Grading using thermal image changes in vibration loading.

木材在弯曲时，包括裂纹或缺口，在曲柄尖端或缺口拐角处有应力集中。结本身并不总是应力集中的一部分。但是，节点中或其周围通常包含一个裂纹，并且该裂纹通常在低载荷下扩展到最终破坏。应力集中在裂纹尖端，其周围的应力-应变关系呈现滞后曲线。根据磁滞回线的损耗能量产生热量。可以通过重复加载来增加所产生的热量。产生热量的部分的位置可以通过热成像设备(红外热像仪)来检测。如果温升太小而无法察觉，则增加负载的重复次数可以得到清晰的图像。在磁滞能量损失较大的地方，例如在边缘节点和裂纹尖端，观察到较高的温升。为了将这种热像仪应用于木材的无损评价或分级，需要大行程高速加载的弯曲机充分升温，因为产生的热大部分在加载缓慢时迅速释放到大气中。热像仪还观察了指节和单板木材的粘合缺陷以及弯曲和压缩过程中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.