Mark:                                                    GB_000004

Author(s):                                            Dívos, Ferenc ; Sugiyama, Michelle K.

Title:                                                     Longitudinal stress waves for detecting the locahon and size of defects in wood and wood based composites

Source:                                                Erdészeti és Faipari Tudományos Közlemények, Erdészeti és Faipari Egyetem Sopron ; 1994-1995. év. 40-41 évfolyam. p.: 185 – 191, Egyetemi Jegyzetsokszorosító, Sopron, 1996.,

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Keywords: fast Fourier transformation, fault localization, nondestructive testing

Abstract

Stress wave techniques have long been employed in the non destructive testing fNDT) of wood and wood products. By introducing artificial defects in wood samples we have found that not only the presence but also the location of these defects can be determined simply by longitudinal stress wave analysis. Naturally occurring defects such as knots can be located in wooden samples in the same manner. In addition, a relationship was detected between defect size and the frequency spectra in particle board samples. However, the procedure was highly sensitive to impact energy, impact location, and impact direction. The method proved unsuitable ir. samples under one meter in length. This study indicates that through continued development of these techniques it may be possible to detect the presence, location, and size of defects in beams in-situ.

Defect-free (clear) spruce samples and particle board samples were tested under freely supported conditions. Saw-cut defects were introduced and systematically enlarged. At each stage, frequency spectra were obtained using a microphone or a piezofilm vibration sensor, a Fast Fourier Transform (FF1) Analyzer, and a glass ball to initiate the longitudinal stress wave. By comparing a board's clear frequency spectrum with its defect spectrum a peak due to the artificial defect was evident. Spruce samples with knots were also tested. Assuming the stress wave propagation speed to be constant in a particular beam, the position of the defect could be determined based on the length of the board and its first natural vibration frequency. In the particle board trials, a ratio of the area of the defect peak to the area of the first natural frequency peak correlated with the size of the artificial defect.