·Table of Contents
Quantification of Defects in the wood by Use of ultrasonics in Association with Imagistic Method
Lelioara Iancu, Cristian Iancu
National Institute of Wood, Bucharest ,7 Street Fabrica de Glucoza, Sect.2, Romania
tel. 40-1-242.06.40, fax 40-1-242.35.50
Raimod Grimberg, Adrian Lupu
National Institute of Technical Physics, Iasi , 47, Street D. Mangeron Ave, Romania, cod 6600
This paper shows the results obtained from the samples of wood.The main purpose is the detection 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 woods. The results demonstrate a good reproductibility of defects.
The determination on the elastic properties of a material by the mesurement of the velocity of ultrasonic in material is a long established science. Its application to the propagation of ultrasonic waves in anisotropy solid, having three mutually perpendicular axes of symmetry.The longitudinal axis of the tree is allong the axis of the trunk or stem of the tree . The radial axis of symmetry follws a line from pith to bark of the tree. The tangential axis of symmetry is locally defined as the direction perpendicular to the radial axis and the longitudinal axis of the tree.
Presence of cellulose inside the structure of wood gives it piezoelectric features. Position of cellulose crystals inside the mass structure of wood is very complex. Wood is thought as a system which containes numberless crystals , having an any possible way orientation-these are known and called by speciality literature, as a piezoelectric pattern.
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. 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 150 kHz transducers and a PC Pentium 250. Our transducers are equipped with wave guides to make sure we have a pointlike source for emussion-reception.
The experiments have been conducted using 350x150 mm samples of solid wood of beech, with thicknesses 35mm and moisture 14%.
From the analysis of experimental data, it results that above nondestructive method for detention of faults in the wood beech species, has favorable results in:
Fig 1: Graphic reprezentation of flying times for a solid wood,
L = 350 mm, l = 150 mm, q= 35 mm.
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 .
- identifications of defects in structure subjected to measurements (see fig. 1);
- scanning of he whole surface, as shown on fig.2.The light areas are zones with a good structure 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 faults (suface of knot).
For an approximative reproduction of defects in the structure we have used one of imagistic methods for form reconstitution, using the process of maximization in Baysian posterior probability, which is equivalent with bi-dimensional deconvolution extended to the mattrix of measurement points.
At the end , we have established the correlation between some nondestructive physical properties of solid wood (ultrasound velocity) and destructive mechanical properties (static modules of elastcity and density of wood).
Fig 2: Map of flying times of signal processing surface containing a knot
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