Delamination and decay are common structural defects in old glued laminated timber (glulam) buildings, which, if left undetected, could cause severe structural damage. This paper presents a new damage detection method for glulam inspection based on moment analysis and wavelet transform (WT) of impact acoustic signals. Acoustic signals were collected from a glulam arch section removed from service through impact testing at various locations. The presence and positions of internal defects were preliminarily determined by applying time centroid and frequency centroid of the first moment. Acoustic signals were then decomposed by wavelet packet transform (WPT) and the energy of the sub-bands was calculated as characteristics of the response signals. The sub-bands of 0–375 Hz and 375–750 Hz were identified as the most discriminative features that are associated with decay and delamination and therefore are indicative of the presence of delamination or decay defects. A defect diagnosis algorithm was tested for its ability to identify internal decay and delamination in glulam. The results show that depth of delamination in a glulam member can be determined with reasonable accuracy.
International Nondestructive Testing and Evaluation of Wood Symposium
September 24-27, 2013, Madison, Wisconsin, USA
The purpose of this study was to explore the possibilities of using existing nondestructive evaluation (NDE) methods to assess delamination and decay of glulam structures. A glulam arch removed from a research building after more than 75-year service was used as a test specimen. The glulam arch section was tested using stress wave timing, ultrasonic wave propagation, and resistance microdrilling methods at a series of locations. The arch was subsequently cut open for visual inspection and small compression and shear samples were obtained for strength testing. It was found that wave propagation times or wave velocities measured across the laminations were good indicators of internal decay. Stress wave timing and ultrasonic propagation methods were able to detect moderate to large delamination, but not micro-delamination. Resistance micro-drilling was found not effective in detecting delamination. Further research is planned to evaluate the possibility of using pulse-echo method to detect internal delamination of glulam members. Key words: glued laminated timber (glulam), stress wave, ultrasonic wave, resistance micro-drilling, strength, modulus of elasticity.