BENDING CREEP-BEHAVIOR AND ACOUSTIC-EMISS ION CHARACTERISTICS OF WOODUNDER CHANGING MOISTURE CONDITIONS

This paper describes the relationships between the bending creep behav iors and the acoustic emission (AE) characteristics of wood under chan ging moisture conditions. The bending creep tests were conducted under air-dry, moisture-regain, high moisture, and moisture-loss conditions at constant stresses of 20, 35, 50, 65, and 75% of the ultimate short -time strength, and at the same time AEs were measured during the cree p tests. AE generation was observed at stress levels above 50% under t he air-dry condition. AEs were generated at certain times, but they we re not generated at other times during the creep tests. For the moistu re-regain condition. AE generation was observed at stress levels above 50%, but few AEs were generated with increased creep strain probably due to the loosening in the non-crystalline region by moisture sorptio n. However, when creep of a specimen was accompanied with a fracture, many AEs including high-amplitude waves (over 80 dB) were generated su ddenly from just before creep fracture. Under the high moisture condit ion, AEs were generated in the same manner as in the air-dry condition , but AE generation was observed at stress levels above 35%. In contra st with the moisture-regain condition, almost no high-amplitude AEs we re recognized even for the specimens that failed. For the moisture-los s condition, AE generation was observed at stress levels above 20%. At a stress level of 65%, AEs were generated at frequent intervals durin g the creep test, but the AE event count decreased with elapsed time, and creep fracture did not occur. The AEs did not have such high-ampli tude waves as those for the moisture-regain condition. Under the moist ure-loss condition, local stress concentration could occur as the non- recovery creep strain was being accumulated, and AEs were considered t o be generated during the creep process when strain energy was release d by the stress concentration. It was found that creep strain was not related closely to the cumulative AE event counts for all of the moist ure conditions.