like a GLT beam based on 12 planks, solid timber grading is becoming more accurate by the system effect. Actually, the relationship between the explicate variable, X, and the continuous dependent variable, Y, is not affected by the system effect, but the residual error of the regression model is becoming small, like it appears on figure 3, showing the relationship between the ultrasound index (average speed of the constitutive plank) versus the MoE of the glued laminated timber beam . Effectively, the usual correlation coefficient between solid timber ultrasound speed and MoE is generally 0,5 c r2 < 0,65. Integrating the system effect, the industrial efficiency of the ultrasound grading allows an evaluation accuracy of more or less 5%, which is extremely significant in wood sciences, and directly applicable to timber industry.
Because attenuation is proportional to the wave frequency  and because the industrial applications have to be developed without additive coupling material, new
piezo-electric transducers, low frequencies have been set up. The reduction of the attenuation function have been solved by using low frequencies, in the range of 20 kHz, together with a high vibration energy obtained by using an excitation function similar to a condenser. The alimentation condenser is progressively loaded until a maximum of 700 V, and then drive the excitation of the piezo-electric cells. At that breakdown tension, the cells are vibrating in the X-axis. In order to have a directional emission, the transducers are built with including a brass mass at the rear of the transducer. Then mono-directional emission is obtained. In order to concentrate the energy between transducer and material specimen, the transducer is profiled following a conical form, in order to be like punctual at the head, for a small contact surface, coupled by a single 20 bars pressure.
Coupling and energy optimisation can then allow to detect the wave transit time in a very good condition, like it is shown on figure 4.
The final transducer applied for industrial measurements is schematically presented on figure 5. We can see eight piezo-electric cells compressed on the Xaxis (vibrating axis) and with the brass mass at the rear and the conical transducer head for the energy concentration effect.
Using this kind of new technologies, ultrasonic measurements can be driven by simple devices, like Sylvatest®, a portable unit, or by an automatic machine, like Sylvamatic, for continuous and automatic measurements in the industrial process.
The ultrasonic wood evaluation is fundamentally defined in and out of the proper axis of wood material. From the application point of view, the transverse measurement is basically interesting for tree analysis, like decay detection. On the longitudinal axis, the ultrasonic propagation method is effective for elasticity evaluation and becomes more and more a grading method working at any step of the industrial wood transformation.
In this paper, efficiency of log grading has been proved by comparison of final timber properties regarding an ultrasonic log classification. For this kind of application, the working frequency is quite low, in the range of 20 kHz, to facilitate coupling and to reduce signal attenuation.