Grading using thermal image changes in vibration loading.

在弯曲过程中，包括裂纹或切口的木材在曲柄尖端或切口拐角处具有应力集中。结本身并不总是应力集中的一部分。但一个结往往包括一个裂纹在它或它周围，裂纹往往延伸到最终失败，在低负荷。裂纹尖端应力集中，其周围的应力应变关系呈现滞后回线。根据磁滞回线的损失能量产生热量。通过重复加载，可以增加所产生的热量。热产生部分的位置可以通过热图像设备（红外热成像）来检测。如果温度上升太小而无法检测到，则增加负载重复将给出清晰的图像。在滞后能量损失大的地方，例如，在边缘节点和裂纹尖端，观察到高的温升。为了将这种热成像技术应用于木材的无损评价或分级，大行程高速加载的弯曲机必须充分升温，因为在缓慢加载时，产生的大部分热量会迅速释放到大气中。指接和单板层积材的粘结缺陷用热像法观察了单板层积材在弯曲和压缩时金属连接件的应力分布。

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.