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

Abstract

Table of contents
1 Introduction
2 TREE DETECTIONS
3 EFFICIENCY OF LOG GRADING
4 INDUSTRIAL ULTRASONIC MEASUREMENTS
5 Conclusion
6 References

1. INTRODUCTION

Wood is a natural raw material based on microstructural definitions given both its orthotropic anisotropy and its variability. The variability is strongly dependent of the quality of the grain and of the grain sequence. Physical appearance of the variability will be ring thickness average, density and transverse density profile. For industrial applications, like structural applications, or in a more general sense, timber construction, the microstructural basic variability will be associated to a macroscopical variability induced by singularities, like knots, cracks, local decay or even local dynamic compression failure.

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 [3] 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.

2. TREE DETECTIONS

The ultrasonic propagation is based on the vibration of the wood microstructure and fundamentally on the elastic properties of the propagation axis [6]. On the transversal axis, eq. (1) is giving the link between speed of sound and the radial elastic properties:
(1) V²_R = C_R/d = E_R/d V_R: radial ultrasonic speed
C_R: rigidity term of the radial axis
E_R: elasticity modulus of the radial axis wood density
d : wood density

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):

(2) Degradation ratio ={ ( V_r - V _m ) /V_r} 100 [%] V_r : referential ultrasonic speed
V _m measured ultrasonic speed

Because of the influence of the moisture content and temperature on the elastic wood properties, two constants, k_MC, k_T, 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.


Figure 1: General view of radial tree measurements using ultrasonic transducers for decay detection.

3. EFFICIENCY OF LOG GRADING

On the longitudinal axis, the ultrasonic speed of propagation is in dependency of the longitudinal rigidity term. Then, a good correlation between speed of ultrasound of the longitudinal axis and modulus of elasticity, MoE, of a timber beams is observed [4, 6].

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.


Figure 2: Ultrasonic speed measured on the planks derived from the three log quality samples
(group 1: weakest, group 5: highest).

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.

Table 1 Ultrasonic quality evaluation at the different steps logs, planks and beams, related to the final mechanical properties.

Group	Number	Log ultrasonic speed [m/s]	Dry plank ultrasonic speed [m/s]	MoE [N/mm2]
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 [7]. 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.

4. INDUSTRIAL ULTRASONIC MEASUREMENTS

One of the main problems limiting the transfer of the ultrasound method to industrial applications was historically the coupling between transducers and material specimen. For quite a long time, high frequency transducers were used, together with a coupling paste. Both coupling material and attenuation level were limiting the performance of the method, at least for industrial size material.

Because attenuation is proportional to the wave frequency [6] and because the industrial applications have to be developed without additive coupling material, new


Figure 3: Relationship between the ultrasonic speed (constitutive plank average [7]) and the final modulus of elasticity of glued laminated timber beams (sizes 120/300/600 mm, included 10 planks finger jointed).

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.


Figure 4: Loading, emission and reception time, of the piezo-electric transducers for ultrasonic wood applications.

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.


Figure 5: Ultrasonic transducers developed for a punctual coupling and with high energy for industrial wood specimen measurements.

5. CONCLUSIONS

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.

References

1. Lindgren, O., 1991: Medical CAT-Scanning-X-Ray Absorption coefficients; CT Numbers and their Relation to Wood Density. 8'h Int. Symp. on Nondestructive Testing of Wood. Sept. 23-25. Vancouver, WA, USA.
2. Madsen, B., 1994: Radiological Density Scanning. A portable gamma camera based on back-scatter tomography. 9th Int. Symp. on Nondestructive Testing of Wood. Sept. 22-24. Madison, Wl, USA.
3. Perstorper, M., 1994: Dynamic Modal Tests of timber evaluation according to the Euler and Timoshenko theories. 9th Int. Symp. on Nondestructive Testing of Wood. Sept.22-24. Madison, Wl, USA.
4. Sandoz, J. L., 1989: Grading of construction timber by ultrasound. Wood Sci. and Technology 23: 95-108.
5. Schwaldt, D.; Duke, J.; Marrone, M.; Jennings, Ch., 1994: Application of ultrasound nondestructive evaluation to grading pallets parts. 9th Int. Symp. on Nondestructive Testing of Wood. Sept.22-24. Madison, Wl, USA.
6. Bucur, V., 1995: Acoustics of wood. CRC Press. Inc.
7. Sandoz, J. L.; Rastogi, P.; Walgenwitz, M., 1994: Grading and Reliability of Glued Laminated Timber. Pacific Timber Eng. Conf., Gold Coast, Australia

Author:

Sandoz Jean Luc, Swiss Federal Institute of Technology DGC-IBOIS,
GCH" Ecublens CH-1015 Lausanne Switzerland