Use of ulttrasonic method in association with the imagistic method for detection of structural defects in gluing of the plywood

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1 Use of ulttrasonic method in association with the imagistic method for detection of structural defects in gluing of the plywood Iancu, L. 1, Grimberg, R. 2, Mitişor, Al. 3, Lupu A. 2, Iancu, C. 1 ABSTRACT This paper shows the results obtained from the samples of plywood, efects simulated in various glue lines. The main purpose is the detection of gluing defects by nondestructive method, using ultrasounds in association with the imagistic method for the form reproduction by direct measurement of flying times. Based on these measurements, we have drawn up a map of flying times, which have been processed through the numeric algorithms, aiming to the maximization of Baysian posterior probabilities. On the basis of the matrix of measured times, we are able to express graphically the distribution of defects in the structure of studied boards. The results demonstrate a good reproductibility of defects. At the end, we have established the true correlation between some nondestructive physical properties and destructive mechanical properties. INTRODUCTION The plywood is produced by 90 cross gluing of veneer sheets by the principle that, on either side of central plane of symmetry, there will be the same number of layers with the grain equidistant, and in the same direction, so that central plane of symmetry shall coincide with the plane of symmetry of core veneer layer. Compared with the solid wood, the wood composites are characterized by alow grade of anisotropy resulted from the densification of veneers by boot pressing, and the key role is played by the adhesive, specially at the glue-wood interface. The adhesive penetrates the wood structure through the pores and the intercellular spaces, keeping after polymerization the wood compressed, (Curtu et. al 1983). DESCRIPTION OF METHOD This method make use of observation that, when determining ultrasound propagation time by direct transmission process, despite the presence of air gap in defective area should reflect completely the incidental ultrasonic beam they arrive, however, at the back of defective area, due to a diffraction phenomenon characteristic for all the waves (Caprtti et. al. 1994). As a result of "apparent bypass" of discontinuity by the ultrasonic beam the length of path increases and consequently increases also the propagation time named, by specialty literature, "flying time". The flying time is measured in various points on the specimens, resulting the flying time map as a matrix, where the position of matrix element represents the coordinates of measurement time and the numeric value of this element represents the value of flying time in respective point. EQUIPMENT AND PRINCIPLE The experimental determination is conducted within the laboratory of INCD-IFT, Iasi, using the specific equipment for ultrasound tests. The equipment we used consists of an ultrasonic apparatus, with 60 khz transducers and a PC Pentium 250. Our transducers are equipped with wave guides to make sure we have a pointlike source for emission-reception. 1 1 Drd.Engineer. National Institute of Wood, 7, Fabrica de Glucoză Street, Bucharest, Romania 2, office@inl.ro 2 Prof. Dr. Engineer. National Institute of Tehnical Physics, 47, D. Mangeron Ave Street, 6600, Iaşi, Romania 3 Prof. Dr.Engineer. Transilvania University, 20, Eroilor Street, Braşov, 2200, Romania

2 OPERATING METHOD The experiments have been conducted using 250x250 mm samples with a structure of 3, 5, 7, 9, and 15 layers of beech. The gluing is carried out using ureo-formaldehyde resins and other adhesives, such as aracet, phenolic films and polyurethane. The thicknesses of boards are between 3, 10 and 15 mm. The testings have been carried out by simmulation of various defects (see fig.1). EXPERIMENTAL RESULTS From the analysis of experimental data, it results that above nondestructive method for detention of gluing faults in the plywood beech species, has favorable results in: -identifications of defects in board structure subjected to measurements, as shown in fig. 2 and 3; -scanning of he whole surface, which allows for searching of gluing condition throughout the board structure, as shown on fig.2. The light areas are zones with a good gluing from the point of new of physical-mechanical strength; the grey areas are those zones with weaker physical-mechanial strengths, and the white areas are zones with gluing faults, due to glue line inerruption, glue insufficiency and gaps in veneer joints (Iancu et. al 1999). The experimental results have been completed with imagistic methods depicting the tested area and allowing for separation of geometrical indications from real indications of defects. For an approximative reproduction of defects in the board structure we have used one of imagistic methods for form reconstitution, using the process of maximization in Baysian posterior probability, which is equivalent with bidimensional deconvolution extended to the mattrix of measurement points. At the end, we have established the correlation between some nondestructive physical properties (ultrasound velocity) and destructive mechanical properties (strength shear, dinamic shear modulus of elasticity end density of plywood). The ultrasound testing method is more sensible than destructive method for the testing of shear strength (the most used method for this purpose). The used method is able to detect unglued areas with miniml size of 8 mm for plywood thicknesses between 3, 2 and 12 mm at the same processing frequency with a precision of ± 5%. At the end has been estblished same corelation between nondestructive and destructive growth, see resulte from Tab. 1, 2, 3, and fig.4, 5 and 6. It is specified that density was calclated based on sizes and weigh of tested specimens. The shearing test was accomplished according with CE Transversal shear modulus was calculated with rule (Făcăoaru, 1965): G T = 2 γ v T g Where: v T -transversal wave velocity γ - specifc weight g - gravity constant,

3 The relative statistical parameter of density (ρ), strenght shear (τ), end velocity ultrasounds (v US ) Table 1 The propriety Parameter P l y w o o d 3 layerrs 5 layerrs 7 layerrs 9 layerrs ρ (kg/mc) X S v US m/s τ MPa Cv(%) X S Cv(%) X S Cv(%) The corelation coefficients between ultrasuond velocity (v US ) end density (ρ), strenght shear of gluing end dinamic shear modulus (G T ) Table 2 The corelation P l y w o o d 3 layerrs 5 layerrs 7 layerrs 9 layerrs v US =f(ρ a ) v US =f(g tus ) v US =f(τ ) v US =f(g ) 0, The regression line equation, the basic value deviation (S) from the regresion line and the equation of spread area values for an accuracy of 95% for the relations between ultrasuond velocity end density (ρ), strenght shear of gluing (τ) end dinamic shear modulus (G T ) Table 3 The plywood Eqaution S P=5% (α=1.96) 3,5, 7 and 9 v US = ρ ± ± layerrs 3,5,7 şi 9 v US = G T ± ± layerrs 3 layerrs v US = τ ± ± layerrs v US = τ ± ± layerrs v US = τ ± ± layerrs V US = ,4629 τ ± 14,550 ± 8,5186 3,5,7 and 9 layerrs v US = 1202,277+55,41958 τ ± 5,3472 ±49,6801

4 Fig. 1. Shape of specimens and schematic presentation of defects in a board of 9 beech veneer layers, q board =12, 21 mm Fig. 2 Graphic reprezentation on flying times for plywood of veneer layerrs

5 Fig. 3 Map of flying as a result of signal processing Fig 4. Relationship between the ultrasonic velocity and density Fig. 5 Relationship between the ultrasonic velocity and strenght shear of plywood

6 Fig. 6 Relationship between the ultrasonic velocity and dinamic shear modulus REFERENCES Capretti,S., Del Senno M., Facaoaru, I., Lugnani, C Ultrasonic Waves Attenuation in Quantitative Control of Glulam Beams, Proc. of the 6 th European Conference on Nondestructive Testing, oct., Nice, pp Curtu, I., end Ghelmeziu,N Mecanica lemnului şi a materialelor pe bază de lemn.editura Tehnică, Bucureşti, pag.271. Dimache, M., Capretti,S., Del Seno,M., and Făcăoaru, I Validation of Theoretical Approach for the Detection of Delamination in Glued Lamined Beams. First European Symposium On Nondestructive Evaluation of Wood Sopron Hungary, pp.250. Grimberg,R., Radu, E.,Lupu,A., Iancu, L Ultrasonic control ov layered timber unsoldering. Proceedings 2 th International Conference Computer Methods and Inverse Problems in Nondestructive Testing and Diagnostics, Minsk, Belarus, pp Iancu, L., Grimberg, R..,Mitişor, Al., Lupu, A.,and Iancu,C Role of crystalin features of pulp expused to nondestructive forces of the plywood. Twelfth International Symposium on Cellulose Chemistry and Technology. Iasi, Romania. september 21-23, pp. 61.