Microscopic processes of shearing fracture of old wood, examined using the acoustic emission technique

We examined the process of microscopic fracturing peculiar to old wood, based on the generation characteristics of acoustic emission (AE) events and fracture surface analysis. The shearing tests of old wood obtained from construction-derived lumber and new wood within 3 years after lumbering were performed in accordance with the Japanese Industrial Standards (JIS Z 2401-1994). The species of wood used in this study was Japanese red pine. The old wood had been used as a beam in a building for 270 years. The number of the occurrences of AEs at low load levels was larger in the old wood than in the new wood. As a result of analyzing the AE amplitude distributions, we found that the period in which AEs with small amplitudes were frequently generated was longer in the old wood than in the new wood. Also, the fracture surfaces after the final rupture under scanning electron microscope showed more uneven and complicated surfaces in the old wood. Based on the above findings, we presume that during the shearing test the old wood underwent a relatively long and stable progress of microcracking before the final fracture.