Grading using thermal image changes in vibration loading.

点击翻译按钮获取中文摘要

Lumber in bending including a crack or a notch has a stress concentration at the crank tip or the notch comer. A knot itself is not always a portion of stress concentration. But a knot often includes a crack in it or around it and the crack often extends to ultimate failure at low load. Stress concentrates at a crack tip and the stress-strain relationship around it shows a hysteresis loop. Heat is generated according to the loss energy of the hysteresis loop. The amount of the generated heat can be increased by repetition of loading. The location of the heat generated portion can be detected by a thermal image equipment (infrared thermography). If the rise of the temperature is too small to detect, increase of repetition of load will give a clear image. High temperature rise was observed where the hysteresis energy loss is large, for example, at edge knots and crack tips. To apply this thermography to practical use for non-destructive evaluation or grading of lumber, bending machine with large stroke and high speed loading is necessary to rise temperature adequately because most of the generated heat is rapidly release to atmosphere when loading slowly.Defects of adhesion in finger joint and LVL (laminated veneer lumber), and stress distribution in the metal connecter in bending and compression were also abserved thermographically.