FR2820204A1 - Non-destructive monitoring of mechanical properties of manufactured panels uses measurement of vibration following single impact while supported at four points - Google Patents

Abstract

The procedure consists of placing the panel to be tested on four rigid or support point supports (3a, b, c, d). The panel is weighed and its dimensions are measured; a sensor (4) is placed in contact with the center of its upper surface, and a single impact is delivered to its surface either adjacent to the sensor or on the opposite side, causing the panel to vibrate. The signal received by the sensor is analysed, e.g. by specially-developed digital software to extract various vibration parameters such as frequencies and damping coefficients, and the elastic properties of the panel are evaluated from these.

Description

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TECHNICAL FIELD The subject of the present invention is a method intended for the non-destructive evaluation of the mechanical properties and for the non-destructive on-line control of the production of panels, such as composite panels, particle boards, fiber boards, laminate panels, veneers, etc. The panels concerned by the present invention are obtained from various methods, in particular reconstitution methods, and can be intended for structural or non-structural applications.

Technical basis The mechanical properties of panels of the type described in the first paragraph are important in the context of their use, in particular when they are used as constructive elements in structures. Mechanical properties must be taken into account when designing and calculating the structure. The measurement of these mechanical properties is therefore of great importance.

Currently, the standard procedures for evaluating the mechanical properties of panels are strictly defined by standards. In Europe, in the case of plywood panels for example, test pieces are cut from the panel and tested according to the procedure defined by standard EN310. This solution has the disadvantage of being destructive, the panel having to be cut before carrying out the tests, and cannot therefore be applied to all the panels produced every day within the framework of an industrial production.

The natural frequencies of vibration of a panel depend on its shape, its mass, its volume and also on its mechanical properties (elastic moduli and shear moduli). The mechanical properties of panels can therefore be obtained by measuring the natural frequencies of vibration, the dimensions and the mass of the elements, provided that the relationship between the mechanical properties and the variables mentioned above is known. This relationship depends on the boundary conditions of the panel.

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The use of such a method is therefore only possible if the boundary conditions of the panel are strictly controlled, which can represent a real problem in an industrial environment. One of the simplest solutions consists, for example, in placing the panel in conditions of free vibration. Even in this case, the speed of the method depends on the technological configuration adopted for the test device. A poor technological choice can result in a substantial increase in measurement time, due to the obligation to perform the measurement in several stages.

indicateur de la qualité d'un panneau. Le suivi en continu, sur la chaîne de production, des fréquences propres du signal ou d'autres paramètres permet de faire du contrôle non destructif de la production et d'affiner en temps réel les réglages machine pour conserver un réglage optimal du procédé de fabrication. More broadly, the whole vibration signal can be used as a

indicator of the quality of a sign. Continuous monitoring, on the production line, of the natural frequencies of the signal or other parameters allows non-destructive control of production and fine-tuning of machine settings in real time to maintain optimal adjustment of the manufacturing process .

OBJECTS OF THE INVENTION The object of the present invention is to provide a method of rapid non-destructive evaluation of the mechanical properties of panels of the type described in the first paragraph.

propres des panneaux. Un autre objet est de fournir une méthode permettant d'avoir un contrôle non destructif de la qualité de panneaux du type décrit dans le premier paragraphe 1-p sur une ligne de production industrielle. Cet objet peut être réalisé en considérant le signal vibratoire dans son ensemble. This object can be achieved using a vibratory method to measure frequencies

clean panels. Another object is to provide a method enabling non-destructive quality control of panels of the type described in the first paragraph 1-p on an industrial production line. This object can be achieved by considering the vibration signal as a whole.

The invention According to a first aspect of the invention, one of the most general embodiments is a method of the type defined above, characterized in that:
1-the panel is positioned on one or more supports,
2-the panel is weighed and its dimensions are measured,

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3-a sensor is placed in contact with the surface of the panel, 4-an impact is given to the surface of the panel to make it vibrate,
5-the vibrations created by the impact are measured by the sensor, and
6-the signal obtained in step 5 is analyzed to extract particular vibration modes and obtain an evaluation of the mechanical quality of the panel.

Steps 1 to 3 can be reversed.

According to the most general variants of the invention, the supports can have any shape and rigidity. For example, rigid supports made of steel balls, rotating or not around their axes, are perfectly suited to the process, because of their low cost and the ease of use which characterizes them on an industrial site. Another example is flexible rubber supports, whose effect on the vibration of the panel could be overlooked. Support means a physical means comprising an area of contact with the panel. Let us cite as nonlimiting examples, a linear support formed by a roller, an embedding ... One-off support means a physical means comprising a local contact area with the panel, which corresponds to a negligible part of a surface of the panel . The panel is preferably placed on these supports, without the application of external forces other than gravity, but different alternatives are possible for the positioning of the supports, for example at the edges, in the corners or above the panel.

The sensor can be an accelerometer or a microphone. The sensor must have sufficient sensitivity, so as to deliver a good quality signal which can be analyzed later. It can be positioned anywhere on the surface of the panel. Wax, glue, adhesive tape or any other product or system can be used to ensure perfect transmission of vibrations from the panel to the sensor.

The impact given to the surface of the panel must be of sufficient intensity to create vibrations whose amplitude is sufficient to be measured by a sensor of the type described.

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dans le paragraphe précédent L'impact peut être créé par quelque moyen que ce soit, de façon manuelle ou automatique sur la face supérieure ou inférieure du panneau.

in the previous paragraph The impact can be created by any means whatsoever, manually or automatically on the upper or lower side of the panel.

The appended figures illustrate the invention: FIG. 1 represents the deformations undergone by a panel (1) vibrating according to the vibration modes 02 and 20 and the position of the nodal lines (2a, b) of these modes (points of the panel where the deformation is zero).

FIG. 2 illustrates the deformations undergone by a panel (1) vibrating according to 12 different vibration modes.

FIG. 3 represents a measurement bench based on a particularly advantageous embodiment of the first aspect of the invention in which a panel (1) is placed on four rigid point supports (3a, b, c, d) located at the intersection of the nodal lines of modes 02 and 20 (2a, b), with 1 defined as the width of the panel and L as the length of the panel, and in which a sensor (4) is placed in contact with the surface of the panel (1 ) in the center of it.

According to a particularly advantageous embodiment of the first aspect of the invention, the method is characterized in that: 1-the panel (1) is positioned on four rigid point supports (3a, b, c, d) placed along the Figure 3, or on flexible supports, 2-the panel (1) is weighed and its dimensions are measured, 3-a sensor (4) is placed in contact with the surface of the panel (1) in its center, 4-a impact is given to the surface of the panel (1), either near the sensor or preferably in the center of the surface opposite to that on which the sensor is positioned, to make it vibrate. As stated previously for the more general variants of the first aspect of the invention, steps 1 to 3 can be reversed. The following steps are identical to those of the various more general variants of the first aspect of the invention mentioned above. The vibrations created by the impact are measured by the sensor (4). The signal obtained is analyzed, for example by digital processing carried out by specially developed software, to extract different vibration parameters. Let us cite by way of nonlimiting examples of

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 vibration parameters; the natural frequencies, the damping coefficients of particular modes of vibration or of the signal, etc. According to a particular embodiment of this variant, only certain natural frequencies of the panel are considered in order to obtain a non-destructive evaluation of the mechanical properties El (modulus of elasticity in the machine direction) and E2 (modulus of elasticity in the transverse direction) of the panel (1).

où p est la densité du panneau (1), h l'épaisseur, L la longueur et 1 la largeur. This particular embodiment is distinguished by a simple signal processing, making this particular embodiment perfectly suited to the automated processing necessary for industrialization. Indeed, in the case of an orthotropic panel in free vibrations (1), the Rayleigh method provides a good estimate of the relationship between the mechanical properties, the different natural frequencies f (i, j) of vibration, the dimensions and the mass of the panel (1). The natural frequencies corresponding to the modes of vibration in pure bending in the machine direction (f2, 0) and in the transverse direction (fro, 2) depend respectively on the rigidities Dal 1 and D22 and consequently on the modules El and E2 respectively (see formula 1).

where p is the density of the panel (1), h the thickness, L the length and 1 the width.

Thus, the measurement of the natural frequencies f (2, 0) and f (0, 2) makes it possible in particular to obtain an estimate of the modulus of elasticity E1 in the machine direction and of the modulus of elasticity E2 in the transverse direction. The modes of vibration in pure bending corresponding to the frequencies f (2, 0) and f (0, 2) result in a deformation of the panel (1) illustrated by FIG. 1 appended. The position of the nodal lines (2a) (where the deformation is zero) of mode 20 corresponds to the lines of equation y = L / 4. The nodal lines (2b) of mode 02 are located at x = l / 4.

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 Consequently, if rigid point supports (3ab, cd) are positioned at the intersection of the nodal lines of modes 02 and 20 (2a, b), they will not prevent the deformations of the panel (1) induced by these modes and the panel (1) can always vibrate freely according to modes 02 and 20. In the same way, flexible supports, whatever their position, will not prevent the deformations induced by these modes, provided that their rigidity is fairly low so that their effect on the vibration of the panel (1) is negligible. In addition, if the sensor (4) is placed in the center of the panel (1) at the intersection of the diagonals and if the impact is given near the center of the panel, the two modes 20 and 02 are favored (the center of the panel being for these two modes a belly of vibration; zone of maximum deformation), while the first intermediate modes of vibration are not detected, the center of the panel (1) being for these intermediate modes (modes 11, 21, .. .) a nodal point (figure 2 attached). This particular embodiment therefore avoids having to identify on the frequency spectrum the order of appearance of the different vibration modes, the order of appearance depending on the properties and dimensions of each panel. Indeed in these particular conditions, whatever the type and dimensions of the panel, the first two frequencies appearing on the frequency spectrum are always the frequencies corresponding to modes 20 and 02, the first intermediate modes being as described previously annihilated by the conditions of measurement. This particular embodiment therefore allows a simple and above all automatable analysis of the signal.

The validity of this particular variant of the invention has been demonstrated in particular on wood-based panels by comparing the results obtained on the same panels using, firstly this variant of the invention, and secondly the conventional destructive testing methods. The results obtained by the two methods are very close: the coefficients of determination are of the order of 0. 84-0. 89 for the correlations between the elastic modules El and E2 obtained by the variant of the invention and the modules El and E2 obtained by carrying out the tests according to the European standard EN310.

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 The process of the invention has several advantages. It can be used to obtain a non-destructive estimate of the mechanical properties of panels of the type described in the first paragraph, but also to do online quality control. The equipment required to perform the measurements is very simple and inexpensive compared to other measurement equipment. The particularly advantageous variant of the invention mentioned above has the further advantage of requiring only a very simple support structure, only one impact, and only a very simple signal processing procedure. The process can thus be easily automated and used in the factory, whether at the end of the production line or anywhere else.

As specified in the first paragraph, the invention can be used on any type of panel, whatever its dimensions. For example, composite panels, laminate panels, OSB (Oriented StrandBoard) panels, synthetic or natural fiber panels, particle boards, etc., can be tested using the invention.

A second aspect of the invention is a kit or an assembly for carrying out the method of the invention. This kit or this assembly is characterized by the fact that it includes one or more supports to support the panel to be tested, a sensor, and optionally an integrated weighing system and impactor as well as computer software allowing the extraction. different vibration parameters necessary for the non-destructive evaluation of the mechanical properties of the panel and for online monitoring of production. This kit can also include instructions for measuring the properties as described above in the first aspect of the invention. The assembly is further characterized by the way it is assembled, which is described above for the first aspect of the invention.

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In order to better understand the invention, several examples of implementation are given by way of illustration. Each of them supports the fact that the principles on which the invention is based work.

3 sont 2500*1250*40 mm. Il est envisagé de mesurer les propriétés des panneaux dans l'usine, sur chaque panneau, juste après la phase de pressage. Chaque panneau (1) est positionné manuellement sur le banc d'essai, composé de quatre billes d'acier servant d'appuis ponctuels rigides (3a, b, c, d) et disposées comme décrit par la figure 3. Le panneau est pesé et ses dimensions sont mesurées. Un impact est donné par l'opérateur au centre du panneau, près de l'accéléromètre (4), collé précédemment à la surface du panneau en utilisant de la cire. Le signal récupéré par l'accéléromètre (4) est enregistré et traité en utilisant un logiciel informatique permettant d'extraire les fréquences propres du panneau (1) et de calculer les propriétés mécaniques El et E2. Le panneau (1) est alors retiré du banc d'essai, et une marque y est apposée pour assurer la traçabilité et l'information du client concernant les propriétés du panneau. Example 1: The mechanical properties E1 and E2 of okoumé plywood panels must be measured to ensure the marketing of the panels. The dimensions of the panels

3 are 2500 * 1250 * 40 mm. It is planned to measure the properties of the panels in the factory, on each panel, just after the pressing phase. Each panel (1) is positioned manually on the test bench, composed of four steel balls serving as rigid point supports (3a, b, c, d) and arranged as described in Figure 3. The panel is weighed and its dimensions are measured. An impact is given by the operator in the center of the panel, near the accelerometer (4), previously glued to the surface of the panel using wax. The signal recovered by the accelerometer (4) is recorded and processed using computer software allowing the natural frequencies of the panel (1) to be extracted and the mechanical properties E1 and E2 to be calculated. The panel (1) is then removed from the test bench, and a mark is affixed to it to ensure traceability and information for the client concerning the properties of the panel.

3 panneaux sont 1250*1250*22 mm3. Il est envisagé de mesurer les propriétés des panneaux dans l'usine, sur chaque panneau, juste après la phase de pressage. Chaque panneau (1) est positionné manuellement sur le banc d'essai, composé de quatre ressorts en caoutchouc servant d'appuis souples et disposés dans chacun des coins du panneau. Ses dimensions sont mesurées par l'opérateur, son poids étant enregistré par un système de pesée intégré. L'impacteur intégré sollicite le panneau au centre de la surface inférieure sous l'accéléromètre (4) collé précédemment au centre de la surface supérieure du panneau (1) Example 2: The elastic properties El and E2 of OSB panels (oriented strandboard) based on maritime pine must be measured to ensure their marketing. The dimensions of

3 panels are 1250 * 1250 * 22 mm3. It is planned to measure the properties of the panels in the factory, on each panel, just after the pressing phase. Each panel (1) is positioned manually on the test bench, composed of four rubber springs serving as flexible supports and arranged in each of the corners of the panel. Its dimensions are measured by the operator, its weight being recorded by an integrated weighing system. The integrated impactor biases the panel in the center of the lower surface under the accelerometer (4) previously glued to the center of the upper surface of the panel (1)

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 using wax. The signal recovered by the accelerometer (4) is recorded and processed using computer software allowing the natural frequencies of the panel (1) to be extracted and the mechanical properties E1 and E2 to be calculated. The panel (1) is then removed from the test bench, and a mark is affixed to it to ensure traceability and information for the client concerning the properties of the panel.

Example 3: We want to do quality control of MDF (Medium Density Fiberboard) panels at the output of the production line in order to control the optimal adjustment of the manufacturing process. The dimensions of the panels are 2800 * 1850 * 12 mm3. A panel (1) is taken every 10 panels, and positioned manually on the test bench, composed of four rigid point supports (3a, b, c, d) arranged as described in Figure 3. Its dimensions are measured by the operator, his weight being recorded by an integrated weighing system. The integrated impactor biases the panel at a point near the accelerometer (4) previously glued to the center of the panel (1) using wax.

The signal recovered by the accelerometer (4) is recorded and processed using computer software making it possible to extract the damping coefficients of the vibration modes (02 and 20). The panel (1) is then removed from the test bench, and the information obtained is compared with the data previously measured on other panels in order to verify whether the best production conditions are maintained.

Claims (3)

CLAIMS 1-A non-destructive evaluation method of the elasticity module E1 in the machine direction and of the elasticity module E2 in the transverse direction of panels and of non-destructive control of the quality of the panels, characterized in that the panel (1) is positioned on four rigid point supports (3a, b, c, d) placed according to Figure 3, or on flexible supports; that the panel (1) is weighed and its dimensions are measured; that a sensor (4) is placed in contact with the surface of the panel (1) at its center; and then that a single impact is given to the surface of the panel (1), either near the sensor or preferably in the center of the surface opposite to that on which the sensor is positioned, to make it vibrate. 2-The method according to claim 1, characterized in that steps 1 to 3 can be reversed. 3-The method according to any one of the preceding claims, characterized in that the signal measured by the sensor (4) is analyzed, for example by digital processing carried out by specially developed software, to extract various vibration parameters such as for example the natural frequencies f (2, 0) and f (0.2) of the panel (1) and the damping coefficients of these modes, and provide a non-destructive evaluation of the elastic properties El and E2 of the panel (1) and allow online monitoring of production.